High-speed vehicular environments require optical systems capable of joint sensing, positioning, and communication (JSPC) without mechanical tracking. Existing optical and integrated sensing-communication approaches often rely on point-source emitters or camera-based receivers, limiting spatial coverage and update rate under highway dynamics. This work introduces a new class of tracking-free optical JSPC systems that combine structured line-laser illumination with modulating retroreflector (MRR) arrays on vehicles. Two orthogonal line lasers perform synchronized longitudinal and transverse scanning to provide continuous, wide-area coverage across the roadway. A coverage-driven analytical framework models the coupling between beam divergence, scan geometry, and dwell-time allocation, enabling joint evaluation of sensing reliability and communication quality. An optimization scheme is developed to adapt scanning and divergence parameters for uniform coverage and power efficiency. Simulation results demonstrate significant improvements in spatial coverage uniformity, link stability, and reliability within a fixed scan period. These results establish a practical pathway toward scalable, turbulence-resilient optical architectures for next-generation vehicular JSPC networks.

The evolution of communication network architectures is steering towards more sustainable, flexible, and lightweight designs, particularly with the advent of sixth-generation (6G) mobile communications. Spectrum-rich optical integrated systems are anticipated to play a crucial role in this transformation, offering significant advantages such as high data rates, reduced interference, and improved energy efficiency. This paper introduces and experimentally demonstrates a novel optical integrated communication, sensing, and power transfer (O-ICSPT) system. The proposed system integrates optical wireless communication, sensing, and wireless power transfer into a multifunctional framework, addressing the limitations of existing systems in terms of flexibility and resource utilization. The experimental setup investigates the effects of bias current, peak-to-peak voltage, and light source wavelength on the performance of each functional module. Experimental results indicate that the O-ICSPT system achieves a maximum data rate of approximately 632.58 Mbps, a best ranging root mean square error approaching 0 m, and a peak energy harvesting capability of about 10.02 mW. These findings underscore the potential of the O-ICSPT system in future 6G integrated communication networks, marking the first experimental validation of such a system.

Integrated satellite–terrestrial networks (ISTNs) are envisioned as a key component of 6G to support global connectivity for massive Internet of Things (IoT) devices. However, while terminals operate under strict energy and computational constraints and face additional challenges from satellite dynamics, current designs rarely integrate orbital information with device energy states in a unified system design, leaving significant room for improvement. Motivated by this, we propose a joint trajectory- and energy-aware framework for ISTNs operating in dynamic environments, supported by a low-complexity algorithm tailored for energy-constrained IoT devices. Specifically, we first develop a system model centered on a day–night separated hybrid energy-harvesting (DNEH) mechanism, where each device harvests light energy during the daytime and radio frequency (RF) energy from the gateway (GW) at night. Meanwhile, the GW broadcasts simplified orbital information to assist device operations, thereby forming an integrated communication–energy transfer framework that enhances system sustainability. Analytical characterizations are subsequently derived for the joint coverage probability, and simulations capture orbital dynamics such as the real-time distance variations. Building on the semi-analytical model, the proposed drift-regularized per-frame (DRPF) algorithm determines transmission opportunities that optimize the average Age of Information (AoI) by capturing the coupling between trajectory dynamics and energy availability. Numerical evaluations confirm the theoretical advantages of the proposed framework, showing substantial performance gains over the Immediate and Minimum Distance transmission baselines. Moreover, the optimized TD-SWIPT configuration and transmission scheduling translate these findings into actionable design guidelines for energy-constrained ISTNs.

This paper proposes a novel indoor optical wireless positioning (IOWP) framework that aims to enhance localization precision and robustness through an advanced machine learning (ML)-driven fusion technique. Unlike traditional single-model approaches, the proposed framework uses received signal strength (RSS) data to intelligently combine multiple lightweight ML algorithms, including K-Nearest Neighbors (KNN), Random Forest (RF), and Gaussian Process Regression (GPR). In the training phase, our system utilizes a performance-optimized weight allocation strategy to identify the optimal weights, harnessing the complementary strengths of individual models while mitigating their limitations to achieve exceptional generalization in complex indoor environments. A comprehensive evaluation is conducted under a realistic ray-traced channel model that incorporates typical light distributions, high-order multipath reflections from walls and objects, and mixed diffuse-reflective surface interactions. Performance is assessed in terms of mean positioning error (MPE), 90th percentile (P90) error, and computational complexity. Results demonstrate that the proposed method achieves an MPE of 0.5 cm and a P90 error below 1 cm, offering a practical and scalable solution for next-generation IOWP applications in smart environments.

Free space optics (FSO) has emerged as a key technology for high-data-rate wireless communication, thanks to the availability of mature transceiver designs and unlicensed spectrum. Although FSO links are highly directive, atmospheric turbulence introduces random scintillation effects analogous to RF multipath fading, degrading system performance. Traditional models often treat turbulence as an intensity-based stochastic process, providing limited insight into phase distortions and wavefront aberrations such as beam wander. While split-step propagation methods can capture these effects accurately, their computational cost is prohibitive for large-scale simulations. In this paper, we propose a single-phase screen channel model using Zernike polynomials, effectively representing the turbulence-induced phase aberrations without resorting to full multi-screen wave propagation. Because it preserves the full complex wavefront, the proposed method captures both phase and intensity distortions, enabling evaluation of beam-shaping and adaptive optics design and providing an accurate performance baseline for coherent FSO links. Simulation results prove the value of the proposed model, as interesting insights can be derived regarding the intensity distribution at the receiver and the impact of the receiver lens.

As 6G and beyond aim for sustainable, high-capacity wireless connectivity, optical wireless communication (OWC) has emerged as a compelling solution. Recent advances in vertical-cavity surface-emitting laser (VCSEL) arrays have significantly enhanced OWC performance, enabling high-speed, low-power data transmission. However, dense VCSEL deployments introduce challenges related to interference and energy efficiency (EE). This paper proposes a scalable precoding framework for EE maximization in fully cooperative VCSEL-based OWC broadcast systems. We formulate a non-convex optimization problem to design the precoding matrix under practical optical constraints while guaranteeing minimum user rates. To solve this, we apply Dinkelbach’s method to handle the fractional objective and the inner approximation technique to iteratively convexify and solve the problem. Simulation results show that our approach consistently outperforms regularized zero-forcing in terms of EE, particularly in large-scale deployments, demonstrating its potential for next-generation sustainable dense OWC networks.

Achieving high-speed optical wireless communication (OWC) with efficient energy usage and full coverage in dynamic environments remains a significant challenge, particularly due to misalignment issues caused by user mobility and random receiver orientations. To address these challenges, this study introduces an innovative reconfigurable intelligent surfaces (RIS)-assisted laser-based light-fidelity (LiFi) system enhanced for energy efficiency and comprehensive coverage. An algorithm is developed to optimize the placement of RIS, reducing the need for continuous real-time adjustments and decreasing system complexity. Moreover, this study introduces a novel power allocation algorithm for multi-tier access points (APs) designed to reduce power consumption. Numerical results demonstrate the superiority of the proposed algorithm over previous designs in terms of transmitted power and outage probability.

The next generation of light fidelity (LiFi) networks will be based on laser-based optical wireless communication (OWC) to meet the demands for multi-Gigabit/s ($\text{Gb} / \mathrm{s}$) per user data rates. In this paper, we consider a double-tier access point (AP) architecture using an array of arrays of vertical cavity surface emitting lasers (VCSELs) to provide grid-of-beam (GoB) multi-user access for sixth generation (6G) laser-based optical wireless networks. In particular, we develop a novel dynamic beam clustering (DBC) algorithm based on graph theory with the aim to perfectly cancel inter-cluster interference (ICI) by dynamically adapting the clustering strategy according to network variations. We then carry out performance analysis for key performance metrics including the multi-user sum rate and fairness, as well as energy efficiency (EE). Simulation results demonstrate remarkable performance gains of DBC with respect to static beam clustering (SBC) strategies using fixed cluster sizes especially in terms of the sum rate and EE.

In optical wireless communication (OWC) networks, managing interference and enhancing data rates are critical challenges, particularly in environments with multiple users. This paper investigates the impact of the receiver's field of view (FoV) on interference reduction and signal-to-interference-plus-noise ratio (SINR) improvement in light-fidelity (LiFi), which is a networked OWC system. A narrower FoV can effectively limit the reception of unwanted signals, thereby mitigating inter-user interference and enhancing SINR. This work explores how FoV optimization contributes to interference suppression while maximizing SINR. Additionally, the integration of liquid crystal lenses (LCL) to the receiver permits a dynamic FoV for achieving interference-resistant communication. LCLs adjust the focus by applying an electric field to them, and have faster response time in comparison to the coherence time of LiFi. By incorporating realistic scenarios, including both line-of-sight (LoS) and non-LoS (NLoS) propagation, as well as random device orientations, and formulating an optimization problem to determine the optimal FoV, this study provides valuable insights for designing interference-resistant and high-performance LiFi systems. The results emphasize that employing a dynamic FoV enhances the average SINR by approximately 4 dB across the entire room, even under random device orientations. Moreover, configuring the receiver with a dynamic FoV range between 40° to 55° consistently yields high SINR throughout the room.

Laser-based optical wireless communication (OWC) can provide ultra-high-speed mobile connectivity for future networks. To address the misalignment challenges inherent in laser-based OWC systems, this paper introduces a novel intelligent reflecting surface (IRS)-assisted indoor OWC system. This system employs a multi-cell architecture and leverages passive IRS elements to enhance user coverage. The concept of angular coverage in indoor laser-based communication is introduced, and an analytical framework is proposed to calculate the angular coverage probability of users. Additionally, an innovative IRS deployment strategy, inspired by circle-packing principles, is proposed to enhance the angular coverage of users with varying locations and orientations. Numerical results validate the proposed IRS deployment strategy, demonstrating its superior performance compared to a conventional laser-based OWC architecture, achieving at least a threefold improvement in angular coverage probability.

Visible Light Positioning and Communication (VLPC) is a promising candidate for implementing Integrated Sensing And Communication (ISAC) in the unlicensed 400 THz to 800 THz band. The current Visible Light Positioning (VLP) systems mainly operate based on the Received Signal Strength (RSS) of the Line-of-Sight (LoS) path. However, its accuracy is degraded by interferences from Non-LoS (NLoS) paths. Furthermore, in Visible Light Communication (VLC) systems, the estimation of Channel State Information (CSI) also becomes challenging, when the optical channel becomes dispersive. Against this background, we propose a new VLPC scheme using Direct Current (DC) biased Optical Orthogonal Frequency-Division Multiplexing (VLPC-DCO-OFDM), where OFDM-based sensing is applied for the sake of improving the resolution of the estimated Channel Impulse Response (CIRs) exploited for positioning functionality. The CIRs estimated by sensing are further exploited to provide enhanced CSI for communication data detection. Moreover, we propose a hybrid Radar-RSS based solution, where the conventional RSS-aided VLP method is invoked for the sake of refining OFDM radar. Our simulation results demonstrate that the proposed VLPC-DCO-OFDM scheme – which simultaneously supports the triple functionalities of illumination, bi-static sensing and communication – is capable of achieving centimeter-level positioning accuracy and Giga-bits-per-second data rate.

In this paper, we put forward a proof of concept for sixth generation (6G) Terabit infrared (IR) laser-based indoor optical wireless networks. We propose a novel double-tier access point (AP) architecture based on anarray of arraysof vertical cavity surface emitting lasers (VCSELs) to provide a seamless grid-of-beam (GoB) coverage with multi-Gb/s per beam. We present systematic design and thorough analytical modeling of the AP architecture, which are then applied to downlink system modeling using non-imaging angle diversity receivers (ADRs). We propose static beam clustering with coordinated multi-beam joint transmission (CoMB-JT) for network interference management and devise various clustering strategies to address inter-beam interference (IBI) and inter-cluster interference (ICI). Non-orthogonal multiple access (NOMA) and orthogonal frequency division multiple access (OFDMA) schemes are also adopted to handle intra-cluster interference, and the resulting signal-to-interference-plus-noise ratio (SINR) and achievable data rate are derived. The network performance is studied in terms of spatial distributions and statistics of the downlink SINR and data rate through extensive computer simulations. The results demonstrate that data rates up to 15 Gb/s are achieved within the coverage area and a properly devised clustering strikes a balance between the sum rate and fairness depending on the number of users.

Optical wireless communication (OWC) has recently received massive interest as a new technology that can support the enormous data traffic increasing on daily basis. In particular, laser-based OWC networks can provide terabits per second (Tbps) aggregate data rates. However, the emerging OWC networks require a high number of optical access points (APs), each AP corresponding to an optical cell, to provide uniform coverage for multiple users. Therefore, inter-cell interference (ICI) and multi-user interference (MUI) are crucial issues that must be managed efficiently to provide high spectral efficiency. In radio frequency (RF) networks, rate splitting (RS) is proposed as a transmission scheme to serve multiple users simultaneously following a certain strategy. It was shown that RS provides high data rates compared to orthogonal and non-orthogonal interference management schemes. Considering the high density of OWC networks, the application of RS within each optical cell might not be practical due to severe ICI. In this paper, a novel strategy is derived, referred to as blind interference alignment-rate splitting (BIA-RS), to fully coordinate the transmission among the optical APs, while determining the precoding matrices of multiple groups of users formed beforehand. Therefore, RS can be implemented within each group to manage MUI. The proposed BIA-RS scheme requires two layers of power allocation to achieve high performance. Given that, a max-min fractional optimization problem is formulated to optimally distribute the power budget among the groups and the messages intended to the users of each group. Finally, a power allocation algorithm is designed with multiple Lagrangian multipliers to provide practical and sub-optimal solutions. The results show the high performance of the proposed scheme compared to other counterpart schemes.

Low Earth Orbit (LEO) satellite networks are capable of improving the global Internet service coverage. In this context, we propose a hybrid beamforming design for holographic metasurface based terrestrial users in multi-altitude LEO satellite networks. Firstly, the holographic beamformer is optimized by maximizing the downlink channel gain from the serving satellite to the terrestrial user. Then, the digital beamformer is designed by conceiving a minimum mean square error (MMSE) based detection algorithm for mitigating the interference arriving from other satellites. To dispense with excessive overhead of full channel state information (CSI) acquisition of all satellites, we propose a low-complexity MMSE beamforming algorithm that only relies on the distribution of the LEO satellite constellation harnessing stochastic geometry, which can achieve comparable throughput to that of the algorithm based on the full CSI in the case of a dense LEO satellite deployment. Furthermore, it outperforms the maximum ratio combining (MRC) algorithm, thanks to its inter-satellite interference mitigation capacity. The simulation results show that our proposed holographic metasurface based hybrid beamforming architecture is capable of outperforming the state-of-the-art antenna array architecture in terms of its throughput, given the same physical size of the transceivers. Moreover, we demonstrate that the beamforming performance attained can be substantially improved by taking into account the mutual coupling effect, imposed by the dense placement of the holographic metasurface elements.

The performance of full-duplex (FD) communications relying on antenna isolation, spatial beamforming, and/or self-interference (SI) reconstruction and subtraction, is limited by residual SI. For degrading the residual SI, we propose a novel FD transceiver design for SI cancellation (SIC), based on the mathematical modelling of pseudo-Doppler shift systems. Our pseudo-Doppler aided transceiver design manifests itself as an endogenous SIC mechanism, which removes SI from the center frequency of a transceiver’s desired signal. A low-pass filter can then be applied to pass on the clean desired signal. Since our design is essentially different from conventional SIC approaches, the SIC capability is defined as the improvement in the output signal-to-SI-plus-noise power ratio, which is taken as a metric for evaluating the intrinsic SIC performance. In terms of the SIC capability and operational complexity, our design outperforms conventional FD systems, even if the latter have managed very high SI suppression factors. To promote the practical application of our design in cellular networking, four main challenges are addressed, including the impact of synchronisation errors, the equivalence of SI suppression, the principles of resource allocation, and the investigation of diversity gain.

To support performance requirements for smart services in 6G, user positioning is a crucial component. Indoor user position and orientation estimation based on Light Fidelity (LiFi) system is considered as a promising technology, due to its high precision, along with its ease of installation. The main bottleneck of user position and orientation estimation in LiFi is a non-linearity between the metrics, such as the received signal strength (RSS), position and orientation. A deep learning-based estimation methodology holds promise for addressing this issue, because it can learn complex propagation features dependent on user position and orientation. To fully capitalize on this advantage in the time-domain, we propose utilizing both time-series RSS and its received time, i.e. time-of-arrival (ToA) fingerprints, along with a novel neural network architecture named Deep RSS-ToA Fusion Network (DRTFNet). Simulation results demonstrate that the proposed DRTFNet achieves positioning accuracy of less than 3 cm and orientation accuracy of less than 3 degrees, outperforming both the basic Convolutional Neural Network (CNN) architecture using only RSS data and other baseline systems with more light sources.

Photodiode (PD) arrays do not violate the second law of thermodynamics. In this paper, we explore the trade-off between PD array photo current and PD size when arrays with the same geometrical fill factor are considered. For this study, we assume a square matrix arrangement of PD. We apply the principles of energy conservation and, by extension, the second law of thermodynamics to demonstrate that reducing the size of individual PD in the array results in a decrease in photocurrent. This decrease is anticipated even when the geometrical fill factor of the array remains constant, i.e., even if the number of PDs in the array is increased. The photocurrent and size trade-off leads to the photocurrent bandwidth trade-off. The trade-off indicates that the PD array photocurrent is inversely proportional to its bandwidth. The derivation of this trade-off relation assumes a uniform irradiance distribution across the photodetector array. However, our findings show that this trade-off holds true for nearly uniform irradiance distributions and even for non-uniform distributions, such as those encountered with a compound parabolic concentrator (CPC). Our simulation results align closely with theoretical predictions. The implications of this study highlight the importance of considering the trade-off when designing optical wireless communication (OWC) or visible light communication (VLC) links for high data throughput.

Light-fidelity (LiFi) is a networked optical wireless communication (OWC) solution to achieve high-speed mobile communications. To address the misalignment challenges encountered in laser-based LiFi, this study introduces an innovative full-coverage indoor LiFi system with integrated communication and positioning capabilities, leveraging intelligent reflected surfaces (IRSs). By design, the proposed system ensures successful wireless downlink connectivity, irrespective of the user's random location and orientation status. An algorithm is developed to ascertain the optimal deployment of both access points (APs) and IRS layers. Moreover, this study introduces a machine learning (ML)-based OWC positioning approach designed to enhance the accuracy of the user positioning, thereby effectively boosting the performance of the IRS-assisted communication system. Numerical results demonstrate the superiority of the proposed positioning approach over traditional methods in terms of average data rate.

We demonstrate that the proposed optical orthogonal time frequency space (O-OTFS) is capable of improving the bandwidth-/power-/energy-efficiencies of optical orthogonal frequency-division multiplexing (O-OFDM). The bandwidth-efficiency is improved because only a single cyclic prefix (CP) is needed for an entire O-OTFS frame. The power-efficiency is enhanced thanks to the diversity gain achieved by its symplectic finite Fourier transform (SFFT), which also leads to a reduced peak-to-average power ratio (PAPR), hence improving its energy-efficiency. These features are facilitated by the proposed layered asymmetrically clipped O-OTFS (LACO-OTFS), which is capable of removing the direct current (DC) bias while retaining the full optical throughput. Nonetheless, there exists an inherent trade-off, where increasing the O-OTFS frame size leads to a commensurately reduced CP percentage at the cost of an increased PAPR. In order to mitigate this, we propose to perform discrete Fourier transform based spreading (DFT-S) in the delay-Doppler (DD)-domain. Furthermore, we demonstrate that regardless of the choice of domain in which the information is modulated (i.e. O-OFDM/O-OTFS with/without DFT-S), the frequency-selectivity of the quasi-static but dispersive optical channel can always be equalized by single-tap frequency-domain equalization (FDE). Moreover, the channel estimation techniques are conceived to operate in the time-/frequency-/DD-domains for both O-OFDM and O-OTFS. Our simulation results demonstrate that for a multi-user optical wireless system associated withM= 64 subcarriers and the OTFS frame length ofN= 64, LACO-OTFS is capable of achieving a 7 dB power-efficiency gain over LACO-OFDM, where the CP overhead is reduced by a factor ofN= 64. DFT-S-LACO-OTFS is also capable of providing a 7 dB power-efficiency gain over DFT-S-LACO-OFDM, where the low PAPR of single-carrier transmission is retained.

To achieve multi-Gb/s data rates in 6G optical wireless access networks based on narrow infrared (IR) laser beams, a high-speed receiver with two key specifications is needed: a sufficiently large aperture to collect the required optical power and a wide field-of-view (FOV) to avoid strict alignment issues. This paper puts forward the systematic design and optimisation of multi-tier non-imaging angle diversity receivers (ADRs) composed of compound parabolic concentrators (CPCs) coupled with photodiode (PD) arrays for laser-based optical wireless communication (OWC) links. Design tradeoffs include the gain-FOV tradeoff for each receiver element and the area-bandwidth tradeoff for each PD array. The rate maximisation is formulated as a non-convex optimisation problem under the constraints on the minimum required FOV and the overall ADR dimensions to find the optimum configuration of the receiver bandwidth and FOV, and a low-complexity optimal solution is proposed. The ADR performance is studied using computer simulations and insightful design guidelines are provided through various numerical examples. An efficient technique is also proposed to reduce the ADR dimensions based on CPC length truncation. It is shown that a compact ADR with a height of$\leq 0.5$cm and an effective area of$\leq 0.5$cm2 reaches a data rate of 12 Gb/s with a half-angle FOV of 30° over a 3 m link distance.

To enable intelligent network management and various 6G smart services, the precise estimation of user location and device orientation is required. Light fidelity (LiFi) based on vertical cavity surface emitting lasers (VCSELs) can not only respond to the needs of 6G communication networks in terms of ultra-high data rate, connection density and area capacity, but also enable high precision position and orientation estimation. However, this problem of joint position and orientation estimation is a non-convex optimization problem. Therefore, in this paper, we design deep neural networks (DNNs) for joint position and orientation estimation of user devices in a VCSEL-based LiFi access network. Simulation results demonstrate that the proposed framework outperforms state-of-the-art methods by significantly reducing position and orientation estimation errors while maintaining a lower complexity. We illustrate the effectiveness of the proposed DNN solution by considering two types of network deployment including distributed VCSELs and collocated VCSELs. In addition, we present the convergence and complexity analysis for the proposed learning framework. It is shown that the proposed DNN provides at least 69% and 27.9% improvements in the mean estimation error for position and orientation, respectively, over the baseline method.

We propose a multi-beam access point (AP) design comprising a vertical cavity surface emitting laser (VCSEL) array followed by a plano-convex lens as a cost-effective and scalable angle diversity transmitter (ADT) solution to realize multi-Gb/s downlink data rates in sixth generation (6G) optical wireless networks. The AP structure is elaborated using the VCSEL array and lens parameters including inter-element spacing in the array, the array-to-lens distance, and the diameter, thickness and radius of curvature of the lens. The design parameters are properly configured to ensure that the beam spots are sufficiently separated from one another on the receiver plane. To address the eye safety for infrared (IR) laser emission, we conduct a thorough eye safety analysis of a VCSEL array when combined with a lens to compute the maximum permissible transmit power (MPTP) for each transmitter element. The coverage performance of the AP is evaluated as a function of the array-to-lens distance based on a non-imaging angle diversity receiver (ADR), identifying a tradeoff between the MPTP value and the coverage performance. Simulation results demonstrate downlink data rates > 11 Gb/s while achieving a 100% coverage over an area of$1\times 1\ \mathbf{m}^{2}$on the receiver plane at 3 m under the AP based on a 2 GHz bandwidth and coordinated multi-beam joint transmission (CoMB-JT).

In this paper, we propose an imaging angle diversity receiver (ADR) that can support multi-Gb/s data rates for 6G optical wireless networks. We derive accurate mathematical expressions for the optical gain and field-of-view (FOV) of the imaging receiver in terms of its various geometrical parameters. Additionally, we identify the design tradeoffs and formulate a non-convex optimisation problem to maximise the data rate under the required FOV constraint. We also propose a low-complexity optimal solution for the optimisation problem, based on which we demonstrate the design of an imaging ADR with a$10^{\circ}$half-angle FOV, providing a data rate of 12 Gb/s for a 3 m link distance.

The rise of intelligent reflecting surfaces (IRSs) is opening the door for unprecedented capabilities in visible light communication (VLC) systems. By controlling light propagation in indoor environments, it is possible to manipulate the channel conditions to achieve specific key performance indicators. In this paper, we investigate the role that IRSs can play in boosting the secrecy capacity of non-orthogonal multiple access (NOMA) VLC systems. More specifically, we propose an IRS-based physical layer security (PLS) mechanism that mitigates the information leakage risk inherent in NOMA. Our results demonstrate that the achieved secrecy capacity can be enhanced by up to 105% for a number of 80 IRS elements. To the best of our knowledge, this is the first paper that examines the PLS of NOMA-based IRS-assisted VLC systems.

Mobile network operators (MNOs) require intelligent schemes for agile access points (APs) channel bandwidth slicing among mobile virtual network operators (MVNOs). These also require effective resource allocation and schedulers to enforce their quota guarantees, resource or data rate targets, across the network to support multiple services. Inline with the sixth generation (6G) vision, this paper develops a deep multi-agent reinforcement learning (DMARL) scheme that supports autonomic multi-tenant LiFi network APs downlink channel spectrum slicing. The scheme is coordinated with a utility scheduler-based network slicing (UBNS) approach to enforce the quota guarantees of MVNOs subject to their service-level agreement (SLA) with the MNO. The performance of the proposed deep Q-network (DQN) with UBNS is compared to UBNS and fixed network slicing (FNS) approaches. The simulation results demonstrate that the DQN with UBNS achieves an average data rate gain around 10% and 20%, compared to UBNS and FNS, respectively.

Light-fidelity (LiFi) is a networked optical wireless communication (OWC) solution to achieve multi-Gbps data rates for mobile optical communications. Unlike typical radio frequency (RF) wireless systems, the OWC channel is not isotropic, which means that the device orientation considerably affects the channel gain, especially for mobile users. In this study, a practical uplink LiFi scenario is analysed in which a number of mobile devices equipped with eye-safe laser-based transceivers transmit multi-Gbps data streams wirelessly to receivers on the ceiling. Ten models for five different types of user mobility with and without stabiliser have been developed on the basis of experimental measurements. Using the proposed models, the capability of stabilisers to mitigate the pointing error due to user mobility is investigated. It is shown that the device orientation may be modelled as Laplace, Gaussian, uniform distributions, or sinusoidal functions. Moreover, an orientation-based random waypoint (ORWP) mobility model is used to assess the performance of the LiFi system in terms of outage probability. The results show that using a stabiliser on mobile devices improves outage performance significantly, and it is feasible to reduce the number of uplink receivers while retaining the required outage performance.

A wireless fidelity (Wi-Fi) access point (AP) can provide signal coverage for light blocked users moving around light fidelity (LiFi) APs, while these can enhance the data rate transmission coverage of Wi-Fi APs. An experimental software-defined LiFi and Wi-Fi networks integration testbed has been developed to experimentally validate software-defined networking (SDN)-based user mobility and traffic load engineering algorithms, as well as, support the SDN applications development. A LiFi access and traffic engineering application has been developed to provide network monitoring and management, user mobility, and network load balancing services. The developed platform is also incorporated into a cloud network as a service testbed to support remote experimentation activities. It allocates in an elastic manner the different networking, computing, and storage resources according to the experimenter demands, which can be programmed based on experiment time cycles.

In this paper, multi-tier LiFi networks are studied in terms of energy efficiency (EE) and spectral efficiency (SE), which are crucial metrics for LiFi system design. We derived a closed-form expression of the user association probability for different tiers using stochastic geometry based Poisson Voronoi Tessellation (PVT) LiFi network. The performance metrics of the network, EE and SE, are analyzed in terms of different parameters such as transmit power and Lambertian index. Performance evaluations and numerical results show that multi-tier LiFi networks have an optimum transmit power in which EE is maximized. Besides, increasing the transmit power does not increase SE after passing a threshold point. The resulting trade-off between EE and SE is presented.

The design of a compact high-speed and wide field of view (FOV) receiver is challenging due to the presence of two well-known trade-offs. The first one is the area-bandwidth trade-off of photodetectors (PDs) and the second one is the gain-FOV trade-off due to the use of optics. The combined effects of these two trade-offs imply that the achievable data rate of an imaging optical receiver is limited by its FOV, i.e., a rate-FOV trade-off. In this paper, we propose an imaging receiver design in the form of an array of (PD) arrays. To control the area-bandwidth trade-off, small PDs are used in an array of arrays structure instead of a single large PD. Moreover, to achieve a reasonable receiver FOV, we use an array of focusing lenses that focus the light individually on each inner PD array. The proposed array of arrays structure provides an effective method to control both gain-FOV trade-off (via an array of lenses) and area-bandwidth trade-off (via arrays of small PDs). We first derive a tractable analytical model for the signal-to-noise ratio (SNR) of an array of PDs that is equipped with a focusing lens assuming maximum ratio combining (MRC). Then, we extend the model to the proposed array of arrays structure and the accuracy of the analytical model is verified based on several Optic Studio-based simulations. Next, we formulate an optimization problem to maximize the achievable data rate of the imaging receiver subject to a minimum required FOV. The optimization problem is solved for two commonly used modulation techniques, namely, on-off keying (OOK) and direct current (DC) biased optical orthogonal frequency division multiplexing (DCO-OFDM) with variable rate quadrature amplitude modulation (QAM). Our results show the limits of high speed wide-FOV imaging receivers that can support mobility. For example, it is demonstrated that a data rate of$\sim 24$Gbps with a FOV of 15° is achievable using OOK with a total receiver size of 2 cm$\!\times \!\,\,2$cm.

The sixth generation (6G) pervasive channel model (6GPCM) can characterize channels for all spectra from the sub-6 GHz band to the visible light communication (VLC) band and all scenarios, such as maritime, (ultra-)massive multiple-input multiple-output (MIMO), and industrial Internet of things (IIoT) communication scenarios in 6G wireless systems. The unified channel model can enable us to analyze channel statistical properties in systems using different scales of antenna arrays, different frequency bands, and different scenarios with different movement speeds. In this paper, we conduct a complete study on space-time-frequency (STF) statistical properties of the 6GPCM. Mathematical derivations and simulations are provided, including STF correlation function (STFCF), spatial/temporal/frequency correlation functions, angular/Doppler/delay power spectral densities (PSDs), root mean square (RMS) angular/Doppler/delay spreads, coherence distance/time/bandwidth, stationary distance/time/bandwidth, and level-crossing rates (LCRs)/average fade durations (AFDs) in STF domains. In addition, we classify these statistical properties according to their definitions and then reveal the complex relationships between them and channel model parameters. This work will lay a solid foundation and offer useful guidelines for research on 6G wireless communication systems.

A hybrid Wi-Fi and light fidelity (LiFi) network combines the best of two worlds with the ubiquitous coverage of Wi-Fi and the high peak data rate of LiFi as the radio spectrum does not interfere with the light spectrum. This hybrid network might be realized by using multipath TCP (MPTCP), where Wi-Fi and LiFi paths can be simultaneously employed to potentially boost the total throughput of Wi-Fi while increasing the resilience towards network failure of LiFi due to, for example, blockage. However, naively implementing MPTCP in a hybrid Wi-Fi and LiFi network can yield an unexpected result, such as a lower throughput compared to the single-path TCP due to a Head-of-Line delay during the slow start phase of the TCP congestion control. Even though this problem can be avoided by improving the existing flow control or congestion control of TCP, these solutions still lack intelligent decision making that can improve the adaptability of MPTCP. Therefore, in this paper, we propose a model-augmented deep reinforcement learning (DRL) approach to intelligently steer MPTCP subflows (i.e., TCP connections) by using a close-to-reality scenario emulated by considering random orientation, random blockage, and random mobility of Wi-Fi-and-LiFi-enabled mobile devices. As a result, we will show later that a performance gain can be achieved compared to the state-of-the-art while maintaining ease implementation to existing MPTCP implementations.

To achieve multi-Gb/s data rates in 6G optical wireless networks based on narrow infrared (IR) laser beams, a high-speed receiver with two key specifications is needed: a sufficiently large aperture to collect the required optical power and a wide field of view (FOV). This paper investigates the design tradeoffs for a non-imaging angle diversity receiver (ADR) coupled with photodiode (PD) arrays for laser-based optical wireless communication (OWC) with the aim to achieve data rates higher than 10 Gb/s and a half-angle FOV of$30^{\circ}$. The design tradeoffs include the gain-FOV tradeoff for each element of the receiver and the area-bandwidth tradeoff for each PD array. Taking both tradeoffs into account, the rate maximisation is formulated as a non-convex optimisation problem to find the optimal configuration of the receiver bandwidth and FOV under the minimum FOV constraint. A low-complexity optimal solution is proposed and the ADR performance is studied using computer simulations.

The emerging intelligent reflecting surface (IRS) technology introduces the potential of controlled light propagation in visible light communication (VLC) systems. This concept opens the door for new applications in which the channel itself can be altered to achieve specific key performance indicators. In this paper, for the first time in the open literature, we investigate the role that IRSs can play in enhancing the link reliability in VLC systems employing non-orthogonal multiple access (NOMA). We propose a framework for the joint optimisation of the NOMA and IRS parameters and show that it provides significant enhancements in link reliability. For example, the bit-error-rate of the NOMA user in the first decoding order is reduced to the order of 10−6using the proposed framework compared to an error floor of 10−2under fixed power allocation and fixed IRS reflection coefficients. The enhancement is even more pronounced when the VLC channel is subject to blockage and random device orientation.

A multi-service light fidelity (LiFi) attocellular net-work should be resource engineered to efficiently support fifth generation (5G) services on customized network slices. Mobile virtual network operators (MVNOs) request network slices to offer customized services according to their service level agreement (SLA) constraints in terms of resource guarantees or data rate targets. However, there are two main challenges to meet these network resource and services slicing guarantees in a sliced LiFi attocellular network: i) conflicting services requirements and ii) dynamic resource quota allocation to slices. A multi-policy resource scheduler is developed to support dynamic LiFi network slicing based on network resource and service utility optimization. The scheduler runs on a utility-based heuristic scheduling algorithm, which can simultaneously support multiple MVNOs to offer delay and throughput sensitive services on different LiFi network slices. This enables MVNOs to deploy their network slice quota customization policies that support different traffic profiles and specific-slice service management policies. The obtained results demonstrate that the weights of utility functions can be adjusted automatically according to the LiFi AP channel conditions and the SLA constraints of MVNOs.

In this paper we investigate the impact of the passband shift (PS) phenomenon on optical wireless communication (OWC) systems based on wavelength division (WD). We first introduce the associated challenges, then we discuss the mathematical framework needed to evaluate the performance of systems based on WD when the impact of the PS phenomenon is taken into account. We introduce the concept of spectral overlap (SO) and discuss its role in the design of WD solutions. Results show that this design phase has to take the SO into account, and that its careful balance with the channel gain is essential when multiple colours are used for parallel communication in OWC.

In this paper, a vehicular light-fidelity (LiFi) system, which supports mobility for seamless broadband data transmission is proposed for multipoint-to-point (M2P) communication. To this end, a novel digital mirror device (DMD) based receiver (RX) is investigated that consists of two functionalities, to sense the transmitters and sense mobility. Relying on the proposed hybrid waveform, the proposed RX has the ability to detect and distinguish multiple transmitter (TX) sources under free movement conditions within three-dimensional space. Computer simulation results for the ‘TX sensing platform’ show that a high TX detection and identification performance is achieved at an extremely low channel signal-to-noise-ratio (SNR) of −11 dB and a bit error ratio (BER) of 10−4. Furthermore. the simulation results for the ‘mobility sensing platform’ was able to trace the positions of light sources simultaneously with the error rate of below 2% relative to the diameter of light spots on the DMD for all considered mobility scenarios.

In this paper, we present our research on modeling a synthetic light fidelity (LiFi) channel model that uses a deep learning architecture called generative adversarial networks (GAN). A research in LiFi that requires the generation of many multipath channel impulse responses (CIRs) can benefit from our proposed model. For example, future developments of autonomous (deep learning-based) network management systems that use LiFi as one of its high-speed wireless access technologies might require a dataset of many CIRs. In this paper, we use TimeGAN, which is a GAN architecture for time-series data. We will show that modifications are necessary to adopt TimeGAN in our use case. Consequently, synthetic CIRs generated by our model can track long-term dependency of LiFi multipath CIRs. The Kullback–Leibler divergence (KLD) is used in this paper to measure the small difference between samples of synthetic CIRs and real CIRs. Lastly, we also show a simple demonstration of our model that can run on a small virtual machine hosted over the Internet.

Considering great benefits brought by massive multiple-input–multiple-output (MIMO) technologies in the Internet of Things (IoT), it is of vital importance to analyze new massive MIMO channel characteristics and develop corresponding channel models. In the literature, various massive MIMO channel models have been proposed and classified with different but confusing methods, i.e., physical versus analytical method and deterministic versus stochastic method. To have a better understanding and usage of massive MIMO channel models, this work summarizes different classification methods and presents an up-to-date unified classification framework, i.e., artificial intelligence (AI)-based predictive channel models and classical nonpredictive channel models, which further clarify and combine the deterministic versus stochastic and physical versus analytical methods. Furthermore, massive MIMO channel measurement campaigns are reviewed to summarize new massive MIMO channel characteristics. Recent advances in massive MIMO channel modeling are surveyed. In addition, typical nonpredictive massive MIMO channel models are elaborated and compared, i.e., deterministic models and stochastic models, which include the correlation-based stochastic model (CBSM), geometry-based stochastic model (GBSM), and beam-domain channel model (BDCM). Finally, future challenges in massive MIMO channel modeling are given.

Light amplification by stimulated emission of radiation (laser) sources has many advantages for use in high-data-rate optical wireless communications (OWCs). In particular, the low-cost and high-bandwidth properties of laser sources, such as vertical-cavity surface-emitting lasers (VCSELs), make them attractive for future indoor OWCs. In order to be integrated into future indoor networks, such lasers should conform to eye safety regulations determined by the International Electrotechnical Commission (IEC) standards for laser safety. In this article, we provide a detailed study of beam propagation to evaluate the received power of various laser sources, based on which and the maximum permissible exposure (MPE) defined by the IEC 60825-1:2014 Standard, we establish a comprehensive framework for eye safety analyses. This framework allows us to calculate the maximum allowable transmit power, which is crucial in the design of a reliable and safe laser-based wireless communication system. Initially, we consider a single-mode Gaussian beam and calculate the maximum permissible transmit power. Subsequently, we generalize this approach for higher mode beams. It is shown that the$M$-squared-based approach for analysis of multimode lasers ensures the IEC eye safety limits; however, in some scenarios, it can be too conservative compared to the precise beam decomposition method. Laser safety analyses with consideration of optical elements, such as lens and diffuser, as well as for the VCSEL array, have been also presented. Skin safety, as another significant factor of laser safety, has also been investigated in this article. We have studied the impacts of various parameters, such as wavelength, exposure duration, and the divergence angle of laser sources on the safety analysis by presenting insightful results.

In this paper, the design of a multiple-input multiple-output (MIMO) optical wireless communication (OWC) link based on vertical cavity surface emitting laser (VCSEL) arrays is systematically carried out with the aim to support data rates in excess of 1 Tb/s for the backhaul of sixth generation (6G) indoor wireless networks. The proposed design combines direct current optical orthogonal frequency division multiplexing (DCO-OFDM) and a spatial multiplexing MIMO architecture. For such an ultra-high-speed line-of-sight (LOS) OWC link with low divergence laser beams, maintaining alignment is of high importance. In this paper, two types of misalignment error between the transmitter and receiver are distinguished, namely, radial displacement error and orientation angle error, and they are thoroughly modeled in a unified analytical framework assuming Gaussian laser beams, resulting in a generalized misalignment model (GMM). The derived GMM is then extended to MIMO arrays and the performance of the MIMO-OFDM OWC system is analyzed in terms of the aggregate data rate. Novel insights are provided into the system performance based on computer simulations by studying various influential factors such as beam waist, array configuration and different misalignment errors, which can be used as guidelines for designing short range Tb/s MIMO OWC systems.

Optical wireless communication (OWC) is considered to be a promising technology which will alleviate traffic burden caused by the increasing number of mobile devices. In this study, a novel vertical-cavity surface-emitting laser (VCSEL) array is proposed for indoor OWC systems. To activate the best beam for a mobile user, two beam activation methods are proposed for the system. The method based on a corner-cube retroreflector (CCR) provides very low latency and allows real-time activation for high-speed users. The other method uses the omnidirectional transmitter (ODTx). The ODTx can serve the purpose of uplink transmission and beam activation simultaneously. Moreover, systems with ODTx are very robust to the random orientation of a user equipment (UE). System level analyses are carried out for the proposed VCSEL array system. For a single user scenario, the probability density function (PDF) of the signal-to-noise ratio (SNR) for the central beam of the VCSEL array system can be approximated as a uniform distribution. In addition, the average data rate of the central beam and its upper bound are given analytically and verified by Monte-Carlo simulations. For a multi-user scenario, an analytical upper bound for the average data rate is given. The effects of the cell size and the full width at half maximum (FWHM) angle on the system performance are studied. The results show that the system with a FWHM angle of 4° outperforms the others.

The visible light communication (VLC) technology has attracted much attention in the research of the sixth generation (6G) communication systems. In this paper, a novel three dimensional (3D) space-time-frequency non-stationary geometry-based stochastic model (GBSM) is proposed for indoor VLC channels. The proposed VLC GBSM can capture unique indoor VLC channel characteristics such as the space-time-frequency non-stationarity caused by large light-emitting diode (LED) arrays in indoor scenarios, long travelling paths, and large bandwidths of visible light waves, respectively. In addition, the proposed model can support special radiation patterns of LEDs, 3D translational and rotational motions of the optical receiver (Rx), and can be applied to angle diversity receivers (ADRs). Key channel properties are simulated and analyzed, including the space-time-frequency correlation function (STFCF), received power, root mean square (RMS) delay spread, and path loss (PL). Simulation results verify the space-time-frequency non-stationarity in indoor VLC channels. In addition, the influence of light source radiation patterns, receiver rotations, and ADRs on channel characteristics have been investigated. Finally, the accuracy and practicality of the proposed model are validated by comparing the simulation result of channel 3dB bandwidth with the existing measurement data. The proposed channel model will play a supporting role in the design of future 6G VLC systems.

Wireless backhaul communication and power provision to fifth-generation small cells (SCs) is expected to decrease their installation cost significantly. In this paper, hybrid solar/laser-based energy harvesting and storage are investigated for the self-sufficient year-round operation of outdoor SCs. The required electrical power for a SC is assumed to be 10 W according to state-of-the-art millimeter-wave phased-array transceivers. Here we show through simulations that hybrid solar/laser-based energy harvesting with storage and exclusive laser-based energy harvesting enable the SC operation not only in sunnier parts of the world but also in ‘darker’ places. Since the highest monthly power required from the laser is shown to be 10.1 W, a highly directive optical wireless link is designed using the simulation software Zemax. The operation wavelength of 1310 nm is selected because of the highest maximum permissible exposure of 1 W/cm2. The designed system is shown to be capable of harvesting 10.4 W at the distance of 100 m with transmitter-receiver dimensions of the order of cm3and meeting eye-safety regulations for Class 1M.

This paper presents a color dimming (CD) method for Light Fidelity (LiFi), enabling the ability to change the color of a light-emitting-diode (LED) and adjust the brightness of the light. The proposed CD scheme targets both high spectral efficiency and high signal-to-noise ratio (SNR). In order to achieve a high spectral efficiency, a superposition modulation scheme called augmented spectral efficiency discrete multi-tone (ASE-DMT) is used with multi-layer modulation and iterative detection. In order to achieve a high SNR, different modulation layers of ASE-DMT are loaded onto different LEDs. This is called a multi-layer multi-output modulation (MLMO) scheme. The proposed MLMO scheme supports single photo diode (PD) demodulation, thus reducing the receiver complexity. Results show that red/green/blue channels can adjust their brightness levels independently within a brightness range of 16 % to 84 % to produce a mixed color. Within the operating color plane, the proposed MLMO ASE-DMT provides a consistently high SNR of up to 33 dB, which is 8.3 dB higher than the typical ASE-DMT with multi-layer single-output modulation (MLSO).

This paper introduces a hybrid system that simultaneously provides two communication links, a Light Fidelity (LiFi) link and an Optical Camera Communications (OCC) link. It aims to take advantage of OCC to support LiFi in a mobile scenario with reduced alignment constraints. Novel codes are proposed to enable OCC modulation and dimming control, while Augmented Spectral Efficiency Discrete Multitone (ASE-DMT) is selected as a high spectral-and-energy efficient LiFi modulation technique. An experimental demonstration, supported by computer simulations, shows the high performance of the system. Particularly, during the simultaneous OCC-LiFi transmission and dimming control, a signal-to-noise ratio (SNR) of at least 25 dB is achieved in various measurements from both OCC and LiFi decoders. Also, a data rate gain of 18.75 kbps is achieved by the OCC link in the hybrid system when compared to the LiFi link at the communication bandwidth of 20 MHz.

Abstract The fifth generation (5G) wireless communication networks are being deployed worldwide from 2020 and more capabilities are in the process of being standardized, such as mass connectivity, ultra-reliability, and guaranteed low latency. However, 5G will not meet all requirements of the future in 2030 and beyond, and sixth generation (6G) wireless communication networks are expected to provide global coverage, enhanced spectral/energy/cost efficiency, better intelligence level and security, etc. To meet these requirements, 6G networks will rely on new enabling technologies, i.e., air interface and transmission technologies and novel network architecture, such as waveform design, multiple access, channel coding schemes, multi-antenna technologies, network slicing, cell-free architecture, and cloud/fog/edge computing. Our vision on 6G is that it will have four new paradigm shifts. First, to satisfy the requirement of global coverage, 6G will not be limited to terrestrial communication networks, which will need to be complemented with non-terrestrial networks such as satellite and unmanned aerial vehicle (UAV) communication networks, thus achieving a space-air-ground-sea integrated communication network. Second, all spectra will be fully explored to further increase data rates and connection density, including the sub-6 GHz, millimeter wave (mmWave), terahertz (THz), and optical frequency bands. Third, facing the big datasets generated by the use of extremely heterogeneous networks, diverse communication scenarios, large numbers of antennas, wide bandwidths, and new service requirements, 6G networks will enable a new range of smart applications with the aid of artificial intelligence (AI) and big data technologies. Fourth, network security will have to be strengthened when developing 6G networks. This article provides a comprehensive survey of recent advances and future trends in these four aspects. Clearly, 6G with additional technical requirements beyond those of 5G will enable faster and further communications to the extent that the boundary between physical and cyber worlds disappears.

Light-fidelity (LiFi) is a fully-networked bidirectional optical wireless communication (OWC) technology that is considered as a promising solution for high-speed indoor connectivity. In this paper, the joint estimation of user 3D position and user equipment (UE) orientation in indoor LiFi systems with unknown emission power is investigated. Existing solutions for this problem assume either ideal LiFi system settings or perfect knowledge of the UE states, rendering them unsuitable for realistic LiFi systems. In addition, these solutions consider the non-line-of-sight (NLOS) links of the LiFi channel gain as a source of deterioration for the estimation performance instead of harnessing these components in improving the position and the orientation estimation performance. This is mainly due to the lack of appropriate estimation techniques that can extract the position and orientation information hidden in these components. In this paper, and against the above limitations, the UE is assumed to be connected with at least one access point (AP), i.e., at least one active LiFi link. Fingerprinting is employed as an estimation technique and the received signal-to-noise ratio (SNR) is used as an estimation metric, where both the line-of-sight (LOS) and NLOS components of the LiFi channel are considered. Motivated by the success of deep learning techniques in solving several complex estimation and prediction problems, we employ two deep artificial neural network (ANN) models, one based on the multilayer perceptron (MLP) and the second on the convolutional neural network (CNN), that can map efficiently the instantaneous received SNR with the user 3D position and the UE orientation. Through numerous examples, we investigate the performance of the proposed schemes in terms of the average estimation error, precision, computational time, and the bit error rate. We also compare this performance to that of the k-nearest neighbours (KNN) scheme, which is widely used in solving wireless localization problems. It is demonstrated that the proposed schemes achieve significant gains and are superior to the KNN scheme.

Light-fidelity (LiFi) is recognised as a promising technology for next generation wireless access networks. However, limited research efforts have been spent on the uplink (UL) transmission system in LiFi networks. In this article, a wireless infrared (IR)-based LiFi UL system is investigated. In particular, we focus on the performance of a single static user under the influence of random device orientation and link blockage. Simulations and mathematical analysis have been used to evaluate the UL system performance. The analytical expressions for the UL optical wireless channel and signal-to-noise ratio (SNR) statistics under the effects of random device orientation and link blockage are derived. The results show that the effects of random orientation and link blockage may lead to a decrease in coverage probability by 10% - 40% with various SNR thresholds.

We consider the physical layer security (PLS) of multi-user (MU) multiple-input-multiple-output visible light communication (VLC) systems with an eavesdropper (Eve) and propose a novel spatial constellation design technique based on generalized space shift keying (MU-GSSK-SCD). The received signals of the legitimate users are optimized jointly, such that their bit error ratios (BERs) are minimized and Eve's BER is significantly degraded. The emission power of randomly selected light-emitting diodes is adjusted, by exploiting users' channel state information at the transmitter. Our strategy ensures that legitimate users receive confidential messages fully in an undistorted fashion, while any meaningful leakage to Eve is strongly prohibited, without any artificial noise addition. Every user can decode only its information, hence inter-user security is also guaranteed. The PLS improvements are presented in terms of both BERs and achievable secrecy rates in practical VLC scenarios. For various user configurations, it is shown that MU-GSSK-SCD increases the BER at Eve to the 0.5 level, while providing minimized BERs to the legitimate users. The achievable secrecy rate region is derived for MU-GSSK-SCD and it is shown that full secrecy can be achieved at 0 dB signal-to-noise ratio (SNR) level with a user separation as small as 90 cm.

Coherent light fidelity (LiFi) systems which uses coherent detection at the receiver are attracting more and more interest as they provide many advantages over the conventional intensity modulation / direct detection (IM/DD) LiFi systems. Recent efforts to boost the performance of IM/DD LiFi systems include the usage of multiple-input-multiple-output (MIMO) technology, among which spatial multiplexing (SMX) has gained certain attention. This paper combines coherent detection and spatial multiplexing, and proposes a novel scheme called CD SMX (coherent-detection spatial-multiplexing) LiFi systems to improve LiFi performances. Compared with CD single-input-single-output (SISO) LiFi systems and CD spatial modulation (SM) LiFi systems, the proposed scheme provides a spectral efficiency gain. Unlike IM/DD or radio frequency (RF) SMX systems, the proposed CD SMX LiFi system is free from inter-channel interferences (ICI) due to the matching properties of coherent detection. The performance of the proposed CD SMX system is studied via both theoretical analysis and computer simulations. It is reported that the proposed system achieves a large gain over its IM/DD counterpart, and a moderate gain over a coherent system with SM.

Light-fidelity (LiFi) is a fully-networked bidirectional optical wireless communication (OWC) technology that is considered as a promising solution for high-speed indoor connectivity. Unlike in conventional radio frequency wireless systems, the OWC channel is not isotropic, meaning that the device orientation affects the channel gain significantly. However, due to the lack of proper channel models for LiFi systems, many studies have assumed that the receiver is vertically upward and randomly located within the coverage area, which is not a realistic assumption from a practical point of view. In this paper, novel realistic and measurement-based channel models for indoor LiFi systems are proposed. Precisely, the statistics of the channel gain are derived for the case of randomly oriented stationary and mobile users. For stationary users, two channel models are proposed, namely, the modified truncated Laplace (MTL) model and the modified Beta (MB) model. For mobile users, two channel models are proposed, namely, the sum of modified truncated Gaussian (SMTG) model and the sum of modified Beta (SMB) model. Based on the derived models, the impact of random orientation and spatial distribution of users is investigated, where we show that the aforementioned factors can strongly affect the channel gain and the system performance.

In this paper, a low cost optical wireless communications link with a data rate of 74 Mb/s at a bit-error ratio of 3.3×10-3is demonstrated. The components are all low cost, off-the-shelf parts, including the laser transmitter and silicon solar panel receiver. This performance is achieved by extending the usable bandwidth using analog equalization and digital techniques such as adaptive bit and power loading. Also, the performance of two spectrally efficient modulation schemes, namely direct-current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and pulse amplitude modulation discrete multitone (PAM-DMT) is evaluated and compared. It is shown that DCO-OFDM outperforms PAMDMT because of efficient bandwidth usage with adaptive bit loading and pre-distortion introduced by adaptive power loading. Furthermore, it is shown that PAM-DMT can avoid complex system designs and still achieve comparable data rates to that of OFDM with higher link reliability.

This paper introduces a novel dimming method for Light Fidelity (LiFi) based on index modulation (IM). A time-domain sample-index modulation (TIM) is proposed for indexed dimming (iDim). The aim is to maintain a high communication performance measured in signal to noise ratio (SNR) and a high transmission rate for a wide light emitting diode (LED) brightness range. Direct current optical orthogonal frequency division multiplexing (DCO-OFDM) is used. The system performance is experimentally validated by an implementation on a National Instruments (NI) PXIe-1085 and NI-7966R Field Programmable Gate Array (FPGA). The proposed iDim offers a wider dimming range and an improved SNR/symbol when compared to amplitude-modulation dimming (AM dimming). In particular, the iDim system provides a SNR/symbol of 22.5 dB for all brightness levels. The lowest optical power is measured at 20 μW which is 10 times lower than the measured limit of AM dimming. This reduces the cost of the optical power per bit. Therefore, iDim is a promising dimming method for applications targeting extremely low illumination levels.

The limited bandwidth of optical wireless communication (OWC) front-end devices motivates the use of multipleinput- multiple-output (MIMO) techniques to enhance data rates. It is known that very high multiplexing gains can be achieved by spatial multiplexing (SMX) at the cost of prohibitive detection complexity. Alternatively, in spatial modulation (SM), a single light emitting diode (LED) is activated per time instance where information is carried by both the signal and the LED index. Since only one LED is active, both the transmitter (TX) and receiver (RX) complexity reduce significantly while retaining the information transmission in the spatial domain. However, this simplified TX utilization approach leads SM to suffer from significant spectral efficiency losses compared to SMX. In this paper, we propose a technique that benefits from the advantages of both systems. Accordingly, the proposed flexible LED index modulation (FLIM) technique harnesses the inactive state of the LEDs as a transmit symbol. Therefore, the number of active LEDs changes in each transmission, unlike conventional techniques. Moreover, the system complexity is reduced by employing a linear minimum mean squared error (MMSE) equalizer and an angle perturbed receiver. Numerical results show that FLIM outperforms the reference systems by at least 6 dB in the low and medium/high spectral efficiency regions.

This paper presents the design of a multiple input multiple output (MIMO) optical wireless communication (OWC) system based on vertical cavity surface emitting laser (VCSEL) arrays. The objective is to realize data rates in excess of 1 Tb/s for the backhaul of next generation indoor networks. The proposed design combines direct current optical orthogonal frequency division multiplexing (DCO-OFDM) with spatial multiplexing to attain the target data rate subject to eye safety limitations. Following a bottom-up approach, the signal-to-interference-plus-noise (SINR) per channel is modeled using a Gaussian emission profile for laser beams by taking into account crosstalk in the MIMO channel. According to the SINR expression, the aggregate data rate is derived. The performance of the system is studied using computer simulations, and insightful remarks are provided on the configuration of various system parameters including the beam waist, the MIMO size and array dimensions.

This paper presents a proof-of-concept for Tb/s infrared (IR) indoor optical wireless networks. We introduce a novel double tier access point architecture based on array of arrays of vertical cavity surface emitting lasers (VCSELs) to deliver beyond Tb/s aggregate capacity. For a given indoor environment, the optimal access point architecture is designed. The downlink performance is analysed throughout the coverage area and the spatial distribution of signal-to-interference-plus-noise ratio (SINR) and data rate are obtained. Numerical results demonstrate that with a single access point in a 25 m2indoor area, data rates of at least 10 Gb/s per beam are achieved almost everywhere and the aggregate data rate can exceed 2 Tb/s.

Energy efficiency (EE) is a major performance metric for fifth generation (5G) and beyond 5G(B5G) wireless communication systems, especially for ultra dense networks. This paper proposes an end-to-end (e2e) power consumption model and studies the energy efficiency for a heterogeneous B5G cellular architecture that separates the indoor and outdoor communication scenarios in ultra dense networks. In this work, massive multiple-input-multiple-output (MIMO) technologies at conventional sub-6 GHz frequencies are used for long-distance outdoor communications. Light-Fidelity (LiFi) and millimeter wave (mmWave) technologies are deployed to provide a high data rate service to indoor users. Whereas, in the referenced non-separated system, the indoor users communicate with the outdoor massive MIMO macro base station directly. The performance of these two systems are evaluated and compared in terms of the total power consumption and energy efficiency. The results show that the network architecture which separates indoor and outdoor communication can support a higher data rate transmission for less energy consumption, compared to non-separate communication scenario. In addition, the results show that deploying LiFi and mmWave IAPs can enable users to transmit at a higher data rate and further improve the EE.

This paper investigates practical solutions to control the brightness of a visible light-based Light Fidelity (LiFi) system. The main objective is to achieve a wide dimming range while maintaining a high communication performance. Experimental studies are presented on the integration of dimming techniques into a communication system using a National Instruments (NI) PXIe-1085 and a NI-7966R Field Programmable Gate Array (FPGA). Pulse-width-modulation (PWM) dimming and amplitude-modulation (AM) dimming are applied to direct current (DC)-biased optical frequency division multiplexing (DCO-OFDM) and augmented spectral efficiency discrete multi-tone (ASE-DMT) modulation schemes. A reliable communication link with a measured Q-factor over 15 dB is realized at dimming levels between 5% and 95%. The performance comparison demonstrates that ASE-DMT is more suitable for dimming applications with up to a 7 dB higher Q factor than that of DCO-OFDM.

Self-driving cars have become a popular research topic since the 1990s. Autonomous vehicles significantly benefit from seamless wireless connections between cars or between cars and the infrastructure. Vehicular visible light communications (V-VLC) can be considered as a complementary technology to radio frequency-based (RF-based) communications. V-VLC can use existing headlights and tail lights in cars. Moreover, it benefits from enhanced security and draws upon the unregulated bandwidth in the visible light spectrum. There have been many studies in this area, but to the best of our knowledge, there has been no investigation on the impact of road surface irregularities or the road terrain on the variations in the received signal-to-noise ratio (SNR). In this paper, a channel model is developed which includes the car suspension system regarding the angular relations and the vertical movement because of the car's vibrations on irregular road surfaces. The SNR ratios from different scenarios are compared using the model.

In conventional visible light communication (VLC) systems with multiple light-emitting diodes (LEDs) and multiple photodiodes (PDs), high data rate transmission with limited modulation bandwidth can be achieved via spatial multiplexing (SMP) or wavelength division multiplexing (WDM). However, the number of multiplexing channels is limited by the strong spatial correlation in SMP and by the inter-colour crosstalk in WDM. In this paper, we propose a multiple-input multiple-output (MIMO) hybrid multiplexing (HMP) VLC system which avoids the disadvantages of SMP/ WDM and explores the degrees-of-freedom (DoFs) in space and wavelength domains jointly. With appropriate system configuration, a MIMO channel matrix with a better channel condition in HMP can be obtained. Eventually, it is able to increase the number of multiplexing channels and support higher data rate transmission.

A task group named IEEE 802.11 Light Communications Amendment-Task Group “bb” (TGbb) was established in July 2018. By bringing light-fidelity (LiFi) technology into the WiFi ecosystem, LiFi can take advantage of the globally recognized WiFi brand, while also improving its capability due to the fact that LiFi does not interfere with WiFi. Early discussions in the task group focused on the physical (PHY) layer. There are two major proposals for the PHY layer. The first one is to use the existing IEEE 802.11 chipsets with LiFi analog front-ends. This is done by means of the frequency up and down-conversions and adding a DC bias. The second proposal is to redefine a whole PHY layer and optimize it by means of adopting adaptive bit loading in order to combat the low-pass filter characteristics of the non-line-of-sight wireless optical channels. Each approach has advantages in terms of the low-entry barrier to the mass market and better performance, respectively. The root question in determining the common mode PHY between the two approaches is how frequent LiFi encounters flat channels. That is, if the channel is flat, then the gain of the adaptive bit loading is not significant. Therefore, this paper aims to investigate the flatness of many samples from the reference channel models defined in the TGbb. We find that the majority of the channels are flat if the signal bandwidth is 20 MHz.

In July 2018, a new IEEE task group focusing on light communications was formed, namely IEEE 802.11bb (TGbb). The primary motivation of this task group is to standardize mobile, networked light communications, i.e., light fidelity (LiFi). At the time of writing, discussions in TGbb still focus on the physical (PHY) layer, and recently a common mode PHY has been agreed. The common mode PHY is defined based on the frequency upconversion of the signal outputs of existing WiFi chipsets. In addition, a DC bias is used to enable the intensity-modulation and direct-detection (IM/DD) over a light emitting diode (LED). In this paper, we compare it with another method, namely in-phase and quadrature wavelength division multiplexing (IQ-WDM). IQ-WDM refers to a method where both I and Q baseband signal from existing WiFi chipsets are transmitted independently at different wavelengths. A comprehensive comparison is done, and our error performance results indicate that IQ-WDM significantly outperforms the frequency upconversion method by 6 dB. We also show that IQ-WDM is robust againsts IQ imbalance.

In this paper the information transfer rate of a single-photon avalanche diode (SPAD) array is investigated. The SPAD array is modelled as a discrete-time Gaussian channel with signal-dependent mean and variance. The SPAD dead time is a parameter which affects the extent of this signal dependency. The SPAD array channel capacity and the properties of the capacity-achieving input distributions are studied. Using a numerical algorithm, the capacity and the optimal input distributions subject to peak and average power constraints are obtained for various array sizes, dead times and background count levels.

Spatial modulation (SM) is a promising technique that reduces inter-channel interference while providing high power efficiency and detection simplicity. In order to ensure the secrecy of SM, precoding and friendly jamming are widely adopted in the literature. However, neither of those methods can take advantage of SM. In this paper, a novel spatial constellation design (SCD) technique is proposed to enhance the physical layer security (PLS) of optical generalized space shift keying (GSSK), which can retain some benefits of SM. Due to the lack of small-scale fading, the quasi-static characteristics of the optical channel is used to tailor the received signal at the legitimate user's (Bob's) side. The PLS of the system is guaranteed by the appropriate selection of the power allocation coefficients for randomly activated light emitting diodes (LEDs). With the aid of Bob's channel state information at the transmitter, the bit error ratio (BER) of Bob is minimized while the BER performance of the potential eavesdroppers (Eves) is significantly degraded. Monte-Carlo simulation results show that the proposed SCD-zero forcing precoding (ZFP) forces Eve to experience a BER of around 0.5 by outperforming both the conventional and ZFP based GSSK for all practical signal-to-noise-ratio regimes and Bob-Eve separations.

A novel modulation scheme for an indoor optical wireless communication (OWC) system referred to as generalized time slot index modulation (GTIM) is proposed for single-carrier frequency domain equalization (SC-FDE) systems. This scheme is further referred to as SC-GTIM. In SC-GTIM, information bits are flexibly encoded onto both the amplitudes and positions of pulses. A set partitioning algorithm is proposed to construct a codebook for the SC-GTIM system. In this paper, SC-GTIM will be compared with the conventional pulse amplitude modulation-based SC-FDE (SC-PAM) and time slot index modulation-based SC-FDE (SC-TIM). By considering a multipath optical wireless channel and the limited dynamic range of a light emitting diode (LED), a considerable bit error ratio (BER) gain can be achieved by SC-GTIM. It is concluded that SC-GTIM is a promising technique for low to medium spectral efficiency transmission and uplink schemes.

In this paper the photon counting characteristics, the information rate and the bit error performance of single-photon avalanche diode (SPAD) arrays are investigated. It is shown that for sufficiently large arrays, the photocount distribution is well approximated by a Gaussian distribution with dead-time-dependent mean and variance. Because of dead time, the SPAD array channel is subject to counting losses, part of which are due to inter-slot interference (ISI) distortions. Consequently, this channel has memory. The information rate of this channel is assessed. Two auxiliary discrete memoryless channels (DMCs) are proposed which provide upper and lower bounds on the SPAD array information rate. It is shown that in sufficiently large arrays, ISI is negligible and the bounds are tight. Under such conditions, the SPAD array channel is precisely modelled as a memoryless channel. A discrete-time Gaussian channel with input-dependent mean and variance is adopted and the properties of the capacity-achieving input distributions are studied. Using a numerical algorithm, the information rate and the capacity-achieving input distributions, subject to peak and average power constraints are obtained. Furthermore, the bit error performance of a SPAD-based system with on-off keying (OOK) is evaluated for various array sizes, dead times and background count levels.

Combining the high-speed data transmission of light fidelity (LiFi) and the ubiquitous coverage of wireless fidelity (WiFi), hybrid LiFi and WiFi networks (HLWNets) are recently proposed to improve the system capacity of indoor wireless communications. Meanwhile, load balancing becomes a challenging issue due to a complete overlap between the coverage areas of LiFi and WiFi. User mobility and light-path blockages further complicate the process of load balancing, since the decision for a horizontal or a vertical handover in a mobile environment with ultra-small cells is non-trivial. These issues are managed separately in most conventional methods, which might cause frequent handovers and compromise throughput. A few studies address these issues jointly for selecting access points at each time instant but require excessive computational complexity. In this paper, a joint optimisation problem is formulated to determine a network-level selection for each user over a period of time. A novel algorithm based on fuzzy logic is also proposed to reduce the computational complexity that is required to solve the optimisation problem. Results show that compared to the conventional method, the proposed approach can improve system throughput by up to 68%, while achieving very low computational complexity.

The increasing number of mobile devices challenges the current radio frequency (RF) networks, e.g. wireless fidelity (WiFi) networks. Light Fidelity (LiFi) is considered as a promising complementary technology, which operates within the visible light spectrum and infrared spectrum. In an indoor scenario, a hybrid LiFi/WiFi network (HLWN) provides a potential solution to future wireless communications where LiFi augments WiFi in providing ultra-high speed and low latency wireless connectivity. In this paper, dynamic load balancing (LB) with handover in HLWNs is studied. The orientation-based random waypoint (ORWP) mobility model is considered to provide a more realistic framework to evaluate the performance of HLWNs. Based on the low-pass filtering effect of the LiFi channel, we firstly propose an orthogonal frequency division multiplexing access (OFDMA)-based resource allocation (RA) method in LiFi systems. Also, an enhanced evolutionary game theory (EGT)-based LB scheme with handover in HLWNs is proposed. In the EGT scheme, each user adapts their strategy to improve the payoff until LB is achieved across LiFi and WiFi. Then, the LiFi system uses the proposed OFDMA-based RA method while the WiFi system applies the carrier sense multiple access with collision detection (CSMA/CA). Simulation results show that in the LiFi system the OFDMA-based RA scheme outperforms the time division multiple access (TDMA) scheme in terms of both user data rate and fairness. Regarding LB in HLWNs, the proposed EGT scheme can achieve a remarkable enhancement in throughput compared to benchmark schemes, such as hard threshold (HT) scheme and random access point assignment (RAA) scheme.

This paper proposes a novel spatial-modulated multicarrier sparse code-division multiple access (SM/MC-SCDMA) system for achieving massive connectivity in device-centric wireless communications. In our SM/MC-SCDMA system, the advantages of both MC signalling and SM are amalgamated to conceive a low-complexity transceiver. Sparse frequency-domain spreading is utilized to mitigate the peak-to-average power ratio (PAPR) of MC signalling, as well as to facilitate low-complexity detection using the message passing algorithm. We then analyze the single-user bit error rate performance of SM/MC-SCDMA systems communicating over frequency-selective fading channels. Furthermore, the performance of SM/MC-SCDMA systems is evaluated based on both Monte-Carlo simulations and analytical results. We demonstrate that our low-complexity SM/MC-SCDMA transceivers are capable of achieving near-maximum likelihood (ML) performance even when the normalized user-load is as high as two, hence constituting a variable solution to support massive connectivity in device-centric wireless systems.

The concept of massive spatial modulation (SM) assisted vertical bell labs space-time (V-BLAST) (SM-VBLAST) system [1] is proposed, where SM symbols (instead of conventional constellation symbols) are mapped onto the VBLAST structure. We show that the proposed SM-VBLAST is a promising massive multiple input multiple output (MIMO) candidate owing to its high throughput and low number of radio frequency (RF) chains used at the transmitter. For the generalized massive SM-VBLAST systems, we first derive both the upper bounds of the average bit error probability (ABEP) and the lower bounds of the ergodic capacity. Then, we develop an efficient error correction mechanism (ECM) assisted compressive sensing (CS) detector whose performance tends to achieve that of the maximum likelihood (ML) detector. Our simulations indicate that the proposed ECM-CS detector is suitable both for massive SM-MIMO based point-to-point and for uplink communications at the cost of a slightly higher complexity than that of the compressive sampling matching pursuit (CoSaMP) based detector in the high SNR region.

The performance of multiple input single output (MISO) and multiple input multiple output (MIMO) systems is limited by spatial channel correlation. This limitation becomes particularly severe in light fidelity (LiFi) systems, which use intensity modulation/direct detection (IM/DD) for data transmission, because light emitting diodes (LEDs) can only emit incoherent light. This paper proposes a novel transmitter structure that provides spatially separated channels and enables optical MISO/MIMO spatial modulation (SM) transmission without the need for a power allocation algorithm or a transmit precoding technique. A single LED array, which consists of multiple LEDs with different characteristics, is considered in an indoor environment. Different numbers of transmit LEDs are chosen for SM transmission based on the relation between channel correlation and bit error probability. It is shown that the proposed structure provides reliable SM transmission when the modulation order for M-ary pulse amplitude modulation (PAM) is considered as 2 and 4. Computer simulations also show that higher modulation order, e.g., 8-PAM, can be supported but at a slightly degraded bit error ratio (BER) performance.

In this paper, a cyclic-prefixed (CP) system with pulse amplitude modulation (PAM) for uplink transmission in light-fidelity (LiFi) is discussed. LiFi supports bi-directional transmission, i.e., downlink and uplink transmissions. However, there have been only a few studies on uplink LiFi despite the fact that the uplink traffic demand increases due to frequent use of, for example, video call and live streaming services. The uplink transmission is designed such that a transmitter has a low peak-to-average ratio as well as a lower computational complexity using a CP in conjunction with PAM. We investigate three different schemes, namely linear equalization (LE) where only a feedforward filter is applied, decision feedback equalization (DFE) where a feedback filter is added at the receiver and Tomlinson-Harashima precoder (THP) where a feedback filter is added at the transmitter. Both feedforward and feedback filters are jointly optimized. In addition, we consider an optical wireless channel with reflections, randomly-located and randomly-oriented user equipment, a human body acting as a reflector and a limited linear dynamic range of a light emitting diode. We conclude that a DFE system is suitable in a low spectral efficiency region, where, compared to an LE system, up to 4 dB gain is achieved. A THP system is suitable for a high spectral efficiency region, where up to 5 dB gain compared to the LE and DFE systems is achieved.

This paper compares an intensity modulation/direct detection (IM/DD) light fidelity (LiFi) system using direct current biased optical orthogonal frequency-division multiplexing (DCO-OFDM) and a coherent LiFi system using homodyne detection. The performance metrics of bit error ratio (BER) and the power efficiency are compared between the two systems. In addition, the channel direct current (DC) gain characteristics of the two systems are investigated. Simulation results show that the coherent system achieves a better BER performance than the IM/DD system, but the transmitter and the receiver of the coherent system need to be perfectly matched. Also, within the same coverage area, the coherent system is more power efficient than the IM/DD system.

As an emerging technology for wireless communications based on the visible light communication (VLC), Light-fidelity (LiFi) is considered as a complementary building block for fifth generation (5G) mobile networks. In this paper, we investigate the optimum field of view (FOV) for the angle diversity receiver (ADR) in LiFi cellular networks. The probability of visibility and the visible area of the ADR is defined. There is a trade off between the channel gain and visibility. An optimisation problem is formulated to maximise the channel gain, and the lower bound of the FOV of photodiodes (PDs) mounted on an ADR is derived. The simulation results show that the data rate is maximised when the FOV is minimised and the structure for the pyramid receiver (PR) achieving highest data rate is the one consisting of 6 PDs each with a FOV of 30 degree.

Light fidelity (LiFi), which occupies part of the optical spectrum, is a promising technology to meet the high-speed wireless data demand. For an additional throughput gain along with low energy consumption, index modulation (IM) has begun to be combined with LiFi systems. The main advantage of IM is its ability to encode information in both a conventional transmit signal as well as the index of the transmit entity such as through the transmitter, subcarrier and time-slot domain indexes. Time-slot IM (TIM) offers additional degrees of freedom to single-carrier (SC) single-input-single-output systems to encode information onto pulse activation patterns. In this paper, we propose a generalized TIM (GTIM) transmission method for SC frequency domain equalization (SC-FDE)-based systems to fully harness the benefits of TIM. In GTIM, both the number and locations of active time-slots are flexible such that they are different for each transmit symbol. Moreover, the active time-slots carry pulse amplitude modulation (PAM) symbols in the signal domain. The proposed system also benefits from SC-FDE which simplifies the transmission and detection complexity at both sides. We also propose a set partitioning algorithm that chooses the optimal transmission symbols set and labels them concurrently. The bit error ratio (BER) performance of GTIM is compared to conventional PAM and TIM under a limited dynamic range constraint and frequency-selectivity. It is shown that the proposed GTIM achieves up to a 3 dB BER performance gain.

The performance of visible light communication (VLC) systems are limited by the modulation bandwidth of light emitting diodes (LEDs), despite the availability of the wide optical spectrum. A multiple-input, multiple-output (MIMO) scheme can be employed to further improve the data rate by means of harnessing the spatial multiplexing gain. Nevertheless, it is known that the MIMO VLC channel can be rank-deficient, and consequently its performance can be degraded significantly. The rank of the MIMO VLC channel is highly affected by the geometries of light emitting diodes and photodiodes (PDs). In this paper, the effects of PD configurations on the MIMO performances are investigated. It will be shown that certain PD configurations can lead to a rank-deficient channel. We propose Irregular PD configurations (IPC) to overcome this issue. Moreover, bi-objective problems are formulized to obtain the optimal IPC. A significant gain in terms of the condition number of the channel is shown with respect to regular PD configurations, which further suggests a benefit of IPC in MIMO VLC.

Sojourn time is an important parameter in the analysis and design of mobile cellular networks. It shows the expected time that a user equipment (UE) remains connected to the serving access point (AP) while moving in the network, i.e. stays within one cell. Therefore, an accurate estimation of the sojourn time is an essential element for mobility management. In this study, an analysis of the sojourn time for indoor light-fidelity (LiFi) cellular networks is presented based on the random waypoint (RWP) mobility model. It is initially assumed that the UE is oriented vertically upward and closed-form analytical expressions are derived. Monte-Carlo simulations are also provided to validate the analytical derivations and to gain more insight into the performance with different orientations. It is shown that the sojourn time mainly varies depending on the speed of the user. However, other factors such as the device orientation and the coverage area of cells also play a role in determining the sojourn time. The results offer several insights and design guidelines in consideration of this parameter for indoor LiFi networks.

As a key enabling technology for next generation wireless connectivity, light fidelity (LiFi) is envisioned to support seamless multi-user access. In this paper, we propose a novel time-domain based multiple access scheme referred to as index time division multiple access (I-TDMA). Our results indicate that I-TDMA provides significant spectral efficiency enhancement compared to ordinary TDMA in a two-users scenario, with minimal added hardware and computational complexity.

Light fidelity (LiFi) is a recently proposed wireless communication technology that employs the light wave as a medium to transmit data. With the coverage range of a few meters, LiFi access points (APs) comprise an ultra-dense attocell network. As a result, users may quickly traverse the coverage area of one LiFi AP, even with a moderate speed. This would cause frequent handovers and a subsequent decrease in the user's throughput. Handover skipping (HS), which enables handovers between two non-adjacent APs, is able to reduce the handover rate. Conventional HS methods rely on the user's trajectory, which is difficult for APs to acquire in practice. In this paper, we propose a novel HS scheme on the basis of received signal strength (RSS). Specifically, the rate of change in RSS is exploited to indicate whether the user is moving towards a certain AP. Since RSS is already used in standard handover schemes, the proposed method does not require extra feedback. Results show that compared to the signal strength strategy, the proposed HS approach can effectively reduce the handover rate, especially for fast- moving users. With respect to the user's throughput, the new method can achieve an increase of up to 67% against the signal strength strategy.

Light-Fidelity (LiFi) has been considered as a complementary technology to radio frequency (RF) communications. The reliability of a LiFi channel highly depends on the availability and alignment of line-of-sight (LOS) links. In this study, we investigate the effect of receiver orientation including both polar and azimuth angles on the LOS channel gain in a LiFi system. The optimum tilt angle is calculated, which depends on both the user's location and direction. The probability density function (PDF) of signal-to-noise ratio (SNR) is derived for on-off keying (OOK) modulation. Using the derived PDF of SNR, the bit-error ratio (BER) of OOK in an additive-white Gaussian noise (AWGN) channel with random orientation of the receiver is evaluated. It is shown that the effect of random orientation is negligible if the optimum tilt angle is chosen. Finally, we assess the effect of random orientation on the Shannon-Hartley upper bound capacity.

Light Fidelity (LiFi) is an emerging technology for future high-speed indoor wireless communications. Co-channel interference (CCI) caused by the dense deployment of LiFi access points (APs) can be effectively mitigated by using angle diversity receivers (ADRs). ADRs require signal combining where the combining weights depend on the selection of serving APs. In this paper, the AP selection (APS) strategy considering handover is studied. A novel APS scheme based on evolutionary game theory (EGT) is proposed for the LiFi network using ADRs. The performance of the proposed scheme is comprehensively analysed and compared with the APS scheme based on signal strength strategy (SSS). The result shows that, in terms of ADRs with SBC/MRC, the EGT-based APS scheme achieves more than 5% improvement in quality of service (QoS) compared with the SSS-based APS scheme. With the sub-optimum weights of maximum ratio combining (MRC) for ADRs, the EGT-MRC scheme can achieve more than 20 Mbps data rate improvement compared with LiFi systems using single photodiode (PD) receiver.

Among the challenges of realizing the full potential of light-fidelity (LiFi) cellular networks are user mobility, random device orientation, and blockage. In this paper, we study the impact of those challenges on the performance of LiFi networks in an indoor environment using measurement-based channel models, unlike existing studies that rely on theoretical channel models. In our paper, we adopt spatial modulation (SM) and consider two configurations for the user equipment (TIE). A multidirectional receiver (MDR) structure is proposed, in which the PDs are located on different sides of the TIE, e.g., a smartphone. This configuration is motivated by the fact that conventional structures exhibit poor performance in the presence of random device orientation and blockage. In fact, we show that the MDR outperforms the benchmark structure by over 10 dB at bit-error ratio (BER) of 3.8 × 10-3. Moreover, an adaptive access point (AP) selection scheme for the SM is considered, where the number of APs is chosen adaptively in an effort to achieve the lowest energy requirement for a target BER and spectral efficiency. The user performance with random orientation and blockage in the entire room is evaluated for sitting and walking activities, for which the orientation-based random waypoint (ORWP) mobility model is invoked. Furthermore, we demonstrate that the proposed adaptive technique with SM outperforms the conventional spatial multiplexing system. We also study the performance of the underlying system on the uplink channel where we apply the same techniques used for the downlink channel. It is shown analytically that the multidirectional transmitter (MDT) with adaptive SM is highly energy efficient.

Light-fidelity (LiFi) is a networked optical wireless communication (OWC) solution for high-speed indoor connectivity for fixed and mobile optical communications. Unlike conventional radio frequency wireless systems, the OWC channel is not isotropic, meaning that the device orientation affects the channel gain significantly, particularly for mobile users. However, due to the lack of a proper model for device orientation, many studies have assumed that the receiver is vertically upward and fixed. In this paper, a novel model for device orientation based on experimental measurements of 40 participants has been proposed. It is shown that the probability density function (PDF) of the polar angle can be modeled either based on a Laplace (for static users) or a Gaussian (for mobile users) distribution. In addition, a closed-form expression is obtained for the PDF of the cosine of the incidence angle based on which the line-of-sight (LOS) channel gain is described in OWC channels. An approximation of this PDF based on the truncated Laplace is proposed and the accuracy of this approximation is confirmed by the Kolmogorov-Smirnov distance. Moreover, the statistics of the LOS channel gain are calculated and the random orientation of a user equipment (UE) is modeled as a random process. The influence of the random orientation on signal-to-noise-ratio performance of OWC systems has been evaluated. Finally, an orientation-based random waypoint (ORWP) mobility model is proposed by considering the random orientation of the UE during the user's movement. The performance of ORWP is assessed on the handover rate and it is shown that it is important to take the random orientation into account.

A novel multiple-input and multiple-output (MIMO) transmission scheme termed as generalized quadrature spatial modulation (G-QSM) is proposed. It amalgamates the concept of quadrature spatial modulation (QSM) and spatial multiplexing for the sake of achieving a high throughput, despite relying on a low number of radio frequency (RF) chains. In the proposed G-QSM scheme, the conventional constellation points of the spatial multiplexing structure are replaced by the QSM symbols, hence the information bits are conveyed both by the antenna indices as well as by the classic amplitude/phase modulated (APM) constellation points. The upper bounds of the average bit error probability (ABEP) of the proposed G-QSM system in high throughput massive MIMO configurations are derived. Furthermore, an efficient multipath orthogonal matching pursuit (EM-OMP)-based compressive sensing (CS) detector is developed for our proposed G-QSM system. Both our analytical and simulation results demonstrated that the proposed scheme is capable of providing considerable performance gains over the existing schemes in massive MIMO configurations.

In this paper, an analytical model for the coverage analysis of multiuser visible light communication (VLC) networks is presented, taking into account the cooperation among access points (APs). Specifically, the cooperation is realized through joint transmission (JT), where the coordinated APs jointly transmit data to users in a noncoherent or coherent manner to reduce the inter-cell interference and enhance the useful signal power. Using a second-order moment matching approach, we find approximate distribution functions of the signal power and interference, and derive tractable results for the network coverage probability based on the signal-to-interference-plus-noise ratio (SINR). For both noncoherent and coherent JT, the derived coverage probabiklity are further simplified into closed forms when the communication link is interference-limited. We validate the derived results through Monte Carlo simulations and apply them to study behaviors and trends of the network under various parameter settings. Results show that JT can improve the coverage performance of the network and coherent JT can provide higher performance gains than its noncoherent counterpart, at the cost of higher implementation complexities.

The problem of backhauling for optical attocell networks has been approached by a number of wired solutions such as in-building power line communication (PLC), Ethernet, and optical fiber. In this paper, an alternative solution is proposed based on the wireless optical communication in visible light and infrared (IR) bands. A thorough analysis of signal-to-noise-plus-interference ratio (SINR) is elaborated for a multi-user optical attocell network based on the direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and decode-and-forward (DF) relaying, taking into account the effects of inter-backhaul and backhaul-to-access interferences. Inspired by concepts developed for radio frequency (RF) cellular networks, full-reuse visible light (FR-VL) and in-band visible light (IB-VL) bandwidth allocation policies are proposed to realize backhauling in the visible light band. The transmission power is opportunistically minimized to enhance the backhaul power efficiency. For a two-tier FR-VL network, there is a technological challenge due to the limited capacity of the bottleneck backhaul link. The IR band is employed to add an extra degree of freedom for the backhaul capacity. For the IR backhaul system, a power-bandwidth trade-off formulation is presented. Closed form analytical expressions are derived for the corresponding power control coefficients. Finally, the network sum rate performance is studied using extensive Monte Carlo simulations.

Light-fidelity (LiFi) is a wireless communication technology that employs both infrared and visible light spectra to support multiuser access and user mobility. Considering the small wavelength of light, the optical channel is affected by the random orientation of user equipment (TIE). In this paper, a random process model for changes in the TIE orientation is proposed based on the data measurements. We show that the coherence time of the random orientation is in the order of hundreds of milliseconds. Therefore, an indoor optical wireless channel can be treated as a slowly varying channel as its delay spread is typically in the order of nanoseconds. A study of the orientation model on the performance of direct-current-biased orthogonal frequency-division multiplexing (DC-OFDM) is also presented. The performance analysis of the DC-OFDM system incorporates the effect of a diffuse link due to reflection and blockage by the user. The results show that the diffuse link and the blockage have significant effects, especially if the TIE is located relatively far away from an access point (AP). It is shown that the effect is notable if the horizontal distance between the TIE and the AP is greater than 1.5 m in a typical 5 × 3.5 × 3 m3indoor room.

Doppler effect is a fundamental phenomenon that appears in wave propagation, where a moving observer experiences dilation or contraction of wavelength of a wave. It also appears in radio frequency (RF) wireless communication when there exists a relative movement between the transmitter and the receiver, and is widely considered as a major impairment for reliable wireless communication. The current paper proposes a wireless receiver that carefully exploits Doppler effect for cochannel interference mitigation. The proposed system-termed as DAIM to stand for Doppler Assisted Interference Mitigation-exploits the propagation environment and the network topology, and consists of an access point with a rotating drum antenna. The rotating drum receive antenna is designed in such a way that it shifts the interference signals away from the desired signal band. This paper includes a detailed system model and a simulation study. The results show that under favorable fading conditions, co-channel interference can be significantly reduced if not eliminated. Therefore, it is anticipated that more sophisticated wireless systems and networks can be designed by extending the basic system proposed herein.

In this paper, a novel physical layer security technique is presented for indoor visible light communications (VLC) systems based on optical spatial shift keying (OSSK). Transmitters are equipped with light emitting diode (LED) arrays and in OSSK information is carried only by the LED indices rather than the transmitted symbols themselves. Assuming that the source has the channel state information (CSI) of the optical channel gains between the LEDs and a legitimate user, a pre-equalizer is designed for the transmitter, which transforms the actual channel gains into a new channel realization in which equalized channel coefficients are widely apart from each other in multiple LED fixtures within the power constraint. Since the eavesdropper's channel is not equalized, its bit error rate performance is profoundly degraded. In addition, it is shown that the proposed technique does not need to have the CSI of the eavesdropper. Also an analytical expression is derived to evaluate the capacity of OSSK exactly from which the achievable secrecy capacity of the proposed scheme is obtained easily by computer simulations. In the computer simulations, the channels for different scenarios are generated by Zemax©, which is an optical design software with ray-tracing capabilities. Simulation results show that, as excellent bit error rate (BER) performance is achieved by the legitimate user, the performance of the eavesdropper degrades to a level that it is not possible to receive any meaningful information.

The backhaul of tens and hundreds of light fidelity (LiFi)-enabled light bulbs constitutes a major challenge. In this paper, a multi-hop wireless backhaul configuration is investigated for optical attocell networks. The backhaul system proposed in this work uses visible light communication (VLC) to realize wireless connections between adjacent LiFi base stations (BSs). By using a tree topology for the backhaul system, the central BS is collocated with the gateway, and the other BSs are connected to the core network via multi-hop wireless backhaul links with the gateway based on decode-and-forward (DF) relaying. The system-level modeling and analysis is presented for the end-to-end sum rate of multiple user equipment (UE) devices with random coordinates in the network. The closest backhaul link to the gateway needs to support multiple independent data flows at the same time. To this end, novel bandwidth scheduling policies are proposed and evaluated. In addition, in order to opportunistically reduce the transmission power of the backhaul system, novel power control schemes are derived. Performance gains of the proposed schemes and their impact on the average sum rate of the downlink optical attocell network are studied using Monte Carlo simulations.

With the significant increase in wireless data traffic in recent years, the scarce radio frequency (RF) spectrum can no longer guarantee the capacity needed to meet the future growth in mobile traffic demand. New applications such as virtual reality and augmented reality further increase this demand. The emergence of high speed visible light communication (VLC) offers an opportunity to mitigate the looming spectrum crisis. However, the power consumption of a VLC link due to incoherent transmission is a major concern for system designers. Only a few studies have addressed this issue. In this paper, a theoretical study on the relationship between the power consumption and the data rate of a high speed VLC link is presented. The considered VLC link is based on direct-current-biased optical orthogonal frequency division multiplexing (DCO - OFDM) and on-off keying (OOK). The effects of spatial modulation (SM) are also investigated. The presented results show that the optimal operating bandwidth can be determined for a SM DCO-OFDM system to achieve the highest power efficiency possible. The study provides a guideline to configure a VLC system in order to optimize power efficiency for different data rate requirements.

A single photon avalanche diode (SPAD) receiver can provide a significantly improved detection sensitivity over conventional photodiodes. However, upon detecting a photon, the SPAD receiver is unable to respond to subsequent incident photons for a certain period of time called dead time. When the SPAD receiver is counting in consecutive intervals and its dead time is relatively long compared to the counting interval, the dead time in one interval may overlap the next intervals, causing counting losses in those intervals. In this paper, the photocount statistics of an active quenching SPAD receiver with a long dead time is investigated. While detecting in consecutive time intervals, the SPAD receiver can be modeled as a Markov chain. It is shown that the SPAD experiences a transient period, before eventually reaching its steady state. The duration of the transient period increases with both the dead time and the signal photon rate. The bit error performance of the system with an on-off keying (OOK) modulated signal is also evaluated. Our results show that the SPAD's dead time significantly degrades the error performance of high data rate links.

In this paper, a novel LED power control algorithm is proposed for an OSM based communications system. The proposed algorithm constantly controls the power of the LEDs employed at the transmitter side to provide the minimum overall SER for a mobile user. An exact analytical SER expression is obtained for the general MISO OSM system. It is shown that the proposed algorithm solves a convex optimization problem where linear constraints are used to minimize the SER. Finally, a 2x1 MISO-OSM example is presented to prove the effectiveness of the proposed algorithm. The computer simulations show that the power adaptive MISO-OSM system with the proposed algorithm achieves the average SER of 3.87E-6 for 30 dB SNR.

The key requirement of fifth generation (5G) systems is seamless connectivity for anyone and anything. The current form of terrestrial mobile systems are mostly mature enough to provide connectivity for any terrestrial user and device. Nowadays, several projects seek to enhance the terrestrial systems to provide such connectivity to airborne users. However, providing connectivity for every airborne user needs a special design for different aircraft types. Due to the shape and structure of an aircraft, widely used system and channel models in terrestrial systems cannot be directly applied to onboard communication systems. In this paper, the realistic throughput performance of three different wireless communications technologies namely, Long Term Evolution (LTE), Wireless-Fidelity (WiFi) and Light-Fidelity (LiFi) is investigated inside a medium-sized commercial aircraft. It is shown that the average user throughput performance is 1.6 Mbps, 2.7 Mbps and 16.6 Mbps for onboard WiFi, LTE and LiFi systems, respectively.

This paper proposes a coordinated multi-point transmission (CoMP) scheduling technique in combination with inter-cell interference coordination (ICIC) techniques, almost blank subframes (ABS) and reduced-power ABS (RP-ABS). It investigates the scheduler performance in the context of an aircraft in- cabin Long Term Evolution (LTE) system deployment. The scheduling problem is solved for an ideal case and a real-world case where LTE system constraints such as common modulation and coding scheme (MCS) assignment per user, channel quality indicator (CQI) reporting and LTE resource allocation (RA) types are considered in the scheduling problem in order to provide a meaningful comparison between real-world and ideal performance. The tradeoff between fairness, which is defined as the lowest variation in user data rates across a large user population, and system throughput is considered as the performance evaluation metric of the system. It is shown that when the LTE constraints are employed, the ideal user data rate and system throughput performance is drastically degraded regardless of the used ICIC technique. In the ideal case, where the LTE constraints are not considered, RP-ABS becomes the optimum tradeoff between fairness and system throughput which achieves a variation of 4.4 Mbps between the lowest and highest user rate, an average user rate of 3 Mbps and an average system throughput of 500 Mbps. However, when the LTE system constraints are taken into account, ABS becomes the optimum technique for the considered onboard system which achieves a variation of 1.7 Mbps, an average user rate of 1.5 Mbps and an average system throughput of 265 Mbps.

This paper focuses on the handover probability as a result of the random rotation of a user equipment (UE) in hybrid light-fidelity (LiFi) and radio frequency (RF) networks. A received signal strength indicator (RSSI)-based handover algorithm is considered in this study. Using RSSI as the user association rule does not guarantee that the randomly-oriented UE is always associated with the nearest access point (AP). In some cases, depending on the orientation of the UE, the received signal powers from the LiFi APs are very weak and unreliable. Therefore, a vertical handover from the LiFi AP to the RF AP is required to maintain the user quality of service. Hence, it is essential to study the handover probability due to the change of orientation. A theoretical analysis of the handover probability based on a Markov chain model is provided. The analytical results are confirmed by the Monte Carlo simulation. The effects of some parameters, such as the threshold, hysteresis level and the trade-off between the frequency and the delay of handover are presented in this paper.

Hybrid light fidelity (LiFi) and wireless fidelity (WiFi) networks (HLWNets) have been recently proposed as a promising technique for indoor wireless communications. Such a network combines the high-speed transmission of LiFi and the ubiquitous coverage of WiFi. Meanwhile, in addition to user mobility, intermittent light-path blockages make the handover issue intricate in HLWNets. Also, load balancing (LB) is necessary because the coverage areas of LiFi and WiFi networks are completely overlapped. This further increases the difficulty in handover as the decision for a vertical or a horizontal handover in a mobile environment with ultra-small cells is non-trival. Aimed at providing an optimal solution to the access point selection (APS) with given channel state information (CSI), the conventional LB method might cause frequent and unnecessary handovers. In this paper, a joint optimisation problem is formulated to simultaneously consider LB and handover in HLWNets. Results show that compared with the conventional LB method, the proposed method can improve the system throughput by up to 60%.

Visible light communications (VLC) and Millimetre wave (mmWave) systems are two emerging technologies for short-range mobile communications. One of the limiting factors of both VLC and mmWave systems is the random orientation of mobile devices that can significantly affect the channel gain in both systems. Since there is no proper model for device orientation, many studies have assumed that device orientation is fixed or modelled as a uniform distribution. To address this issue, an experimental study of mobile user behaviour is conducted and a statistical orientation model is proposed in this paper. The results show that the probability density function (PDF) of the elevation angle follows a Laplace distribution. Based on the statistical orientation model, Monte-Carlo simulations are carried out to analyse the system performance of VLC and mmWave systems. The statistics of signal-to-noise-ratio (SNR) are compared with the experiment-based simulation results in both VLC and mmWave systems.

Visible light communication and its use in wireless networks, which is referred to as light fidelity (LiFi), can potentially provide ubiquitous indoor broadband connections. Most of the existing studies on LiFi consider a device that always faces upward, disregarding the importance of the orientation of the device. This paper presents experimental results on the randomly-oriented devices and a statistical model of the LiFi channel considering the random orientation. The distribution of the cosine of the incidence angle is shown to have a Laplacian form. This distribution is then applied to study the signal-to-noise ratio of the randomly-oriented and the upward-facing devices. Finally, we show that neglecting the random orientation can lead to inaccurate conclusions. Hence, it is important to take the random orientation into account in future LiFi network analyses.

Light fidelity (LiFi), using light emitting devices such as light emitting diodes (LEDs) which are operating in the visible light spectrum between 400 and 800 THz, provides a new layer of wireless connectivity within existing heterogeneous radio frequency wireless networks. Link data rates of 10 Gbps from a single transmitter have been demonstrated under ideal laboratory conditions. Synchronization is one of these issues usually assumed to be ideal. However, in a practical deployment, this is no longer a valid assumption. Therefore, we propose for the first time a low-complexity maximum likelihood-based timing synchronization process that includes frame detection and sampling clock synchronization for direct current-biased optical orthogonal frequency division multiplexing LiFi systems. The proposed timing synchronization structure can reduce the high-complexity two-dimensional search to two low-complexity one-dimensional searches for frame detection and sampling clock synchronization. By employing a single training block, frame detection can be realized, and then sampling clock offset (SCO) and channels can be estimated jointly. We propose three frame detection approaches, which are robust against the combined effects of both SCO and the low-pass characteristic of LEDs. Furthermore, we derive the Cramér–Rao lower bounds (CRBs) of SCO and channel estimations, respectively. In order to minimize the CRBs and improve synchronization performance, a single training block is designed based on the optimization of training sequences, the selection of training length, and the selection of direct current (DC) bias. Therefore, the designed training block allows us to analyze the trade-offs between estimation accuracy, spectral efficiency, energy efficiency, and complexity. The proposed timing synchronization mechanism demonstrates low complexity and robustness benefits and provides performance significantly better than achieved with existing methods.

As a complementary technology for conventional radio frequency communication, visible light communication (VLC) is a potential form of the optical wireless communication, which can provide both communication and illumination simultaneously. Since load balancing and power control for interference management are key challenges in the network deployment, we consider a joint user association and power allocation scheme in a cell-free VLC network to improve the system performance. It is mathematically formulated as a non-convex network utility maximization problem in consideration of the user fairness, load balancing, and power control. To tackle this non-convex problem, we divide it into two subproblems (i.e., the user association subproblem and the power allocation subproblem) and solve them with the dual projected gradient algorithm and successive convex approximation algorithm iteratively until a stationary point is found. Simulation results verify that significant gain can be achieved with the proposed scheme compared with the user association schemes without consideration of the power control.

In this paper, we study the physical-layer security in a 3-D multiuser visible light communication (VLC) network. The locations of access points (APs) and mobile users are modeled as two 2-D, independent and homogeneous Poisson point processes at distinct heights. Using mathematical tools from stochastic geometry, we provide a new analytical framework to characterize the secrecy performance in multiuser VLC networks. Closed-form results for the outage probability and the ergodic secrecy rate are derived for networks without AP cooperation. Considering the cooperation among APs, we give tight lower and upper bounds on the secrecy outage probability and the ergodic secrecy rate. To further enhance the secrecy performance at the legitimate user, a disk-shaped secrecy protected zone is implemented in the vicinity of the transmit AP. Based on the obtained results, it is shown that cooperating neighboring APs in a multiuser VLC network can bring performance gains on the secrecy rate, but only to a limited extent. We also show that building an eavesdropper-free protected zone around the AP significantly improves the secrecy performance of legitimate users, which appears to be a promising solution for the design of multiuser VLC networks with high security requirements.

In this paper, a comprehensive analytical approach is presented for modeling the counting statistics of active quenching and passive quenching single-photon avalanche diode (SPAD) detectors. It is shown that, unlike ideal photon counting receiver for which the detection process is described by a Poisson arrival process, photon counts in practical SPAD receivers do not follow a Poisson distribution and are highly affected by the dead time caused by the quenching circuit. Using the concepts of renewal theory, the exact expressions for the probability distribution and moments (mean and variance) of photocounts in the presence of dead time are derived for both active quenching and passive quenching SPADs. The derived probability distributions are validated through Monte Carlo simulations and it is demonstrated that the moments match with the existing empirical models for the moments of SPAD photocounts. Furthermore, an optical communication system with on-off keying and binary pulse position modulation is considered and the bit error performance of the system for different dead time values and background count levels is evaluated.

Channel adaptive signaling, which is based on feedback, can result in almost any performance metric enhancement. Unlike the radio frequency channel, the optical wireless communication (OWC) channel is relatively deterministic. This feature of OWC channels enables a potential improvement of the bidirectional user throughput by reducing the amount of feedback. Light-Fidelity (LiFi) is a subset of OWCs, and it is a bidirectional, high-speed, and fully networked wireless communication technology where visible light and infrared are used in downlink and uplink, respectively. In this paper, two techniques for reducing the amount of feedback in LiFi cellular networks are proposed: 1) limited-content feedback scheme based on reducing the content of feedback information and 2) limited-frequency feedback scheme based on the update interval. Furthermore, based on the random waypoint mobility model, the optimum update interval, which provides maximum bidirectional user equipment throughput, has been derived. Results show that the proposed schemes can achieve better average overall throughput compared with the benchmark one-bit feedback and full-feedback mechanisms.

Spatial modulation (SM) is a unique single-stream, multiple-input multiple-output (MIMO) transmission technique. One key property of the SM is that a single transmit antenna is activated at any given time, which completely avoids inter-channel interference in a single-user scenario. In the context of multi-user cellular networks, inter-cell interference (ICI) mitigation becomes the main challenge for the SM. Network MIMO is a technique that employs precoding to enable ICI cancellation among neighbouring cells. However, the cell-edge users might still experience a high level of the ICI due to the significant channel attenuation. In this paper, we propose a novel cooperative scheme based on the SM, named cooperative SM (CoSM). The basic concept is to reschedule the transmit antennas of multiple base stations for multiple co-channel users, so as to maximize the antenna diversity gain. Unlike the traditional antenna selection for a single user, the involvement of multiple users might cause conflict due to the selection of the same antenna. An antenna rescheduling scheme (ARS) is thus proposed to address this issue. The performance of the ARS is theoretically analyzed, and closed-form expressions are derived for the probability distribution of the received SNR. Also, a novel three-tier cellular architecture is proposed to accommodate the CoSM within the context of the network MIMO. Results show that the CoSM can improve signal-to-interference-plus-noise ratio by up to 4 dB over the network MIMO. In addition, compared with spatial multiplexing using the same ARS, the CoSM can halve the energy consumption while achieving the same bit error rate.

LiFi attocell access networks will be deployed everywhere to support diverse applications and service provisioning to various end-users. The LiFi infrastructure providers will need to offer LiFi access points (APs) resources as a service. This, however, requires a research challenge to be solved to dynamically and effectively allocate resources among service providers (SPs) while guaranteeing performance isolation among them and their respective users. This paper introduces an autonomic resource slicing (virtualization) scheme, which realizes autonomic management and configuration of virtual APs, in a LiFi attocell access network, based on SPs and their users service requirements. The developed scheme comprises of traffic analysis and classification, a local AP controller, downlink and uplink slice resources manager, traffic measurement, and information collection modules. It also contains a hybrid medium access protocol and an extended token bucket fair queueing algorithm to support uplink access virtualization and spectrum slicing. The proposed resource slicing scheme collects and analyzes the traffic statistics of the different applications supported on the slices defined in each LiFi AP and distributes the available resources fairly and proportionally among them. It uses a control algorithm to adjust the minimum contention window of user devices to achieve the target throughput and ensure airtime fairness among SPs and their users. The developed scheme has been extensively evaluated using OMNeT++. The obtained results show various resource slicing capabilities to support differentiated services and performance isolation.

In this paper, a new non-stationary regular-shaped geometry-based stochastic model (RS-GBSM) is proposed for vehicular visible light communications (VVLC) channels. The proposed model utilizes a combined two-ring model and a confocal ellipse model, in which the received optical power is constructed as a sum of single-bounced (SB) and double-bounced (DB) components, in addition to the line-of-sight (LoS) component. Using the proposed RS-GBSM, the channel impulse response is generated and utilized to investigate VVLC channel characteristics, such as channel gain and root-mean-square (RMS) delay spread. The received optical power is computed considering the distance between the optical transmitter (Tx) and the optical receiver (Rx). Moreover, the impact of the Rx height on the received power is considered for the LoS scenario. The results show that the LoS power highly depends on the distance, Rx height, and the optical source pattern. For the SB components, it is confirmed that the channel gain in dB and the RMS delay spread follow Gaussian distributions. Finally, the results indicate that the detected optical power from the DB components is low enough to be overlooked.

The co-deployment of radio frequency (RF) and visible light communication (VLC) technologies has been investigated in indoor environments to enhance network performances and to address specific quality-of-service (QoS) constraints. In this paper, we explore the benefits of employing both technologies when the QoS requirements are imposed as limits on the buffer overflow and delay violation probabilities, which are important metrics in designing low latency wireless networks. Particularly, we consider a multi-mechanism scenario that utilizes RF and VLC links for data transmission in an indoor environment, and then propose a link selection process through which the transmitter sends data over the link that sustains the desired QoS guarantees the most. Considering an ON-OFF data source, we employ the maximum average data arrival rate at the transmitter buffer and the non-asymptotic bounds on data buffering delay as the main performance measures. We formulate the performance measures under the assumption that both links are subject to average and peak power constraints. Furthermore, we investigate the performance levels when either one of the two links is used for data transmission, or when both are used simultaneously. Finally, we show the impacts of different physical layer parameters on the system performance through numerical analysis.

In this paper, a new mathematical framework for the coverage probability analysis of multiuser visible light communication (VLC) networks is presented. It takes into account the idle probability of access points (APs) that are not associated with any users and hence do not function as the source of interference. The idle probability of APs is evident especially in underloaded networks as well as general networks that operate with an AP sleep strategy to save energy and/or minimize the co-channel interference. Due to the absence of the “multipath fading” effect, the evaluation of the distribution function of the signal-to-interference-plus-noise ratio (SINR) is more challenging in VLC networks than in radio frequency-based cellular networks. By using the statistical-equivalent transformation of the SINR, analytical expressions for the coverage probability are derived and given in tractable forms. Comparing the derived results with extensive Monte Carlo simulations, we show that assuming a thinned homogeneous Poisson point process for modeling active APs is valid in general, and it gives close results to the exact ones when the density of users is no less than the density of APs in the network. Both analytical and simulation results show that, for typical receiver noise levels (~-117 dBm), approximating the SINR by the signal-to-interference ratio is sufficiently accurate for the coverage analysis in VLC networks.

In this paper, a radically new anticipatory perspective is taken into account when designing the user-to-access point (AP) associations for indoor visible light communications (VLC) networks, in the presence of users' mobility and wireless-traffic dynamics. In its simplest guise, by considering the users' future locations and their predicted traffic dynamics, the novel anticipatory association prepares the APs for users in advance, resulting in an enhanced locationand delayawareness. This is technically realized by our contrived design of an efficient approximate dynamic programming algorithm. More importantly, this paper is in contrast to most of the current research in the area of indoor VLC networks, where a static network environment was mainly considered. Hence, this paper is able to draw insights on the performance tradeoff between delay and throughput in dynamic indoor VLC networks. It is shown that the novel anticipatory design is capable of significantly outperforming the conventional benchmarking designs, striking an attractive performance trade-off between delay and throughput. Quantitatively, the average system queue backlog is reduced from 15 to 8 [ms], when comparing the design advocated to the conventional benchmark at the peruser throughput of 100 [Mbps], in a 15 × 15 × 5 [m3] indoor environment associated with 8 × 8 APs and 20 users walking at 1 [m/s].

We propose in this paper a physical‐layer security (PLS) scheme for dual‐hop cooperative networks in an effort to enhance the communications secrecy. The underlying model comprises a transmitting node (Alice), a legitimate node (Bob), and an eavesdropper (Eve). It is assumed that there is no direct link between Alice and Bob, and the communication between them is done through trusted relays over two phases. In the first phase, precoding‐aided spatial modulation (PSM) is employed, owing to its low interception probability, while simultaneously transmitting a jamming signal from Bob. In the second phase, the selected relay detects and transmits the intended signal, whereas the remaining relays transmit the jamming signal received from Bob. We analyze the performance of the proposed scheme in terms of the ergodic secrecy capacity (ESC), the secrecy outage probability (SOP), and the bit error rate (BER) at Bob and Eve. We obtain closed‐form expressions for the ESC and SOP and we derive very tight upper‐bounds for the BER. We also optimize the performance with respect to the power allocation among the participating relays in the second phase. We provide examples with numerical and simulation results through which we demonstrate the effectiveness of the proposed scheme.

An aircraft cabin can be considered as an unusual propagation environment due to the construction materials and interior furniture of the aircraft. In this paper, a measurement and modeling procedure is proposed to characterize radio propagation inside an aircraft. The proposed procedure is divided into three phases in order to characterize (1) the propagation direction; (2) the reflections from the cockpit and tail ends of the cabin; and (3) the reflections from the sidewalls of the cabin. The first phase of the proposed measurement procedure is conducted in an Airbus A321 aircraft for the 1.8 GHz frequency band. In the measurements, the receive antenna is located in 24 different passenger seats and three different transmit antenna locations are considered to investigate how the propagation characteristics change according to the propagation direction. According to the measured values, we found that the path loss exponent of the distance dependent path loss model (decay constant) differs according to the propagation direction and distance. The path loss exponent is found equal to 1.5 and 2.6 for both the propagation directions when the transmitter-receiver separation is shorter than 5 meters and longer than 14 meters, respectively. However, when the distances between 5 and 14 meters are considered, the path loss exponent is found equal to 2 for in- flight propagation direction and 2.3 for in-rear propagation direction. In light of the obtained radio propagation characteristics for the first phase of the proposed procedure, it is worthwhile to follow the remaining phases in order to improve channel propagation modeling inside an aircraft.

Light-fidelity (LiFi) extends visible light communication into high-speed and multiuser wireless networks. As in cellular networks, co-channel interference (CCI) stemming from other access points (APs) sharing the same transmission resources is the main limiting factor in the performances of the LiFi networks; therefore, it is important to study characteristics of CCI. This paper studies the statistical characteristics of aggregate signal interference iaggin downlink LiFi networks where LiFi-enabled APs are randomly deployed following a Poisson point process (PPP). One of the advantages of considering iaggcompared to the aggregate power interference is that parameters of modulation schemes and room dimensions can both be incorporated. The paper's key finding is that the energy of iaggis not monotonically decreasing as the vertical distance of user of interest and the AP increases. In addition, we show that the distribution of iaggrapidly approaches a Gaussian distribution as PPP intensity increases.

Light Fidelity (LiFi) is a recently proposed technology that combines illumination and high speed wireless communication using light emitting diodes (LEDs). Unlike radio frequency (RF) channels, LiFi channels do not exhibit fading characteristics as the detector size is much larger than the wavelength. Another distinguishing feature is that LiFi channels are mainly affected by the low-pass filtering effect caused by LEDs, while the multipath effect is inconspicuous. Therefore, unlike in RF systems, LiFi systems can hardly achieve the conventional multi-user diversity gain when using orthogonal frequency division multiplexing access (OFDMA). Due to the LED characteristics, users with high direct current (DC) signal-to-noise ratio (SNR) may be able to use a large modulation bandwidth in LiFi systems. This means that users with good channel conditions can be more likely to utilise high-frequency resources. In this paper, resource allocation (RA) in OFDMAbased LiFi systems is investigated and an optimal RA scheme and a low- complexity RA scheme are proposed. Simulation results show that by efficiently using high- frequency bandwidth resources, the RA schemes in OFDMA systems outperform those in TDMA systems in terms of both data rate and user fairness. Also, the low-complexity RA scheme is able to achieve nearoptimal performance at a reduction of 90% in computational complexity.

In this paper, physical layer security for optical attocell networks is studied. The secrecy throughput of four different types of optical attocell deployment, hexagonal (HEX), square, poisson point process (PPP) and hard-core point process (HCPP) are compared. In order to improve system confidentiality, angle diversity transmitters are implemented in optical attocell networks. An angle diversity transmitter usually consists of multiple narrow-beam light-emitting diode (LED) elements with different orientations. Angle diversity transmitters are very suitable for secure transmission since data transmission via narrow light beams can effectively avoid the leakage of messages. Results show that HEX deployment performs the best in terms of secrecy throughput while PPP deployment performs the worst. With the use of angle diversity transmitters, secrecy throughput of optical attocell networks can be significantly improved. Compared with single-element transmitter, over 6 times secrecy throughput improvement can be obtained by using 18-element angle diversity transmitter.

Light-fidelity (Li-Fi) is an emerging technology for wireless optical networking using the principle of visible light communication (VLC). Li-Fi attocells are smaller in size than the radio frequency (RF) femtocells, suitable for deploying ultradense cellular networks. In this paper, a novel wireless backhaul solution is proposed for indoor Li-Fi attocell networks using VLC, which is already embedded in the Li-Fi base station (BS) units. Since the backhaul links operate in the visible light spectrum, two methods are proposed for bandwidth allocation between the access and backhaul links, namely, full frequency reuse (FR) and in-band (IB). In order to realize dual-hop transmission over the backhaul and access links, both amplify-and-forward (AF) and decode-and-forward (DF) relaying protocols are analyzed. Considering a direct current optical orthogonal frequency division multiplexing (DCO-OFDM)-based multiple access system, novel signal-to-interference-plus-noise ratio (SINR) and spectral efficiency expressions are then derived for user equipment (UE) randomly distributed in each attocell. Downlink performance of the optical attocell network is assessed in terms of the average spectral efficiency using Monte Carlo simulations. Guidelines are given for the design of the proposed wireless backhaul system.

In the National Broadband Plan released by the Federal Communications Commission (FCC) in 2010, it was predicted that with the ever increasing demand for data, the spectrum in 2009 will no longer be sufficient in 2013. The spectrum need was obtained based on the predicted mobile data traffic, number of cell sites and achievable spectral efficiency for the US in the FCC's report. In this study, predictions made by the FCC are compared with the achieved values from 2009 to 2016 in order to validate the required spectrum prediction. Furthermore, trends seen from 2009 to 2016 are used to predict when the required spectrum that satisfies the mobile data demand will exceed the entire radio frequency (RF) spectrum. According to the predicted mobile data traffic, number of cell sites and achievable spectral efficiency for the US, the entire RF spectrum will be fully used around 2035. Therefore, unlocking the visible light spectrum and deploying indoor/outdoor optical wireless systems would be a necessity for the next generation communication technologies in order to alleviate the spectrum crunch.

This work aims to debunk the myth that visible light communication (VLC) systems cannot work under the presence of sunlight. It provides an investigation of an outdoor VLC system model in the presence of solar irradiance. Worst-case scenarios are considered in terms of location, link orientation and choice of photodiode. The effect of solar irradiance is investigated in terms of data rate and bit error rate (BER) degradation. An optical bandpass blue filter is shown to compensate for 53% of the reduced data rate in the presence of sunlight. Reliable communications above 1 Gb/s can be achieved using simple optical filtering under the effect of solar irradiance.

The access point (AP) in many cases requires information about the channel condition of user equipment (UE) for better resource allocation and scheduling purposes. Although this feedback information can contribute to enhanced performance, it is at a cost of uplink throughput degradation. Limited feedback (LF) techniques offer the ways to reduce the amount of feedback at the AP. Light-Fidelity (Li-Fi) is a new high-speed and fully networked optical wireless technology where its channel is fairly static. Considering this feature, we proposed a new LF method based on the channel update interval. The optimum value for the update interval has been derived and it can provide maximum sum throughput of uplink and downlink. Results confirm that the proposed LF method can yield enhanced overall throughput compared to the traditional LF techniques.

We propose in this paper a physical layer security (PLS) scheme for dual-hop cooperative networks in an effort to enhance the communications secrecy. The underlying model comprises a transmitting node (Alice), a legitimate node (Bob) and an eavesdropper (Eve). It is assumed that there is no direct link between Alice and Bob, and the communication between them is done through trusted relays over two phases. In the first phase, precoded spatial modulation (PSM) is employed, owing to its low interception probability, while simultaneously transmitting a jamming signal from Bob. In the second phase, the selected relay detects and transmits the intended signal, whereas the remaining relays transmit just the jamming signal received from Bob. We analyze the performance of the proposed scheme in terms of the ergodic secrecy capacity (ESC) and secrecy outage probability (SOP) where we obtain closed form expressions for those metrics. We also optimize the performance with respect to the power allocation among the participating relays in the second phase. We provide examples with numerical and simulations results through which we demonstrate the effectiveness of the proposed scheme. We also provide a comparison of the bit error rate (BER) at Bob and Eve by simulation.

Light-fidelity (Li-Fi) is an emerging technology for wireless networking based on visible light communication (VLC). As a licence free, high speed, bidirectional and secure wireless access solution, Li-Fi is a complementary building block for fifth generation (5G) heterogeneous mobile networks. By modulating data on the light intensity emitted by light emitting diode (LED) luminaires which already exist in indoor lighting infrastructure, Li-Fi cellular networks are formed. Such networks are termed optical attocell networks, where the optical attcells are smaller in size than the radio frequency (RF) femtocells. This paper focuses on the problem of handover for downlink in an indoor optical attocell network. A fundamental approach is proposed for handover modeling by taking into account the effects of both mobility and rotation for a connected user equipment (UE). By using a random waypoint (RWP) model for the receiver movement and a geometric model for the receiver orientation, the probability of handover and the handover rate are calculated. Novel insights are provided into the handover performance in indoor optical attocell networks using Monte Carlo simulations.

Recently a promising concept of hybrid networks based on light fidelity (LiFi) and wireless fidelity (WiFi) emerged. The idea is to combine ultra-small cell LiFi networks with ubiquitous coverage radio frequency (RF) communication systems. In such a hybrid network, WiFi access points (APs) serve a relatively large coverage area with limited bandwidth, and are thus susceptible to traffic overload. This issue is magnified with an increasing number of users because of the inefficient medium access control (MAC) in WiFi systems. LiFi can alleviate this issue by providing additional capacity. LiFi cells, however, have a limited coverage and this could result in significant handover overhead. A conventional load balancing (LB) method optimises the network throughput when the signal-to-noise ratio (SNR) of each user is known and fixed. Although this method delivers maximum throughput at a given time instance, it fails to consider the throughput loss due to handover, especially in an indoor scenario where users may frequently switch between APs. Taking the handover overhead into account, in this paper we propose a novel LB method that focuses on optimising the network throughput over a period of time. Simulation results show that the proposed method can increase the system throughput by up to 70% compared to existing LB methods.

In this paper, a simple energy-efficient physical layer modulation scheme termed as random number modulation (RNM) is proposed. It is a class of new systems that harness the randomness of pseudo random number generators (RNGs) for efficient communication, and adds a new degree of freedom to digital communication systems. It is also highly adaptable to high-rate, low-latency, and low-rate, high-energy efficiency scenarios. This paper includes a detailed system model, preliminary performance analysis, and extended discussions. The performance of the proposed system is analyzed in terms of symbol and block error probabilities, energy efficiency, and latency. It is shown that there is a fundamental tradeoff between the energy efficiency and the latency of the proposed system, and they can be easily software programmable allowing devices to adapt to different circumstances and environments rapidly. Based upon the basic system model herein, it is anticipated that more sophisticated RNM systems can be developed.

In this paper, a hybrid radio frequency (RF) and visible light communication (VLC) system is considered. A hybrid system with multiple VLC access points (APs) and RF APs is designed and analyzed. In indoor environments, VLC APs provide very high data rates whilst proving illumination, and RF APs offer ubiquitous coverage with moderate data rates. Since VLC networks piggyback on existing lighting infrastructures, they may not always be able to provide full coverage despite supporting very high data rates in some areas. Hence in practical deployments, the standalone VLC networks should be augmented in order to improve the per user data rate coverage. In this context, RF APs can be used to improve the per user rate coverage of VLC networks as well as to provide the ubiquitous control functionalities. In this paper, a simple RF deployment is proposed in order to improve the per user outage data rate performance of standalone VLC networks. It is assumed that the VLC system resources are fixed, and this paper quantifies the minimum spectrum and power requirements for a RF system, which after introduction to the VLC system, the hybrid RF/VLC system achieves certain per user rate coverage performances.

In this paper, the performance analysis of Receive Space Modulation (RSM) in the shadowing Multiple-Input Multiple-Output (MIMO) broadcast channel is presented. This is undertaken by considering Zero Forcing (ZF) precoding and both small and large scale fading. In particular, a closed form and accurate framework for the evaluation of the Symbol Error Rate (SER) is derived. In addition, the diversity order and the coding gain of the new architecture are also obtained. The derived framework can be directly extended to the conventional spatially multiplexed shadowing MIMO broadcast channel. It is shown that RSM achieves the same diversity order and, in certain scenarios which are well defined, higher coding gain than spatially multiplexing (SMX). Also, the performance difference between RSM and SMX in the shadowing broadcast channel is mathematically quantified. Finally, numerical results that verify the new framework and conclusions are provided.

Light fidelity (LiFi) uses light emitting diodes (LEDs) for high-speed wireless communications. Since an LED lamp covers a small area, a LiFi system with multiple access points (APs) can offer a significantly high spatial throughput. However, the spatial distribution of data rates achieved by LiFi fluctuates because users experience inter-cell interference from neighboring LiFi APs. In order to guarantee a quality of service (QoS) for all users in the network, an RF network is considered as an additional wireless networking layer. This hybrid LiFi/RF network enables users with low levels of optical signals to achieve the desired QoS by migrating to the RF network. With regard to moving users, the hybrid LiFi/RF system dynamically allocates either a LiFi AP or an RF AP to users based on their channel state information. In this paper, a dynamic load balancing scheme is proposed, which considers the handover overhead in order to improve the overall system throughput. Joint optimization algorithm (JOA) and separate optimization algorithm (SOA), which jointly and separately optimize the AP assignment and resource allocation, respectively, are proposed. Simulation results show that SOA can offer a better performance/complexity tradeoff than JOA for system load balancing.

Hybrid light fidelity (Li-Fi) and wireless fidelity (Wi-Fi) networks are an emerging technology for future indoor wireless communications. This hybrid network combines the high-speed data transmission offered by visible light communication and the ubiquitous coverage of radio-frequency techniques. While a hybrid network can improve the system throughput and users' experience, it also challenges the process of access point selection (APS) due to the mixture of heterogeneous access points. In this paper, the differences between homogeneous and heterogeneous networks regarding APS are discussed, and a two-stage APS method is proposed for hybrid Li-Fi/Wi-Fi networks. In the first stage, a fuzzy logic system is developed to determine the users that should be connected to Wi-Fi. In the second stage, the remaining users are assigned in the environment of a homogeneous Li-Fi network. Compared with the optimisation method, the proposed method achieves a close-to-optimal throughput at significantly reduced complexity. Simulation results also show that our method greatly improves the system throughput over the conventional methods, such as the signal strength strategy and load balancing, at slightly increased complexity.

Visible light communications (VLC) is a promising and uncharted new technology for the next generation of wireless communication systems. This paper proposes a novel generalized light emitting diode (LED) index modulation method for multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM)-based VLC systems. The proposed scheme avoids the typical spectrum efficiency losses incurred by time- and frequency-domain shaping in OFDM signals. This is achieved by exploiting spatial multiplexing along with LED index modulation. Accordingly, real and imaginary components of the complex time-domain OFDM signals are separated first, then resulting bipolar signals are transmitted over a VLC channel by encoding sign information in LED indexes. As a benchmark, we demonstrate the performance analysis of our proposed system for both analytical and physical channel models. Furthermore, two novel receiver designs are proposed. Each one is suitable for frequency-flat or selective channel scenarios. It has been shown via extensive computer simulations that the proposed scheme achieves considerably better bit error ratio versus signal-to-noise-ratio performance than the existing VLC-MIMO-OFDM systems that use the same number of transmit and receive units [LEDs and photo diodes (PDs)]. Compared with the single-input single-output (SISO) DC biased optical (DCO)-OFDM system, both spectral efficiency and DC bias can be doubled and removed respectively simply by exploiting a MIMO configuration.

Light Fidelity (LiFi) is a recently proposed technology that uses 300 THz visible light spectrum for high speed wireless communications as well as providing illumination. Basically, a LiFi access point (AP) covers only a few square meters, enabling a dense deployment of LiFi APs to improve the network throughput. However, channel blockage and shadowing in conjunction with inter-cell interference compromise the connectivity and system throughput of LiFi networks. In this paper, a network structure that combines LiFi with the conventional radio frequency (RF) system is considered. Users experiencing strong blockages can be switched to the RF system to achieve a higher data rate. In this paper, blockages, random orientation of LiFi receivers, and the user data rate requirement are characterised to model a practical communication scenario. A novel load balancing (LB) scheme based on evolutionary game theory is proposed for hybrid LiFi/RF networks. The performance of the proposed scheme is comprehensively analyzed. Results show that compared to state-of-the-art LB algorithms, the proposed scheme greatly improves user satisfaction levels at reduced computational complexity. Also, an optimal orientation of LiFi receivers and blockage density in hybrid networks would maximize the users quality of service.

Capacity analysis for millimeter-wave (mmWave) quadrature spatial modulation (QSM) multiple-input multiple-output (MIMO) system is presented in this paper. QSM is a new MIMO technique proposed to enhance the performance of conventional spatial modulation (SM) while retaining almost all its inherent advantages. Furthermore, mmWave utilizes a wide-bandwidth spectrum and is a very promising candidate for future wireless systems. Detailed and novel analysis of the mutual information and the achievable capacity for mmWave-QSM system using a 3-D statistical channel model for outdoor mmWave communications are presented in this paper. Monte Carlo simulation results are provided to corroborate derived formulas. Obtained results reveal that the 3-D mmWave channel model can be closely approximated by a log-normal fading channel. The conditions under which capacity can be achieved are derived and discussed. It is shown that the capacity of QSM system can be achieved, by carefully designing the constellation symbols for each specific channel model.

In this paper, the information rate of single photon avalanche diode (SPAD) receivers with active quenching circuits for optical communication systems is studied. The SPAD receiver is modeled as a discrete memoryless channel (DMC), and the information transfer rate is studied using an information theoretic approach. To assess the input-output information transfer rate of the SPAD optical receiver, a channel capacity metric is proposed which is a function of the physical parameters including average signal photon count, SPAD dead time, and SPAD average background counts. The proposed metric can be used for theoretical optimization of the device structure and operating conditions. Performance results are presented based on the proposed metric.

Compared with the conventional amplitude phase modulation (APM), spatial modulation (SM) is a low- complexity, yet energy-efficient transmission technique, whereby transmit antenna (TA) indices are utilized to convey the information. However, the number of the required TAs grows exponentially with the number of transmitted bits, which leads to unacceptable pilot overhead for channel estimation in practical systems. To reduce the number of TAs whereas keep the data rate unchanged, virtual spatial modulation (VSM) is proposed in the first time. Specifically, by activating multiple TAs with their corresponding analog phase shifters (APSs), massive equivalent channel vectors could be constructed based on the combinations of original channel vectors from different TAs and their phase rotations. By way of mapping each equivalent channel vector to a virtual transmit antenna (VTA) index which might convey the information, the number of the required TAs could grow linearly with the number of transmitted bits. Furthermore, the selection of a VTA subset from all available VTAs is formulated as a combinatorial optimization problem to maximize the minimal Euclidean distance (ED) among the equivalent channel vectors. A spatial constellation optimizing (SCO) algorithm is proposed to obtain a near-optimal solution to this problem with low-complexity. Simulation results demonstrate that the proposed VSM is able to achieve lower bit error rate (BER) under the same transmit rate compared with the conventional SM and APM schemes.

In this paper, we propose a generalized light emitting diode (LED) index modulation scheme for multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) visible light communications (VLC) systems. The proposed scheme generalizes the LED index modulation concept by using the spatial multiplexing principle to transmit complex OFDM signals through VLC channels by separating these signals into their real-imaginary and positive-negative parts. The maximum a posteriori (MAP) estimator of the proposed scheme, which relies on quadratic programing (QP) problem, is presented for flat VLC channels. It is shown via computer simulations that the proposed scheme achieves considerably better error performance than the existing VLC-MIMO-OFDM systems due to its power efficiency and improved transceiver structure.

One of the important issues in an indoor visible light communication (VLC) system is the multi-path (MP) effect due to reflections. Generally, the MP effect can be characterised by the channel impulse response (CIR). Methods based on ray-tracing/Monte Carlo simulations are primarily used to obtain these impulse responses, but this generally leads to time consuming computer simulations. In this study, an analytical approach is proposed to directly and accurately calculate the non-line-of-sight (NLOS) components of the CIR due to the interior surface reflections in a room. The proposed method could be used in system-level simulations of a networked VLC system (also referred to as a LiFi attocell network), which requires a large number of channels with different transmitter and receiver deployments to be generated in a computationally efficient manner.

In this paper, downlink cooperation is proposed for indoor visible light communication (VLC) cellular networks, also termed as optical attocell networks, with the aim of improving the spectral efficiency performance. An orthogonal frequency division multiple access (OFDMA) system based on direct current optical orthogonal frequency division multiplexing (DCO-OFDM) with fractional frequency reuse (FFR) planning is considered, and a system level analysis is carried out. A line-of-sight (LOS) light propagation model is used for the indoor VLC channel. The downlink cooperation is realized by means of a relay-assisted transmission with the aid of neighboring base stations (BSs) using a non-orthogonal amplify-and-forward (NAF) protocol. The links between the source BS and the relay BSs are provided by perfectly aligned VLC connections. Four relaying schemes with a different number of relays are considered, and for each scheme analytical signal-to-interference-plus-noise ratio (SINR) and spectral efficiency expressions for a user equipment (UE) with random coordinates in an optical attocell are derived. Also, average spectral efficiency performance of the system is studied using Monte-Carlo simulations.

Light Fidelity (LiFi) is a recently proposed technology that combines illumination and high speed wireless communication using light emitting diodes (LEDs). Since the used electromagnetic spectrum does not overlap with the radio frequency (RF) spectrum, a small cell LiFi attocell network can be added to the conventional RF network as an additional networking layer in order to mitigate the data traffic bottlenecks in high density environments. In such a hybrid LiFi/RF network where the LiFi attocell covers a few square meters, user movement may prompt frequent handovers, and the handover overhead would degrade the system throughput. The goal is to reduce the handover overhead by appropriately assigning users to either the RF or the LiFi access point (AP). In this study, a fuzzy logic (FL) based dynamic handover scheme is proposed. This FL scheme uses not only the channel state information (CSI), but also the user speed and desired data rate to determine whether a handover needs to be prompted. Simulation shows that the proposed scheme outperforms the conventional handover algorithms, and the performance improvement is approximately 40% in terms of both data rate and user satisfaction level.

Light Fidelity (LiFi) uses light emitting diodes (LEDs) for high speed wireless communications. Since employing a different range of the electromagnetic spectrum from radio frequency (RF) communications, LiFi can significantly alleviate the traffic bottlenecks in high density RF scenarios, typically present in an indoor environment. Hence, a combination of LiFi and RF networks becomes a promising candidate for future indoor wireless communications. In a practical indoor scenario, the optical interference from neighbouring LiFi access points (APs) and the blockages of line-of-sight (LoS) optical channels induced by people and objects are the main factors that cause significant optical channel variations. In this study, the effect of these two factors on the system throughput of a hybrid LiFi/RF network is investigated. In order to offer a fair comparison, area data rate, which is defined as the system throughput in a unit area, is used for performance evaluation. The simulation shows that there is an optimal distance between two neighbouring LiFi APs to achieve the highest area data rate. In addition, the area data rate increases with the density of blockages when the blockage density is below a certain threshold.

High performance of visible light communication (VLC) systems requires overcoming the limitations imposed by the optical wireless channel distortions resulting from path loss and temporal dispersion. In order to design techniques to combat the effects of channel distortions, an accurate VLC channel model is needed. In this paper, we propose a new regular-shaped geometry-based multiple bounce model (RS-GBMB) for VLC channels. The proposed model employs a combined two-ring model and ellipse model, where the received signal is constructed as a sum of the line-of-sight (LoS), single-, double-, and triple bounced rays of different powers. This makes the model sufficiently generic and adaptable to a variety of indoor scenarios. Based on the proposed RS-GBMB model, statistical properties are then investigated, such as the channel DC gain, mean excess delay, root mean square (RMS) delay spread, and Rician factor.

The cellular Light-Fidelity (Li-Fi) network is considered as a promising approach for high speed indoor data access. The conventional metric, based on which an access point (AP) is selected for each user, is signal strength. This metric offers the best channel quality for each user but does not guarantee the achievable data rate, since the resource of an AP is limited. In this paper, we propose a new metric for AP selection to improve the load balancing among APs by considering both the received signal-to-interference-plus-noise-ratio (SINR) and the traffic of AP. The orientation of mobile stations (MS) is also taken into account and its effect on users' performance is evaluated. In reality, receivers have random angles with the coordinate axes. We consider three standard angles similar to those used in mobile devices to model the device orientation. Based on this model, the effect of arbitrary orientation on user's throughput and satisfaction is investigated. Simulation results show that when the orientation of users is considered, the proposed AP selection metric outperforms the conventional metric.

This work studies the issue of access point selection (APS) in a hybrid wireless network accommodating visible light communication (VLC) and radio-frequency (RF) technologies. A hybrid network constructs multiple layers of coverage, and thus a user possibly acquires a high level of receiving signal strength (RSS) from more than one access point (AP). This fact undermines the effectiveness of conventional RSS-based APS methods. Another challenging factor is the dissimilarity between heterogeneous APs in terms of coverage area and capacity. In general, RF offers larger coverage area but lower capacity than VLC, and therefore the RF system is susceptible to overload. Although the issue of APS can be formulated as an optimisation problem, this approach requires a prohibitive amount of processing power. In this paper a two-stage APS method is proposed on the basis of fuzzy logic, with very low computational complexity. The new method first determines the users that should be connected to the RF system, and then assigns the remaining users as if in a stand-alone VLC network. Results show that when achieving the same amount of throughput, the proposed method can support up to 25% and 56% more users than the load balancing (LB) and signal strength strategy (SSS) methods, respectively.

The performance of visible light communication (VLC) system may potentially be degraded by wireless optical channel distortions resulting from path loss and temporal dispersions. In order to devise techniques to combat the effects of channel distortions, an accurate channel model is needed. In this paper, we propose a new field of view (FOV) geometry-based single bounce (GBSB) model for VLC channels. Statistical properties of the proposed GBSB model are then studied, such as the channel gain, mean excess delay, root mean square (RMS) delay spread, Rician factor, and time correlation. Also, the required illuminance for the proposed scenario and the number of light emitting diodes (LEDs) per LED lamp are estimated.

In this paper, novel spectral efficient cooperative transmission schemes are proposed for downlink in indoor light-fidelity (Li-Fi) cellular networks, also termed as optical attocell networks. Downlink cooperation builds upon dual-hop decode-and-forward (DF) relaying with the aid of one or two neighboring base stations (BSs). The connections between the source BS and the relay BSs are provided by perfectly aligned visible light communication (VLC) links. Two cooperation protocols are introduced and evaluated, namely, non-orthogonal DF (NDF) and joint transmission with DF (JDF). A multiple access system based on direct current optical orthogonal frequency division multiplexing (DCO- OFDM) with fractional frequency reuse (FFR) is considered. Also, a line-of-sight (LOS) Lambertian propagation model is used for the indoor VLC channel. For each scheme, the signal-to- interference-plus-noise ratio (SINR) and spectral efficiency for user equipment (UE) with random coordinates in an attocell are derived. The average spectral efficiency within an optical attocell is studied using Monte Carlo simulations.

In this paper, a promising wireless communication concept which is termed as Spatial Modulation (SM) is considered. In particular the focus is on an interference scenario between a SM multiple-input- multiple-output (MIMO) system and a conventional spatial multiplexing (SMX) MIMO system. It has been shown that the interference caused by SM has a peculiar behavior. Since SM exploits the spatial position of the active antenna as an additional dimension for data transmission, the interference caused by SM system has no fixed interference subspace which can not be estimated accurately. Hence, conventional interference suppression techniques can not be applied accordingly. In this paper, we discuss this unique problem generated by SM systems, and propose three methods to overcome the adverse effects caused by such interference.

In this paper, resource allocation (RA) and load balancing (LB) are jointly investigated in a visible light communication (VLC) system. With RA, the resource of access points (APs) is intelligently allocated to their users, while LB enables a user to select an AP other than the one offering the highest signal strength. To date, most research on VLC has treated those two issues separately, leading to an insufficient usage of system resource. In this study, a novel concept termed bidirectional allocation game is proposed, where allocating the AP resource to users and assigning users to APs are carried out together. Also, a fuzzy logic system is developed to cope with the complexity challenge introduced by the relatively small coverage area of a single VLC AP. Results show that the proposed method greatly outperforms conventional schedulers in terms of both throughput and fairness, while also achieving a fast convergence.

In this paper, an optical space division multiple access (SDMA) scheme is proposed for optical attocell networks. In the system, a conventional single-element transmitter in each optical cell is substituted by an angle diversity transmitter which can simultaneously serve multiple active users at different positions. This type of configuration can increase the available bandwidth resource and also mitigate inter-cell interference (ICI) in optical attocell networks. The results indicate that the optical SDMA scheme significantly outperforms the conventional optical time division multiple access (TDMA) scheme. The SDMA schemes is shown to improve the average spectral efficiency of the system by a factor of 26 for a 37-element light-emitting diode (LED) angle diversity transmitter. Finally, the study is extended to account for user position errors. The Monte-Carlo simulation results show that the system is very robust to user position errors.

In this paper, we design an energy efficient indoor visible light communications (VLC) system from a radically new perspective based on an amorphous user-to-network association structure. Explicitly, this intriguing problem is approached from three inter-linked perspectives, considering the cell formation, link-level transmission and system-level optimisation, critically appraising the related optical constraints. To elaborate, apart from proposing hitherto unexplored amorphous cells (A-Cells), we employ a powerful amalgam of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and transmitter pre-coding aided multi-input single-output (MISO) transmission. As far as the overall system-level optimisation is concerned, we propose a low-complexity solution dispensing with the classic Dinkelbach's algorithmic structure. Our numerical study compares a range of different cell formation strategies and investigates diverse design aspects of the proposed A-Cells. Specifically, our results show that the A-Cells proposed are capable of achieving a much higher energy efficiency per user compared to that of the conventional cell formation for a range of practical field of views (FoVs) angles.

In this paper, two major multiple-input-multiple-output (MIMO) transmission techniques such as spatial modulation (SM) and spatial multiplexing (SMX) are considered. In particular, the focus is on an interference scenario, where an SM MIMO system interferes with a conventional SMX MIMO system. The interference caused by SM has a peculiar behavior. In SMX MIMO networks, the interference subspace is fixed in a given coherence time interval. It can be estimated accurately, and interference suppression techniques can be applied accordingly. However, the interference caused by an SM system has no fixed interference subspace, and it changes in every single channel use. Therefore, SM interference could pose some unique challenges to SMX MIMO systems in some scenarios. In this paper, the effect of SM interference on SMX MIMO systems and methods to overcome/reduce the adverse effects caused by SM interference are studied. Furthermore, the effect of SMX interference on SM MIMO systems is also studied.

In this paper, the performance of non-orthogonal multiple access (NOMA) is characterized in a downlink visible light communication system for two separate cases. In the case of guaranteed quality of service (QoS) provisioning, we derive an analytical expression of the system coverage probability and show the existence of optimal power allocation coefficients on two-user paired NOMA. In the case of opportunistic best-effort service provisioning, we formulate a closed-form expression of the ergodic sum rate, which is applicable for arbitrary power allocation strategies. The probability that NOMA achieves higher individual rates than OMA is derived. Also, we give an upper bound of the sum rate gain of NOMA over OMA in the high signal-to-noise ratio regime. Both the theoretical and simulation results prove that the performance gain of NOMA over OMA can be further enlarged by pairing users with distinctive channel conditions. We also find out that the choice of light emitting diodes (LEDs) have a significant impact on the system performance. In the case of guaranteed QoS provisioning, the LEDs with larger semi-angles have better performance; while in the case of opportunistic best-effort service provisioning, the LEDs with 35° semi-angle give nearly optimal performance.

In this paper, Multi-Stream Receive-Spatial Modulation (MSR-SM) for application to the Multiple-Input Multiple-Output (MIMO) broadcast channel is introduced and studied. MSR-SM is a closed-loop transmission scheme, which applies the concept of multistream space modulation at the receiver side. An accurate mathematical framework for the evaluation of the Bit Error Rate (BER) is proposed. In addition, the diversity order and coding gain of the new architecture are derived. Note that the proposed analytical framework takes into account both the small-scale fading and the system topology, and is directly applicable to the conventional MIMO broadcast channel. Compared with the state-of-the-art MIMO transmission in the broadcast channel, it is mathematically shown that MSR-SM achieves the same diversity order and a better coding gain, in the high Signal-to-Noise Ratio (SNR) regime. Finally, the proposed mathematical framework and the new findings are validated via Monte Carlo simulation results.

This paper studies complex modulation schemes, including orthogonal frequency-division multiplexing (OFDM), received by a single photon avalanche diode (SPAD) array integrated circuit (IC). A SPAD operates in the Geiger mode, and is able to detect single photons. This feature enables order of magnitude receiver sensitivity in intensity modulation (IM) / direct detection (DD) Visible Light Communication (VLC) systems. The tradeoff between received power and bit error ratio (BER) using both pulse-amplitude modulation (PAM) and OFDM is shown. A first order model of the noise in a digital SPAD receiver is derived. The noise in the experimental receiver chip approaches the predicted noise in our model, and we achieve receiver sensitivity of $-$64 dBm with a 100 kbit/s signal at a BER of 10^-5. It is concluded that future improvements in SPAD VLC receiver architecture will allow sensitivity to approach the quantum limit.

In this paper, the downlink signal-to-interference- plus-noise ratio (SINR) statistics are analysed in a direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) based optical attocell network with a Poisson point process (PPP) cell deployment. An optical attocell system utilises existing lighting fixtures in an indoor environment to function as a small-cell cellular network. It uses each luminary as a base station (BS) to serve multiple nearby mobile users. Similar to a conventional radio frequency (RF) cellular system, the grid cell deployment is restricted by many practical issues in an optical attocell network. Therefore the performance of the system with PPP cell deployment is considered in this study in order to identify a lower bound for practical attocell networks with irregular cell deployment. An analytical framework is presented and compared with the computer simulations. Also the SINR statistics with different cell deployments are compared and discussed.

In visible light communications (VLC), the transmitted signal is non-negative and real valued, and these characteristics constrain the signal space and provide new challenges for waveform design in a multi-user system. In Light Fidelity (Li- Fi), a VLC technology, and similar to radio frequency (RF) systems, precoder designs can be used to serve multiple users. However, a signal shaping algorithm is required to address the non-negativity of the transmitted signal in Li-Fi. In this study, a power allocation algorithm is proposed to achieve a multi-user Li-Fi system. A compound eye transmitter consisting of several LEDs pointing in different directions is used. This means that the process of the obtaining the inverse of the channel matrix in precoder design is not required. Results show that when a 30° semi-angle is used at the transmitter and the system has 4 users, the proposed transmission model can achieve 10-5average bit error rate (BER) performance at 1 dBm average transmission power.

In this paper, a novel architecture for the Multiple-Input Multiple-Output (MIMO) broadcast channel is proposed and studied. The new architecture is based on the concept of Multi- Stream Receive-Spatial Modulation (MSR-SM). MSR-SM is a closed-loop transmission scheme, which applies the concept of multi-stream space modulation at the receiver side. A new and accurate framework for computing the Average Bit Error Probability (ABEP) of the new architecture is proposed. In addition, the new architecture is compared against the state- of-the-art MIMO transmission in the broadcast channel and it is shown to: i) provide superior Bit Error Rate (BER) performance in the high Signal-to-Noise-Ratio (SNR) regime and ii) reduce the signal processing complexity at the transmitter.

A coordinated multi-point visible light communication (CoMP-VLC) downlink is investigated, where the time-frequency (TF) resources are shared by a group of users relying on space division multiple access (SDMA). Linear zero-forcing (ZF) transmit precoding (TPC) is tailored to the CoMP-VLC system in order to eliminate the inter-user interference while accommodating the linear operating region of the light emitting diodes (LEDs). A SDMA user-group sharing the same TF has to host users having sufficiently different user-signatures, i.e. channel gains in a non-dispersive VLC channel. Hence we develop efficient resource allocation (RA) and SDMA user-grouping algorithms in the context of VLC systems that use intensity modulation and direct detection (IM/DD). Finding the optimal SDMA user-group requires an exhaustive search (ES), which has exponentially increasing complexity as the number of users increases. Therefore, a low-complexity user-grouping method is conceived and compared to three existing algorithms in terms of the attainable area spectral efficiency (ASE) and throughput fairness. Our simulation results demonstrate that the proposed algorithm gives a better performance-fairness trade-off than existing benchmarks.

Multiple-input multiple-output (MIMO) systems can increase wireless link capacity without requiring additional bandwidth and power. On the one hand, MIMO systems heavily depend on channel state information (CSI) at the receiver. It has also been shown that CSI at the transmitter (CSIT) can improve the link capacity and reliability considerably but with the disadvantage of extra feedback and computational costs. On the other hand, energy efficiency of MIMO systems could adversely increase with the number of antennas in the link. Therefore, there are considerable interests in energy efficient practical transmission schemes for MIMO links without CSIT. In this study, an emerging wireless communication concept which is termed as spatial modulation (SM) is considered for large scale MIMO. The results show that in information-theoretic viewpoint, SM achieves capacity comparable to the open-loop MIMO capacity even though a subset of transmit antennas is activated in every channel use. The reason is both the channel coefficients and input symbols carry information in SM. As a result, SM regains (compensates) the loss of information capacity due to activating a subset of antennas by modulating information in the antenna index. This means that the sum information rate remains high.

In this paper, a visible light communication (VLC) system adopting space division multiple access (SDMA) is proposed. In the optical SDMA system, a conventional single-element transmitter is substituted by an angle diversity transmitter which can simultaneously serve multiple active users in different positions. In the simulation, two spatial partition schemes, random grouping and optimal grouping, are implemented which demonstrate the lower bound and the upper bound of the system throughput gain for the optical SDMA system, respectively. The results indicate that the optical SDMA significantly outperforms conventional optical time division multiple access (TDMA). In specific, it has been found that an optical SDMA can achieve over 10 times higher system throughput compared with optical TDMA.

The performance of a multi-cell, multi-user Long Term Evolution (LTE) deployment in a medium-sized aircraft cabin is studied. Due to the structure and construction materials of the aircraft, channel propagation behaviour is different from conventional indoor channel models. Also, depending on the seating plan of the aircraft, user density is higher than other indoor environments. Thus, firstly, a proper channel model is adapted for an aircraft, and then the deployed LTE system is optimized using Taguchi's Method (TM). TM is a recently used optimization method for electromagnetic (EM) problems and for the optimization of communication networks. The results show that multi-cell, multi-user LTE deployment can achieve 4.33 Mbps average user data rate and 676 Mbps total system data rate with two evolved NodeBs (eNBs) deployed.

A number of unipolar optical orthogonal-frequency-division-multiplexing (OFDM) schemes have been proposed as a solution to the high energy consumption in the widely adopted direct-current-biased optical OFDM (DCO-OFDM) modulation scheme. These schemes have a reduced spectral efficiency due to the restrictions imposed on their frame structure. The enhanced unipolar OFDM (eU-OFDM) modulation scheme was recently introduced to compensate for the reduced spectral efficiency in unipolar OFDM (U-OFDM). The concept exploits the frame structure of U-OFDM and allows for multiple U-OFDM information streams to be combined, thus increasing the overall spectral efficiency of the communication system. In this paper, the concept of the enhanced U-OFDM scheme is generalized for arbitrary combinations of U-OFDM data streams with various constellation sizes and various power allocations. A closed-form theoretical bound on the bit error rate (BER) performance of the GeneRalizEd EnhaNcEd unipolaR OFDM (GREENER-OFDM) is derived and verified by comparison with the results of Monte Carlo simulations. The proposed scheme has an improved power efficiency compared with a spectrally equivalent DCO-OFDM. The GREENER-OFDM allows the gap in spectral efficiency between DCO-OFDM and the inherently unipolar optical OFDM schemes to be completely closed.

In this paper an energy efficient light emitting diode (LED) driver circuit for visible light communications (VLC) and results from the electrical and optical characterization are presented. A current steering digital to analogue converter (DAC) based LED driver circuit supporting 4-channels with an 8-bit resolution is realized in a 0.18 μm complimentary metal oxide semiconductor (CMOS) technology. Each channel delivers a full-scale current of 255 mA and achieves up to 67% electrical efficiency when driving 250 MS/s orthogonal frequency division multiplexing (OFDM) signals. Optical differential drive capability is demonstrated by using two output branches of a single channel to drive two different LEDs with an SNR improvement (>5 dB). The chip is also capable of full digital control of color shift keying (CSK) using multi-color LEDs enabling lighting color-temperature adjustment, dimming and multiple-input and multiple-output (MIMO) optical communications and these aspects will be the subject of future research.

For the same spectral efficiency, pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) has a lower power efficiency in comparison with direct current (DC)-biased optical orthogonal frequency division multiplexing (DCO-OFDM). A power efficient spectrally enhanced PAM-DMT (ePAM-DMT) is proposed in this paper. Superposition modulation of multiple PAM-DMT streams is used to improve the spectral efficiency of PAM-DMT. The ePAM-DMT allows the spectral efficiency gap between DCO-OFDM and other unipolar OFDM schemes to be completely closed. The proposed system is compared with DCO-OFDM in the context of an additive white Gaussian noise (AWGN) channel. A closed-form theoretical bound on the bit error rate (BER) performance of the proposed scheme is derived and verified by Monte Carlo simulations. The proposed scheme is energy efficient in terms of the electrical power consumption. The high optical energy dissipation of the proposed scheme makes it suitable for the illumination-based visible light communications (VLC) applications.

This paper presents the characterization of single photon avalanche diodes (SPADs) for optical communication applications. SPAD-based optical receivers can provide a significantly improved single-photon detection sensitivity compared with conventional photodiodes. However, an undesirable dead time introduced by the quenching circuit decreases the performance of these receivers. Using a precise analysis, we show that in the presence of dead time, the photon arrival process does not follow a Poisson process. Also, the effective count rate is evaluated and shown to depend critically on dead time. The effect of SPAD dead time on the error performance of an on-off keying (OOK) modulation optical communication system is also investigated. It is shown that, when background counts due to dark current and afterpulsing are small, the performance degradation of the SPAD-based receiver due to dead time losses is negligible. However, for systems with considerable background counts, the bit error rate (BER) degrades rapidly with increasing dead time.

The Internet of Things (IoT) is a new generation of network that can remotely and intelligently control distributed objects. Due to the large number of objects in the IoT, a high data traffic for the object communications is required, which is mostly routed through wireless links. However, the available spectrum for radio frequency (RF) wireless communications is exhausted so that each user in the IoT can only achieve very low data rate. In order to offer a better service to users, a light fidelity (Li-Fi) and radio frequency (RF) hybrid network is considered, where Li-Fi uses the large spectrum of visible light to achieve a high data rate, and the RF system guarantees a seamless coverage. In this study, a load balancing (LB) algorithm for the Li-Fi/RF hybrid IoT network is proposed based on evolutionary game theory (EGT). A key feature of the proposed algorithm is that users autonomously select the APs and adapt their strategies. Thus, compared with the conventional centralised algorithm, the computation load of the central unit (CU) can be reduced by using the EGT algorithm. Moreover, simulation results show that the proposed algorithm outperforms the conventional centralised algorithms in terms of the user satisfaction.

This research addresses the issue of resource and power allocation in small cell networks, and focuses on two aspects: i) the interference coordination among cells; and ii) the resource allocation among the users served by the same cell. Due to the density of small cells, centralised interference coordination schemes require an enormous level of communication among cells. Fuzzy logic (FL) is a promising low-complexity approach to realise autonomous interference coordination that does not need communication between cells. In this paper, we propose a novel FL method and associated decision-making algorithm for tackling resource allocation in small cell networks. Unlike the traditional FL method using the values of two states `yes' and `no' to describe how much a resource block (RB) should or should not be allocated, the proposed method employs a 3-state criterion that distinguishes high-quality RBs from medium-quality RBs. Also, the issue of allocating the RBs of a single cell to multiple users is studied in the FL method. Simulation results show that the proposed method can notably improve the performance of the traditional FL method in terms of both throughput and user satisfaction, without requiring extra processing power.

In this paper, the performance of non-orthogonal multiple access (NOMA) is characterized in a downlink visible light communication (VLC) system. Analytical expressions of the system performance are derived for two separate scenarios. In the scenario of achieving guaranteed quality of service (QoS), the outage probability of each user is studied and the effect of power allocation coefficients on the system coverage probability is investigated. In the scenario of providing opportunistic best-effort service, system ergodic sum rate is formulated based on a fixed power allocation (FPA) strategy. Both simulation and analytical results demonstrate that, in the first scenario, the maximum coverage probability can be achieved through the exhaustive search (ES) method to find the optimum set of power allocation coefficients. In the second scenario, it is shown that unlike orthogonal multiple access (OMA) techniques, NOMA can achieve a higher system capacity for a larger number of users. Also, the performance of NOMA can be further enhanced by choosing LEDs with a suitable semi-angle. When compared with OMA, NOMA can increase the system capacity by 125% if LEDs with 30° semi-angle are used.

A hybrid radio frequency (RF) and visible light communication (VLC) system is considered. A hybrid system with multiple VLC access points (APs) and RF APs is designed and compared. In indoor environments, VLC APs can provide very high data rates whilst satisfying illumination requirements, and RF APs can offer ubiquitous coverage. Even though very high throughput is supported by VLC APs, such coverage area may be spatially limited. This gives an opportunity to introduce a new small cell layer to current heterogeneous wireless networks, and it is referred as optical attocellular layer. However since VLC piggy-backs on the existing lighting infrastructure, it may not be possible to achieve full coverage. Hence in practical deployments, the spatial throughput distribution of standalone VLC systems should be augmented. In this context, RF APs can be used to improve the overall rate performance of the heterogeneous wireless system. In this study, the focus is on improving the per user average and outage throughput. It is assumed that the VLC system resources are fixed, and the study quantifies the spectrum and power requirements for a RF system, which after introduction to the VLC system, achieves better per user rate performance.

A new concept for indoor positioning using visible light communication (VLC) is presented in this paper. The feasibility of uplink localization is verified so that the proposed system can work in conjunction with downlink positioning systems to improve overall localization accuracy. We propose to use an angle diversity transmitter (ADT) associated with accelerometers for uplink three-dimensional localization. Unlike other techniques using VLC for positioning, the proposed system can achieve indoor localization without making assumptions about the height or orientation angle of a mobile user. The received signal strength (RSS) method is used to minimize implementation costs while achieving satisfactory positioning accuracy. Simulation results show that an average localization error of less than 0.15 m can be achieved even when the receiver is tilted by 45°. Also, compared with a single light emitting diode (LED), the ADT is found to be more robust to accelerometer measurement errors.

Visible light communications (VLC) has the potential to play a major part in future smart home and next generation communication networks. There is significant ongoing work to increase the achievable data rates using VLC, to standardize it and integrate it within existing network infrastructures. The future of VLC systems depends on the ability to fabricate low cost transceiver components and to realize the promise of high data rates. This paper reports the design and fabrication of integrated transmitter and receiver components. The transmitter uses a two dimensional individually addressable array of micro light emitting diodes (μLEDs) and the receiver uses an integrated photodiode array fabricated in a CMOS technology. A preliminary result of a MIMO system implementation operating at a data rate of 1 Gbps is demonstrated. This paper also highlights the challenges in achieving highly parallel data communication along with the possible bottlenecks in integrated approaches.

A number of inherently unipolar orthogonal frequency division multiplexing (OFDM) modulation schemes have been introduced recently in an attempt to improve the energy efficiency of OFDM-based intensity modulation and direct detection (IM/DD) systems. All such algorithms, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone modulation (PAM-DMT) and unipolar orthogonal frequency division multiplexing (U-OFDM), experience an inherent loss in spectral efficiency caused by the restrictions imposed on the OFDM frame structure required for the generation of a unipolar signal. The current paper presents a modified modulation approach, termed enhanced U-OFDM (eU-OFDM), which compensates the spectral efficiency loss in U-OFDM. At the same time, it still allows for the generation of an inherently unipolar modulation signal that achieves better performance in terms of both electrical power and optical power dissipation compared to the conventional state-of-the-art technique direct current (DC)-biased optical OFDM (DCO-OFDM). To the best of the authors' knowledge, the current work also presents the first experimental proof-of-concept demonstration of both U-OFDM and eU-OFDM, and clearly demonstrates the significant energy advantages that these two schemes can introduce in an optical wireless communications (OWC) system.

This paper proposes a novel design of an optical wireless communications (OWC) receiver using a solar panel as a photodetector. The proposed system is capable of simultaneous data transmission and energy harvesting. The solar panel can convert a modulated light signal into an electrical signal without any external power requirements. Furthermore, the direct current (DC) component of the modulated light can be harvested in the proposed receiver. The generated energy can potentially be used to power a user terminal or at least to prolong its operation time. The current work discusses the various parameters which need to be considered in the design of a system using a solar panel for simultaneous communication and energy harvesting. The presented theory is supported with an experimental implementation of orthogonal frequency division multiplexing (OFDM), thus, proving the validity of the analysis and demonstrating the feasibility of the proposed receiver. Using the propounded system, a communication link with a data rate of 11.84 Mbps is established for a received optical signal with a peak-to-peak amplitude of 0.7 × 10-3W/cm2.

The recent development of high-speed trains (HSTs), as a high-mobility intelligent transportation system, and the growing demands of broad-band services for HST users, introduce new challenges to wireless communication systems for HSTs. The deployment of mobile relay stations on top of the train carriages is one of the promising solutions for HST wireless systems. For a proper design and evaluation of HST wireless communication systems, we need accurate channel models that can mimic the underlying channel characteristics for different HST scenarios. In this paper, a novel nonstationary geometry-based stochastic model (GBSM) is proposed for wideband multiple-input multiple-output HST channels in rural macrocell scenarios. The corresponding simulation model is then developed with angle parameters calculated by the modified method of equal areas. Both channel models can also be used to model nonstationary vehicle-to-infrastructure channels in vehicular communication networks. The system functions and statistical properties of the proposed channel models are investigated based on a theoretical framework that describes nonstationary channels. Numerical and simulation results demonstrate that the proposed channel models have the capability to characterize the nonstationarity of HST channels. The statistical properties of the simulation model, verified by the simulation results, can match those of the proposed theoretical GBSM. An excellent agreement is achieved between the stationary intervals of the proposed simulation model and those of relevant measurement data, demonstrating the utility of the proposed channel models.

Cognitive radio (CR) is considered one of the prominent techniques for improving the utilization of the radio spectrum. A CR network (i.e., secondary network) opportunistically shares the radio resources with a licensed network (i.e., primary network). In this work, the spectral-energy efficiency trade-off for CR networks is analyzed at both link and system levels against varying signal-to-noise ratio (SNR) values. At the link level, we analyze the required energy to achieve a specific spectral efficiency for a CR channel under two different types of power constraint in different fading environments. In this aspect, besides the transmit power constraint, interference constraint at the primary receiver (PR) is also considered to protect the PR from a harmful interference. Whereas at the system level, we study the spectral and energy efficiency for a CR network that shares the spectrum with an indoor network. Adopting the extreme-value theory, we are able to derive the average spectral and energy efficiency of the CR network. It is shown that the spectral efficiency depends upon the number of the PRs, the interference threshold, and how far the secondary receivers (SRs) are located. We characterize the impact of the multi-user diversity gain of both kinds of users on the spectral and energy efficiency of the CR network. Our analysis also proves that the interference channels (i.e., channels between the secondary transmitter and PRs) have no impact on the minimum energy efficiency.

In this paper, we propose an optimum transmit structure for spatial modulation (SM), a unique single-stream multiple-input multiple-output (MIMO) transmission technique. As a three-dimensional modulation scheme, SM enables a trade-off between the size of the spatial constellation diagram and the size of the signal constellation diagram. Based on this fact, the novel method, named transmission optimized spatial modulation (TOSM), selects the best transmit structure that minimizes the average bit error probability (ABEP). Unlike the traditional antenna selection methods, the proposed method relies on statistical channel state information (CSI) instead of instant CSI, and feedback is only needed for the optimal number of transmit antennas. The overhead for this, however, is negligible. In addition, TOSM has low computational complexity as the optimization problem is solved through a simple closed-form objective function with a single variable. Simulation results show that TOSM significantly improves the performance of SM at various channel correlations. Assuming Rayleigh fading channels, TOSM outperforms the original SM by up to 9 dB. Moreover, we propose a single radio-frequency (RF) chain base station (BS) based on TOSM, which achieves low hardware complexity and high energy efficiency. In comparison with multi-stream MIMO schemes, TOSM offers an energy saving of at least 56% in the continuous transmission mode, and 62% in the discontinuous transmission mode.

This paper proposes a novel non-stationary wideband multi-confocal ellipse two dimensional (2-D) channel model for massive multiple-input multiple-output (MIMO) communication systems. Spherical wavefront is assumed in the proposed channel model, instead of the plane wavefront assumption used in conventional MIMO channel models. In addition, the birth-death process is incorporated into the proposed model to capture the dynamic properties of clusters on both the array and time axes. Statistical properties of the channel model such as the space-time-frequency correlation function and power imbalance on the antenna array are studied. The impact of the spherical wavefront assumption on the statistical properties of the channel model is investigated. Furthermore, numerical analysis shows that the proposed channel model is able to capture specific characteristics of massive MIMO channel as observed in measurements.

The cellular systems with base station (BS) coordination are known for increasing the cell edge user throughput considerably. They process signals captured by geographically distributed BSs coherently. In contrast, noncooperative BSs with large number of antennas are also known for achieving both high spectral and energy efficiency. The fully cooperative BSs have several drawbacks such as ideal backhaul requirements, need for BS synchronization, and channel state information (CSI). These drawbacks hinder fully cooperative cellular systems considerably. Therefore, in this paper, the prospect of designing noncooperative cellular systems with comparable performance in some sense to ideal cooperative cellular systems with no additional power or spectrum is systematically pursued. The approach is based on the information theoretic rate characterization for finite size systems. The size of systems is in terms of the number of BS antennas and users. We show in this paper that under certain conditions, such noncooperative cellular systems can be designed. We devise a framework to compare two systems and obtain necessary conditions for making the performance of two seemingly unrelated access technologies closely comparable.

In this paper, an indoor visible light communication (VLC) cellular network, referred to as an optical attocell network, is analysed at system level. A line-of-sight (LOS) ray-tracing model is used to characterise the light propagation and its effect on the performance of an intensity modulation (IM) and direct detection (DD) communication system. Orthogonal frequency division multiple access (OFDMA) based on direct-current optical orthogonal frequency division multiplexing (DCO-OFDM) is used as a multi-user access scheme. The signal-to-interference-plus-noise ratio (SINR) for a user with a random location in an optical attocell is studied. An analytical approach to calculate the statistics of the SINR is presented and verified by Monte Carlo simulations. Moreover, average spectral efficiency is also studied in order to estimate the downlink wireless capacity of the optical attocell network. The spectral efficiency of the system has been found to be strongly dependent on the radius of an optical attocell and on the half-power semi-angle of the light transmission profile. Guidelines for the configuration of the relevant attocell parameters are provided. An optical attocell with an average spectral efficiency of 5.9 bits/s/Hz is demonstrated for an appropriate set of attocell parameters.

In this paper, a novel double-source cell configuration for indoor optical wireless cellular networks is proposed. In this configuration, each optical cell consists of two access points (APs) which transmit the same information signals, but with opposite polarity. When the optical sequences from the same optical cell are added up, they interfere destructively. This characteristic is beneficial for an angle diversity receiver (ADR) which enables the destructive interference to be exploited in order to reduce inter-cell interference. The results clearly indicate that the double-source cell configuration significantly outperforms the conventional single-source cell configuration when an ADR receiver is used. In this study, four signal combination schemes, the equal gain combining (EGC), select best combining (SBC), maximum ratio combining (MRC) and optimum combining (OPC) are implemented to enhance the performance of the ADR.

A mobile receiver with angular diversity detectors in Visible Light Communication (VLC) Multiple Input Multiple Output (MIMO) channels is considered. The objective is to improve the rank of the channel matrix and hence system throughput. Repetition Coding, Spatial Multiplexing and Spatial Modulation concepts are used to evaluate throughputs across multiple locations in a small room scenario. Since the receiver is mobile, the channel gains are weak in some locations of the room due to the lack of Line of Sight (LOS) paths between transmitters and receivers. Therefore we use Adaptive Modulation and Per Antenna Rate Coding (PARC) are used to improve spectral efficiency. The throughputs for fixed transmitters and receivers are compared with the oriented/inclined detectors for different cases. The results shows that for the mobile receivers, oriented/inclined detectors with Adaptive modulation and PARC provides capacity improvement when compared with the fixed and vertically oriented receivers.

In this paper, it is demonstrated for the first time that the problem of continuous downhole monitoring in the oil and gas industry is effectively addressed by the use of visible light communication (VLC). As a reliable, flexible and low-cost technique, VLC can fulfill a critical need of operators to maintain production efficiency and optimize gas well performance. The proposed VLC system makes use of a light emitting diode (LED) transmitter and a high sensitivity single photon detecting receiver referred to as single-photon avalanche diode (SPAD). The latter is instrumental in achieving long range communications, and the fact that ambient light is not present in a gas pipe is exploited. Specifically, the lack of ambient light enables high signal to noise ratio (SNR) at the receiver which operates in a photon counting mode. In this study, the bit error ratio (BER) performance of the system is simulated for a 4 kilometres long metal pipe. It is shown that the proposed system has superior power efficiency over conventional methods, which is important as it is assumed that the transmitter is battery operated. In addition, the theoretical BER performance is calculated and compared to the simulation results.

In this paper, we first investigate the negative impact of light reflections and then introduce an angle diversity receiver (ADR) to mitigate the interference effect resulting from light reflections in an indoor cellular optical wireless communications (OWC) network. As the ADR consists of multiple photodiodes (PDs), a proper signal combining scheme is essential to optimize the system performance. Therefore, four different combining schemes, the select best combining (SBC), the equal gain combining (EGC), the maximum ratio combining (MRC) and the optimum combining (OPC), are investigated. The results indicate that the OPC scheme achieves the best performance among these combination schemes. In particular, when we use a seven-PD ADR as the optical detector, the OPC scheme can significantly suppress the interference signal resulting from light reflections and can achieve a signal to interference plus noise ratio (SINR) performance that is close to the SINR performance of the same configuration that is free of light reflections.

This paper addresses the assignment of transmission and sleep time slots between interfering transmitters with the objective of minimal power consumption. In particular, we address the constructive alignment of Discontinuous Transmission (DTX) time slots under link rate constraints. Due to the complexity of the combinatorial optimization problem at hand, we resort to heuristic assignment strategies. We derive four time slot alignment solutions (sequential alignment, random alignment, p-persistent ranking and DTX alignment with memory) and identify trade-offs. One of the proposed solutions, namely, DTX alignment with memory addresses identified issues of the other three solutions by maintaining memory of past alignment and channel quality to buffer short term changes in channel quality. All strategies are found to exhibit similar convergence behavior, but different power consumption and retransmission probabilities. DTX alignment with memory is shown to achieve up to 40% savings in power consumption and more than 20% lower retransmission probability than the State-Of-The-Art (SotA).

Unipolar orthogonal frequency division multiplexing (U-OFDM) has recently been introduced for intensity modulation and direct detection (IM/DD) systems. The scheme achieves higher power efficiency than the conventional direct-current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) at the expense of half the spectral efficiency for the same M-ary quadrature amplitude modulation (M-QAM) order. This paper presents a modulation approach which doubles the spectral efficiency of U-OFDM and still allows it to achieve better performance in terms of both electrical power and optical power dissipation compared to DCO-OFDM. The simulation results and the theoretical analysis suggest that the performance improvement of the proposed scheme over DCO-OFDM increases with the modulation order. This trend is different from the inherently unipolar state-of-the-art techniques such as U-OFDM, asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and pulse-amplitude-modulated discrete multitone modulation (PAM-DMT). It is typical for these schemes to exhibit a loss in the power efficiency as the spectral efficiency is increased. The novel approach is very promising for the achievement of high data rates in IM/DD systems. To the best of the authors' knowledge, this is the first design of a strictly unipolar orthogonal frequency division multiplexing (OFDM) scheme which requires no biasing and is able to demonstrate significant energy advantage over DCO-OFDM without sacrificing spectral efficiency.

In this paper, we experimentally demonstrate the feasibility of optical wireless communication (OWC) systems with a solar panel as a photo-detector. The advantage of a solar panel is that it is a passive device, which does not require an additional power supply for converting the received light signal into an electrical signal. The frequency response of a solar panel shows that its 3-dB modulation bandwidth is 350 kHz. The results demonstrate that for a 1-Mbit/s on-off keying signal, a bit error rate of less than 2 × 10-3could be achieved when the average irradiance on the solar panel is 3.5 × 10-4W/cm2. In the current experimental setup, this corresponds to a transmission distance of 39 cm. A data rate of 7.01 Mbit/s is achieved by using orthogonal frequency division multiplexing. In addition, the feasibility of using the solar panel for simultaneous communication and energy harvesting is investigated. A load is connected in parallel to the receiver circuit in order to simulate the conditions of charging a battery by using the received signal's DC component. It is shown that the load does not hamper the communication capabilities. Hence, an OWC system with a solar-panel-based receiver can satisfy the requirements of simultaneous communication and energy harvesting.

This work investigates the impact of power allocation for channel estimation (CE) in spatial modulation (SM). SM is a unique single-stream multiple-input multiple-output (MIMO) transmission technique that achieves spatial multiplexing gains. While SM completely avoids inter-channel interference, a single active antenna leads to a challenge for CE, i.e., all transmit antennas have to be activated sequentially to send the pilot signal. More time as well as energy is therefore consumed for CE. Driven by this motivation, we propose a closed-form optimal power allocation (OPA) for SM in this paper. Regardless of the SM transceiver structure, the optimal solution only depends on the pilot sequence length and the ratio between the number of pilots and the number of total symbols. Simulations results show that the bit error ratio performance of OPA-based SM tightly approaches the case of perfect channel state information with a gap of less than 0.8 dB.

The outdoor deployment of small cells (SCs) in heterogeneous cellular networks for mobile communications is advantageous from both the capacity and the power consumption point of view. In practice, however, the network operators are limited by the high costs associated with the provisioning of backhaul communication and power supply to the SCs. This paper addresses these operational challenges by using optical links at the same time for high-capacity backhaul and wireless power supply. In particular, we present an experimental design of an optical wireless link for energy harvesting (EH). Our system consists of (i) a high-brightness white light-emitting diode (WLED) with an output luminous flux of 2200 lm, and (ii) an amorphous silicon (a-Si) solar panel. The distance of the experimental link ranges from 0.5m to 5m. The visible-light EH from the solar panel is negatively affected by the Lambertian radiation pattern of the WLED. In order to increase the collimation of the source light, we use three optical elements - a spherical lens, a reflector, and a parabolic mirror. The latter is shown to achieve a gain of 28.4 dB in EH over a distance of 5 m compared to the non-collimated case.

In this paper, the performance of a visible light communication (VLC) network exploiting power control is considered. The achievable rate region of two VLC communication pairs is characterized. The system employs optical orthogonal frequency division multiplexing (O-OFDM) with power control performed at the transmitters. The optical signal clipping effect is taken into consideration which results from the physical limitations at the transmitters. Also, the system is constrained by the desired illumination power for each transmitter. We prove that the achievable rate region is defined as the union of two regions where the boundary of each of these regions can be obtained by a single variable optimization problem. Our numerical results show the effects of the system parameters including direct current (DC) bias, the desired illumination power, clipping noise and the relative positions of the receivers relative to the LED transmitters on the achievable rate region of the two users. We show the enhancement in the performance resulted from using power control compared to orthogonal resource allocation techniques.

In this paper, a hybrid network combining visible light communication (VLC) with a radio frequency (RF) wireless local area network (WLAN) is considered. In indoor scenarios, a light fidelity (Li-Fi) access point (AP) can provide very high throughput and satisfy any illumination demands while wireless fidelity (Wi-Fi) offers basic coverage. Such a hybrid network with both fixed and mobile users has the problem of variable user locations, and thus large fluctuations in spatially distributed traffic demand. Generally, a handover occurs in such a method when a user is allocated by the central controller unit to a different AP which is better placed to serve the user. In order to be representative of real deployments, this paper studies the problem of load balancing of a dynamic system where we consider the signalling overhead for handover. We propose a scheme for dynamic allocation of resources to users, where the utility function takes into account both throughput and fairness. The simulation results show that there is a trade off between the aggregate throughput and user fairness when handover overhead is considered. The proposed dynamic scheme always outperforms the considered benchmarks in terms of fairness and can achieve better aggregate throughput in the case of low user density.

In this paper, we investigate the benefits of an angle diversity receiver (ADR) in an indoor cellular optical wireless communications (OWC) network. As the ADR consists of multiple photodiodes (PDs), a proper signal combining scheme is essential to optimise the system performance. Therefore, three different combination schemes, the equal gain combining (EGC), the select best combining (SBC) and the maximum ratio combining (MRC), are investigated. The results indicate that the ADR significantly outperforms the single-PD receiver in terms of both signal to interference plus noise ratio (SINR) and area spectral efficiency (ASE). In particular, the ADR implementing the MRC scheme achieves the best performance, where over 40 dB SINR improvement is attained compared to the single-PD receiver.

In this paper, we propose two novel receiver designs for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO- OFDM). The first one, termed 'Proposed Receiver I', exploits the structure of the clipping noise which improves the power efficiency over the state-of- the-art ACO-OFDM receiver. The second one, termed 'Proposed Receiver II', exploits the benefit of bit error correction which achieves a further improvement over the state-of-the-art ACO-OFDM receiver at the expense of computational complexity.

Spatial modulation (SM) is a relatively new multiple-input multiple-output (MIMO) technology that can provide high data rate with reasonable spectral efficiency. In this paper, the bit error rate (BER) performance of SM systems under vehicle-to-vehicle (V2V) channel models is investigated. The theoretical BER expression is given. The impact of some V2V channel model parameters on the underlying space-time correlation function (STCF) and the BER performance of SM systems are also studied. Simulation results indicate that modulation schemes, maximum Doppler frequency, the distance between the transmitter (Tx) and receiver (Rx), and antenna element spacings can affect the performance of SM systems.

In this paper we propose a hybrid visible light communications (VLC) and radio frequency (RF) femtocell system and investigate user association issues to achieve effective load balancing. We propose a scheme for cell association based on the minimum distance to the closest access point (AP) which involves a simple and fully uncoordinated decision. The performance of the three tier (macro-, femto- and optical attocells) heterogeneous network is analysed in terms of area spectral efficiency (ASE). We compare the performance of the minimum distance user association, the maximum signal to interference and noise ratio (SINR) based association and a decentralized iterative algorithm. The latter scheme results in a near optimal solution by considering the load of the AP in the cell association process. Simulation results show that the simple min-distance based user association nearly achieves the optimal load-aware performance. Moreover the average ASE of the hybrid VLC/RF system may be increased by at least two orders of magnitude over the stand-alone RF network.

Spatial modulation (SM) is a unique single-stream, multiple-input multiple-output (MIMO) transmission technique. Unlike multi-steam MIMO schemes, a single transmit antenna is activated in SM at any given time. Therefore, inter-channel interference is completely avoided. In addition, SM requires only one radio-frequency (RF) chain, regardless of the number of transmit antennas used. This key property results in a significant saving in quiescent power of power amplifiers. However, a challenge occurs when channel estimation (CE) is implemented for SM: the transmit antennas have to send pilots sequentially. Thus, in order not to compromise the throughput, the pilot number of each transmit antenna is restricted. This means we focus on improving the CE performance without increasing the number of pilots. In this paper, we propose a novel CE method for SM, in which the channel is jointly estimated across receive antennas. Simulation results show that the proposed approach outperforms the conventional method for various channel correlations between the receive antennas.

Spatial Modulation (SM) is a recently proposed single-RF multiple-input-multiple-output (MIMO) technique, which is capable of outperforming many conventional MIMO transmission schemes with low implementation and computational complexity. Recently, there have been some attempts in understanding the performance of SM in multi-user environments. However, most of the work has been oriented towards uplink multi-access scenarios. Also, conventional downlink/broadcast MIMO precoding techniques such as Zero Forcing (ZF) or Minimum Mean Square Error (MMSE) cannot be used in Multi-User SM (MU-SM), as part of the data in SM is also encoded into the Channel Impulse Responses (CIRs). In this paper, a novel precoding scheme for single-cell downlink MU-SM systems is proposed with a two-fold objective: i) the precoder needs to be able to completely eliminate the Multi-User Interference (MUI) by taking advantage of the Channel State Information (CSI) at the transmitter and ii) it needs to allow the users to use a single-user Maximum Likelihood (ML) optimum detector while achieving the same performance as interference-free point-to-point SM transmission. Finally, we also develop an interference-aware multi-user detection scheme, which does not require any CSI at the transmitter, and compare its performance with that of single-user detection schemes based on precoding.

This paper presents results of a practical implementation of a spatial shift keying (SSK) visible light communication (VLC) system. This is the first practical proof-of-concept realtime implementation of SSK for VLC to the best knowledge of the authors. The system uses four transmitter light emitting diodes (LEDs) to encode information, and four receiver photo diodes (PDs) to decode the spatial signatures and decode the incoming data signal. The achieved bit error ratio (BER) of less than 2 × 10-3allows for error-free communication if forward error correction (FEC) is to be applied. The main challenge with practical implementations of SSK in VLC is identified, namely maintaining symbol separation in the received constellation, and solutions are proposed.

We propose a new radio resource management algorithm which aims at minimizing the base station supply power consumption for multi-user MIMO-OFDM. Given a base station power model that establishes a relation between the RF transmit power and the supply power consumption, the algorithm optimizes the trade-off between three basic power-saving mechanisms: antenna adaptation, power control and discontinuous transmission. The algorithm comprises two steps: a) the first step estimates sleep mode duration, resource shares and antenna configuration based on average channel conditions and b) the second step exploits instantaneous channel knowledge at the transmitter for frequency selective time-variant channels. The proposed algorithm finds the number of transmit antennas, the RF transmission power per resource unit and spatial channel, the number of discontinuous transmission time slots, and the multi-user resource allocation, such that supply power consumption is minimized. Simulation results indicate that the proposed algorithm is capable of reducing the supply power consumption by between 25% and 40%, dependend on the system load.

A key challenge of future mobile communication research is to strike an attractive compromise between wireless network's area spectral efficiency and energy efficiency. This necessitates a clean-slate approach to wireless system design, embracing the rich body of existing knowledge, especially on multiple-input-multiple-ouput (MIMO) technologies. This motivates the proposal of an emerging wireless communications concept conceived for single-radio-frequency (RF) large-scale MIMO communications, which is termed as SM. The concept of SM has established itself as a beneficial transmission paradigm, subsuming numerous members of the MIMO system family. The research of SM has reached sufficient maturity to motivate its comparison to state-of-the-art MIMO communications, as well as to inspire its application to other emerging wireless systems such as relay-aided, cooperative, small-cell, optical wireless, and power-efficient communications. Furthermore, it has received sufficient research attention to be implemented in testbeds, and it holds the promise of stimulating further vigorous interdisciplinary research in the years to come. This tutorial paper is intended to offer a comprehensive state-of-the-art survey on SM-MIMO research, to provide a critical appraisal of its potential advantages, and to promote the discussion of its beneficial application areas and their research challenges leading to the analysis of the technological issues associated with the implementation of SM-MIMO. The paper is concluded with the description of the world's first experimental activities in this vibrant research field.

The cellular systems with network coordination are known for increasing the cell edge user throughput by processing signals captured by geographically distributed base stations coherently. This concept is also known as macrodiversity or coordinated multipoint (CoMP) in the literature. It is now well known that large-scale fading has a significant influence on the link-level performance of users in multiuser (MU) multicell systems. In this paper, we investigate a channel distribution information (CDI)-based MU multiple-input-multiple-output (MIMO) power control problem in the uplink with joint processing to overcome the large-scale fading in macrodiversity systems. There are many power control algorithms for different variants of this problem, but the CDI-based problem in MU scenario has not been well addressed in the literature. Perhaps, the lack of understanding of the performance of macrodiversity MU systems, in terms of CDI, may have prohibited advanced power control solutions. Despite such analytical difficulties, we propose a simple algorithm for this uplink power control problem, and its accuracy is proved using Monte Carlo simulation.

In this paper, we propose a novel Virtual Multiple-Input-Multiple-Output (VMIMO) architecture based on the concept of Spatial Modulation (SM). Using a dual-hop and Decode-and-Forward protocol, we form a distributed system, called Dual-Hop Hybrid SM (DH-HSM). DH-HSM conveys information from a Source Node (SN) to a Destination Node (DN) via multiple Relay Nodes (RNs). The spatial position of the RNs is exploited for transferring information in addition to, or even without, a conventional symbol. In order to increase the performance of our architecture, while keeping the complexity of the RNs and DN low, we employ linear precoding using Channel State Information (CSI) at the SN. In this way, we form a Receive-Spatial Modulation (R-SM) pattern from the SN to the RNs, which is able to employ a centralized coordinated or a distributed uncoordinated detection algorithm at the RNs. In addition, we focus on the SN and propose two regularized linear precoding methods that employ realistic Imperfect Channel State Information at the Transmitter. The power of each precoder is analyzed theoretically. Using the Bit Error Rate (BER) metric, we evaluate our architecture against the following benchmark systems: 1) single relay; 2) best relay selection; 3) distributed Space Time Block Coding (STBC) VMIMO scheme; and 4) the direct communication link. We show that DH-HSM is able to achieve significant Signal-to-Noise Ratio (SNR) gains, which can be as high as 10.5 dB for a very large scale system setup. In order to verify our simulation results, we provide an analytical framework for the evaluation of the Average Bit Error Probability (ABEP).

In this paper, a novel channel estimation (CE) method is proposed for spatial modulation (SM), a unique single-stream multiple-input-multiple-output transmission technique. In SM, there is only one transmit antenna being active at any time instance. While this property completely avoids inter-channel interference, it results in a challenge to estimate the channel information. In conventional CE (CCE) methods for SM, all transmit antennas have to be sequentially activated for sending pilots. Therefore, the time consumed in CE is proportional to the number of transmit antennas, which significantly compromises the throughput. By exploiting channel correlation, the proposed method, named transmission cross CE (TCCE), has the following characteristics: i) the entire channel is estimated by sending pilots through one transmit antenna; ii) it requires no overhead or feedback; and iii) it achieves a low computational complexity at the receiver. In addition, we propose an analytical framework to compute the distribution of the CE errors over time-varying fading channels. The corresponding average bit error probability (ABEP) bound of SM is also derived for the proposed method. Results show that the proposed ABEP bound matches with the simulations very well. When compared with CCE, the new method obtains a signal-to-noise ratio gain of up to 7.5 dB for medium and high correlations between the transmit antennas. Moreover, an adaptive CE technique can be readily implemented for SM via switching between CCE and TCCE.

In this paper, we strive to improve the energy efficiency (EE) of downlink base station (BS) cooperative transmission systems by switching off some BSs and antennas and allocating power to the users according to the required data rate and channel of each user. To show the potential of EE gain, we first propose an optimal dynamic switching off scheme using instantaneous channel information that maximizes the EE under per-user data rate and per-BS power constraints. To provide a feasible scheme for practical systems, we proceed to propose a semi-dynamic scheme where the BS-antenna operation pattern selection is based on average channel gains. Low complexity iterative algorithms are provided to find solutions for the dynamic and semi-dynamic strategies. Simulation results with typical circuit power consumption show that the semi-dynamic scheme performs close to the dynamic scheme and both schemes provide a substantial EE gain over the scheme with all antennas at all BSs active, when the data rate requirement is low. By comparing the overall power consumption for the systems of two extreme cases in deploying the active antennas, we find that the scheme only switching off BSs is more cost-effective in terms of energy than the scheme switching off antennas when the data rate requirement is low and the number of active antennas far exceeds that of the users.

Spatial Modulation (SM) is a combined multiple-input-multiple-output (MIMO) and digital modulation technique. Like in any MIMO scheme, the performance of SM is related to the differentiability, i.e. the correlation, of the multiple channels. In this paper, we analyse the performance of optical SM for a wireless indoor environment. It is shown that the use of colour light-emitting diodes (LEDs) can improve the performance of SM by more than 10 dB. This is due to the fact that the responsivity of photo-diodes is a function of the optical wavelength. As a consequence, using LEDs with different colours induces specific optical power levels at the receiver. These distinctive power levels directly affect the channel characteristic since they reduce the channel correlation. Moreover, we analyse the computational complexity of optical SM and compare it to the complexity of other common MIMO techniques. It is found that SM requires a much lower computational complexity compared to Repetition Coding (RC) and Spatial Multiplexing (SMP).

In this paper the average area spectral efficiency (ASE) of indoor visible light communication (VLC) wireless network is investigated for various room geometries (2.5m×5m×3m, 5m×5m×3m and 10m×10m×3m). A comparison is made against state-of-the-art radio frequency (RF) systems employing femtocells indoors. A modified version of the wireless world initiative new radio (WINNER) indoor deployment scenario (office building) is used as the common setup where femtocell access points (APs) and VLC APs are installed alternatively. For the RF system the minimum number of femtocell APs per floor area is four as specified. The requirement for the VLC system is to fulfill the lighting conditions inside the room, characterized by the illuminance distribution in rooms with several possible dimensions. The ASE gains of the VLC system compared to the femtocell network range between 12 and 924 (depending on the number of femtocell APs and floor geometry).

Energy-efficiency (EE) has been considered as an important goal for designing future high throughput cellular networks. In this paper, we strive to improve the EE of downlink coherent cooperative transmission systems by adaptively selecting the transmitting mode, i.e., by switching off some antennas and base stations (BSs) according to the required data rate of each user and then by allocating powers to the users. To show the potential gain in EE, we first propose an optimal dynamic mode selection scheme to select the most energy-efficient active antenna and BS pattern based on the instantaneous channels, where per-user date constraint and the per-BS-power constraint are taken into account. To provide a feasible scheme for practical systems, we proceed to propose an optimal semi-dynamic mode selection scheme where the antenna and BS switching is based on the average channel gains. Simulation results with typical circuit power consumptions show that the semi-dynamic scheme performs closely to the dynamic scheme when the cell-edge signal to noise ratio is high, and both mode selection schemes provide substantial EE gain over the traditional scheme with all antennas at all BSs active.

Interference coordination in optical wireless cellular networks using different frequency reuse techniques are discussed and compared in this paper. On the one hand, full frequency reuse maximises the system throughput at the cost of poor cell-edge user performance. On the other hand, cluster-based static resource partitioning offers good cell-edge user performance at the cost of low system throughput. Fractional frequency reuse (FFR) is introduced as a compromise between cell-edge user performance and the system throughput with low system complexity. Simulation results show that a guaranteed user throughput of 5.6 Mbps and an average area spectral efficiency (ASE) of 0.3389 bps/Hz/m2are achieved by the FFR optical wireless system with appropriate power control factors. These results show considerable throughput improvement compared to both benchmark systems. It is also shown that by adjusting the LED transmission optical power of a system using visible light spectrum, the illumination requirement for an office room can be satisfied without extra lighting facilities.

In this paper, a comparison between direct-current-biased optical orthogonal frequency-division multiple-access (DCO-OFDMA) and synchronous one-dimensional (1-D) optical code-division multiple-access (OCDMA) is performed for optical wireless communication (OWC). Line-of-sight (LOS) channel conditions are considered in an indoor downlink scenario. The bit-error ratio (BER) performance is investigated in an additive white Gaussian noise (AWGN) channel under the conditions of equal spectral and power efficiency. For DCO-OFDMA and 1-D OCDMA, the negative-level clipping effect and multiple-access interference (MAI) are considered, respectively. For the OCDMA scheme, the categories of optical orthogonal codes (OOCs) and unipolar Walsh-Hadamard codes (WHCs) are employed. The time-division multiple-access (TDMA) is also represented in the context of synchronous OCDMA by the use of identity matrices. In order to enhance the 1-D OCDMA spectral efficiency, unipolar M-ary pulse amplitude modulation (M-PAM) is employed. The BER performance versus the average electrical signal-to-noise ratio (SNR) per bit is illustrated both for the single and the multiple-user communication using Monte Carlo simulations.

In this paper, a novel optical Orthogonal Frequency Division Multiplexing (OFDM) modulation approach is presented. This method uses the Optical Spatial Modulation (OSM) technique to obtain positive and real-valued signals which are required by an Optical Wireless Communication (OWC) system. In comparison to existing OFDM methods applied to the OSM system, the new scheme, Non-DC-biased OFDM (NDC-OFDM), has significant advantages. Compared to DC-biased Optical OFDM (DCO-OFDM), NDC-OFDM avoids DC-biasing and, thus, improves the power efficiency. Moreover, the spectral and power efficiency of the new approach are better than the well-known unipolar optical modulation scheme, Asymmetrically Clipped Optical OFDM (ACO-OFDM). The bit-error ratio (BER) performances of these three methods are compared. Compared to ACO-OFDM and DCO-OFDM, NDC-OFDM has an energy saving gain of at least 5 dB for the same spectral efficiency. The improvement comes at the expense of additional hardware at the transmitter and receiver. However, visible light communication (VLC) systems typically are equipped with multiple low-cost Light Emitting Diodes (LEDs) to fulfill minimum indoor lighting conditions.

This paper presents an initial study on how to model user mobility in a wireless cellular network scenario. The model provides realistic patterns of movement between a number of locations. The aforementioned mobility model is built upon to introduce a base station (BS) energy efficient power state control algorithm (EESC) that is able to provide significant energy savings for the simulated scenario. Erratic subsequent on and off BS behavior is avoided by referring to an average loading curve, generated for each BS during a learning period, when making decisions. Moreover, BSs are turned off only when the users can be handed off to nearby cells with a predefined maximum allowed predicted loss in data rate. The resulting system is able to deliver an energy consumption reduction between 36% and 11% over an always on benchmark for the simulated scenarios. There is negligible loss in terms of achieved data rate per user.

In single-stream multiple-input multiple-output (MIMO) schemes, such as spatial modulation (SM) and space shift keying (SSK), a single transmit antenna is activated at any given time. Therefore, unlike multi-stream MIMO transmitters, simultaneous pilot transmissions are prohibitive because of a single radio-frequency (RF) chain. In state-of-the-art literature, the channels of different transmit antennas are individually estimated. As a result, more time is required to transfer pilots and the effective data rate is compromised. In this paper, we propose a novel channel estimation (CE) technique for single-stream MIMO systems. Given a pilot ratio, the proposed scheme achieves the same estimation period as multi-stream MIMO systems without needing any additional information or feedback. Simulations are implemented under a practical base station environment. Results show that for various speeds of the mobile user, the proposed approach significantly improves the performance of SM in comparison to the conventional CE method.

We propose the use of echo state networks for the task of wireless channel characterisation to select the most similar channel to the current observed channel from a pre-defined set, based entirely on received signal information. This allows the system to select the optimal resource allocation scheme and transmission parameters from pre-computed solutions. Using suitable training data, the neural network was able to learn to characterise a signal correctly 68% of the time, which can be further improved to 72% by adding some simple location data to the signals being examined. Our system out- performs a comparable statistical method by a factor of two, demonstrating echo state networks' ability to infer information from their training data which other systems can not.

In this paper we present a method for predicting a wireless channel using an efficient sum of sinusiods (SOS) method. Our method models a channel as the sum of a discrete number of rays, and extrapolates these rays forward along a trajectory when given accurate measurements of the component rays at distinct spatial points. In simulations, we have been able to eliminate the need for regular pilot signals in an OFDM system over a distance of 10 cm, increasing system data throughput by 17% in the case of a moving receiver.

In this paper, we extend the previously proposed power-saving Resource Allocation using Power control and Sleep (RAPS) algorithm to the interference environment. The RAPS algorithm allocates resources, power and Discontinuous Transmission (DTX) time slots jointly for Orthogonal Frequency Division Multiplexing (OFDMA) downlink resource scheduling such that Base Station (BS) supply power consumption is reduced while providing guaranteed link rates. The RAPS scheduler was previously proposed and studied only as a single-cell mechanism. It is here adapted to and tested in the multi-cell interference setting. The resulting dynamical system is analyzed with respect to convergence, power consumption and load dependence. It is found that the Signal-to- Interference-and-Noise-Ration (SINR) distribution in the network is greatly improved and average power consumption can be reduced by around 50% in comparison to a Proportional Fair (PF) scheduling benchmark.

A cooperative diversity protocol for multi-relay wireless networks is proposed. Its distinguishable feature is to protect the transmission of the source through distributed diversity while simultaneously serving the data traffic of the relays. Therefore, the performance of the source is improved without reducing the aggregate throughput. The protocol exploits a recently introduced concept for multiple- antennas wireless systems, which is known as Spatial Modulation (SM). The contribution of this paper is threefold: i) the optimal receiver that takes into account demodulation errors at the relays is developed; ii) its achievable diversity is studied analytically; and iii) it is shown, with the help of Monte Carlo simulations, that it outperforms state- of-the-art cooperative diversity protocols.

In this paper, we aim to study the Energy Efficiency (EE) of Spatial Modulation (SM) at different Base Stations (BSs) taking into account the total power consumption. Compared to conventional Multiple-Input Multiple-Output (MIMO) schemes, SM benefits from a single Radio Frequency (RF) chain which results in decreased power supply (W), higher EE (Mbits/J), and reduced complexity. Using the fundamental limits of Shannon capacity, we show that SM achieves a range of average data rates with only a fraction, which can be as low as 24% for four transmit antennas, of the total power supply of conventional MIMO. In addition, we demonstrate that the EE of the studied schemes is maximized for a certain average data rate and that SM achieves the highest EE among them. Finally, we note that a BS employing SM can be up to 67% more energy efficient compared to a BS under a conventional MIMO transmission scheme, for four transmit antennas.

In this paper the optimum placement of light emitting diode (LED) arrays is investigated for indoor visible light communication (VLC) subject to maximization of average area spectral efficiency (ASE). Design parameters of LED array such as the distance between two neighboring LEDs and the precise location on the ceiling of the LED arrays can be obtained by solving an optimization problem. The formulated optimization problem is applied to a room of 2.5m×5m×3m dimension where two square LED arrays provide the required illumination. Due to the complexity of the optimization problem, a numerical optimization procedure is employed and the maximization is carried out by using algorithms from Matlab optimization toolbox. The results show that when the users employ a wide field of view (FOV) detector (e.g. 85°) the optimum solution involves placing the two LED arrays close to the opposite walls in the direction of the highest dimension of the room. It is also shown that when employing receivers with a smaller FOV (45°) the optimum solution allows the two LED arrays to be placed farther away from the opposite walls in the room. The average ASE achieved in this case is twice as high as the average ASE obtained when employing receivers with 85° FOV. Moreover, when the illumination constraint is also fulfilled, the ASE performance degradation can be quite significant (e.g. 33% for detectors with 85° FOV) as compared with the case where the VLC system is used only for communication purposes.

Multiple-input-multiple-output systems provide a potential approach for achieving high communication capacities in optical wireless. A small amount of publications based on different scenarios are available. The current work will illustrate that a very good distribution of communication capacity can be achieved for certain configurations. In particular, we look at angle diversity receivers which are able to successfully identify transmitter directions. The investigated scenarios include 4x4 and 16x16 multiple-input-multiple-output configurations.

To improve the performance of spatial modulation (SM) over correlated MIMO channels, transmit optimized spatial modulation (TOSM) has been proposed recently. It trades off traditional signal constellation diagrams with spatial constellation diagrams to minimize the average bit error probability (ABEP). After the optimum number of transmit antennas is determined, the specific antennas need to be carefully chosen from the entire array to provide a minimum ABEP. Like in conventional transmit antenna selection (TAS) schemes, the problem can be solved by an exhaustive search. However, this results in an unaffordable complexity especially when the spectral efficiency is high. In this paper, we propose a creative TAS approach for TOSM. Given a required number of antennas, the novel technique determines the selection solution based on circle packing. Simulation results show that for various channel correlations and spectral efficiencies, the proposed method achieves performance results close to exhaustive search with a gap of less than 0.3 dB. The complexity is reduced to an extremely low level.

In this paper, for the first time real--world channel measurements are used to analyse the performance of spatial modulation (SM), where a full analysis of the average bit error rate performance (ABER) of SM using measured urban correlated and uncorrelated Rayleigh fading channels is provided. The channel measurements are taken from an outdoor urban multiple input multiple output (MIMO) measurement campaign. Moreover, ABER performance results using simulated Rayleigh fading channels are provided and compared with a derived analytical bound for the ABER of SM, and the ABER results for SM using the measured urban channels. The ABER results using the measured urban channels validate the derived analytical bound and the ABER results using the simulated channels. Finally, the ABER of SM is compared with the performance of spatial multiplexing (SMX) using the measured urban channels for small and large scale MIMO. It is shown that SM offers nearly the same or a slightly better performance than SMX for small scale MIMO. However, SM offers large reduction in ABER for large scale MIMO.

In this paper, a distributed and autonomous technique for resource and power allocation in orthogonal frequency division multiple access (OFDMA) femto-cellular networks is presented. Here, resource blocks (RBs) and their corresponding transmit powers are assigned to the user(s) in each cell individually without explicit coordination between femto-base stations (FBSs). The “allocatability” of each resource is determined utilising only locally available information of the following quantities: the required rate of the user; the quality (i.e., strength) of the desired signal; the frequency-selective fading on each RB; and the level of interference incident on each RB. Using a fuzzy logic system, the time-averaged values of each of these inputs are combined to determine which RBs are most suitable to be allocated in a particular cell, i.e., which resources can be allocated such that the user requested rate(s) in that cell are satisfied. Fuzzy logic presents a completely novel, low-complexity methodology for inter-cell interference coordination (ICIC). A comprehensive study of this system in a femtocell environment is performed, yielding system performance improvements in terms of throughput, energy efficiency and coverage over state-of-the-art ICIC techniques.

In this paper, we compare the performance of multiple-input-multiple-output (MIMO) techniques applied to indoor optical wireless communications (OWC) assuming line-ofsight (LOS) channel conditions. Specifically, several 4 × 4 setups with different transmitter spacings and different positions of the receiver array are considered. The following MIMO algorithms are considered: Repetition Coding (RC), Spatial Multiplexing (SMP) and Spatial Modulation (SM). Particularly, we develop a framework to analytically approximate the bit error ratios (BERs) of these schemes and verify the theoretical bounds by simulations. The results show that due to diversity gains, RC is robust to various transmitter-receiver alignments. However, as RC does not provide spatial multiplexing gains, it requires large signal constellation sizes to enable high spectral efficiencies. In contrast, SMP enables high data rates by exploiting multiplexing gains. In order to provide these gains, SMP needs sufficiently low channel correlation. SM is a combined MIMO and digital modulation technique. We show that SM is more robust to high channel correlation compared to SMP, while enabling larger spectral efficiency compared to RC. Moreover, we investigate the effect of induced power imbalance between the multiple transmitters. It is found that power imbalance can substantially improve the performance of both SMP and SM as it reduces channel correlation. In this context, we also show that blocking some of the links is an acceptable method to reduce channel correlation. Even though the blocking diminishes the received energy, it outweighs this degradation by providing improved channel conditions for SMP and SM. For example, blocking 4 of the 16 links of the 4 × 4 setup improves the BER performance of SMP by more than 20 dB, while the effective signal to noise ratio (SNR) is reduced by about 2 dB due to the blocking. Therefore, MIMO techniques can provide gains even under LOS conditions which provide only little channel differences.

In this paper the error performance analysis of a low complexity, multiple transmitter generalised space shift keying (GSSK) signalling technique for short range indoor visible light communications is presented. In an N_t transmitter system, this signalling technique is capable of achieving a spectral efficiency of N_t bits/s/Hz. The GSSK system has a higher spectral efficiency than the conventional on-off keying (OOK) and pulse position modulation (PPM) techniques. The GSSK transmitter is much simpler than that of an equivalent regular pulse amplitude modulation (PAM) system with similar spectral efficiency. An analytical expression for the symbol error rate (SER) is presented and verified using simulations. The receiver is based on the maximum likelihood criterion and using multiple photodetectors (PDs) in the receiver improves the error performance. Within the room and the transmitter-receiver configuration under consideration, with four PDs, a gain of 6 dB is attainable over the single PD system for the case of 2 and 3 bits/s/Hz. Considering a fixed link range with N_t = 2 and at a symbol error rate of 10-6, using wide half angle LEDs with Φ1/2= 60° requires about 12 dB less in electrical signal-to-noise ratio compared with Φ1/2= 30°. The beam directivity is however of much lower effect when the room under consideration is equiped with sufficient LEDs to provide full coverage. The GSSK technique can also support dimming control by using different pulse widths. This, however, is at a price of reduced spectral efficiency.

In this paper, Sphere Decoding (SD) algorithms for Spatial Modulation (SM) are developed to reduce the computational complexity of Maximum—Likelihood (ML) detectors. Two SDs specifically designed for SM are proposed and analysed in terms of Bit Error Ratio (BER) and computational complexity. Using Monte Carlo simulations and mathematical analysis, it is shown that by carefully choosing the initial radius the proposed sphere decoder algorithms offer the same BER as ML detection, with a significant reduction in the computational complexity. A tight closed form expression for the BER performance of SM—SD is derived in the paper, along with an algorithm for choosing the initial radius which provides near to optimum performance. Also, it is shown that none of the proposed SDs are always superior to the others, but the best SD to use depends on the target spectral efficiency. The computational complexity trade—off offered by the proposed solutions is studied via analysis and simulation, and is shown to validate our findings. Finally, the performance of SM—SDs are compared to Spatial Multiplexing (SMX) applying ML decoder and applying SD. It is shown that for the same spectral efficiency, SM—SD offers up to 84% reduction in complexity compared to SMX—SD, with up to 1 dB better BER performance than SMX—ML decoder.

This paper presents a wireless multiple-input/multiple-output (MIMO) testbed for the UK-China Science Bridges Project: R&D on Beyond Fourth Generation (B4G) Wireless Mobile Communications (UC4G). An introductory review of other approaches is first presented and then subsequently the motivation, architecture and specifications of the UC4G testbed are outlined. The flexibility of operation, versatility of function and modular design of the UC4G testbed are shown to be its key benefits.

A unique characteristic of spatial modulation (SM) is the three dimensional constellation diagram. This enables to trade-off traditional signal constellation diagrams with spatial constellation diagrams where the latter is defined by the physical antenna array. In this paper we investigate the optimum pairs of signal and spatial constellation sizes with respect to average bit error probability (ABEP) and energy efficiency. The analysis is performed for varying antenna correlations and channel conditions. Both numerical and closed-form results are presented which show that a significant performance gain can be obtained when the optimal constellation pairs are used.

In this paper, a novel scheduling mechanism that enhances both network spectral and energy efficiency is presented. In Pareto Optimal Scheduling (POS), mobile stations (MSs) are scheduled based on path gains such that the sufficient conditions for Pareto optimal power control (POPC) are fulfilled. This is performed in such a manner to maximise the number of concurrently transmitting MSs. Furthermore, a Stepwise Removal (SR) algorithm is introduced for the situation where links do not meet the sufficient conditions for power control. In this case, links are removed in order for other MSs to achieve their signal-to-interference-plus-noise ratio (SINR) targets. The targets of these remaining MSs are updated to prevent losses in system spectral efficiency caused by the link removals. Large network simulation results show that significant gains in spectral efficiency can be achieved over standard power control techniques, while additionally providing substantially improved energy efficiency.

In this paper, Pareto Femto-Cell Scheduling (PFCS), a novel scheduling mechanism for randomly deployed femto-cells, is presented. Here, the signal-to-interference-plus-noise ratio (SINR) targets of femto-users are adapted such that the sufficient conditions for Pareto optimal power control (POPC) are fulfilled. Furthermore, interference from full bandwidth users is managed such that as many mobile stations (MSs) as possible can transmit. Due to the random nature of femto-base station (FBS) deployment, interference graphs are used to group femto-cells (and hence, users) such that target spectral efficiencies can be achieved at Pareto optimum power. Simulation results show that PFCS achieves significant system capacity gains over other SINR-target-based power allocation techniques, while maximising coverage in dense mobile environments. Furthermore, substantial power savings can be achieved.

In this paper, a framework for optimum signal shaping in multi-carrier modulation is presented for optical wireless communications (OWC). The two fundamental multi-carrier transmission schemes based on orthogonal frequency division multiplexing (OFDM), direct-current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM), are studied. The optimum signal shaping is defined as optimum biasing and optimum scaling of the time domain signal within the optical power constraints of the transmitter front-end. These include the boundaries of the limited linear dynamic range, such as minimum and maximum radiated optical power, and the desired average optical power level. As a result, the minimum required electrical signal-to-noise ratio (SNR) to maintain a target bit-error ratio (BER) is obtained for a desired multi-level quadrature amplitude modulation (M-QAM) scheme and a given combination of optical power constraints. The average optical power is varied over dynamic ranges of 10 dB, 20 dB and 30 dB. With the increase of the dynamic range and for a major portion of the average optical power levels, DCO-OFDM demonstrates a lower minimum electrical SNR requirement for a target BER as compared to ACO-OFDM for modulation orders with similar spectral efficiencies.

This paper proposes a non-stationary wideband geometry-based stochastic model (GBSM) for multiple-input multiple-output (MIMO) high-speed train (HST) channels. The proposed model has the ability to investigate the non-stationarity of HST environment caused by the high speed movement of the receiver. Based on the proposed model, the space-time-frequency (STF) correlation function (CF) and STF local scattering function (LSF) are derived for different taps. Numerical results show the non-stationarity of the proposed channel model.

Energy efficiency is becoming increasingly important for wireless communication systems in order to minimize carbon footprint of wireless networks and to increase the battery life of mobile terminals. The spectral-energy efficiency trade-off is of primary significance to determine how much energy per bit is required in a wireless communication system to achieve a specific spectral efficiency. In this paper, we study energy-efficient resource allocation scheme for Orthogonal Frequency- Division Multiple Access (OFDMA) systems with multiple users. The trade-off between spectral and energy efficiencies is analyzed under the constraint of maintaining the fairness among users. We first formulate the energy-efficient optimization problem as an integer fractional programming. We then apply an iterative fractional method to simplify the problem to integer linear programming (ILP) problem. Simulation results demonstrate that the impact of user's quality of service (QoS) is minor on the energy efficiency when a large spectral efficiency is required.

It is known that Coordinated Multiple Point (CoMP) transmission can increase data rates as more than one copies of signals are coordinated transmitted by multiple points. For the wideband communication system with frequency selective fading channel, the optimal power allocation will be different for each fading block. The Water-Filling (WF) power allocation is optimal for single transmission point. However, it is hard to get closed-form optimal solution for the CoMP transmission as the power allocation for fading blocks of one point is simultaneously affected by other coordinated points. This paper analyzes the power allocation of coherent coordinated transmission in CoMP block fading communication system with two transmission points, and the optimal solution is given by Lagrange method. The numerical simulations show that it has a near constant performance gap between the optimal allocation and equal power allocation, and the performance of separate WF power allocation is between optimal allocation and equal power allocation.

Spatial Modulation (SM) is a relatively new tech- nique to include information bits in the spatial domain, combining multiple-input multiple-output (MIMO) and digital modulation techniques. The increase on the overall data rate is achieved by activating only one transmitter at any time, whereas the index of the active emitter represents data bits. At the receiver side the active transmitter index is estimated based on the channel gain for the different paths. In this work we show the behaviour of an optical SM wireless communications system. A fully operative test system consisting of four transmitters and one receiver has been implemented and tests have been conducted to obtain the symbol error rate (SER) for different link distances. Results show the strong relation between the error rate achieved and the relative channel gain differences between all channels rather than the individual signal-to-noise ratio (SNR).

Spatial modulation (SM) and space shift keying (SSK) use only one out of several transmit antennas at a time to transmit data via an antenna index. In such a system, the information is encoded by exploiting channel randomness i.e. the fact that channels between different transmit and receive antennas are random. This difference is used to distinguish among the transmit antennas. While SSK uses only antenna index to transmit data, SM also uses ordinary signal modulation. In wireless secrecy systems, one of the key performance measures is secrecy capacity. It specifies the rate at which the transmitter can communicates on the main link to the desired receiver while this information cannot be decoded by the eavesdropper. We investigate SM and SSK in the context of wireless secrecy capacity when the underlying modulation and the difference between the legitimate and eavesdropper signal to noise ratios (SNRs) are varied.

In this paper, motivated by the relatively new concept of Spatial Modulation (SM), we are addressing the problem of Receive-Spatial Modulation (R-SM) under two cases of imperfect channel knowledge at the transmitter side. In the first case, we adopt a statistical model for the channel uncertainties, whereas in the second one a worst-case approach is followed and the channel uncertainties are expressed as a bounded set. Based on Zero-Forcing (ZF) precoding and using standard tools from optimization theory, we derive closed form solutions that turn out to be robust. Simulation results show that the proposed schemes have a performance close to the perfect channel knowledge scenario in low and mid-low SNRs. Furthermore, these designs can be applied to wide range of channels with different correlation states combined with a transmit power gain.

A novel modulation technique coined U-OFDM is proposed. U-OFDM uses different time sample states and an innovative rearrangement of the Orthogonal Frequency Division Multiplexing (OFDM) frame which allow for the creation of unipolar OFDM signals required for Optical Wireless Communication (OWC) with Light Emitting Diodes (LEDs). In comparison to similar techniques like DC-biased Optical OFDM (DCO-OFDM) and Asymmetrically Clipped Optical OFDM (ACO-OFDM), U-OFDM is both optically and electrically more power efficient in an Additive White Gaussian Noise (AWGN) channel, which is prevalent in an optical wireless system.

The majority studies on resource allocation are based on continuous power allocation. However, the transmit power can be assumed as a finite set of discrete power levels only, especially for practical digital cellular systems. By simply rounding up or down, the conventional continuous power allocation algorithm will not suffice. In this paper, via transmitting signal on discrete power levels, multi-cell power allocation is modeled as a combinatorial optimization problem, which proved to be NP-hard. Ant colony optimization (ACO) is applied to get near-optimal solution of the problem. In particular, a multiple power level based searching graph is designed, and conventional ACO is improved that ant colonies in each cell cooperate to maximize system throughput. Simulation results indicate that with only 4 power levels, the proposed algorithm can achieve a significant rate gain over the existing schemes.

This paper constructs a model for Optical Spatial Modulation Orthogonal Frequency Division Multiplexing (O-SM-OFDM). It investigates the effects of different power allocation schemes on bit error ratio (BER) performance. Optical features such as the DCO technique, the LED non-linearity and the indoor optical channel are considered. Under power constraint, when making power difference between transmitters, we discussed trade-off between the accuracy of the spatial domain bits detection and that of signal domain bits detection. We investigated the relationship between transmitter power surplus, spatial BER, signal BER, system total BER and signal to noise ratio (SNR). The optimal point of transmission power difference is found to be around 11.5 dB.

Spatial Modulation (SM) is a combined multiple-input-multiple-output (MIMO) and digital modulation technique which besides common signal modulation conveys additional information bits in the spatial domain. To this end, only one transmitter is active at any time instance. The actual index of each emitter represents a unique spatial constellation point and thus conveys additional information. As a consequence, SM completely avoids inter-channel interference (ICI) and provides low detection complexity. Like for any MIMO scheme, the performance of SM is degraded in the presence of high channel correlation. Therefore, Trellis Coded Spatial Modulation (TCSM) applies coding techniques to the bits conveyed in the spatial domain to assist the detection of the active transmitter. As optical wireless communications (OWC) in indoor environments is subject to high spatial correlation and low channel distinctness, we evaluate the performance of coded SM applied to indoor OWC. For this purpose, we propose an enhanced coded SM technique which jointly encodes the bits conveyed in the signal and spatial domains. It is found in this paper that our enhanced coded SM technique can achieve gains in signal to noise ratio (SNR) of about 1-3 dB compared to the originally proposed TCSM scheme.

In this paper, the performance of indoor OFDM (orthogonal frequency division multiplexing) optical wireless (OW) communication systems in the presence of LED (light emitting diode) clipping distortions is analyzed. Two indoor OW OFDM techniques are considered. An ACO-OFDM (asymmetrically clipped optical OFDM) system produces half-wave symmetry time signal at the output of the OFDM modulator by special assignment of subcarriers. Therefore, bipolar-unipolar conversion is achieved by clipping the signal at the zero level at the expense of data rate reduction. The second technique, called DCO-OFDM (DC-biased optical OFDM), assigns data to all possible subcarriers to increase the data rate. However, half-wave symmetry signals cannot be achieved and a DC bias is needed to convert the bipolar signal to a unipolar signal before modulating the LED intensity. The performance of these systems, in terms of average electrical OFDM signal power versus bit-error-ratio (BER), DC power consumption, and transmitted optical power in the presence of additive white Gaussian noise (AWGN) channel and considering a practical LED model, are studied. Analytical results are validated through Monte Carlo simulation results and the results demonstrate close match. The results clearly highlight that LED clipping has significant impact on the performance of these systems and system design should consider optimizing the OFDM signal power, DC bias point, and LED dynamic range.

In this paper, the performance of an asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) indoor wireless system is investigated. In particular, the illuminance distribution inside a room (5m×5 m×3m) under different brightness conditions, i.e. dimming levels, and the electrical signal-to-noise ratio (SNR) at the receiver are considered. The system performance is also assessed in terms of bit-error-ratio (BER) as a function of average transmitted optical power. Brightness control is achieved using the continuous current reduction (CCR) technique to assure higher luminous efficacy compared to the pulse-width modulation (PWM) dimming technique. In addition, the impact of the induced distortions due to the nonlinear behavior of the light-emitting diode (LED) as a light source is included in the simulation model. For a circular area of about 1m radius located directly below one of the four LED illumination modules utilized in the room, having illuminance values above 300 lx (10% brightness), a 9 dB average electrical SNR is achieved at the receiver located on a desktop.

In this work, the performance of binary phase shift keying (BPSK) subcarrier intensity-modulated terrestrial free-space optical (FSO) communication link is analysed in the presence of noise and atmospheric turbulence. To ameliorate channel fading caused by the atmospheric turbulence, we propose a subcarrier time delay diversity (STDD) scheme in which the delayed versions of the original data are re-transmitted on different subcarriers. In short range links, a single re-transmission scheme results in an estimated gain of up to 4.5 dB in received irradiance. With sufficient link margins, short range FSO links might be able to cope without the use of any diversity techniques but for links spanning over 1 km, turbulence induced fading effect must be compensated for. In this study, the authors show that the subcarrier STDD is a viable scheme for this purpose.

The authors take a look at the existing 3G systems in service and investigate the capabilities of 4G, and while the theoretical throughput of these cellular systems is expected to be high, the future promises to offer more technological improvements and innovations.

The Myth: Sixty years of research following Shannon's pioneering paper has led to telecommunications solutions operating arbitrarily close to the channel capacity—“flawless telepresence” with zero error is available to anyone, anywhere, anytime across the globe.

In this paper, the impact of clipping noise on optical wireless communication (OWC) systems employing orthogonal frequency division multiplexing (OFDM) is investigated. The two existing optical OFDM (O-OFDM) transmission schemes, asymmetrically clipped optical OFDM (ACO-OFDM) and direct-current-biased optical OFDM (DCO-OFDM), are studied. Time domain signal clipping generally results from direct current (DC) biasing and/or from physical limitations of the transmitter front-end. These include insufficient forward biasing and the maximum power driving limit of the emitter. The clipping noise can be modeled according to the Bussgang theorem and the central limit theorem (CLT) as attenuation of the data-carrying subcarriers at the receiver and addition of zero-mean complex-valued Gaussian noise. Analytical expressions for the attenuation factor and the clipping noise variance are determined in closed-form and employed in the derivation of the electrical signal-to-noise ratio (SNR). The validity of the model is verified through a Monte Carlo bit-error ratio (BER) simulation. Finally, the BER performance of ACO-OFDM with DCO-OFDM is compared for different clipping levels and multi-level quadrature amplitude modulation (M-QAM) schemes.

In this paper, we study the performance of space modulation for Multiple-Input-Multiple-Output (MIMO) wireless systems with imperfect channel knowledge at the receiver. We focus our attention on two transmission technologies, which are the building blocks of space modulation: i) Space Shift Keying (SSK) modulation; and ii) Time-Orthogonal-Signal-Design (TOSD-) SSK modulation, which is an improved version of SSK modulation providing transmit-diversity. We develop a single-integral closed-form analytical framework to compute the Average Bit Error Probability (ABEP) of a mismatched detector for both SSK and TOSD-SSK modulations. The framework exploits the theory of quadratic-forms in conditional complex Gaussian Random Variables (RVs) along with the Gil-Pelaez inversion theorem. The analytical model is very general and can be used for arbitrary transmit- and receive-antennas, fading distributions, fading spatial correlations, and training pilots. The analytical derivation is substantiated through Monte Carlo simulations, and it is shown, over independent and identically distributed (i.i.d.) Rayleigh fading channels, that SSK modulation is as robust as single-antenna systems to imperfect channel knowledge, and that TOSD-SSK modulation is more robust to channel estimation errors than the Alamouti scheme. Furthermore, it is pointed out that only few training pilots are needed to get reliable enough channel estimates for data detection, and that transmit- and receive-diversity of SSK and TOSD-SSK modulations are preserved even with imperfect channel knowledge.

The sum rate of spectrum-sharing in decentralized and self-organizing wireless networks is investigated in this paper. Such networks pose the following two fundamental challenges: 1) cochannel interference and 2) the hidden node problem. For a slotted shared wireless medium, where resources are partitioned into time-frequency slots, time-multiplexed receiver initiated busy burst (BB) transmissions solve these problems by establishing an exclusion region around an active receiver by means of receiver feedback. The size of this exclusion region is controlled by an interference threshold that determines whether a user is allowed to transmit on a specific time-frequency resource unit. We propose a novel approach for setting the interference thresholds based on a heuristic derived for a two-link network. First, for two-links, the optimum threshold value is derived that maximizes the sum rate. Second, for multiple links, the new heuristic threshold that only relies on locally available information is derived. It is demonstrated via simulations that heuristic thresholding achieves superior sum rate compared to a fixed system-wide threshold. To complement simulation results, an analytical approach is developed to approximate the probability density function (pdf) of the sum of the interferers in BB setting with fixed threshold with a cumulant-based shifted log normal fitting method.

In this paper, we study the performance of Spatial Modulation (SM) applied to optical wireless communications (OWC) in indoor environments with line-of-sight (LOS) characteristics. To this end, we consider setup scenarios with different numbers of optical transmitters and receivers which are arranged within a room. SM is compared to Repetition Coding (RC). Because RC is known to achieve very good performance in OWC systems due to the use of intensity modulation and the resulting constructive superposition of the power signals. The results show that SM can outperform RC when high spectral efficiencies are desirable, e.g. 4 bit/s/Hz and greater, since it can operate with reduced signal modulation orders by conveying additional data bits in the spatial domain. We also demonstrate that SM benefits from receive-diversity to a larger extent while at the same time requiring less computational complexity. Furthermore, we give a general framework to numerically approximate the average bit error probability of both SM and RC.

This paper investigates a new pedestrian speed estimation method for OFDM systems. Correlation coefficients are calculated from the wideband channel frequency response and the instantaneous coherence time is estimated for a certain envelope correlation coefficient. The performance of the proposed method is verified by means of an experimental system in order to predict the reliability of the estimator. The algorithm gives an average error of less than 2% for pedestrian speeds and its computational complexity is low.

In this paper, we study the sensitivity and robustness of Space Shift Keying (SSK) modulation to imperfect channel knowledge at the receiver. Unlike the common widespread belief, we show that SSK modulation is more robust to imperfect channel knowledge than other state-of-the-art transmission technologies, and only few training pilots are needed to get reliable enough channel estimates for data detection. More precisely, we focus our attention on the so-called Time-Orthogonal-Signal-Design (TOSD-) SSK modulation scheme, which is an improved version of SSK modulation offering transmit-diversity gains, and provide the following contributions: i) we develop a closed-form analytical framework to compute the Average Bit Error Probability (ABEP) of a mismatched detector for TOSD-SSK modulation, which can be used for arbitrary transmit-antenna, receive-antenna, channel fading, and training pilots; ii) we perform a comparative study of the performance of TOSD-SSK modulation and the Alamouti code under the same imperfect channel knowledge, and show that TOSD-SSK modulation is more robust to channel estimation errors; iii) we point out that only few pilot pulses are required to get performance very close to the perfect channel knowledge lower-bound; and iv) we verify that transmit- and receive-diversity gains of TOSD-SSK modulation are preserved even for a mismatched receiver.

In this paper, the impact of double-sided signal clipping on optical wireless communication (OWC) systems employing asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is investigated. Time domain signal clipping is introduced due to biasing issues or physical limitations of the front-ends. These include insufficient forward biasing of the emitter and the maximum driving power limit of the emitter and/or the detector. The clipping noise can be modeled according to the Bussgang theorem and the central limit theorem as attenuation of the subcarriers at the receiver and addition of zero-mean complex Gaussian noise. The attenuation factor and the clipping noise variance are determined semi-analytically and employed in the derivation of the signal-to-noise ratio (SNR) at the receiver. Finally, the theoretical SNR is verified through a bit-error ratio (BER) simulation.

Spatial Modulation (SM) is a recently proposed low--complexity modulation scheme for multiple-antenna wireless systems. Recent works have shown that state-of-the-art SM provides a multiplexing gain with respect to single-antenna systems by avoiding inter-channel interference, but it is inherently unable to achieve transmit-diversity. The aim of this paper is to shed light on the design of multiple-antenna wireless systems that exploit the SM concept and can achieve transmit-diversity gains. More specifically, we propose a novel modulation concept, which in a unique fashion combines the high multiplexing gain offered by SM and the transmit-diversity gain provided by Space Time Block Codes (STBCs) technology. This modulation scheme is introduced here for the first time and referred to as "Spatially-Modulated STBCs" (SM-STBC). By using analysis and simulation, we show that SM-STBC achieves multiplexing and diversity gains, as well as still retains a single-stream receiver implementation. By using numerical simulations, we show that the proposed code can provide better performance than state-of-the-art Alamouti and rate-3/4 STBCs.

Space Shift Keying (SSK) is a low-complexity modulation scheme for multiple-antenna wireless systems. In this paper, we analyze the transmit-diversity/multiplexing trade-off of SSK modulation with the main objective of developing practical solutions to achieve transmit-diversity. More specifically, the contributions of this paper are as follows: i) we propose a practical scheme that achieves transmit-diversity equal to two for any number of antennas at the transmitter. The solution is based on the so-called Time-Orthogonal-Signal-Design (TOSD) principle introduced in [1], and adopts time-orthogonal shaping filters at the transmitter; ii) we show that the TOSD principle with orthogonal shaping filters can be applied to the so-called Generalized SSK (GSSK) modulation scheme in [2], and that a transmit-diversity equal to two can still be obtained while increasing the data rate with respect to SSK modulation; and iii) we propose a general encoding scheme that allows us to get transmit-diversity greater than two. The solution combines TOSD and GSSK principles in a unique fashion, and is flexible enough to accommodate various transmit-diversity gains by trading-off the number of transmit-antenna, the number of simultaneously-active transmit-antenna, and the achievable data rate. Furthermore, proposed methods and findings are substantiated via analysis and numerical simulations.

This paper presents a novel adaptation of the score-based scheduling principle along with a method for trading-off bandwidth for energy efficiency for use in Long Term Evolution (LTE) systems. The score-based principle is adapted to make use of both a relative and absolute energy efficiency metric. This results in flexible energy efficient scheduling that preserves the quality of service (QoS), as well as reduces the interference in the system. The general principle behind bandwidth expansion is to extend a user's bandwidth by a factor of α. The target throughput is maintained by switching to a lower order modulation scheme and adjusting any other link parameters as necessary. Through a theoretical derivation, it is established that the higher the user's employed modulation order is, the greater the possible gains are from employing such a transmission mode. The calculated improvement in expended energy ranges from approximately 43% to 98%. Simulations are used to empirically validate the theoretical results. When an expansion of bandwidth by a factor of 2 is simulated, the bandwidth expansion mode (BEM) is more efficient in 93% of the time, and the expended energy is reduced by 44%, while improving on the QoS delivered by the benchmark system.

In this paper, Sphere Decoding (SD) algorithms for Spatial Modulation (SM) are developed to reduce the computational complexity of Maximum--Likelihood (ML--) optimum detectors, which foresee an exhaustive search of the whole search space and have a complexity that linearly increases with the product of number of transmit--antenna, receive--antenna, and size of the modulation scheme. Three SDs specifically designed for SM are proposed and analyzed in terms of Bit Error Probability (BEP) and computational complexity. By judiciously choosing some key parameters, e.g., the radius of the sphere centered around the received signal, it is shown that the proposed algorithms offer the same BEP as ML--optimum detection, with a significant reduction of the computational complexity. Also, it is shown that none of the proposed SDs is always superior to the others, but the best SD to use depends on the system setup, i.e., the number of transmit--antenna, receive--antenna, and the size of the modulation scheme. The computational complexity trade--off offered by the proposed solutions is studied via analysis and simulation, and numerical results are shown to validate our findings.

In this paper, self-organising interference management for optical wireless networks deployed inside an aircraft cabin is investigated. A user that has successfully received data in a given frame and intends to continue receiving data in the next frame broadcasts a busy burst (BB) in a time-multiplexed BB slot. A neighbouring access point (AP) that intends to reuse the reserved resource listens to the BB slot and infers (prior to transmission) the amount of co-channel interference (CCI) it could cause towards the user that has reserved the resource, provided that channel reciprocity holds. This is a vital information for an AP to decide without any central supervision whether to transmit or to defer the transmission to another time or frequency slot so as to limit CCI caused to the active link to a threshold value. Compared to using static resource partitioning for interference coordination, the proposed approach improves the median system throughput by 17%whilst delivering roughly the same throughput at the cell-edge. Moreover, it is demonstrated that the threshold parameter can be adjusted to tradeoff aggregate system throughput for user throughput at the cell-edge.

This paper analyses the impact of imperfect synchronisation in optical orthogonal frequency division multiplexing (OFDM) systems utilising intensity modulation (IM) / direct detection (DD). The use of IM/DD technique inherently eliminates the problem of carrier frequency offset. Therefore, time-frequency synchronisation in optical OFDM system reduces to frame synchronisation and sampling clock synchronisation. Sampling clock offset causes irreducible intercarrier interference (ICI) and symbol timing offset. A technique to mitigate the impact of sampling clock offset is proposed in this paper. In the proposed approach, the received signal is oversampled at twice the Nyquist rate and the odd samples are punctured. The symbol timing offset is estimated using pilots and when the symbol timing offset exceeds ±0.5 of sample duration, the puncturing pattern flips between puncturing the odd series and the even series. This in effect, constraints the symbol timing offset within ±1 sample duration. The residual symbol offset can be perfectly corrected using linear phase equaliser. The results show that the proposed method attains a performance comparable to the system that is ideally synchronised provided that the sampling clock offset is lower than 50 ppm.

We consider single cell multi-user OFDMA downlink resource allocation on a flat-fading channel such that average supply power is minimized while fulfilling a set of target rates. Available degrees of freedom are transmission power and duration. This paper extends our previous work on power optimal resource allocation in the mobile downlink by detailing the optimal power control strategy investigation and extracting fundamental characteristics of power optimal operation in cellular downlink. We find that only a system wide allocation of transmit powers is optimal rather than on link level. The allocation strategy that minimizes overall power consumption requires the transmission power on all links to be increased if only one link degrades. Furthermore, we show that for mobile stations with equal channels but different rate requirements, it is power optimal to assign equal transmit powers with proportional transmit durations. To relate the effectiveness of power control to live operation, we take the power model into consideration which maps transmit power to supply power. We show that due to the affine mapping, the solution is independent of the power model. However, the effectiveness of power control measures is completely dependent on the underlying hardware and the load dependence factor of a base station (instead of absolute consumption values). Finally, we conclude that power control measures in base stations are most relevant in macro stations which have load dependence factor of more than 50%.

This paper analyzes a new dead-reckoning method for pedestrian navigation. In traditional approaches, accelerometers are used to detect steps and to estimate the step length. However, these algorithms must be calibrated for each user and fail in practice when the user moves with an irregular posture like crawling or carrying a victim. In the proposed method the speed is determined from coherence time measurements of the wideband frequency response in Orthogonal Frequency Division Multiplexing (OFDM) systems. The presented algorithm does not require user calibration and it is not based on a particular type of movement. Moreover, it is shown by means of an experimental system that the positioning accuracy matches that of state-of-the-art step length estimation algorithms.

Cognitive Radio systems require detailed feedback about their environment, and detecting anomalies is core to this task. The KLD metric can be used to detect a variety of anomalies in radio signals, and has been previously demonstrated to be both effective, and efficient enough to run in real-time. In tests, it was observed that some anomalous signals caused the KLD to increase monotonically for long time periods, while others did not. After analysing the KLD equation and comparing the findings with the results from the tests, we present a hypothesis for how such monotonic sequences could occur and demonstrate that this agrees very closely with results in observed signals.

In this paper, a novel decentralised coordinated beamforming technique for interference mitigation in multi-user multiple-input-multiple-output (MIMO) networks is proposed. When the available spectrum is fully reused in all cells, the base station (BS) can potentially activate a beam which causes detrimental co-channel interference (CCI) towards mobile stations (MSs) served by neighbouring cells. This problem is solved by requiring each active MS to transmit a busy burst (BB) in a time-multiplexed feedback channel using the spatial precoder utilised for receiving data. Likewise, the BS must scan the feedback channel using the same precoder which it intends to use for transmitting data in the next slot. Provided that channel reciprocity holds, the feedback channel and the data channel become reciprocal. This enables the BS to listen to the feedback channel and ascertain the amount of CCI it potentially causes towards the active MSs if it were to apply the particular spatial precoder. The BS is permitted to utilise only such precoders that cause CCI lower than a predetermined threshold. The impact of the threshold parameter for selecting spatial precoder in cellular networks is investigated and a novel heuristic for setting the thresholds in the downlink (DL) mode is proposed. It is demonstrated that the proposed heuristic achieves a 14% increase in spectral efficiency and a 6.3-fold increase in cell-edge user throughput compared to a transmit beamforming approach without interference awareness.

We consider resource allocation over a wireless downlink where Base Station (BS) power consumption is minimized while upholding a set of required link rates. A Power and Resource Allocation Including Sleep (PRAIS) method is proposed that combines resource sharing, Power Control (PC), and Discontinuous Transmission (DTX), such that downlink power consumption is minimized, which can be formed into a convex optimization problem. Unlike conventional approaches that aim at minimizing transmit power, in this work the BS mains supply power is chosen as the relevant metric. Based on a linear power model, which maps a certain transmit power to the necessary mains supply power, we quantify the fundamental limits of PRAIS in terms of achievable BS power savings. The fundamental limits are numerically evaluated on link level for four sets of BS power model parameters representative of envisaged future hardware developments. We establish an expected lower limit for PRAIS of 27W to 68W depending on load per link for BSs installed in 2014, which provides a 61% to 34% gain over conventional resource allocation schemes.

In this paper, we describe GREENET (an early stage training network in enabling technologies for green radio), which is a new project recently funded by the European Commission under the auspices of the 2010 Marie Curie People Programme. Through the recruitment and personalized training of 17 Early Stage Researchers (ESRs), in GREENET we are committed to the development of new disruptive technologies to address all aspects of energy efficiency in wireless networks, from the user devices to the core network infrastructure, along with the ways the devices and equipment interact with one another. Novel techniques at the physical, link, and network layers to reduce the energy consumption and carbon footprint of 4G devices will be investigated, such as Spatial Modulation (SM) for Multiple-Input-Multiple-Output (MIMO) systems, Cooperative Automatic Repeat reQuest (C-ARQ) protocols, and Network Coding (NC) for lossy networks. Furthermore, cooperation and cognition paradigms will be exploited as additional assets to improve the energy efficiency of wireless networks with the challenging but indispensable constraint of optimizing the system capacity without degrading the user's Quality-of-Service (QoS).

In this paper, we introduce Dual-hop Spatial Modulation (Dh-SM). We look at the effect that Dh-SM has on the required signal to noise ratio (SNR) at the destination and how it can help alleviate the multi-hop burden. Initial bit-error-ratio (BER) results comparing the performance of Dh-SM with orthogonal decode-and-forward (DF) are presented where Dh-SM is shown to have up to a 10 dB SNR advantage.

Time switched transmit diversity (TSTD) system uses only one out of several transmit antennas at a time in order to exploit the benefit of transmit diversity while keeping the system simple by having only one RF chain. We show that while average rate of such a system remains similar to the capacity of the single transmit antenna, outage secrecy capacity is greatly improved. In particular, as the number of transmit antenna is increased or outage probability decreased, the gains over single antenna system improve significantly - gains of over 20 dB with respect to the single antenna system are achieved for 1% outage rate. Furthermore, the probability of existence of secrecy capacity is improved as well for cases when the main channel has higher SNR than the eavesdropper channel.

Nonlinearities can drastically degrade the performance of OFDM (orthogonal frequency division multiplexing) based optical wireless (OW) communication systems using intensity modulation (IM) of the optical carrier. The light emitting diode (LED) transfer function distorts the signal amplitude and forces the lower signal peaks to be clipped at the LED turn-on voltage (TOV). Additionally, the upper signal peaks can result in optical output degradation. The induced distortion can be controlled by optimizing the bias point (BP) of the LED and/or backing-off the signal power modulating the LED. In this paper, the obtained experimental results using a hardware demonstrator for OW OFDM transmission based on field programmable gate array (FPGA) and off-the-shelf analog components are presented. The conducted measurements for the bit-error performance focus on determining the optimum BP and optimizing the OFDM signal amplitude to obtain best performance. In this context, the experimental bit-error ratio (BER) is obtained as a function of the LED BP and the RMS (root mean square) OFDM signal across the LED.

In this study, the potential of reducing radio base station operational energy consumption is discussed in terms of deploying sleep modes. By periodically switching off the base station transmission, or using fewer transmit antennas, the energy consumption of base station hardware decreases. By delivering less control signalling overhead, the radio frequency energy consumption can also be reduced. Taking the long-term evolution system as an example, up to 90% radiated energy reduction can be obtained in low traffic conditions by employing time-domain optimisation in each radio frame. The optimum on/off duty cycle is derived, enabling the energy consumption of the base station to scale with traffic load. In the spatial domain, an antenna selection criterion is proposed, indicating the most energy-efficient antenna configuration as a function of users' locations and quality of service requirements. Without time-domain sleep modes, using fewer transmitter antennas could outperform full antenna transmission. However, with time-domain sleep modes, using all available antennas is generally the most energy-efficient choice.

In this paper, we study the performance of Space Shift Keying (SSK) modulation for a generic Multiple—Input—Multiple—Output (MIMO) wireless system over correlated Rician fading channels. In particular, our contribution is twofold. i) First, we propose a very general framework for computing the Average Bit Error Probability (ABEP) of SSK—MIMO systems over a generic Rician fading channel with arbitrary correlation and channel parameters. The framework relies upon the Moschopoulos method. We show that it is exact for MIMO systems with two transmit—antenna and arbitrary receive—antenna, while an asymptotically—tight upper—bound is proposed to handle the system setup with an arbitrary number of transmit—antenna. ii) Second, moving from the consideration that conventional SSK—MIMO schemes can offer only receive—diversity gains, we propose a novel SSK—MIMO scheme that can exploit the transmit—antenna to increase the diversity order. The new method has its basic foundation on the transmission of signals with good time—correlation properties, and is called Time—Orthogonal—Signal—Design (TOSD—) assisted SSK modulation (TOSD—SSK). It is shown that the proposed method can increase twofold the diversity order for arbitrary transmit— and receive—antenna. In particular, for MIMO systems with two transmit—antenna and N_r receive—antenna full—diversity equal to 2N_r can be achieved. Analytical frameworks and theoretical findings are substantiated via Monte Carlo simulations for various system setups.

Motivated by the recent concept of Space-Time Shift Keying (STSK), we propose a novel cooperative STSK family, which is capable of achieving a flexible rate-diversity tradeoff, in the context of cooperative space-time transmissions. More specifically, we first propose a Coherent cooperative STSK (CSTSK) scheme, where each Relay Node (RN) activates Decode and-Forward (DF) transmissions, depending on the success or failure of Cyclic Redundancy Checking (CRC). We invoke a bit to-STSK mapping rule, where according to the input bits, one of the Q pre-assigned dispersion vectors is activated to implicitly convey log2Q bits, which are transmitted in combination with the classic log2L-bit modulated symbol. Additionally, we introduce a beneficial dispersion vector design, which enables us to dispense with symbol-level Inter-Relay Synchronization (IRS). Further more, the Destination Node (DN) is capable of jointly detecting the signals received from the source-destination and relay destination links, using a low-complexity single-stream-based Maximum Likelihood (ML) detector, which is an explicit benefit of our Inter-Element Interference (IEI)-free system model. More importantly, as a benefit of its design flexibility, our cooperative CSTSK arrangement enables us to adapt the number of the RNs, the transmission rate as well as the achievable diversity order. Moreover, we also propose a Differentially-encoded cooperative STSK (DSTSK) arrangement, which dispenses with CSI estimation at any of the nodes, while retaining the fundamental benefits of the cooperative CSTSK scheme.

A novel decentralized coordinated multipoint (CoMP) technique facilitated by receiver feedback is proposed. If multiple base stations (BSs) access the same radio resource; downlink transmissions from one BS cause co-channel interference (CCI) towards towards users served by neighboring BSs. To mitigate CCI in a multiple-input-multiple-output (MIMO) network, the mobile station (MS) scheduled in a tagged cell (i.e. the active user) emits a busy burst (BB) on a time-multiplexed feedback slot, applying the precoder used earlier for receiving data. Prior to transmission a neighboring BS must scan the feedback slot using the spatial precoder(s) intended for data transmission. Consequently, in a time division duplex (TDD) system the channels of the data and the feedback slots become reciprocal. Therefore, by listening to the feedback slot, a neighboring BS can predetermine the CCI potentially imposed towards the active user for each of its available precoders. This information enables the BS to select spatial precoders autonomously so that CCI is limited to a predetermined threshold. Simulation results demonstrate that the proposed technique achieves 20% increase in spectral efficiency, 6-fold increase in cell-edge user throughput, and 40% reduction in transmitted energy, compared to transmit beamforming without interference coordination.

In this paper, the clipping noise in an asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) wireless communication system is derived semi-analytically. Clipping noise in ACO-OFDM is introduced either because of insufficient forward biasing of the emitter, or a low power sensitivity of the detector. Following the Bussgang theorem, the non-linear distortion caused by the clipping of the time domain signal attenuates the frequency domain subcarriers at the receiver and adds zero-mean Gaussian noise. The attenuation factor and the clipping noise variance are determined and verified through simulation. Finally, bit-error ratio (BER) and goodput simulations compare the performance of ACO-OFDM with a direct-current-biased optical OFDM (DCO-OFDM) system for different clipping levels and multilevel quadrature amplitude modulation (M-QAM) schemes.

Spatial Modulation (SM) is a recently proposed joint coding and modulation scheme for Multiple-Input-Multiple-Output (MIMO) wireless systems, which is receiving a growing interest. SM offers a low-complexity alternative to the design of MIMO wireless systems, which avoids multiple Radio Frequency (RF) chains at the transmitter and high-complexity interference cancellation algorithms at the receiver, but still guarantees a multiplexing gain that only depends on the number of antennas at the transmitter. This makes this technology especially suitable for the downlink with low-complexity mobile units. So far, the feasibility and performance of SM have been assessed and studied only for point-to-point communication systems, i.e., the single-user scenario. However, the performance achievable by the vast majority of wireless communication networks is interference limited, due to the simultaneous transmission of various users over the same physical wireless channel. Therefore, the adoption of SM in the next generation of wireless communication systems requires a deep understanding of its performance over interference channels. Motivated by this consideration, in this paper we study the performance of SM over the reference multiple-access fading channel composed by two transmitters and one receiver. Two detectors at the receiver are studied, i.e., the single- and the multi-user detector. In particular, analysis and Monte Carlo simulations show that the single-user detector does not offer, in general, good error performance for arbitrary channel conditions, while the multi-user detector achieves error performance very close to the single-user lower-bound. These results clearly highlight that SM can be adopted for enabling data transmission over multiple-access fading channels as well.

In shared wireless medium access, receiver initiated busy burst (BB) transmission mitigates the problem of co-channel interference. Receiver feedback allows potential transmitters to ascertain the amount of interference their transmission would cause to victim receivers in adjacent communication links. The BB concept mitigates excessive co-channel interference, by maintaining an exclusion region around any victim receiver: interferers within the region must refrain from accessing the shared wireless medium. In this paper, an analytical framework for interference modelling of the BB protocol is developed, via cumulant-based shifted log normal approximation of its probability density function (pdf). The interference pdf is then applied to derive the pdf of the signal-to-interference ratio (SIR).

In this paper a novel detection algorithm for spatial modulation (SM) based on sphere decoder (SD) tree search idea is proposed. The aim is to reduce the receiver complexity of the existing optimal decoder while maintaining an optimum performance. The algorithm performs a maximum likelihood (ML) search, only over those points that lie inside a sphere, centered at the received signal, of given radius. It is shown with the aid of analytical derivations, that for a SNR (signal-to-noise ratio) between 2~dB and 18~dB at least 45% and up to 85% reduction in the number of complex operations can be achieved with a close to optimal bit-error-ratio (BER) performance.

In this paper, a multiple-input multiple-output (MIMO) technique for indoor optical wireless (OW) communication is proposed. The technique is referred to as optical spatial modulation (OSM). The key concept is based on spatial modulation (SM). At any given time instant, only one transmitter is active and the others are inactive. A transmitter in space is considered as a spatial constellation point which is assigned a unique bit sequence. Consequently, transmitters are turned on and off depending on the incoming data bits, similar to the activation of constellation points in traditional digital modulation schemes. Hence, a data rate of the base two logarithm of the number of transmit units is achieved. The active transmitter radiates a certain intensity level at a particular time instant. At the receiver side, the optimal SM detector is slightly modified and used to estimate the spatial constellation point. The estimated spatial constellation point is used to arrive at the original bit stream via de-mapping. The upper bound bit-error-ratio (BER) of OSM is analyzed for a MIMO configuration consisting of four transmit units (light emitting diodes (LEDs)) and four receive units (photo diodes (PDs)) in a room. The BER performance is determined for different transmitter and receiver separation distances and different transmitter half power semiangles (Φ1/2). It is shown that the proposed OSM technique achieves twice and four times the data rate as compared to OOK (on-off keying) and PPM (pulse-position modulation), respectively.

In this paper, we develop an analytical framework for analyzing the performance of wireless systems adopting the recently proposed Space Shift Keying (SSK) modulation scheme. More specifically, we investigate the performance of a 2 × 1 MISO (Multiple-Input-Single-Output) system setup with Maximum-Likelihood (ML) detection at the receiver. The exact Average Bit Error Probability (ABEP) over correlated and non-identically distributed Nakagami-m fading channels is computed in closed-form. Numerical results will show that the performance of SSK modulation is significantly affected by the characteristics of fading channels, e.g., channel correlation, fading severity, and power imbalance among the wireless links. Analytical frameworks and findings will also be substantiated via Monte Carlo simulations.

In this paper, we introduce TOSD-SM (TimeOrthogonal Signal Design assisted Spatial Modulation), which is a novel space modulation scheme based on the principle of the recently proposed Spatial Modulation (SM) wireless transmission technique for Multiple-Input-Multiple-Output (MIMO) systems. We show that, unlike other SM-based methods available in the literature, the proposed approach can offer transmit-diversity gains by properly designing the transmitted signal to have a peaky time auto-correlation function. By considering the basic 2 × 1 MISO (Multiple-Input-Single-Output) system setup, the optimal Maximum-Likelihood (ML) detector with full Channel State Information (CSI) at the receiver will be developed, and its performance evaluated in closed-form. Performance comparison with other proposed SM schemes over fading channels will evidence two main benefits of TOSD-SM: i) transmit-diversity gains, and ii) intrinsic robustness to spatial correlation of channel fading. Analytical frameworks and findings will also be substantiated via Monte Carlo simulations.

Nonlinearities can drastically degrade the performance of optical communication systems based on intensity modulation (IM) of the optical carrier using the time domain orthogonal frequency division multiplexing (OFDM) electrical signal variations. The light emitting diode (LED) transfer function distorts the signal amplitude and forces the lower signal peaks to be clipped at the LED turn-on voltage (TOV). Additionally, the upper signal peaks are purposely clipped before modulating the LED to avoid chip overheating. The induced distortion can be controlled by optimizing the bias point and/or backing-off the signal power. In this paper, a model that incorporates amplitude distortion and that provides a parameterized upper clipping to control nonlinearity induced distortion is proposed. Through Monte Carlo simulations, the model can be used to determine the optimum bias point and to optimize the signal power to obtain best performance. In this context, an analytical approach to evaluate symbol error probability is proposed. A comparison with Monte Carlo simulation results is carried out to verify the accuracy of this approach.

The performance of hard and soft convolutional channel coding techniques for optical spatial modulation (OSM) are analyzed in this paper. OSM is a recently proposed multiple-input multiple-output (MIMO) technique for indoor optical wireless (OW) communication. In OSM only one transmitter is active at a time and the others are turned off. The spatially separated transmit units are considered as spatial constellation points, i.e. the incoming bit sequence activates one of the transmit units. The active transmitter radiates a certain intensity level at a particular time instance. At the receiver side, optimal detection techniques are used to estimate the active transmitter index and retrieve the original information bits. In this paper, channel coding is applied to OSM and the performance of hard and soft detections are analyzed analytically and validated through Monte Carlo simulations. It is shown that the performance can be significantly enhanced by applying channel coding techniques and a gain in signal-to-noise-ratio (SNR) of about 5 dB and 7 dB at a bit-error-ratio (BER) of 10-4are achieved for hard and soft decisions, respectively.

In this paper a new speed estimation method for pedestrian dead-reckoning is presented. The proposed approach is based on the estimated channel frequency response of Digital Video Broadcasting-Terrestrial (DVB-T) signals. Correlation coefficients are calculated from the acquired envelopes and the coherence time is estimated for a certain correlation level. The algorithm gives an average error of less than 1% for pedestrian speeds. The scheme does not require a new infrastructure for estimating the speed and its computational complexity is low. Moreover, the performance of the proposed method is verified by means of an experimental system.

Trellis Coded Spatial Modulation (TCSM) is a novel transmission technology for Multiple-Input-Multiple-Output (MIMO) systems, which has been recently proposed to improve the performance of Spatial Modulation (SM) over correlated fading channels. The fundamental principle of TCSM is to use convolutional encoding and Maximum-Likelihood Sequence Estimation (MLSE) decoding to increase the free distance between sequences of spatial constellation points, thus improving, especially over spatially correlated fading channels, the end-to-end system performance. In this paper, we propose tight analytical bounds for performance analysis of TCSM over correlated fading channels. In particular, the contributions of this paper are as follows: i) we propose two asymptotically tight (for high Signal-to-Noise-Ratios, SNRs) upper bounds for the analysis of uncoded SM schemes, which offer a better accuracy than already existing frameworks, ii) we propose a simple Chernoff bound for performance analysis of TCSM, which, although weak, can well capture the diversity order of the system, and iii) we propose an asymptotically tight (for high SNRs) true union bound for the accurate performance prediction of TCSM over correlated fading channels. Analytical frameworks and findings will also be substantiated via Monte Carlo simulations.

In this paper, we offer an accurate framework for analyzing the performance of wireless communication systems adopting the recently proposed Space Shift Keying (SSK) modulation scheme. More specifically, we study the performance of a Ntx 1 MISO (Multiple-Input-Single-Output) system setup with Maximum-Likelihood (ML) detection and full Channel State Information (CSI) at the receiver. The exact Average Bit Error Probability (ABEP) over generically correlated and non-identically distributed Nakagami-m fading channels is computed in closed-form when Nt=2, while very accurate and asymptotically tight upper bounds are proposed to compute the ABEP when Nt> 2. With respect to current literature, our contribution is threefold: i) the ABEP is computed in closed-form without resorting to Monte Carlo numerical simulations, which, besides being computationally intensive, only yield limited insights about the system performance and cannot be exploited for a systematic optimization of it, ii) the framework accounts for arbitrary fading conditions and is not restricted to identically distributed fading channels, thus offering a comprehensive understanding of the performance of SSK modulation over generalized fading channels, and iii) the analytical framework could be readily adapted to study the performance over generalized fading channels with arbitrary fading distributions, since the Nakagami-m distribution is a very flexible fading model, which either includes or can closely approximate several other fading models. Numerical results show that the performance of SSK modulation is significantly affected by the characteristics of fading channels, {e.g.}, channel correlation, fading severity, and power imbalance among the Nttransmit-receive wireless links. Analytical frameworks and theoretical findings are also substantiated via Monte Carlo simulations.

Space Shift Keying (SSK) modulation is a new and recently proposed transmission technology for Multiple-Input-Multiple-Output (MIMO) wireless systems, which has been shown to be a promising low-complexity alternative to several state-of-the-art MIMO schemes. So far, only optimal or heuristic transceivers with Full Channel State Information (F-CSI) at the receiver have been investigated, and their performance analyzed over fading channels. In this paper, we develop and study the performance of the optimal Maximum-Likelihood (ML) detector with unknown phase reference at the receiver (i.e., Partial-CSI, P-CSI, knowledge). A very accurate analytical framework for the analysis and optimization of this novel detector over generically correlated and non-identically distributed Nakagami-{m} fading channels is proposed, and its performance compared to the optimal receiver design with F-CSI. Numerical results will point out that: i) the performance of SSK modulation is significantly affected by the characteristics of fading channels, e.g., channel correlation, fading severity, and, particularly, power imbalance among the transmit-receive wireless links, and ii) unlike ordinary modulation schemes, there is a substantial performance loss when the receiver cannot exploit the phase information for optimal receiver design. This latter result highlights the importance of accurate and reliable channel estimation mechanisms for the efficient operation of SSK modulation over fading channels. Analytical frameworks and theoretical findings will also be substantiated via Monte Carlo simulations.

Trellis coded modulation (TCM) is a well known scheme that reduces power requirements without any bandwidth expansion. In TCM, only certain sequences of successive constellation points are allowed (mapping by set partitioning). The novel idea in this paper is to apply the TCM concept to the antenna constellation points of spatial modulation (SM). The aim is to enhance SM performance in correlated channel conditions. SM considers the multiple transmit antennas as additional constellation points and maps a first part of a block of information bits to the transmit antenna indices. Therefore, spatial multiplexing gains are retained and spectral efficiency is boosted. The second part of the block of information bits is mapped to a complex symbol using conventional digital modulation schemes. At any particular time instant, only one antenna is active. The receiver estimates the transmitted symbol and the active antenna index and uses the two estimates to retrieve the original block of data bits. In this paper, TCM partitions the entire set of transmit antennas into sub-sets such that the spacing between antennas within a particular sub-set is maximized. The scheme is called trellis coded spatial modulation (TCSM). Tight analytical performance bounds over correlated fading channels are proposed in this paper. In addition, the performance and complexity of TCSM is compared to the performance of SM, coded V-BLAST (vertical Bell Labs layered space-time) applying near optimum sphere decoder algorithm, and Alamouti scheme combined with TCM. Also, the performance of all schemes with turbo coded modulation is presented. It is shown that under the same spectral efficiency, TCSM exhibits significant performance enhancements in the presence of realistic channel conditions such as Rician fading and spatial correlation (SC). In addition, the complexity of the proposed scheme is shown to be 80% less than the V-BLAST complexity.

In this paper, the infrared optical wireless path loss inside an aircraft cabin is estimated. To this end, a Monte Carlo ray-tracing (MCRT) simulation is performed in a geometric computer-aided design (CAD) cabin model. Position, azimuth (AZ), elevation (EL) and field of view (FOV) properties of transmitters and receivers are defined. Key path loss parameters such as the path loss exponent and the standard deviation of the shadowing component are determined for particular line-of-sight (LOS) and non-line-of-sight (NLOS) cases. A LOS path loss exponent of 1.92 and a shadowing standard deviation of 0.81 dB are obtained. In NLOS conditions, however, the path loss exponent varies depending on the nature of the particular NLOS case considered. The presented scenarios yield NLOS path loss exponent values of 2.26 and 1.28, and shadowing standard deviation values of 1.27 dB and 0.7 dB, respectively.

Interference aware user scheduling in fixed grid-of-beam (GoB) transmission is envisaged to significantly benefit from the receiver initiated busy burst (BB) protocol. Fixed GoB scheduling relies on the knowledge of the location of the intended users as well as the vulnerable users which effectively is provided by the BB protocol via exploitation of channel reciprocity. This paper studies the hybrid BB and GoB (BB + GoB) approach in a Manhattan environment. The new proposed hybrid interference avoidance scheme with an underlying score based scheduler is evaluated by means of system level simulations, and is compared against a pure GoB approach with the same scheduler as well as the BB based interference avoidance techniques applied to omnidirectional antennas. The results show an improvement in both system throughput and fairness (defined as cell edge user throughput). In particular, system throughput of up to 238.5 Mbps/cell or user throughput of up to 8.88 Mbps/user for the lower 10%-ile of users are shown to be feasible. In particular, the hybrid BB + GoB scheme exhibits a 16-fold improvement at the lower 10%-ile compared to pure GoB technique.

This paper presents a novel approach for setting the thresholds for busy burst (BB) enabled interference avoidance in TDD (time division duplex) systems. One of the key issues in the BB concept is the appropriate choice of the threshold, which determines whether a user is allowed to transmit on a specific time-frequency resource unit. For a two-link network we derive the optimum threshold value that maximises the sum rate. Furthermore, for multiple links, a new heuristic threshold that only relies on locally available information is derived. It is demonstrated via simulations that the heuristic threshold provides superior sum rate than a fixed system-wide threshold.

A new transmission approach, referred to as subcarrier-index modulation (SIM) is proposed to be integrated with the orthogonal frequency division multiplexing (OFDM) systems. More specifically, it relates to adding an additional dimension to the conventional two-dimensional (2-D) amplitude/phase modulation (APM) techniques, i.e. amplitude shift keying (ASK) and quadrature amplitude modulation (QAM). The key idea of SIM is to employ the subcarrier-index to convey information to the receiver. Furthermore, a closed-form analytical bit error ratio (BER) of SIM OFDM in Rayleigh channel is derived. Analytical and simulation results show error probability performance gain of 4 dB over 4-QAM OFDM systems for both coded and uncoded data without power saving policy. Alternatively, power saving policy retains an average gain of 1 dB while using 3 dB less transmit power per OFDM symbol.

In future time division duplex (TDD) wireless systems, where channel reciprocity is maintained, uplink (UL) channel sounding method is considered as one of the most promising feedback methods due to its bandwidth and delay efficiency. Conventional channel sounding only conveys the channel state information (CSI) of each active user to the base station (BS). Due to the limitation in system performance because of co- channel interference (CCI) from adjacent cells in interference- limited scenarios, CSI is only a suboptimal metric for multiuser spatial multiplexing optimization. Interference-weighted channel sounding (IWCS) is a spectrally efficient feedback technique that provides the base station (BS) with implicit knowledge about the interference experienced by each active user. The main contribution of this paper is to propose a novel way to make the IWCS pilots usable for improved UL performance. In this context, it is proposed to weight the metric obtained from the IWCS pilots by the interference experienced at the BS. The resultant new metric, which is implicitly dependent on downlink (DL) and UL interference, provides link-protection awareness and is used to jointly improve spectral efficiency in UL as well in DL. Using maximum capacity scheduling criterion, the link-protection aware metric results in a gain in the median system sum capacity of 26.7% and 12.5% in DL and UL respectively compared to the case when conventional channel sounding techniques are used.

An information theoretic approach to detecting unusual events in radio signals is presented. Anomalies are detected through a measure of the events' information content. Clustering is utilised to reduce false-positives while allowing a lower discrimination threshold to be used for improved anomaly detection. Experiments with a real wireless local area network test signal shows that it is possible to achieve 100% detection rates while maintaining very low false discovery rate of 1%.

Self-configuring, hierarchical, ad hoc, wireless networks are fast becoming the focus of research for spectral efficiency and energy efficiency reasons. Such networks can be considered as graphs where a node represents a transceiving unit and an edge represents a communication link between two nodes. In order to probabilistically analyse connectedness issues in such graphs, the concept of the star-node is introduced here, where a star-node is a node whose path loss to multiple nodes in its vicinity falls below a given threshold, i.e., it is capable of sustaining "healthy" communication links to multiple neighbours. Star-nodes are important in the context of ad hoc and sensor networks since nodes in a self-configuring network must be able to identify their neighbours, i.e., those nodes with which viable communication links can be established. The probability of finding such star-nodes is analytically derived for a uniform user distribution in a circular scenario. The theory is tested against Monte Carlo simulations and a very good match has been found.

This paper presents the integration of power control and the busy burst (BB) technique for a TDD OFDMA (time division duplex orthogonal frequency division multiple access) system in an urban Manhattan grid type of environment. The objective is to combine power control with the BB protocol to develop a power efficient interference-avoidance technique. Previous work has demonstrated that the time multiplexed BB can be used to achieve interference aware medium access. It has also been demonstrated in other work that the implicit feedback mechanism of BB can be exploited to achieve Pareto optimal SINR-based (signal-to-interference-plus-noise ratio) power control. However, these two applications are not compatible the former depends on fixed signal powers across users, while the latter makes use of variable signal power to achieve optimum power control. This paper proposes an amendment to the BB technique that allows the two approaches to co-exist by exploiting the frequency granularity of OFDMA to frequency multiplex two BBs one with variable and one with fixed power. The resulting control protocol exhibits an eight-fold reduction at the 50th percentile of used energy with no additional overhead as compared to the single BB protocol.

In this paper, we discuss resource management in modern wireless communications with the objective of minimizing power consumption, and carry out an analysis of simultaneous transmission and orthogonal transmission schemes in an FDMA (Frequency Division Multiple Access) framework. Based on a two-link line model with path-loss only, the performance of these two schemes for network power minimization under the specified rate constraints is investigated. It is found that ST (Simultaneous Transmission) consumes less power than OT (Orthogonal Transmission) close to the base station, but much more power in the cell-edge area. This illustrates that the ST/OT selection criterion for sum power minimization is location-dependent. Further, numerical results suggest that the ST/OT selection margins are generally dominated by their network rate constraints rather than the rate proportions between the two links. New definitions of power-utility and fairness metrics are further proposed, following by the design of weighted resource allocation approaches based on efficiency-fairness tradeoffs.

In this paper the capacity of decentralized wireless networks is addressed. An exclusion region is introduced that protects active receivers from destructive interference of nearby transmitters. An exclusion range imposes an upper bound on the interference that a transmitter may cause to receivers of competing links. While an exclusion region avoids excessive interference and thus improves capacity per link, the spatial reuse in terms of concurrently served links is compromised. The resulting trade-off is elaborated by computer simulations, so to optimize the exclusion range as a function of the user density and maximum transmit power. We demonstrate that by an appropriately specified exclusion range, the network capacity is substantially enhanced. In this context, it has been found that the exclusion range that maximizes the system capacity does not vary greatly when changing the a priori user density. In addition, the trade-off between maximizing the system capacity and maintaining fairness is investigated.

In this paper, first, path loss models are developed for infrared optical wireless transmission inside an aircraft cabin. Second, a cellular network in the aircraft is considered and signal-to-interference ratio (SIR) maps are determined via simulation. For this purpose, a Monte Carlo ray-tracing (MCRT) simulation is performed in a geometric computer-aided design (CAD) cabin model with defined position, azimuth (AZ), elevation (EL) and field of view (FOV) properties of transmitters and receivers. Mathematical models are developed for line-of-sight (LOS) and non-line-of-sight (NLOS) path losses along particular paths, including estimation of the path loss exponent and the shadowing component. The shadowing is modeled according to a log-normal distribution with zero mean and standard deviation sigma. The validity of this model is confirmed in the paper. It is shown that irradiance distribution under LOS conditions experiences an attenuation with a path loss exponent of 1.92 and a shadowing standard deviation of 0.81 dB. In NLOS conditions, however, the path loss exponent varies, depending on the nature of the NLOS cases considered. The presented NLOS scenarios yield path loss exponent values of 2.26 and 1.28, and shadowing standard deviation values of 1.27 dB and 0.7 dB, respectively. Finally, the cabin is divided into cells and SIR maps are presented for different frequency reuse factors. It is shown that at the edges of the circular cells with diameter of 2.8 m, a SIR of -5.5 dB is achieved in a horizontal cross section of the cabin for frequency reuse of 1, and -2 dB and 3 dB for frequency reuse factors of 2 and 3, respectively. This means that in an aircraft cabin, for reuse factors less than three, viable communication at the cell edges is not feasible without additional interference avoidance or interference mitigation techniques.

Cognitive radio (CR) systems need to be able to adjust the transceiver characteristics in response to stimuli received from the radio environment. Therefore, monitoring the wireless signal in real-time for stimuli such as unexpected changes due to sporadic interference in radio frequency band of operation is at the core of such systems. In this paper, a method of detecting anomalies in a periodic signal by means of statistical analysis of its envelope is described. The proposed scheme makes use of the Kullback-Leibler divergence between probability distributions drawn from analogous segments of the periodic signal to detect anomalous events. Experiments conducted on real wireless signals suggest that the method described is simple, robust and effective for the analysis of periodic signals.

The purpose of this paper is to compare two interference avoidance techniques for time division duplex (TDD) wireless networks, viz.: a novel technique termed asymmetry balancing and the known zone division (ZD) principle. Both asymmetry balancing and ZD strive to reduce the same-entity interference (mobile station (MS)-to-MS and base station (BS)-to-BS) that occurs during crossed slots, i.e. slots which are simultaneously used for uplink (UL) in one cell and for downlink (DL) in a neighbouring cell. Asymmetry balancing eliminates crossed slots by synchronising the TDD switching point (SP) among cells. Cell-specific asymmetry demands are still maintained through cooperation among the entities in the network. ZD, on the other hand, reduces same-entity interference by decreasing transmission range. This study demonstrates that asymmetry balancing achieves more than 100% higher spectral efficiencies than ZD for the considered scenarios.

This paper analyses busy burst (BB) enabled interference avoidance for multi-user orthogonal frequency division multiple access (OFDMA) in a Manhattan grid deployment scenario. Upon successful reception of data, the receiver transmits a BB in a time-multiplexed mini-slot. Exploiting the channel reciprocity of time division duplex (TDD), an exclusion region around a victim receiver is established, where potential transmitters are denied access to reserved resources. The size of the exclusion region is determined by a threshold parameter, known to the entire network. System level simulations compare the system and user throughput of BB enabled interference avoidance with greedy resource allocation that does not attempt to avoid interference. It is shown that by adjusting the BB threshold parameter, system throughput can be traded with fairness in terms of cell-edge user throughput. By tuning the BB threshold, either up to 45% increase in system throughput or up to 7-fold increase in cell-edge user throughput are feasible compared to greedy resource allocation. Moreover, by an appropriate adjustment of the BB power further gains in system throughput without compromising fairness are achieved.

This paper presents a new generalized path loss equation that takes in to account both small-scale multipath fading as well as large-scale path loss including lognormal shadowing. The probability density function (pdf) of the signal to interference ratio (SIR) for wireless networks with Nakagami-m channel model is analytically derived using the new path loss equation. We choose the Nakagami-m channel fading model because it encompasses a large class of fading channels. Using convolution, Fourier or Mellin transforms, the results obtained for a single interference scenario can be extended to multiple interference scenario. The results presented show good agreement between the analytical and Monte Carlo based methods.

In this paper decentralized power control in ad hoc networks is addressed, so to maintain a certain carrier-to- interference ratio (C/I) at the receiver. The objective is to extend the Pareto optimal C/I power control algorithm to time division duplex (TDD) links, where each node in the network transmits and receives data to/from other nodes. By exploiting the channel reciprocity of TDD, we demonstrate that Pareto optimal C/I power control can be implemented without any explicit feedback of the achieved link quality.

This paper presents a spectrum sharing approach which exploits the clustered distribution of users as would be expected in a typical home with several communicating devices (also known as a femto-cell) or in public places such as airports, malls, etc. From each cluster, a mobile station (MS), known as a gateway mobile (GM), acts as a relay between the other users of the cluster and the base station (BS) of the associated cell. The system makes use of frequency division duplexing (FDD) for communication between the BS and the GM. Traffic is assumed to be asymmetric in favour of downlink (DL) due to current trends in user traffic requirements. The unused resources in the uplink band are then used for communication within the cluster using time division duplexing (TDD underlay). This model is compared against one in which the same user distribution is assumed but the GM concept is not made use of, i.e., an FDD system. Based on the results, it has been shown that the proposed enhancements lead to a significant increase in system spectral efficiency through the use of intra-cluster communication.

This paper studies the performance of orthogonal frequency division multiple access - time division duplex (OFDMA-TDD) cellular networks when jointly applying dynamic channel allocation (DCA) and user scheduling under the assumption of asymmetric uplink (UL)/downlink (DL) loads. Specifically, a comparison between a fixed slot allocation (FSA) scheme, where the uplink UL/DL switching is synchronised across the network, and the random time slot opposing (RTSO) technique is made. The RTSO resembles an opportunistic interference mitigation technique. RTSO, however, does not obviate the need for user scheduling algorithms, but the combined use of RTSO and scheduling has not been studied. Therefore, two different scheduling algorithms, greedy and fair, are adapted to suit the OFDMA-TDD architecture. Their performance for various channel asymmetries under RTSO and symmetric FSA is evaluated, based on spectral efficiency and user outage. In order to account for the exposed location of base station (BS) antennas in a cellular environment, the effect of line-of-sight (LOS) propagation among BSs is considered. The results show that LOS among BSs in a system with unsynchronised switching points strongly hampers the network's performance. This effect, however, is demonstrated to be substantially offset by DL-favoured asymmetries (dominant in data-centric networks) in combination with RTSO. Furthermore, it is shown that the greedy algorithm only offers a marginal increase in spectral efficiency as compared to the fair algorithm, while the fair algorithm exhibits up to ~20% lower outage.

This paper proposes a novel approach to interference avoidance via inter-cell relaying in cellular OFDMA-TDD (orthogonal frequency division multiple access - time division duplex) systems. The proposed scheme, termed asymmetry balancing, is targeted towards next-generation cellular wireless systems which are envisaged to have ad hoc and multi-hop capabilities. Asymmetry balancing resolves the detrimental base station (BS)-to-BS interference problem inherent to TDD networks by synchronizing the TDD switching points (SPs) across cells. In order to maintain the flexibility of TDD in serving the asymmetry demands of individual cells, inter-cell relaying is employed. It is demonstrated that asymmetry balancing offers great flexibility in uplink (UL)- downlink (DL) resource allocation. In addition, results show that a spectral efficiency improvement of more than 100% can be obtained with respect to a case where the TDD SPs are adapted to the cell-specific demands.

This paper targets the problem of optimum power allocation and link scheduling in cellular systems. In particular, the uplink performance of OFDMA (orthogonal frequency division multiple access) is investigated. By means of heuristic relaxation, the NP-hard problem of joint power control and scheduling is transformed into one of power controlled random access (PCRA), which is formulated as a convex optimization problem. The PCRA problem formulation in this paper clearly reveals the power v/s delay dilemma associated with wireless multiple access networks. A relative increase in the SINR (signal- to-interference-plus-noise) target of 2dB (from 12dB to 14dB) results in a 100%-130% increase in the Pareto optimum power assuming a cellular full frequency reuse system. In contrast, this increase is only 30%-55% in a single cell scenario. Therefore CCI (co-channel interference) mitigation techniques should be implemented to alleviate the problem of high CCI.

Under the horizontal spectrum sharing scenario where two cognitive radios are able to find an unutilised resource and access to the resource is coordinated between them, it is shown in our earlier work that either single or simultaneous transmission at maximum power maximises system spectral efficiency. While in general suboptimal, sequential transmission, an average of two single transmissions, is also investigated as a more fair alternative to single transmission. In this paper, the system spectral efficiencies of simultaneous, single and sequential transmission schemes are compared for the 2-link scenario where the two communication pairs are chosen from uniformly distributed nodes on a disk. The simultaneous transmission scheme is shown to provide smaller expected system spectral efficiency for higher transmit power than sequential or single transmission schemes. Introducing additional interferers, however, significantly reduces the advantage of single and sequential schemes over simultaneous transmission. However, under the scenario of transmission to the closest neighbour, simultaneous transmission is shown to result in higher average system spectral efficiency than the other two schemes over the wide range of transmit powers.

This paper analyzes the system spectral efficiency of a 2-link ad hoc network. The analysis shows that there are exactly three operating points that maximize system capacity: either both links transmit with maximum power simultaneously; or one single link transmits with maximum power while the other is silent. The impact of the scheduling policy on the system spectral efficiency is also studied: simultaneous transmission or sequential access where the two links share the medium by dedicated time/frequency slots without causing interference. An exhaustive numerical search over a wide range of pathloss parameters shows that sequential transmission is superior to simultaneous transmission in about two third of the cases. Even when simultaneous transmission outperforms sequential transmission, the gains are typically not substantial. Furthermore, we show that in case of simultaneous transmission power control causes only marginal degradation in terms of system spectral efficiency, while transmitting with maximum power pays off for sequential transmission. This highlights the importance of the joint optimization of power and rate adaptation together with scheduling for power constraint ad hoc networks.

This paper presents a new cross-layer approach to achieve active link adaptation and QoS support in ad hoc networks. Upon data reception in a transmission slot, receivers broadcast a feedback signal in a time-multiplexed mini-slot. The level of this feedback or busy signal is made dependent on the received signal from the intended transmitter and the required SINR (signal-to-interference-plus-noise ratio). Potential new interfering transmitters adjust their data transmit power based on the received busy signal power. It is demonstrated that due to the channel reciprocity in TDD (time division duplex) the received busy signal power is sufficient information to be able to maintain the SINR at the already existing links. Since multiple links are coupled via the busy signal, convergence and stability issues are analyzed.

This paper analyses the busy burst (BB) enabled interference avoidance mechanism applied to anadhocindoor scenario with time division duplex (TDD) mode of WINNER parameters. The BB algorithm exploits the channel reciprocity of the TDD mode. Upon successful data reception, a receiver broadcasts a busy signal. A potential interfering transmitter can directly infer the level of interference it would cause to the vulnerable receiver from the received BB signal power. The new transmitter can, thus, autonomously decide whether to transmit, or to refrain from transmission so that interference at the vulnerable receiver is below a given threshold. As a consequence, significant co-channel interference is avoided and the system performance is ameliorated. The results show that using the BB protocol, approximately a three fold increase in throughput and a significant reduction in packet delay and packet expiration rate is achieved compared to uncoordinated random medium access in suchadhocnetworks.

In this paper, the effect of channel imperfections on the performance of spatial modulation (SM) and V-BLAST (vertical-Bell Labs layered space-time) combined with OFDM (orthogonal frequency division multiplexing) transmission is presented. Channel imperfections include Rician fading, spatial correlation and mutual antenna coupling. Recently, SM-OFDM is proposed as a multi-antenna transmission approach that results in an increase in spectral efficiency by systematically mapping a block of information bits to a single transmit antenna (out of a set of antennas) and an information symbol. This principle is applied to each subchannel, i.e. for an entire OFDM symbol at any given subchannel and time instant only one antenna is transmitting. For each subchannel this might be a different antenna, though, depending on the sequence of incoming bits. Consequently, inter-channel interference (ICI) at the receiver input is entirely avoided while high spectral efficient is maintained. In this paper, the effect of channel imperfections on SM-OFDM and V-BLAST-OFDM for coded and uncoded systems are studied. For the same spectral efficiency and a BER (bit error ratio) of 10~3, SM-OFDM is shown to perform 4 dB better than V-BLAST-OFDM in the presence of Rician fading and by 7 dB in the presence of all channel imperfections.

In this paper, the effect of variation in the spatial protection margin, is investigated for multihop wireless networks under the interference avoidance protocol model and compared to single-hop communication network. Under the protocol model, the interference is avoided by the application of the exclusion range concept. This means that a spatial protection region (or a circular exclusion region) is specified around all receiving nodes so that any node other than the intended transmitter is not allowed to reuse the same frequency resource within this region. It is demonstrated that for an asymptotically infinite coverage area, the spatial reuse efficiency for a multihop design increases proportionally with the number of multiple hops per link, and with the spatial protection margin. However, under a finite coverage area, the monotonic increase of spatial reuse efficiency does not remain completely valid. In order to analyze the performance of such a system, a relation for the system throughput is derived from the Shannon capacity equation. It is found that the system throughput attains a peak when the value of spatial protection margin is nearly unity. The exact value of spatial protection margin when the throughput reaches its maximum is found to be a monotonically increasing function of the number of multiple hops and the pathloss exponent.

One of the largest obstacles to effective navigation and positioning using pedestrian dead reckoning is the lack of accurate speed estimation algorithms. Existing methods are either complex or provide results that are unsatisfactory at low velocities associated with pedestrians and other slow moving entities. In contrast, the algorithm proposed in this paper is simple to implement and is able to provide accurate results at low velocities. A one-dimensional and unidirectional two- antenna solution is described where the speed can be easily estimated from a knowledge of the fixed inter-antenna distance and the time it takes for the trailing antenna to experience the same channel conditions (radio frequency (RF) signature) as the leading antenna. Computer simulations of the new adaptive algorithm show that with estimation errors of less than 1.2% around average pedestrian speeds the approach is indeed effective and accurate.

This paper presents a fully decentralized link adaptation algorithm to manage interference in an ad hoc network operating in TDD mode. The algorithm, called busy tone interference tolerance signaling, is of low complexity and easy to implement. Two functions are explored to set the busy tone signal power dependent on the level of tolerable interference, and their performance is compared with a system with fixed transmit powers. Results show that an appropriately chosen function for setting the busy-tone exhibits significantly enhanced power efficiency, implying that battery life can be extended, while providing a similar capacity than a fixed power system.

White LEDs are set to penetrate many areas of everyday life. An interesting property of these devices (in addition to their lightening capabilities) is that they can be utilised for data transmission. In the past, primarily OOK (on-off keying) has been used for digital data modulation of such devices. OOK imposes limitations on the achievable data rates. Therefore, in this paper OFDM is considered in combination with higher order modulation schemes. A hardware demonstrator with an entire link chain (transmitter and receiver) is developed and measured BER (bit error ratio) results are reported. The system uses pilot sub-carriers to correct frequency synchronisation errors, training sequences for channel estimation and time synchronisation routines. Forward error correction (FEC) coding is used. It is shown that for COFDM (coded OFDM) with QPSK (quadrature phase shift keying) modulation and a single LED, a BER of 2 times 10-5is achieved for a distance of 90 cm between transmitter and receiver.

A multihop ad hoc wireless network with an interference avoidance model is analyzed for a time division duplex (TDD) air-interface. In accordance with the non-interference protocol, an exclusion region is defined around every communicating receiver wherein no other node except the desired transmitter can transmit. In a network comprising finite coverage area, the position of the communicating nodes determine the number of simultaneous communicating pairs and hence the overall system throughput. A random data hopping technique for a time slot (TS) partitioned system is proposed in this paper. In this technique, the number of TS(s) in the TDMA frame is increased by reducing the duration of each TS. Then, the time slots over which the transmission pairs of the end-to-end link communicate are randomly assigned over the time frame. This randomness combined with a high TS granularity results in an increased system throughput of up to 31% as compared to a system with low TS granularity

This paper presents a new analytical framework for the analysis of inter-cellular timeslot allocation over TDMA (time division multiple access) based air-interfaces using TDD (time division duplexing). The analysis is used to evaluate the delay- throughput performance of an adaptive decentralized inter- cell interference mitigation technique that exploits the inherent channel reciprocity of TDD. The main principle is that receivers upon successful transmission of a packet transmit a busy burst on a succeeding minislot. Potential transmitters in neighboring cells sense the minislot prior to signal transmission. With this mechanism, inter-cell interference to the existing link is avoided. The performance of this MAC (medium access control) protocol is compared against blind timeslot allocation. The new MAC protocol is shown to facilitate network self-organization, offering superior delay and throughput compared to the state of the art, with modest overhead and complexity requirements.

In this paper a novel decentralized and interference aware medium access control (MAC) protocol combined with a dynamic subchannel selection algorithm for OFDM (orthogonal frequency division multiplexing) is presented. The protocol resolves several drawbacks of the existing radio resource allocation techniques for OFDM systems, such as the hidden and exposed node problem, and the requirement for a time-invariant channel. The proposed scheme enables the transmitter to determine the level of interference it would cause to already active links prior to any transmission. This is achieved through a busyslot signaling that exploits the channel reciprocity offered by the TDD mode. Compared to existing radio resource allocation schemes the required signaling overhead is significantly reduced. The interference awareness allows the system to avoid significant co-channel interference (CCI) and to operate with full frequency reuse. When applied to a broadband OFDM air interface, this principle can also exploit the frequency selective fading channel on the intended as well as the interference links. This results in an autonomous subcarrier allocation algorithm which can dynamically adapt to time-varying channels. Due to its decentralized nature, the algorithm can be utilized in both cellular and ad hoc networks. Simulation results for a cellular system show that the new decentralized medium access and channel assignment algorithm is capable of achieving spectral efficiencies of up to 2.25 bits/Hz/cell using 16 QAM (quadrature amplitude modulation).

Orthogonal frequency division multiplexing (OFDM) is very sensitive to frequency offsets which result in considerable interference. Performance of the system will be exacerbated in the uplink of a cellular deployment with frequency reuse of one. A general mathematical model is developed to calculate the amount of interference in cellular OFDM system considering frequency offset between transmitter (Tx) and receiver (Rx), depending on different multiple access and duplexing techniques. An ideal adaptive modulation scheme is applied to compare the capacity of different cellular OFDM systems. It is found that the capacity of frequency division multiple access (FDMA) system outperforms that of time division multiple access (TDMA) system due to multiuser diversity in FDMA. Moreover, time division duplexing (TDD) system outperforms frequency division duplexing (FDD) system irrespective of the multiple access techniques by exploiting time slot (TS) opposing techniques.

This paper introduces a new technique for efficiently incorporating multihop communications in a wideband code division multiple access (WCDMA) system with a clustered user distribution. For each cluster of users a mobile station (MS) is selected as a gateway to act as a relay between the other MSs and the base station (BS). The under-used resources from the FDD (frequency division duplex) uplink band are utilized for communication within the cluster (TDD underlay). This model is simulated for a number of scenarios and scheduling algorithms. It is demonstrated that the proposed enhancements lead to an improvement in the spectral efficiency of the system of up to 50%

This paper presents a new analytical framework for the analysis of intercellular timeslot allocation over TDMA (time division multiple access) based air-interfaces using TDD (time division duplexing). The analysis is used to evaluate the performance of an adaptive decentralized intercellular interference mitigation technique that exploits the inherent channel reciprocity of TDD. The main principle is that receivers upon successful transmission of a packet transmit a busy burst on a succeeding minislot. Potential transmitters in neighboring cells sense the minislot prior to signal transmission. With this mechanism, intercellular interference to the existing link is avoided. The performance of this MAC (medium access control) protocol is compared against blind timeslot allocation. The new MAC protocol is shown to facilitate network self-organization, offering superior delay and throughput compared to the state of the art, with modest overhead and complexity requirements

In this paper different scheduling algorithms applied to cellular CDMA (code division multiple access) systems with different duplexing modes are studied. In particular, the performance of cellular CDMA-TDD (time division duplex) with random uplink (UL)/downlink (DL) assignment is compared to cellular CDMA-FDD (frequency division duplex) systems. The random UL/DL assignment in TDD is previously reported as random time slot opposing (RTSO), and it was shown that this technique results in interference diversity when used in a cellular system. It is demonstrated in this paper that scheduling algorithms can effectively exploit this interference diversity to enhance the total spectral efficiency. Throughput, outage and average transmit power are used to compare the performance. First, it can be observed that, despite additional interference sources in TDD, TDD and FDD result in about the same throughput and outage performance for a cellular deployment. However, the stepwise rate removal (SRR) algorithm results in significantly improved outage in TDD. Second, regardless of the duplexing technique the results show that a general improvement in throughput is always at the cost of the number of users served.

In this paper, a semi-analytical approach for the performance analysis of the random time slot (TS) hopping (RTSH) algorithm applied to code division multiple access - time division duplex(CDMA-TDD) systems will be given. TDD systems are subject to two independent interference scenarios, giving rise to interference diversity, which is exploited by the RTSH algorithm. Depending on the actual slot assignment, the system either experiences same-entity interference (MS (mobile station)-to-MS and BS (base station)-to-BS interference) or other- entity interference (MS-to-BS and BS-to-MS interference). The RTSH algorithm results in a random switching between these two scenarios, each of which will result in a different level of interference. Thereby, constant severe interference is avoided. It has been shown that the RTSH algorithm results in lowest interference for channel asymmetries in favor of the downlink (DL).

In this paper a model for simulating a CDMA system with clustered and uniform user distributions is implemented. A number of existing results pertaining to CDMA systems such as trade-off between throughput and interference power, decreasing carrier to interference ratio (C/I) as a function of distance and boundary effects in CDMA systems are verified. Further, a spatial analysis is performed to study the effects of these user distributions and two different data rates using higher order moments. It is shown that the two distributions have similar mean values for C/I and outage but the standard deviation for the clustered distribution is significantly higher than that for the uniform distribution

Interchannel interference (ICI) is the key problem in wireless MIMO (multiple-input multiple-output) transmission. Numerous ICI reduction algorithms are reported in literature. It is common to these ICI reduction algorithms that they require considerable signal processing power. In this paper, a new approach is presented which avoids ICI and, thus, results in low complexity receivers whilst maintaining high spectral efficiency. It is demonstrated that this can be achieved for BPSK (binary phase shift keying) and QPSK (quadrature phase shift keying) modulation by compressing the symbols prior to transmission and by exploiting spatial information at the receiver. The V-BLAST (Vertical Bell Labs Layered Space-Time) algorithm and simple, low complexity MMSE (minimum mean squared error) and zero-forcing (ZF) detection without ICI reduction was used for comparison. It is shown that the new MIMO approach outperforms V-BLAST by about 6.5 dB at a signal-to-noise-ratio (SNR) of 15 dB while it outperforms direct MMSE detection by about 12 dB. In addition, it was found that the spectral efficiency compared to SISO (single-input single-output) is improved by a factor of 4 at about the same bit-error performance

In this paper, a novel approach to minimising intercellular interference and maximising spectral-efficiency in a TDMA (time division multiple access) TDD (time division duplexing) cellular network with 100% frequency re-use and unequal traffic load asymmetry in different cells is presented. A new distributed medium access control (MAC) protocol called CSTDMA (carrier sensing time division multiple access) and a modified PRMA (packet reservation multiple access) protocol are applied and their performance is compared against state-of-the-art MAC protocols. CSTDMA TDD employs a modified busy-tone broadcast/channel-sensing mechanism, while PRMA (packet reservation multiple access) employs a handshaking procedure. Results show significant improvements in system capacity and spectral-efficiency from the application of this novel approach compared to the state-of-the-art, especially with the CSTDMA TDD approach, which overall achieves the best performance, offering more than 3 to 4 times the capacity and system spectral-efficiency of the traditional methods

A hybrid cellular and ad hoc network for high data rate wireless access is considered. An air-interface which uses TDD in combination with time division multiple access (TDMA) is assumed. Interference and resource reuse are important issues in hybrid networks. For this purpose analytical models for the reuse efficiency and a capacity bound of such a network are developed. These models are based on an exclusion range concept which is introduced to keep interference at an acceptable level. It is found that with this interference limiting measure, multi hop networks can benefit from a multi hop reuse gain compared to single hop networks. However, it is further found that this gain does not significantly increase for more than 5 hops. Furthermore, the pre-requisite for being able to exploit the multi hop reuse gain is a certain TS granularity. It was observed that with an increasing number of TS per frame, the mean and the variance of the number of supported hops increased almost linearly

A novel strategy for mitigating high intercellular interference in a 4/sup th/ generation (4G) OFDM/TDMA TDD cellular mobile communications network with 100% frequency reuse is proposed. This strategy is denoted as channel-sensing TDMA TDD and it employs a modified busy-tone broadcast/channel-sensing mechanism to avoid and mitigate high intercellular interference scenarios. Results show significant improvements in system capacity and spectral-efficiency from the application of this protocol compared to the state-of-the-art.

In this paper, a coding scheme with M-ary Walsh-Hadamard (WH) orthogonal modulation is considered for multi-carrier (MC) transmission. A novel pilot-assisted channel estimation approach is proposed, that efficiently exploits the properties of WH codes. Firstly, simulation results on the single-user performance show the influence of the energy distribution between pilot and information symbols. Then, the impact of the channel estimation method is assessed for different system loads in a MC code division multiple access (MC-CDMA) uplink scenario.

In this paper the performance of the RTO algorithm at different asymmetries favouring uplink (UL) and downlink (DL) is studied. The RTO algorithm is a dynamic channel allocation (DCA) algorithm that yields a significant interference reduction (min. 3 dB, max. 20 dB) compared to a fixed channel assignment (FCA) algorithm. The FCA algorithm used, emulates an equivalent FDD system which has no same-entity interference. Thus, the existence of different interference modes in TDD is exploited constructively to avoid jamming. The RTO algorithm shows an excellent performance for an asymmetry favouring the DL. This is a very useful result as future high data rate services which, for example, make use of multimedia streaming technologies require significantly higher capacity in the DL than in the UL. If the asymmetry. for example, is 14:2 (14 time slots (TS's) used for DL and 2 TS's for UL). the performance is about 15 dB better than with an asymmetry of 2:12 (2 TS's used for DL and 14 TS's for UL). It is important to note that the FCA algorithm results in greater interference for all rates of asymmetry investigated.

The sparsity of multipath fading channel is introduced, and then the improved MMSE channel estimation for orthogonal frequency-division multiplexing (OFDM) system is presented. With computationally simple algorithm, the proposed channel estimator extracts the channel information from its noisy version accurately by utilizing the correlation of the taps of finite impulse response (FIR) filter channel model. Simulation results show that the proposed channel estimation gives much better performance than LS and DFT-based MMSE channel estimator.

The interference in a TDD system is demonstrated as being comprised of two categories of interference: (1) same entity interference, which is base station (BS) to BS interference and mobile station (MS) to MS interference; (2) other entity interference which is BS/spl rarr/MS and MS/spl rarr/BS interference. Based on this specific TDD property, an equation for capacity in a CDMA/TDD system is derived. Moreover, it is shown that the capacity in a CDMA/TDD system can be significantly higher compared with an equivalent FDD system. This assumes use of a DCA algorithm. A novel DCA is presented which uses a newly developed method termed: time slot (TS) opposing technique. The main advantage of this technique is that a TDD system adopting different asymmetries for uplink and downlink in neighbouring cells does not necessarily suffer from capacity losses.

A novel data multiplexing scheme is presented. This technique uses an antenna array with K transmit antennas. In. total K information bits are multiplexed in an orthogonal fashion. The special property of the multiplexing technique is that, for each symbol duration, only one out of K antennas is transmitting using BPSK modulation. Furthermore, the symbol duration is equivalent to the bit-duration due to the parallelism introduced by the K antennas. The receiver is able to detect the transmitting antenna and by using this information the demultiplexing is carried out. It is demonstrated that the proposed method achieves about the same spectral efficiency as 8PSK, but with a relaxed bit-energy-to-noise ratio of about 2.5 dB. Furthermore, the linear channel capacity gain (error-free transmitted bits) over the single antenna BPSK transmission scheme is found to be in the range of 2.5-3.

Multi-antenna OFDM system calls the consideration of coding across antennas and OFDM tones. Derived from the design criteria for space-time block codes (STBC), the coding scheme termed space-frequency block codes (SFBC) is conceived to achieve the maximum diversity order for a given number of transmit and receive antennas. In this contribution, a practical space-frequency block coded OFDM scheme (SFBC-OFDM) is proposed for fast fading broadband channels. Besides, this novel scheme is rather flexible to combat the frequency-selective fading by decreasing the coding size. Simulation and analysis show that our scheme outperforms the STBC-OFDM scheme in the scenario with high vehicle speed and large delay spread when the number of subcarriers in the OFDM system is large.

This paper analyzes the performance of the adaptive modulation system over Nakagami-m fading channels, which utilizes four modulation modes. The optimization criterion of the setting of the modulation switching levels is to maintain the target BER which is desired by many data services. Under this condition, the performance analysis of the throughput and the transmission outage probability of the adaptive modulation system are presented. We do so by adopting the numerical analysis methods. Finally, the simulation results show that the adaptive modulation system can theoretically achieve good performance with high throughput and low transmission outage probability.

As an important space-time code, the layered space-time (LST) code has been studying widely since it was firstly proposed by Foschini in 1996. To exploit its potential, the Bell Lab Layered Space-Time (BLAST) structure of the LST was proposed by Bell Lab. There are two types of BLAST architectures: vertical BLAST (V-BLAST) and diagonally BLAST (D-BLAST). Detection algorithms of V-BLAST were proposed by Golden (see Electronics Letters, vol.35, no.1, 1999). His detection process uses linear combination nulling and successive symbol cancellation (SSC) based on inversing and ordering. However, inversing (Moore-Penrose pseudoinverse) and ordering operation for each iteration bring a huge computation complexity. To detect M transmit antenna signals, Golden's detection algorithm needs M inversing and M ordering operations. Aiming at this shortcoming, a new detection algorithm for layered space-time code is proposed. This new sub-optimal detection algorithm is based on Greville inversing process, with two inversing and one ordering process. Simulation results show us the proposed sub-optimal scheme still has a good BER performance.

In 3GPP FDD specifications, space time transmit diversity (STTD) employs space-time block code (STBC) to maintain orthogonality between the two antennas in order to avoid self-interference in fading channels. However, the STTD technique is just open loop transmit diversity, and we investigate whether we can obtain further gain if closed loop STBC is used. A novel scheme based on closed loop STBC is proposed first. And then an optimized allocation scheme of a fixed transmitter power budget between two transmit antennas is presented. Finally, COSSAP simulation results show a significant performance improvement between closed loop STBC and traditional STBC. Moreover, these simulation results also show a good agreement with our theoretical analyses.

It has been demonstrated that code division multiple access (CDMA) provides great flexibility by enabling efficient multiuser access in a cellular environment. In addition, time division duplex (TDD) as compared to frequency division duplex (FDD) represents an appropriate method to cater for the asymmetric use of a duplex channel. However, the TDD technique is subject to additional interference mechanisms compared to an FDD system, in particular if neighboring cells require different rates of asymmetry. If TDD is combined with an interference limited multiple access technique such as CDMA, the additional interference mechanism represents an important issue. This issue poses the question of whether a CDMA/TDD air-interface can be used in a cellular environment. The problems are eased if a hybrid time division multiple access (TDMA)/CDMA interface (TD-CDMA) is used. The reason for this is that the TDMA component adds another degree of freedom which can be utilized to avoid interference. This, however, requires special channel assignment techniques. A notable example of a system which uses a TD-CDMA/TDD interface is the Universal Mobile Telecommunications System (UMTS). This paper presents a novel centralized dynamic channel assignment (DCA) algorithm for a TD-CDMA/TDD air-interface. The DCA algorithm exploits a new technique which is termed "TS-opposing technique." The key result is that the new DCA algorithm enables neighboring cells to adopt different rates of asymmetry without a significant capacity loss.

When comparing UTRA-TDD with UTRA-FDD it can be found that in the UTRA-TDD mode additional interference scenarios exist. Mobile stations (MSs) can interfere with each other and so can base stations (BSs). Since the source and sink of this type of interference are the same we call it same-entity interference. It is shown by a novel interference resolving algorithm that same-entity interference can be constructively exploited to enable asynchronous overlaps in a UTRA-TDD network. Asynchronous overlaps exist when any cell A is transmitting while the neighbour cell B is receiving. It was found that in a network where an asynchronous overlap exists the algorithm proposed reduces outage from 14% to 6%. In contrast, the outage of an ideally synchronised network was found to be 3.5%. In this 'ideal' network asynchronous overlaps are disallowed resulting in a significant drawback. This is that the flexibility of a TDD system to easily adopt different channel asymmetries is significantly limited.

We investigated the feasibility of the coexistence of CDMA-TDD micro cells under a CDMA-FDD macro cell network with both sharing the same frequency band. A dynamic channel allocation (DCA) algorithm decides on the basis of mutual interference which FDD band is to use for the micro cell TDD channel. On the assumption of giving the macro cell higher priority we investigated the lower bound probability of total micro cell loss (outage) dependent on the distance between the micro- and macro cell BS. We also considered wall loss at the boundary of the micro cell and studied the effect of different propagation conditions on the assumption of equally distributed macro cell subscribers. A mathematical model for outage is developed and the results are compared with simulations. The results reveal a good match between the theoretical approximation and the simulations. This, in turn, indicates that micro cell outage is mainly caused by a single macro cell mobile within a certain area around the micro cell BS. It can be seen that the outage goes up to about 15% when the micro cell is located at the macro cell boundary. This value exponentially decreases when the micro cell is moved towards the macro cell BS. Walls around micro cells do have a significant influence on outage. It has been shown, that outage converges towards a value determined by the micro cell size.