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.

Integrated sensing and communication (ISAC) is set to transform data transmission and real-time sensing in sixth-generation (6G) networks. By offering high capacity, free-space optical (FSO) links can play a vital role as a backhaul solution, complementing radio frequency (RF) technologies such as millimeter wave (mmWave) to enhance 6G reliability. While adverse weather conditions such as fog can reduce the reliability of FSO links due to atmospheric attenuation, the back-scattered light it generates may also enable real-time sensing of atmospheric channel gain at the transmitter. This paper proposes a novel user offloading scheme utilizing an optical ISAC (O-ISAC) framework within a single-cell network, supported by a ground base station (GBS) and enhanced by an unmanned aerial vehicle (UAV). The UAV connects to the gateway via an FSO backhaul link, estimating the channel gain based on back-scattered light and dynamically optimizing its position to efficiently offload users in the downlink. Numerical results demonstrate the UAV’s effectiveness in optimizing its position under various weather conditions and show that the proposed deployment scheme outperforms existing UAV offloading methods. The framework is evaluated using a real dataset of hourly visibility measurements in Edinburgh, underscoring the significance of optical channel sensing for overall system performance and providing insights into how sensing impacts communication efficiency.

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.

Integrated sensing and communication (ISAC) is expected to transform data transmission and real-time sensing, enhancing sixth-generation (6G) networks. Free-space optical (FSO) communication is a key 6G backhaul solution, complementing radio frequency (RF) technologies like millimeter wave (mmWave) for improved network reliability. However, adverse weather can significantly reduce FSO link reliability due to atmospheric attenuation. Such adverse weather conditions also increase the level of back-scattered light, potentially enabling the real-time sensing of the atmospheric channel gain at the transmitter side. Therefore, this paper proposes a novel optical ISAC (O-ISAC) framework, where the back-scattered light from the FSO communication signal is used as the sensing feedback signal. This O-ISAC framework is analyzed considering a single-cell network aided by an unmanned aerial vehicle (UAV) to support edge users. The UAV is connected to the gateway via a FSO backhaul link while estimating the FSO channel gain based on the back-scattered light and dynamically optimizing its trajectory. The aim of this adaptive O-ISAC system is to maximize the end-to-end network throughput of the edge users while considering FSO backhaul capacity and the UAV's directional antenna beamwidth and bandwidth allocation. Numerical results demonstrate that UAV can effectively optimize its trajectory by adjusting the antenna beamwidth and downlink bandwidth allocation at different weather conditions. The proposed framework is tested using hourly visibility data from Edinburgh, demonstrating that optical channel sensing is crucial for the system's overall performance.

In this paper, we investigate the capacity of the continuous spectrum of nonlinear frequency division multiplexing (NFDM) systems when data is encoded using b-modulation. We prove that the capacity-achieving distribution is unique, discrete in amplitude and uniform in phase, forming a support of finitely concentric shells. An algorithm is proposed to numerically calculate the optimal number of shells, their amplitudes and their probabilities such that the mutual information is maximized. An analytical lower bound on the capacity of the b-modulation channel is presented. Moreover, the mismatch capacity lower bound is calculated based on a set of realistic channel realizations, generated by the transmission of pulses over a long-haul fiber modeled by nonlinear Schrodinger equation (NLSE) and simulated by split step Fourier method. We also calculate the mutual information for discrete-points constellations. The numerical results show that the gap of mutual information is very small between these two approaches which proves that discrete-points distribution can be used as a tight lower bound instead of shell-based optimal distribution since it is a practical constellation. Finally, practical achievable rates are calculated based on some standard amplitude and phase shift keying (APSK) modulation schemes to show the achievable performance gains.

Optical wireless communication (OWC) emerges as a pivotal solution for achieving terabit-level aggregate throughput in next-generation wireless networks.With the mature high-speed transceivers and advanced (de)multiplexing techniques designed for fiber optics, fiber-coupled OWC can be seamlessly integrated into existing ultra-high-speed networks such as data centres. In particular, OWC leveraging spatial mode multiplexing (SMM) and few-mode fiber (FMF) coupling can significantly increase capacity, though misalignment may reduce performance. This paper presents a thorough investigation into the SMM-enabled FMF coupling OWC systems affected by link misalignment, specifically focusing on systems with intensity modulation with direct detection (IM/DD) receivers. A theoretical analysis is conducted to assess the fiber coupling efficiency of the considered system in the presence of both pointing error and angle of arrival (AOA) fluctuations caused by random device vibrations. Our model elucidates the dependence of coupling efficiency to the order of the incident modes, highlighting the critical role of beam properties in system performance. To mitigate the intermodal crosstalk arising from link misalignment, we employ zero-forcing beamforming (ZF) to enhance the overall aggregated data rate. Through extensive numerical results, we identify optimal system configurations encompassing aperture design and mode selection, leading to a capacity boost exceeding 200% when compared with system performance without considering the ZF.

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.

Laser-based optical wireless communication (OWC) is a key enabler for achieving terabit aggregate throughput for future indoor wireless networks. Fiber-coupled optical transceivers are the most mature technology to realize ultra-high-speed transmission links. However, fiber-coupled OWC links are vulnerable to misalignment issues due to device vibrations. To alleviate the impact of misalignment, the single-mode fiber can be replaced by the few-mode fiber (FMF). This paper presents a refined model to assess the impact of FMF coupling on indoor OWC channels in the presence of misalignment. Moving beyond the traditional use of Gaussian beams, our approach encompasses a broader range of spatial beams, offering a more realistic perspective on fiber coupling losses. We diverge from the conventional far-field assumptions and idealized channel conditions, instead offering detailed theoretical insights into the effects of both pointing errors and angle of arrival (AOA) fluctuations caused by random device vibrations. Our simulations reveal varied coupling efficiencies for different incident modes across FMF modes when misalignments occur. Furthermore, we provide insights into the achievable data rates of indoor fiber-coupled OWC systems, showing the variation of the throughput with different transmitted mode orders.

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.

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.

Light fidelity (LiFi) is a potential key technology for future 6G networks. However, its feasibility of supporting mobile communications has not been fundamentally discussed. In this paper, we investigate the time-varying channel characteristics of mobile LiFi based on measured mobile phone rotation and movement data. Specifically, we define LiFi channel coherence time to evaluate the correlation of the channel timing sequence. Then, we derive the expression of LiFi transmission rate based on the m-pulse-amplitude-modulation (M-PAM). The derived rate expression indicates that mobile LiFi communications is feasible by using at least two photodiodes (PDs) with different orientations. Further, we propose two channel estimation schemes, and propose a LiFi channel tracking scheme to improve the communication performance. Finally, our experimental results show that the channel coherence time is on the order of tens of milliseconds, which indicates a relatively stable channel. In addition, based on the measured data, better communication performance can be realized in the multiple-input multiple-output (MIMO) scenario with a rate of 36Mbit/s, compared to other scenarios. The results also show that the proposed channel estimation and tracking schemes are effective in designing mobile LiFi systems.

Indoor optical wireless communication (OWC) systems such as light fidelity (LiFi) face a number of challenges when it comes to providing seamless connectivity between the transmitter and the receiver. For example, establishing a line of sight (LOS) link that achieves uninterrupted optical wireless connectivity in a realistic indoor environment can be challenging. Some of the important factors that contribute to the obstruction of the LOS link include the presence of the human body, random device orientation and the limited field of view (FOV) of optical receivers. In most previous studies, only blockage by the human body are considered. However, in practical scenarios, it is also important to take into account the effect of random orientation of the user equipment (UE) as well as the receiver's limited FOV. In this study, we present an analysis of link blockage by developing an analytical framework to calculate the blockage probability of indoor OWC systems considering all of the aforementioned issues. In particular, we derive novel analytical expressions of the blockage probability for a single source scenario considering the effects of blockage due to the limited FOV of the receiver, random orientation of the UE and blockage by the user body and by other external blockers. Using the proposed analytical framework, we investigate the characteristics of link blockage of indoor OWC by varying the receiver FOV, user directions and the distance between the source and the receiver. The analytical results, that are computed much faster than simulation results, are shown to be perfectly matched with the simulation.

In this paper, we investigate the capacity of the$b- \mathbf{modulated}$nonlinear frequency division multiplexing (NFDM) systems. Recently, a tractable channel model was proposed for such optical fiber communication systems, describing the received$b-\mathbf{Modulated}$signal by an input-dependent complex Gaussian distributed noise. Considering this channel model, we prove that the capacity achieving input distribution has discrete amplitude, uniform independent phase (DAUIP). Noting that this distribution is supported on a finite number of concentric shells, we find a lower bound for the capacity by assuming a single shell support. The corresponding output distribution and mutual information expressions are derived in closed-form.

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.

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.

An approximated channel model is proposed for direct signaling on the continuous spectrum of a nonlinear frequency division multiplexed (NFDM) communication system, describing the effect of noise and nonlinearity at the receiver. The optimal input distribution that maximizes the mutual information of the proposed approximated channel under the peak amplitude constraint is then studied. We present that, considering the input-dependency of the noise, the conventional amplitude-constrained constellation designs can be geometrically shaped to provide significant mutual information gains. However, it is observed that further probabilistic shaping and constellation size optimization can provide only limited additional gains beyond the best geometrically shaped benchmark scheme, i.e., 64 Amplitude Phase Shift Keying. Then, an approximated channel model that neglects the correlation between subcarriers is proposed for the matched filtered signaling system, based on which the input constellation is geometrically shaped. We demonstrate that although the inter-subcarrier interference in the filtered case is neglected in the channel model, the shaping of the matched filtered case can provide promising gains in mismatch capacity over the unfiltered scenario.

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.

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.

In this paper, we investigate the capacity of the continuous spectrum of nonlinear frequency division multiplexing (NFDM) systems when data is encoded using b-modulation. Recently, a tractable channel model is proposed for such optical fiber communication systems, describing the received b-modulated signal with an input-dependent complex Gaussian distributed noise. Considering this channel model, we prove that the capacity-achieving distribution is unique and discrete. A search algorithm is proposed to determine the optimal discrete input distribution that maximizes the mutual information. The numerical results show that the optimal input distributions form ring-based constellations. We compare these optimal designs with some conventional ring-based amplitude and phase shift keying (APSK) modulation schemes including 16 APSK, 32 APSK and 64 APSK, to show the achievable performance gains.

An approximated channel model is proposed for signalling on the continuous spectrum of a nonlinear frequency division multiplexing communication system, describing the effect of noise and nonlinearity at the receiver. The optimal input distribution that maximizes the mutual information of the proposed approximated channel under peak amplitude constraint is then studied. We show that, considering the input-dependency of noise, the conventional amplitude-constrained constellation designs can be shaped geometrically to show significant mutual information gains. However, it is observed that further probabilistic shaping and input distribution size optimization can only provide limited additional gains beyond the best geometrically shaped benchmark scheme, i.e., 64 Amplitude Phase Shift Keying.

Visible light communication (VLC) is an emerging mode of wireless communication that supports both illumination and communication. Dimming control is one essential function for such systems that requires a sensitive receiver for low-light conditions. The use of an array of single-photon avalanche diodes (SPADs) is one promising approach to enhancing receivers’ sensitivity in a VLC system. However, an increase in the light brightness can impair the performance of SPAD-based receivers due to non-linear effects caused by the SPAD dead time. In this paper, an adaptive SPAD receiver is proposed to ensure reliable operation at a wide dynamic range of channel conditions and different dimming levels. In the proposed receiver, a variable optical attenuator (VOA) is used to optimize the SPAD’s incident photon rate at each channel condition. Two dimming control methods of IEEE 802.15.7 standard based on analog and digital dimming for on-off keying (OOK) modulation are studied and compared for the proposed SPAD-based VLC system. Our detailed performance investigation shows that the proposed adaptive receiver outperforms traditional PIN PD and non-adaptive SPAD array receivers in terms of the bit error rate (BER) performance. Moreover, the considered analog dimming can provide a significantly higher achievable data rate compared to that of digital dimming at the expense of lower power efficiency.

In the context of beyond the fifth-generation (B5G), free-space optics (FSO) is a vital technology for high data rate applications because of its low cost and licence-free high bandwidth. However, during adverse weather, the FSO link’s capacity can be significantly impacted, and establishing parallel radio frequency (RF) backup links just for occasional utilization is both complex and costly. In this study, in order to improve the availability of FSO link in adverse weather conditions, we propose to use unmanned aerial vehicles (UAVs) as hybrid relays to intercept an FSO link. The UAVs construct an intermediate millimeter wave (MMW) link, while ensuring that the approach has no impact on the source and destination. The end-to-end system capacity is maximized by optimizing UAVs' location based on the negatively correlated FSO and MMW link length. The performance of the proposed model is compared to benchmark systems such as single FSO, fixed relay-assisted FSO, and fixed hybrid FSO/RF systems. The numerical results clearly demonstrate the proposed scheme’s superiority. Finally, using weather statistics from the cities of Edinburgh and London in the United Kingdom, the performance of the proposed system and its counterparts in realistic scenarios is demonstrated, which illustrates that the proposed system may significantly improve link availability towards the carrier-class criterion.

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 recent years, there has been a growing interest in exploring the application of single-photon avalanche diode (SPAD) in optical wireless communication (OWC). As a photon counting detector, SPAD can provide much higher sensitivity compared to the other commonly used photodetectors. However, SPAD-based receivers suffer from significant dead-time-induced non-linear distortion and signal dependent noise. In this work, we propose a novel SPAD-based OWC system in which the non-linear distortion caused by dead time can be successfully eliminated by the pre-distortion of the signal at the transmitter. In addition, another system with joint pre-distortion and noise normalization functionality is proposed. Thanks to the additional noise normalization process, for the transformed signal at the receiver, the originally signal dependent noise becomes signal independent so that the conventional signal detection techniques designed for AWGN channels can be employed to decode the signal. Our numerical results demonstrate the superiority of the proposed SPAD-based systems compared to the existing systems in terms of BER performance and achievable data rate.

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.

In this paper, we investigate an integrated visible light positioning and communication (VLPC) system with a single LED-lamp. First, by leveraging the fact that the VLC channel model is a function of the receiver’s location, we propose a system model that estimates the channel state information (CSI) based on the positioning information without transmitting pilot sequences. Second, we derive the Cramer-Rao lower bound (CRLB) on the positioning error variance and a lower bound on the achievable rate with on-off keying modulation. Third, based on the derived performance metrics, we optimize the power allocation to minimize the CRLB, while satisfying the rate outage probability constraint. To tackle this non-convex optimization problem, we apply the worst-case distribution of the Conditional Value-at-Risk (CVaR) and the block coordinate descent (BCD) methods to obtain the feasible solutions. Finally, the effects of critical system parameters, such as outage probability, rate threshold, total power threshold, are revealed by numerical results.

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.

Optical wireless communication (OWC) has attracted more and more attention in recent decades. One promising method of improving the sensitivity of the receivers in OWC system is the utilization of the single-photon avalanche diode (SPAD). However, due to the non-linear effects induced by the dead time, the performance of SPAD-based receivers is significantly degraded in high incident power regime. Recently, we proposed a novel hybrid SPAD/PD receiver in which the receiver can adaptively switch between the SPAD- and PD-mode based on the instantaneous received optical power. In addition, a variable optical attenuator (VOA) is applied to optimize the performance of the SPAD unit. In this work, the superiority of the proposed hybrid receiver is experimentally demonstrated. For a data rate transmission of 450 Mbps, it is demonstrated that the proposed receiver significantly outperforms the traditional PD and SPAD array receiver in terms of the BER performance and can greatly extend the receiver dynamic range to reliably operate at both extremes of the incident light levels.

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 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.

Light fidelity (LiFi) is an emerging communication technology that utilizes light intensity modulation in order to transfer data from light-emitting diode (LED) to users. Due to the vast visible light spectrum, LiFi can support high data rates; however, its coverage is limited. In contrast to LiFi, WiFi works in radio frequency and is capable of providing ubiquitous coverage with limited data rates. Since the spectrum of LiFi does not overlap with WiFi, both can co-exist to form a hybrid LiFi and WiFi network. The advantage of hybrid LiFi and WiFi network is that it provides high data rates and better connectivity. The performance of a hybrid LiFi and WiFi network significantly depends upon the load balancing strategies. Therefore, in this paper, gradient descent-based reinforcement learning (RL) has been proposed to determine an optimal access point (AP) assignment policy that aims to maximize the average network throughput while ensuring user's satisfaction. The performance of the proposed method is then compared against conventional signal strength strategy (SSS); the results are presented in terms of the average network throughput, user satisfaction, and outage probability. Based on the results, it was observed that the proposed RL method provides a significant improvement in all the performance metrics over the SSS based method.

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.

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.

In this work, a hybrid RF/FSO system employing a game theoretic spectrum trading process is investigated. Different from the hybrid systems in the literature, in the proposed system no RF spectrum is preallocated to the FSO link. Only under infrequent adverse weather conditions where the availability of the FSO link is severely impaired, the source borrows a portion of the licensed RF spectrum from the surrounding RF nodes. The competition among RF nodes is considered and the Stackelberg game is employed to model the spectrum trading game between the source of FSO link and the RF nodes. Using the leased spectrum, the source can establish a hybrid link containing not only the original FSO link but also an RF link to improve its throughout to the destination. Our performance analysis demonstrates that the proposed scheme improves the average capacity of the system in adverse weather conditions remarkably, thereby enhancing the availability of FSO communications.

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.

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.

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.

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.

Light emitting diodes (LEDs) are increasingly used in automotive exterior lighting. The expected wide availability of LED-based front and back lights makes visible light communication (VLC) a natural vehicular connectivity solution. A major challenge in vehicular VLC systems is their relatively poor performance in adverse weather conditions such as severe fog. In this paper, we propose the use of single-photon avalanche diode (SPAD) for vehicular VLC systems. With their higher sensitivity in comparison to conventional photodetectors, SPADs can be efficiently used to detect weak signals. Under the assumption of on-off keying (OOK), we present the error rate performance of vehicular VLC systems. Since the SPAD output is modelled by Poisson statistics, the bit error rate (BER) takes the form of a semi-infinite summation. Based on Anscombe transformation, we approximate Poisson noise as Gaussian and derive a closed-form BER expression. Using this expression, we obtain a closed-form expression for maximum achievable link distance to ensure a targeted BER. We present an extensive numerical study to validate our derivations and demonstrate the performance of vehicular VLC system under different weather conditions.

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.

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.

Free-space optical (FSO) communication offers wireless connectivity with high data rates and low system complexity; however, it suffers from the infrequent adverse weather conditions. This paper proposes a novel hybrid RF/FSO system based on a game theoretic spectrum trading process to enhance the reliability of FSO links. The proposed system is considered to be both spectrum- and power-efficient. It is assumed that no RF spectrum is preallocated to the FSO link and only when the link availability is severely impaired by the infrequent adverse weather conditions, i.e., fog, and so on, the source can borrow a portion of licensed RF spectrum from one of the surrounding RF nodes. A market-equilibrium-based pricing process is proposed for the spectrum trading between the source and RF nodes. By using the leased spectrum, the source is able to establish a dual-hop RF/FSO hybrid link to maintain its throughput to the destination. Our extensive performance analysis illustrates the effectiveness of the proposed communication system. It is demonstrated that the proposed scheme can significantly improve the average capacity of the system, especially when the surrounding RF nodes are with low traffic loads. In addition, the system benefits from involving more RF nodes into the spectrum trading process by means of diversity. Finally, the application of the proposed system in a realistic scenario is presented based on the weather statistics in the city of Edinburgh, U.K., which demonstrates that the system can substantially enhance the link availability toward the carrier-class requirement.

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.

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.

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 free-space quantum key distribution (QKD) system based on BB84 protocol is studied assuming that the transmitted quantum bits are multiplexed spatially based on orthogonal orbital angular momentum (OAM) carrying beams. The performance of such a system is estimated in terms of quantum bit error rate, outage capacity, and effective data rate taking into account the impact of error correction and privacy amplification. Based on the simulation of OAM-carrying beam propagating through atmospheric channels impaired by turbulence, the crosstalk among the multiplexed channels are investigated and its effect on the performance of the QKD system is evaluated. The presented results provide insight into the efficient design of such multiplexed QKD systems particularly in terms of the choice of the optimal average photon number per pulse and the multiplexing mode set.

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.

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.

In this paper, the spatial-mode multiplexing (SMM) free-space optical communication (FSO) systems with direct detection are considered. Unlike systems studied in several works in the literature where mutually incoherent channels are assumed, we investigate high-speed SMM FSO systems with mutually coherent channels where a single laser source with narrow linewidth is employed at the transmitter. An analytical model for such mutually coherent channels is developed and expression for aggregate achievable rate (AAR) is derived. Through numerical simulations, it is shown that an optimal transmitted mode set can be determined which results in the maximal asymptotic AAR at high transmitted power and for different turbulence conditions, the optimal mode set varies.

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.

This paper investigates spatial-mode multiplexing (SMM) for practical free-space optical communication (FSO) systems using direct detection. Unlike several works in the literature where mutually incoherent channels are assumed, we consider mutually coherent channels that accurately describe SMM FSO systems employing a single laser source at the transmitter with a narrow linewidth. We develop an analytical model for such mutually coherent channels and derive expressions for aggregate achievable rate (AAR). Through numerical simulations, it was shown that there exist optimal transmit mode sets which result in the maximal asymptotic AAR at high transmitted power. Moreover, in order to resolve the reliability issues of such SMM FSO systems in the presence of turbulence, a so-called mode diversity scheme is proposed that can be easily implemented along with SMM FSO systems. It is demonstrated that mode diversity can significantly improve the outage probability and the E-outage achievable rate performance of the multiplexed channels in SMM FSO systems degraded by turbulence.

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 asymmetry and channel correlation are frequently encountered in wireless communication systems. Orthogonal transmission schemes are usually inefficient in dealing with these problems. In this paper, in order to boost the throughput performance for multiple-input multiple-output broadcast communications in the presence of channel asymmetry and/or channel correlation, we study optimization algorithms for multi-user superposition coding beamforming (SCBF). Starting with solving the minimum power optimization problem for the two-user case, we derive the optimal solution structure of the problem and two types of dedicated algorithms that could efficiently find the optimal solutions with all parameter setups. Extensions are then made to the same problem with the signals of more than two users multiplexed in the power domain as well as to the rate region computation problem. Finally, to adapt our algorithms to more general cases, novel hybrid precoding schemes are proposed, where certain user grouping strategy is used to combine zero-forcing beamforming and SCBF. Numerical simulations are provided to show that with our algorithms, a considerable performance gain is achieved by SCBF compared to the other orthogonal transmission methods.

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.

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.

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.

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.

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.

In this paper, the impairments of free-space optical (FSO) communication systems caused by angular fluctuations including beam misalignment and angle-of-arrival (AOA) fluctuations are modeled in the presence of both atmospheric turbulence and transceiver vibrations. In particular, assuming FSO receivers with a limited field-of-view (FOV), the fading caused by AOA fluctuations is studied. The outage probability expressions for both coherent and direct detections are derived in both shot-noise-limited and thermal-noise-limited regimes. For direct detection, the optimal receiver FOV that achieves the minimum outage probability is considered. Furthermore, the issue of imperfect phasefront tracking in practical coherent receivers is investigated.

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.

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.

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, 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, different optical detection schemes are investigated in the presence of turbulence-induced effects. Both log-amplitude fluctuations and angle-of-arrival (AOA) fluctuations introduced by atmospheric turbulence are taken into account in channel modelling. The impact of the angular spread can be thus determined based on the performance of the FSO receivers. For direct detection, we find the optimal receiver field-of-view (FOV) to improve the performance. We further show the benefit of coherent detection over direct detection in the presence of angular spread. Moreover, spatial diversity receivers which are able to significantly improve the performance of atmospheric optical systems are discussed in detail.

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, rate-adaptive free-space optical (FSO) communication is studied using experimental data measured over a 1.87 km terrestrial FSO link in different weather conditions. To accommodate to the channel gain fluctuations induced by atmospheric turbulence and/or weather variations, a rate-adaptive communication system is implemented by puncturing low-density parity-check (LDPC) codes in a rate-compatible fashion. Beside the conventional random puncturing method, an optimized intentional puncturing technique is employed. Using experimental data and on-off-keying (OOK) modulation, the performance of the rate-adaptive FSO system operating at different signaling rates is evaluated. Unlike uncoded OOK, the rate-adaptive technique provides reliable and efficient FSO communication under different weather conditions and scintillation indices. Applying intentional puncturing can further improve the efficiency of the FSO system in terms of throughput.

In this paper, the performance of multi-beam free-space optical (FSO) communication systems are studied through an accurate analytical approach which does not rely on far-field assumptions commonly used in the literature. A framework is presented for analytical and numerical performance analyses of multi-beam FSO systems employed in a diversity or multiplexing scheme. The performance analyses show that the far-field assumptions may not correctly estimate the system behavior in many geometrical scenarios within practical interest. The results demonstrate the degrading effects of diffraction and spatial correlation of atmospheric turbulence in the form of diversity gain reduction in diversity systems and crosstalk in multiplexing systems. These effects have been mostly neglected in the literature by applying far-field assumptions. Our results can thus be useful in the design of practical diversity or multiplexing FSO systems especially when a compact design is desired.

In this paper, the performance of a multiple-input/single-output (MISO) free-space optical (FSO) systems is studied through an analytical approach which more accurately models the atmospheric channel. The diversity gain is evaluated based on measures determined by up to second-order statistics of the received power. The numerical results demonstrate the effects of diffraction and spatial correlation of turbulence-induced fading on the performance of multi-beam diversity systems. These effects have been mostly neglected in the literature by applying far-field assumptions. Our results can thus be useful in the design of practical multi-beam FSO systems especially when a compact design is desired.

Relay-assisted free-space optical (FSO) transmission exploits the fact that atmospheric turbulence fading variance is distance dependent and yields significant performance gains by taking advantage of the resulting shorter hops. In this paper, we first investigate how to determine optimal relay locations in serial and parallel FSO relaying as to minimize the outage probability and then quantify performance improvements obtained through optimal relay placement.

In this paper, we study the outage behavior of a decode-and-forward multi-hop free-space optical (FSO) system over a Poisson channel degraded by atmospheric turbulence. We assume that perfect channel side information (CSI) is available at the receiver side and consider both cases of perfect CSI and no CSI at the transmitter side. We solve the outage probability minimization problem subject to a peak power constraint as well as a short- or long-term average sum power constraint. As a result, optimal power control strategies are presented for different scenarios under consideration. A sub-optimal yet low-complexity solution is further proposed under the short-term power constraint. Our results demonstrate that multi-hop relaying yields significant performance improvements which are particularly important for long-range FSO links.

Although there has been a growing interest on cooperative diversity, the current literature is mainly limited to the results obtained for Rayleigh, Rician, or Nakagami fading channels. In this paper, we investigate the performance of cooperative diversity schemes over log-normal fading channels which provide an accurate channel model for indoor wireless environments. We focus on single-relay cooperative networks with amplify-and-forward relaying and consider three TDMA-based cooperation protocols: which correspond to distributed implementations of MIMO (multi-input multi-output), SIMO (single-input multi-output), and MISO (multi-input single-output) schemes. For each protocol under consideration, we derive upper bounds on pairwise error probability over log-normal channels and quantify the diversify advantages. Based on the minimization of a union bound on the bit error rate performance, we further formulate optimal power allocation schemes which demonstrate significant performance gains over their counterparts with equal power allocation.

In this letter, the authors investigate spatial diversity techniques for free-space optical (FSO) links with intensity modulation and direct detection (IM/DD) over log-normal atmospheric turbulence-induced fading channels. We restrict our attention to the deployment of orthogonal space-time block codes (OSTBCs) and repetition codes both of which have been recently proposed for FSO links. Our performance analysis demonstrates that, although both schemes are able to extract full diversity, repetition codes outperform OSTBCs. The performance gap increases with the increasing number of transmit apertures. Our findings clearly point out that deployment of OSTBCs is not necessary for a FSO IM/DD link.

In this paper, we present relay-assisted transmission as a powerful fading mitigation tool for free-space optical systems operating in atmospheric turbulence channels. We study both serial (i.e., multi-hop transmission) and parallel (i.e., cooperative diversity) relaying encoupled with amplify-and-forward and decode-and-forward modes. We consider an aggregated channel model which takes into account both path-loss and turbulence-induced log-normal fading. Since fading variance is distance-dependent in free-space optical systems, relay-assisted transmission takes advantage of the resulting shorter hops and yields significant performance improvements. We derive outage probability of the relaying schemes under consideration which are further confirmed through Monte-Carlo simulations. Our outage probability analysis demonstrates that an impressive performance improvement of 18.5 dB is possible with the use of a single relay at a target outage probability of 10-6.

This paper proposes a new method of face representation which is used for face recognition by SVM. For face representation we have used a two-step method, first two-dimensional discrete wavelet transform (DWT) is used to transform the faces to a more discriminated space and then principal component analysis (PCA) is applied. The proposed method produced a significant improvement which includes a substantial reduction in error rate and in time of processing during the obtaining PCA orthonormal basis.