Optical wireless communication (OWC) has been recognized as a promising solution for establishing high-speed, low-latency data connectivity between air and water. During the propagation of optical beams across dynamic water surfaces, random scattering and refraction can result in information leakage issues, significantly increasing the probability of an adversary listener (Willie) detecting the signal transmission. In this paper, we investigate the covert performance of bidirectional air-water cross-boundary OWC systems under the detection of arbitrary underwater/aerial Willie for the first time. To characterize the detection accuracy at Willie and communication quality between a legitimate transmitter (Alice) and a legitimate receiver (Bob), the covert performance metrics including the covertness outage probability and covert throughput are computed based on the theoretical derivations of channel gain under dynamic wave, surface refraction, and particle absorption and scattering. The numerical simulation reveals the variation patterns of covert performance metrics with respect to multiple environmental and system parameters, which shows that wave intensity and transmitted beam’s divergence angle have opposite effects on detection performance of Willie locating at different positions, and both the optical wavelength beyond blue-green band and strong ambient light can increase Willie’s detection uncertainty. Furthermore, it can be seen that the upper bound on covertness outage probability reaches 1 and the achievable covert throughput approaches 0 in some cases, while covert threat regions exist in both atmospheric and underwater areas within several meters, which indicates that covert transmission strategy and system parameters are required to be further designed and optimized to enhance communication security.

The following paper presents a framework for DC-biased optical OFDM (DCO-OFDM) in visible light communication (VLC) systems, addressing critical implementation challenges in timing synchronization and channel estimation. We propose a novel preamble design based on modified Zadoff-Chu (ZC) sequences that preserve constant amplitude zero autocorrelation (CAZAC) properties in the frequency-domain while satisfying intensity modulation/direct detection (IM/DD) constraints. The frequency-domain cross-correlation of the modified ZC sequence enables robust timing offset estimation, particularly in low-SNR scenarios. We evaluate the probability of correct timing offset estimation and the normalized mean square error (NMSE) for the proposed channel estimation scheme. Furthermore, we characterize the system-level trade-offs between clipping distortion, quantization resolution, and DC biasing, deriving optimal operating points for power efficiency under the proposed framework. Simulation results demonstrate significant improvements in synchronization accuracy compared to other alternatives, with the proposed method achieving a correct timing estimation probability of ≥ 0.99 even at low SNR values. The channel estimation NMSE performance confirms the scheme’s efficiency, outperforming conventional approaches. The approach presented hereby establishes practical design guidelines for robust timing synchronization and channel estimation for IM/DD-based DCO-OFDM implementations in VLC systems.

Quaternion-based machine learning algorithms have emerged as a promising candidate for four dimensional (4D) signal processing, which utilize 4D signal dimensions’ correlations for estimation purposes. The existing quaternion least mean square (QLMS) and quaternion maximum correntropy criterion (QMCC) algorithms are based on stochastic gradient descent and gradient ascent methods, respectively. Due to their reliance on instantaneous gradient approximations, these approaches exhibit slower convergence characteristics. To circumvent this limitation, quaternion recursive least square (QRLS) algorithm is proposed in the literature, which exhibits fast convergence and achieves lower MSE. However, the QRLS algorithm is based on minimum MSE (MMSE) criterion which optimizes error-energy, and is thus not a sufficient statistic for impulsive noise. In this paper, we propose novel non-MMSE criterion based recursive algorithms, namely quaternion recursive maximum correntropy criterion (QRMCC) and quaternion recursive maximum Versoria criterion (QRMVC). The proposed QRMCC and QRMVC algorithms exhibit robustness against impulsive/non-Gaussian distortions due to the inclusion of higher order error statistics. Simulations are presented for quaternion-valued channel estimation, which indicate that the proposed QRMCC and QRMVC algorithms deliver improved convergence compared to existing approaches. Lastly, the convergence of the proposed algorithms is analyzed with performance-validation using relevant computer simulations.

Free-space optical (FSO) communications provide higher bandwidth and immunity to electromagnetic interference compared to radio communications. To increase FSO system capacity, space-division multiplexing (SDM) with orbital angular momentum (OAM) modes has been explored. However, atmospheric turbulence induces power leakage among modes, leading to significant intermodal interference and performance degradation. Efficient power allocation and mode selection are essential to mitigate these effects. This paper addresses the joint mode selection and power allocation (JMSPA) problem for optimizing data transmission in OAM-multiplexed FSO systems in the presence of atmospheric turbulence. The proposed algorithm extends the Generalized Linear Fractional Programming (GLFP) approach by leveraging its polyblock approximation and projection framework while introducing key modifications to handle the mixed-integer nonlinear fractional structure of JMSPA. Unlike standard GLFP, JMSPA employs a specialized projection step based on a bisection-based search, enabling efficient resolution of the nonlinear rate constraints within the max-min weighted rate (WR) problem. Additionally, instead of a single polyblock approximation, JMSPA constructs distinct polyblocks for each mode subset, effectively managing the mode selection constraints and ensuring a globally optimal solution. The performance of the proposed algorithms is evaluated through channel simulations under different turbulence levels. Results demonstrate significant link performance improvements over uniform power allocation, with notable gains in stronger turbulence conditions.

With its ultra-high-capacity, free space optical (FSO) communication stands out as a powerful connectivity solution for airborne backhauling. In this paper, we consider typical backhauling scenarios for single-layer and multi-layer airborne networks and characterize the underlying multi-hop FSO transmission. In the first scenario, we consider a single-layer airborne backhaul system where a fleet of high-altitude platform stations (HAPSs) continuously rotates on a circular track at a specific altitude. It is assumed that the communication links are always established with the closest HAPS. In such a scenario, a dual-hop FSO transmission is required where the first hop is from the gateway node to HAPS and the second hop is from the HAPS to the base station. In the second scenario, we consider a two-layer system where HAPSs are assisted by rotary-wing unmanned aerial vehicles (UAVs) at lower operation altitudes to ease line of sight (LoS) requirement, which might be especially critical in urban scenarios. This mainly corresponds to a three-hop configuration where the first hop is from the gateway to HAPS, the second hop is from HAPS to UAV and the final hop is from UAV to the base station. For both scenarios under consideration, we first develop a channel model for the airborne link to describe atmospheric attenuation, geometrical loss, and pointing error. The transmission distance of the HAPS-based link is subject to continuous change due to movement and leads to a time-varying atmospheric attenuation loss and geometric loss. These mobility-induced variations in the average received power effectively introduce a fading effect. We develop a probability density function (PDF) to capture this effect. While pointing error can be ignored for HAPS-based links under certain conditions, rotary-wing UAVs introduce displacements in both$\mathcal {X}$and$\mathcal {Y}$directions. We further characterize this phenomenon and quantify the resulting variance of displacements. Based on the developed PDFs, we derive the end-to-end bit error rate (BER) for two-hop and three-hop airborne systems under consideration. We further propose a power allocation scheme to optimize the BER performance, ensuring that the overall performance is not dominated by a single hop.

Visible light communication (VLC) builds upon the dual use of illumination infrastructure for wireless access. The recent commercialization of laser diode (LD)–empowered white light luminaires gives the opportunity to build coherent VLC systems. Coherent VLC systems can leverage the high bandwidth of lasers and advanced digital signal processing techniques to significantly improve the data rates. For coherent reception, a local oscillator in the form of a monochromatic laser is used. In practice, lasers suffer from random fluctuations in phase due to spontaneous emission and carrier density fluctuations. In this paper, we analyze the effects of phase noise on the performance of a coherent VLC system in the presence of atmospheric turbulence as encountered in outdoor and vehicular applications. We assume that a phase locked loop (PLL) is used for phase recovery. Turbulence-induced fading phase is estimated through pilot symbols and provided as input to the PLL. The PLL uses this estimated channel phase to initialize its phase estimate at the beginning of each frame. Based on the statistical distribution of the residual phase, we derive an approximate closed-form expression for the pairwise error probability (PEP) which is then used to obtain a union bound on the error rate. Through comprehensive simulations, we validate the accuracy of the derived expression and quantify the impact of phase noise on the error rate performance.

Laser diode (LD) powered luminaires are already gaining traction in automotive applications and are expected to be increasingly deployed in homes and offices in the near future. This gives a unique opportunity to build visible light communication (VLC) systems with ultra-high-bandwidths offered by the LDs. However, LDs are subject to some inherent noise sources, notably signal-dependent shot noise (SDSN) and relative intensity noise (RIN). RIN basically describes the instability in the power level of a laser and quantifies the output power fluctuations. SDSN arises from variations in the photodetector’s response to optical signals. Our research explores the interplay between SDSN and RIN, revealing their combined adverse effect on channel estimation accuracy in a VLC system. Towards this direction, we first derive the Cramér-Rao lower bound (CLRB) in the presence of the SDSN and the RIN, which gives a lower estimate for the variance of an unbiased estimator. Then, we present the derivation of least square (LS) and maximum likelihood (ML) channel estimators. Furthermore, we present the optimal receiver in ML sense and compare it with a simple threshold detector as a sub-optimal solution, quantifying the impact of channel estimation accuracy on both receivers. Our study results reveal the adverse impact of the joint presence of SDSN and RIN on the channel estimation error; consequently, it degrades the BER of the two proposed receivers. Moreover, RIN emerges as the dominant noise source, especially at higher levels of transmitted power.

Non-terrestrial networks (NTNs) involve the use of unmanned aerial vehicles (UAVs), high-altitude platform stations (HAPSs), and low-earth orbit satellites (LEOs) and have emerged as a powerful tool to enable global connectivity. With their flexible deployment, rotary-wing UAVs are particularly useful to deliver versatile airborne wireless access and backhaul in areas where there is limited terrestrial infrastructure. In this paper, we consider a scenario where free space optical (FSO) terminals are mounted on rotary wing UAV to provide high capacity wireless backhaul for ground base stations. Due to the narrow divergence angle of laser tranmsitters, FSO systems are prone to pointing errors. In our study, we classify and analyze two primary sources of pointing errors in rotary-wing UAV-based links: horizontal movement and rotational stability. The former stems from the semi-fixed hovering nature of UAVs, introducing uncertainties in horizontal displacement that influence the positioning of the received beam along the$x$and$y$directions. Additionally, rotational stability issues in rotary-wing UAVs, including rolling, yawing, and pitching, lead to shifts in the center of the received beam. We classify these pointing errors as altitude-dependent and altitude-independent and analyze their distinct impacts on the bit error rate (BER) of UAV-based FSO links. Our findings contribute to a better understanding of pointing errors in UAV-based FSO communication, paving the way for enhanced airborne connectivity solutions.

With its high capacity and immunity to electro-magnetic interference, free space optical (FSO) communication is positioned to be a key connectivity solution for airborne backhauling. In this paper, we consider a scenario where a fleet of high-altitude platform stations (HAPSs) follow a predefined circular trajectory and provide airborne backhauling to ground base stations. Unlike terrestrial FSO links that are primarily limited by atmospheric turbulence-induced fading, aerial links between a HAPS and a ground station are subject to fading effects induced by mobility. The instantaneous transmission distance continuously changes due to movement, leading to a time-varying atmospheric attenuation loss and geometric loss. These mobility-induced variations in the average received power effectively introduce a fading effect. In this paper, we present a statistical model for the aggregate airborne channel coefficient. First, we derive a probability density function (PDF) to describe the instantaneous changes in the propagation distances as a result of mobility. Then, we derive the PDF for the aggregate channel coefficient that includes both geometric losses and atmospheric attenuation. We present numerical results to corroborate our analytical findings and discuss the effects of several system and channel parameters on the severity of fading.

This paper presents an analysis of the Age of Information (AoI) in a wireless sensor network consisting of multiple IoT sensors. This network consists of three nodes: a wireless power source (WPS), sensors, and an access point (AP). We exploit hybrid visible light communication/radio frequency (VLC/RF) for the sensors with the orthogonal frequency bands. Power domain non-orthogonal multiple access (PD-NOMA) and successive interference cancellation (SIC) are also adopted for the sensors and the AP. Finally, we present two main optimization problems for average AoI with the sensor's transmit power constraint. We analyze the average AoI in a hybrid VLC/RF network, and we obtain the global optimum value for the average AoI of each sensor by considering the probability of sensors being charged, the probability of choosing the links, calculating the probability of successful decoding. According to the optimization results based on the Particle Swarm Optimization (PSO) method, the average AoI for each sensor and the total average AoI in the proposed system are reduced by 10% and 15%, respectively.

This paper introduces an innovative multi-queue fog architecture coupled with an intelligent controller, aimed at enhancing the efficiency and adaptability of fog computing environments. Unlike conventional single-queue fog architectures that typically rely on basic first-in-first-out (FIFO) task execution models in fog servers, our approach offers heightened granularity and flexibility in task scheduling. This feature enables effective task management, catering specifically to Internet of Things (IoT) applications characterized by varying degrees of time-sensitivity and resource requirements. Our proposed deep weighted-fair fog servers (DeepWFFS) scheme comprises two key elements: the weighted-fair fog server (WFFS) framework and an intelligent deep controller (DC) leveraging deep reinforcement learning (DRL) for task prioritization. The WFFS framework adopts multiple queues within fog servers, each assigned a predefined weight representing task priority. This prevents task starvation and promotes equitable task execution. Meanwhile, the DC continuously monitors task workloads and priorities, ensuring optimal task allocation to the most suitable queues within fog and cloud servers. Through simulation, our results exhibit the superior performance of DeepWFFS compared to benchmark schemes. This advancement showcases the potential of our architecture to efficiently manage diverse tasks in fog computing environments.

In this paper, we explore the use of multiple photodetectors for vehicular visible light communication (VLC) systems with a focus on the so-called Angle-Diversity Receiver (ADR). ADR builds upon the principle of using multiple photodetectors oriented at different reception angles to enable multidirectional signal reception. With ADR, it is possible to receive light rays from the vehicle headlight in challenging mobility conditions such as in the cases of U-turn, left-turn, and right-turn. In our work, we present preliminary results of an experimental verification of ADR-based vehicular VLC system implemented using software-defined radio platforms. Our results demonstrate that a packet delivery ratio (PDR) of more than 99 % is achieved even for T-junction road scenarios where the link is likely to get lost in the case of conventional single transmitter/receiver configurations.

The wide availability of light-emitting diode (LED)-based light sources makes possible the use of visible light communication (VLC) for both indoor and vehicular wireless connectivity. Earlier works on VLC have predominantly used photodetectors as receivers. It is also possible to utilize solar panels as receivers. In this paper, we present an experimental performance evaluation of a vehicular VLC system with a truck headlight as the transmitter and a solar panel as the receiver. First, we characterize the frequency response of two different solar panels and measure their bandwidth. Then, we present the performance of the vehicular VLC system in terms of data rate, signal-to-noise ratio (SNR), and eye diagrams.

This paper investigates streetlight-to-vehicle visible light communication (VLC) system performance for outdoor broadcasting applications. We adopt streetlight lamps as optical internet-of-thing (IoT) devices broadcasting internet services and safety messages to road vehicles. With their asymmetrical radiation patterns, Streetlight antennas are exceedingly different from indoor lighting modules, which deploy ceiling luminaries with ideal Lambertian ones. Therefore, a realistic channel modelling for streetlight-to-vehicle VLC system should be deployed for precise performance insights. We consider a streetlight-to-vehicle VLC system in a two-lane road with multiple light poles uniformly distributed on both sides. Based on that, we investigate the system performance of the streetlight-to-vehicle VLC system in terms of the bit-error-rate (BER) and outage distance and explore the effect of different transceivers and system parameters on the performance. These consider the transmission modulation order, receiver size, height of the streetlight poles, and their corresponding intermediate distances.

This article studies the network lifetime optimization problem in a multiuser mobile-edge computing (MEC)-enabled Internet of Things (IoT) system comprising an access point (AP), a MEC server, and a set of$K$mobile devices (MDs) with limited battery capacity. Considering the residual battery energy at the MDs, stochastic task arrivals, and time-varying wireless fading channels, a soft actor–critic (SAC)-based deep reinforcement learning (DRL) lifetime maximization, called DeepLM, is proposed to jointly optimize the task splitting ratio, the local CPU-cycle frequencies at the MDs, the bandwidth allocation, and the CPU-cycle frequency allocation at the MEC server subject to the task queuing backlogs constraint, the bandwidth constraint, and maximum CPU-cycle frequency constraints at the MDs and the MEC server. Our results reveal that DeepLM enjoys a fast convergence rate and a small oscillation amplitude. We also compare the performance of DeepLM with three benchmark offloading schemes, namely, fully edge computing (FEC), fully local computing (FLC), and random computation offloading (RCO). DeepLM increases the network lifetime by 496% and 229% compared to the FLC and RCO schemes. Interestingly, it achieves such a colossal lifetime improvement when its nonbacklog probability is 0.99, while that of FEC, FLC, and RCO is 0.69, 0.53, and 0.25, respectively, showing a significant performance gain of 30%, 46%, and 74%.

Visible light communication (VLC) provides an alternative underwater wireless connectivity solution with its low latency and high data rates albeit at relatively shorter distances in the order of tens of meters. In the context of underwater sensor networks (USNs), VLC is particularly suitable to establish connectivity between “data mule” autonomous underwater vehicles (AUVs) and sensor nodes since communications is enabled only when the sensor node and mule AUV are in close proximity. In this paper, we consider a USN scenario where a solar-powered AUV gathers data from the sensor nodes using VLC signaling. We formulate a three-dimensional trajectory optimization for solar-powered AUVs with the goal of maximizing the harvested energy under constraints imposed by the data transmission. The optimization constraints include the minimum required data transfer rate, therefore a corresponding transmission distance, between the sensors and the AUV. We formulate the problem as a bilevel optimization problem. The lower-level objective function is in the form of traveling salesman problem which determines the optimum sequence order of the sensor nodes to be visited while the upper-level objective function is the optimization of the trajectory between each pair of adjacent nodes for the given order of node visits. Our numerical results demonstrate that the proposed trajectory significantly prolongs the mission time and autonomous operation of the AUV without the need to return to home base. Furthermore, since the proposed trajectory optimization is reactive to ocean currents, it brings reductions in the energy consumption of the AUVs.

Visible light communication (VLC) builds upon the dual use of LED-based lighting infrastructure for data transmission. It is particularly attractive for user-dense environments and mainly positioned as a complementary wireless access technology to radio-frequency (RF) counterparts. The practical implementation of hybrid light/radio access networks requires the design of efficient vertical and horizontal handover mechanisms to exploit their complementary features. In this paper, we propose an event-triggered adaptive handover mechanism for centralized hybrid VLC/millimeter wave (MMW) systems. Unlike distributed architectures where all the functionalities are deployed in each access point (AP), the centralized systems allow better optimization of network parameters due to awareness of global network view. The proposed handover mechanism takes advantage of this architecture and uses offset-enhanced signal-to-interference plus noise ratio levels of APs as performance metrics. To ensure balanced distribution of user equipment (UE) numbers between two underlying technologies, it auto-tunes the offset variables using rule-set based algorithms based on the number of connected UEs and data rates as inputs. Our results demonstrate that the proposed handover mechanism significantly improves hybrid system performance via adaptive tuning of AP selection parameter while minimizing signaling overhead.

This paper investigates different precoding strategies for rate splitting multiple access (RSMA) in the downlink of multi-cell visible light communication (VLC) networks. Since classical Shannon capacity formula does not hold for VLC, we first provide a lower bound on the channel capacity for RSMA in such interfering networks. Then, we formulate a spectral efficiency maximization problem to jointly find the optimal rate-splitting and transmit precoding. Beside that, since cell-edge users suffer from additional inter-cell interference, we propose to design the precoders of different RSMA signals utilizing coordinated beamforming (CB). Subsequently, aiming to improve the performance of the CB design for RSMA, while maintain a reduced complexity, we introduce two enhanced precoding strategies for RSMA. To the best of the authors’ knowledge such a contribution has not been considered before in the open literature. It is shown in the paper that the formulated optimization problem is non-convex and a sub-optimal, yet, a low complexity solution can be obtained efficiently using semi-definite relaxation combined with successive convex approximation. Through analytical results, we illustrate the flexibility and superiority of the proposed precoding strategies for RSMA over conventional coordinated space division multiple access and non-orthogonal multiple access for different scenarios and network loads.

Weight reduction and low power consumption are key requirements in the next generation of unmanned aerial vehicle (UAV) networks. Employing modulating retro-reflector (MRR)-based free space optical (FSO) technology is an innovative technique for UAV-to-ground communication in order to reduce the payload weight and power consumption of UAVs which leads to increased maneuverability and flight time of UAV. In this paper, we consider an MRR-based FSO system for UAV-to-ground communication. We will show that the performance of the considered system is very sensitive to tracking errors. Therefore, to assess the benefits of MRR-based UAV deployment for FSO communications, the MRR-based UAV FSO channel is characterized by taking into account tracking system errors along with UAV’s orientation fluctuations, link length, UAV’s height, optical beam divergence angle, effective area of MRR, atmospheric turbulence and optical channel loss in the double-pass channels. To enable effective performance analysis, tractable and closed-form expressions are derived for probability density function of end-to-end signal to noise ratio, outage probability and bit error rate of the considered system under both weak-to-moderate and moderate-to-strong atmospheric turbulence conditions. The accuracy of the analytical expressions is verified by extensive simulations. Analytical results are then used to study the relationship between the optimal system design and tracking system errors.

We study the performance of a network-coded cooperative (NCC) system in an underlay cognitive radio network (CRN). The primary network (PN) consists of a single transmitter-receiver pair, and the secondary network (SN) is an NCC system with$N$users,$M$relays, and a single destination. The relays employ decode-and-forward (DF) protocol and use network coding (NC). We study the performance of the SN under two types of power constraints: i) the combined peak interference power constraint on the PN and maximum transmit power constraint at the SN; and ii) the single peak interference power constraint on the PN. For the SN, an exact closed-form expression and an asymptotically tight end-to-end outage probability are derived, and the diversity order and coding gain are quantified. Compared to the existing literature, the proposed CRN NCC has four main distinguishable features: i) it applies to general CRN NCC network settings with an arbitrary number of users and relays; ii) it considers general relay selection mechanism and independent and non-identically distributed (i.n.i.d.)$Nakagami-m$fading channels; iii) it assumes secondary-to-primary and primary-to-secondary interference links; and iv) it provides a generalization of previous work and includes existing results in the literature as special cases.

Visible light communication (VLC) building on the existing illumination infrastructure utilizes light emitting diode (LED) luminaries as wireless transmitters. To achieve data rates on the order of gigabits per second in VLC systems, multiple-input multiple-output (MIMO) transmission has been identified as a key technology. In this paper, we consider a centralized network architecture where the ceiling LED luminaries act as transmitter elements of a distributed massive MIMO VLC system. To obtain the full benefits of massive MIMO, channel state information must be available at the transmitter side. Downlink (DL) channel estimation in a massive MIMO system requires huge pilot overhead in DL as well as feedback overhead in uplink. This makes the implementation of massive MIMO system infeasible if not properly designed. In our study, we address the design of massive MIMO VLC systems with a limited number of DL pilot signals. We explore different DL pilot patterns (PPs) in frequency, time, and spatial domains where interpolation is performed to obtain the global channel matrix taking advantage of the indoor environment geometry and layout of luminaries. We then present data rate results for different DL PPs under the consideration of varying number of active user equipments through Monte-Carlo simulations.

Relaying is a key technology for millimeter wave communications to extend the range and to route around blockages. In practical implementation of relaying systems, probing is required to identify proper neighbor terminals which will serve as relays. There is however an inherent trade-off between relay probing and required overhead. In this paper, we consider WiGig (IEEE 802.11ad) devices, which are multiband capable with Wi-Fi support, and propose a Wi-Fi assisted relay probing for WiGig device-to-device networks. In the proposed scheme, Wi-Fi received signal strengths are used to pre-select the WiGig relays expected to maximize the spectral efficiency of the overall system. Then, only these pre-selected relays are used in online relay probing step. Simulation analysis demonstrate improvements in throughput and energy consumptions over existing relay probing schemes.

In vehicular visible light communication (VLC), the reliable communication distance may be relatively short due to its dependence on many factors including the weather condition. This necessitates the use of relay-assisted systems to extend the transmission range. In this paper, we investigate experimentally the performance of vehicular multi-hop VLC systems in outdoor environments. Particularly, we first examine the effect of ambient noise on signal-to-noise ratio (SNR) during a whole day. Then, we conduct two multi-hop experiments, i.e., one at daytime and one at nighttime. We measure the bit error rate (BER) for each individual hop and then obtain the overall performance of the multi-hop system. We further compare experimental BER with theoretical BER to confirm our results.

In this article, we propose a cross-layer resource management mechanism for an indoor multiuser visible light communication (VLC) access network. The physical layer builds upon DC-biased orthogonal frequency division multiple access. Channel state information at the physical layer serves as a performance indicator for better data rate opportunity while queue state information at medium access control layer provides the information about packet urgency. A packet based traffic arrival model is considered such that incoming packets with random inter arrival times join a separate queue dedicated to each user if the admission control accepts them, otherwise packets are dropped. We formulate and solve a stochastic cross-layer optimization problem to optimize the network resources under the constraints of queue stability and power to maximize the average system throughput. By using the Lyapunov optimization technique, we convert the long term time-average optimization problem into series of single time slot online problem. Then, we decompose the problem into independent sub problems and propose packet admission, resource (access point and subcarrier) allocation and power control solutions. We present simulation results to demonstrate the superiority of proposed scheme over existing solutions in the literature.

In this paper, we investigate simultaneous lightwave information and power transfer (SLIPT) for underwater visible light communication systems. We consider three SLIPT methods namely time switching (TS), power splitting (PS) and time switching-power splitting (TS-PS) where the splitting/switching factors are defined as optimization parameters. For each of these methods, we derive closed-form expressions for the average harvested energy, bit error rate and spectral efficiency in the presence of underwater turbulence modeled by lognormal statistics. Using these expressions, we determine the optimal splitting factors to maximize the harvested energy while satisfying a given bit error rate value and a given threshold spectral efficiency value. Our results reveal that, if not optimized, SLIPT methods under consideration are outperformed by the simple AC-DC separation (ADS) method which provides the largest harvested energy versus spectral efficiency (HE-SE) region. Optimization of splitting/switching factors extends the HE-SE regions; hence, optimized versions of TS, PS and TS-PS methods are able to significantly outperform ADS for most cases. We further investigate the effect of various channel and system parameters such as water type, turbulence level, beam divergence, receiver aperture size on the harvested energy and quantify the improvements in battery lifetime through the use of SLIPT methods.

Building on the existing illumination infrastructure, visible light communication (VLC) utilizes light emitting diode (LED) luminaries as wireless access points. The presence of large number of ceiling lights in conference halls, classrooms and similar public places makes possible the implementation of massive multiple-input multiple-output (MIMO) VLC systems in a distributed manner. In order to obtain the full benefits of massive MIMO communications, channel state information must be available at transmitter side. Estimation of downlink channel through pilot signals, if not judiciously designed, requires huge pilot overhead and makes the implementation of massive MIMO system infeasible. In our study, we address the design of massive MIMO VLC systems with a limited number of pilot signals. We explore different pilot arrangements in spatial, frequency, and time domains where interpolation is performed to obtain global channel matrix taking advantage of the indoor environment geometry and layout of luminaries. We consider direct current biased orthogonal frequency division multiplexing as the basis of physical layer and present simulation results to demonstrate the achievable spectral efficiencies for pilot signal arrangements under consideration.

Free space optical (FSO) communication systems provide high bandwidth in unregulated spectrum and act as a powerful line-of-sight wireless connectivity solution. The performance of FSO systems can be seriously impaired by fading as a result of atmospheric turbulence and/or pointing errors due to misalignment. In the context of FSO systems, relaying was proposed as an effective fading mitigation technique due to the fact that the variance is distance-dependent in turbulence channels. In this article, we present a unified performance analysis of multi-hop FSO systems over Double Generalized Gamma (DGG) turbulence channels with pointing error impairments. We assume amplify-and-forward relaying and consider both heterodyne detection and intensity modulation with direct detection. We derive tight closed-form expressions for the outage probability and bit error probability of both fixed-gain and channel state information (CSI)-assisted relaying in terms of the bivariate Fox-H functions and Fox-H functions, respectively. We further analyze asymptotic behavior of the outage probability in terms of simple elementary functions and obtain the achievable diversity orders. Diversity gain is found to be a function of atmospheric turbulence parameters, pointing error, detection type and the number of hops. Monte Carlo simulation results are further provided to verify the accuracy of the derived expressions.

Underwater visible light communication (UVLC) has been introduced to support emerging high data rate applications such as real-time image and video transmission. Initial works on UVLC build upon the assumption of fixed turbulence strength through the transmission range which can be justified only for horizontal links. In vertical underwater links, the gradient of temperature and salinity changes with depth. This effectively results in ocean stratification where water with different values of salinity and temperature form non-mixing layers. In this paper, we first model the vertical underwater link as a cascaded fading channel where fading coefficients associated with different layers are modeled as independent and non-identical distributed. Based on the cascaded lognormal and Gamma-Gamma distributions respectively for weak and moderate/strong turbulence conditions, we first derive closed-form expressions for the bit error rate (BER) performance of UVLC systems. Then, we analyze the asymptotic BER performance and determine the diversity orders. In addition, we derive closed-form expressions for the average ergodic capacity of underwater cascaded fading channels under consideration. We present simulation results to confirm the analytical findings.

We consider a network-coded cooperative (NCC) system that consists of N ≥ 2 sources, M ≥ 1 decode- and-forward (DF) relays, and a single destination. The relays perform network coding (NC) on the received sources' symbols using maximum distance separable (MDS) codes. For this system, we propose the most generalized user-relay selection (GURS) scheme in the literature that selects any arbitrary subsets of K users and any arbitrary subsets of L relays subject to practical constraints such as load balancing conditions and scheduling policy. Our analytical results and design guidelines generalize and subsume all existing results as special cases. To this end, we derive a new closed-form outage probability (OP) expression, assuming non-identically and independently distributed (n.i.i.d.) Rayleigh fading channels. The asymptotic outage expression at high signal-to-noise ratio (SNR) regime is further derived, based on which, the achievable diversity order and coding gain are quantified. The theoretical derivations are also validated through Monte-Carlo simulation.

Visible light communication (VLC) has the potential to serve as a high-speed underwater wireless connectivity solution to support real-time image and video transmission. Underwater propagation medium imposes additional challenges on the design of VLC systems which were originally proposed for indoor applications. Temperature and salinity fluctuations result in fluctuations of the refractive index of seawater and eventually introduce turbulence-induced fading. In this paper, to mitigate the effects of fading, we propose transmit laser selection for a diver-to-diver underwater VLC link where the transmitter has multiple laser sources and the receiver has one photodetector. The source with the highest received instantaneous signal-to-noise ratio is selected for transmission while the remaining sources remain idle. We derive a closed-form expression for asymptotical bit error rate over log-normal distributed underwater turbulence channels and derive the achievable diversity order. We further present simulation results to confirm our derivations and diversity gain analysis.

Visible light communication (VLC) has emerged as a high-capacity connectivity solution for underwater sensor networks. Since water is relatively transparent to blue or green light, visible light lasers or LEDs can be used as transmitters for underwater wireless connectivity with data rates up to hundreds of Mbps. In underwater networks, a critical system design issue is the network lifetime which highly depends on the battery capacity. Since recharging in underwater scenarios is typically very costly and impractical, energy harvesting can be considered as a promising alternative. In this paper, we explore simultaneous lightwave information and power transfer (SLIPT) for VLC-based USNs. We adopt time splitting method where the receiver switches in time between the modes of energy harvesting (EH) and information decoding (ID). We derive a closed-form expression for the average harvested energy over log-normal model underwater turbulence channel. Using this expression, we determine the splitting factor between EH and ID operation modes to maximize the harvested energy while satisfying a given bit error rate value.

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.

In line with the increasing number of high speed train (HST) commuters and their demand for mobile data, railways operators expore means of providing high speed onboard wireless access. The data from all train cars are sent to a gateway terminal mounted at the roof of the train and an efficient backhaul solution is required to connect the train to the nearest base station. In this paper, we propose the use of unmanned aerial vehicles (UAVs) equipped with free space optical (FSO) communication terminal to provide backhauling for HSTs. We present a composite fading model for UAV-to-HST channel where both atmospheric turbulence and pointing errors due to position/orientation deviation are considered. Based on the derived fading model, we present closed form expressions for the bit error and outage probability. We further investigate the effect of several parameters such as beam divergence angle, displacement deviation variance, coverage distance and UAV operation altitude on system performance.

Free space optical (FSO) communication systems provide line-of-sight wireless connectivity in the unlicensed optical spectrum and achieve higher data rates in comparison to their radio frequency (RF) counterparts. FSO systems are particularly attractive for the last mile access problem by establishing a connectivity bridge between optical fiber-based backbone and RF access networks. To address this practical deployment scenario, there has been an increasing attention on the so-called mixed (dual-hop) FSO-RF systems where FSO transmission is utilized at one hop followed by RF transmission at the other one. In this paper, we investigate the performance of mixed FSO-RF transmission systems where the FSO link is modeled by the double generalized Gamma distribution with generalized pointing error impairments and the RF link experiences the extended generalized-K shadowed fading. For both amplify-and-forward and decode-and-forward relaying, we derive closed-form expressions for outage probability, bit error probability, and ergodic capacity in terms of bivariate Fox-H function. Monte Carlo simulation results are provided to verify the accuracy of derived expressions. We further provide an asymptotic analysis and discuss the achievable diversity orders of mixed FSO-RF systems.

In this paper, we investigate the performance limits of underwater visible light communication (UVLC) systems. We first develop a closed-form path loss expression as a function of transceiver parameters and water type. We then utilize this new expression to determine the maximum achievable link distance for UVLC systems in pure sea, clear ocean, coastal water, and harbor water. Our results demonstrate that the maximum achievable distance is limited to a few tens of meters. This necessitates the deployment of relay-assisted UVLC systems to extend the transmission range. We consider both detect-and-forward and amplify-and-forward relaying. For each relaying method, we first consider a dual-hop UVLC system and determine optimal relay placement to minimize the bit error rate (BER). Then, we consider a multi-hop system with equidistant relays and determine the maximum achievable distance for a given number of hops to satisfy a targeted end-to-end BER.

In this paper, we investigate the performance analysis of multi-hop free space optical (FSO) systems over Double Generalized Gamma (DGG) turbulence fading with pointing error impairments assuming both heterodyne detection and intensity modulation with direct detection (IMlDD). We derive closed-form expressions for the outage probability in terms of the Fox-H function. We further analyze asymptotical signal-to-noise-ratio (SNR) behavior of the outage probability and obtain the achievable diversity orders. Monte-Carlo simulation results are provided to verify the accuracy of the derived expressions.

In Intelligent Transportation Systems, visible light communication (VLC) has emerged as a powerful candidate to enable wireless connectivity in vehicle-to-vehicle (V2V) and vehicle-to- infrastructure (V2I) links. While VLC has been studied intensively in the context of indoor communications, its application to vehicular networking is relatively new. In this paper, we carry out a comprehensive channel modeling study to quantify the effect of rain and fog on a V2V link with a high-beam headlamp acting as the transmitter. Taking advantage of advanced ray tracing features, we first develop a path loss model for V2V link as a function of distance under different weather conditions. Then, we use this expression to determine the maximum achievable distance to ensure a given bit error rate. We further investigate the deployment of relay- assisted systems to extend transmission ranges. Extensive numerical results are presented to corroborate our findings.

In this paper, we propose a multiple-input multiple-output (MIMO) uplink transmission scheme for optical wireless communication applications. The transmission is based on optical single-carrier frequency domain equalization (SC-FDE) due to its low complexity where the signal is transmitted over infrared communication channels. Based on non-sequential ray tracing, we first obtained realistic infrared MIMO channel impulse responses including low-pass filter effect of infrared light-emitting-diodes. We then investigate the performance of bit-error-rate (BER) and peak to average power ratio (PAPR) with respect to different modulation orders using spatial multiplexing.

Estimation of the time-varying optical wireless channel response is crucial in order to decode received signals coherently. In this work, we investigate symbol-error-rate and mean absolute error performance of different interpolation techniques including linear, nearest, spline, and piece-wise cubic Hermite interpolating polynomial (pchip), which are used in pilot-aided channel estimation process for visible light communication. The performance of interpolators is evaluated in realistic time-varying channel model, generated on Zemax software and compared with each other under the consideration of different modulation orders, different pilot symbol periods, and different user equipment (UE) speeds through Monte Carlo simulations. The results reveal that spline and pchip techniques are more robust to low pilot symbol transmission rate and fast time-varying channel conditions as a consequence of high UE speeds. However, low complex linear interpolation technique can be chosen for highly rated pilot signal transmission cases or when optical channel varies slowly over the time.

Network coded cooperative (NCC) systems have recently gained increasing attention due to their high power and spectral efficiency. For high speed data transmission over frequency-selective channels, NCC is further combined with orthogonal frequency division multiple access (OFDMA). In this paper, we derive outage probability and diversity gain of NCC-OFDMA systems in the presence of carrier frequency offset (CFO). Our results demonstrate that NCC-OFDMA system is capable to achieve full spatial and frequency diversity in noise limited system where the CFO variance σ2ε→ 0. However, in interference limited case where the CFO variance σ2ε> 0, error floor occurs leading zero diversity gain. Numerical results are further presented to validate our analytical findings.

Strong atmospheric turbulence can highly affect the free space optical (FSO) link. In fact, the strength of turbulence is depending on weather factors where the temperature is considered as the main parameter increasing these turbulences. In this work, an experimental investigation of the effect of Qatar's harsh climate on FSO link in term of packet ratio delivery (PDR) is carried out. Two FSO transceivers, operating June at 1550 nm for 600 m link distance, have been used. Based on Bendersky, Kopeika and Blaunstein (BKB) model and on the measured weather factors we tried to characterise the strength of turbulences under Qatar climate.

Multicarrier transmission in the form of orthogonal frequency division multiplexing (OFDM) is an efficient method to handle inter‐symbol interference resulting from the frequency selectivity of the underwater acoustic (UWA) channels. OFDM can be further combined with cooperative transmission to benefit from the spatial diversity advantages and improve the performance over conventional point‐to‐point UWA communication systems. In this study, the authors consider a cooperative OFDM UWA communication system and investigate the outage performance for both amplify‐and‐forward relaying and decode‐and‐forward (DF) relaying. They derive expressions for bounds on outage probability and outage capacity. Monte Carlo simulations corroborate and quantify the enhanced performance of cooperative OFDM UWA communication compared with direct transmission systems. Through numerical simulations, they investigate the impact of geometry on outage probability and signal‐to‐noise ratio requirements in cooperative OFDM UWA communication with DF relaying. Furthermore, the authors analyse the effects of relay location and carrier frequency on the outage probability for both relaying schemes.

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

Ultraviolet (UV) communication enables non-line-of-sight (NLOS) outdoor wireless connectivity and is particularly desirable to relax or eliminate pointing and tracking requirements of infrared links. In comparison to infrared counterparts, UV links are subject to relatively higher path loss as well as intersymbol interference resulting from frequency selective nature of the channel. In this paper, we propose the powerful combination of relay-assisted transmission and multi-carrier architecture based on orthogonal frequency division multiplexing (OFDM). Specifically, we consider a cooperative diversity system with orthogonal cooperation protocol and use DC-biased optical OFDM as the underlying physical layer. We consider both amplify-and-forward (AF) and detect-and-forward (DF) relaying. We investigate the error rate performance of the proposed relay-assisted OFDM UV system under consideration and demonstrate performance gains over point-to-point OFDM UV systems. We further determine optimal power allocation schemes to improve the performance. We also propose a variable-rate UV OFDM system and improve system throughput via bit loading.

Network-coded cooperation (NCC) is an effective method to improve the throughput efficiency in cooperative wireless networks. The combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been further studied in the literature to exploit the multipath diversity gains. In this paper, we consider orthogonal frequency division multiple access (OFDMA), an extension of OFDM to a multiuser system where subsets of carriers are assigned to different users. We first derive a closed-form expression for the outage probability of NCC-OFDMA over Rician fading channels and then present the asymptotical and finite-SNR DMT expressions. Our results provide insight into the performance mechanisms under practical SNR regime of NCC-OFDMA systems and demonstrate that NCC-OFDMA is able to fully exploit both frequency and spatial diversity. We also show that the derived finite-SNR DMT converges to an asymptotical one as expected. Furthermore, special cases for our derived analytical expressions are presented, to show that the derived expression is a generalized case of the related the state of the art results. Simulation results are further presented to verify our theoretical analyses.

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

Network coded cooperative (NCC) systems have recently attracted attention with their high spectral and power efficiency. In this paper, we investigate the information theoretical limits of NCC systems in the presence of imperfect channel state information (CSI). Specifically, we derive exact outage probability and finite-SNR diversity- multiplexing tradeoff (DMT) for general multisource multirelay NCC wireless networks over Rayleigh fading channels. Numerical results are presented to verify our analytical derivations and provide insight into the impact of imperfect CSI on the NCC system performance.

Vehicular hotspots for on-board Internet access using Long Term Evolution (LTE) as the backhaul network has recently gained popularity. Currently, Wi-Fi is the most common technology to provide in-vehicle access, where data has been relayed through on board LTE receiver. Despite its wide acceptance, coexistence and contention based data rate limitations with Wi-Fi necessitates alternatives for in-vehicle data access schemes. This paper investigates the performance of hybrid LTE and visible light communication (VLC) networks using LTE as the backhaul and VLC as the on-board access network.Under the consideration of vehicle interior unique channel characteristics and light emitting diode (LED) deployment flexibility, best transmitter configuration using repetition coding (RC) and spatial multiplexing (SM) multiple input multiple output (MIMO)modes is determined. Proposed configurations based on direct current biased optical orthogonal frequency-division multiplexing(DCO-OFDM) are compared with respect to their bit-error-rate (BER) performances. Furthermore, the performance of intravehicular VLC networks for single and multi-user scenarios is investigated.

Visible light communication (VLC) has recently gained popularity as a complementary technology to radio frequency (RF) based alternatives for vehicular communications as a low-cost, secure and RF interference free technology. In this paper, we propose IEEE 802.15.7 standard-compliant physical layer (PHY) implementation and experimental evaluation, using commercial off-the-shelf (COTS) automotive light emitting diode (LED) fog light for the purpose of low-latency safety message dissemination. We first show that the standard is applicable to line of sight (LoS) vehicle-to-vehicle (V2V) VLC. We then demonstrate that the proper selection of modulation coding schemes (MCS) plays an important role in order to minimize bit-error- rate (BER) for the reliable transmission with varying inter-vehicle distances. We also addressed the angular limitations of COTS automotive LED light for viable vehicular VLC.

Strong molecular and aerosol scattering in ultraviolet (UV) wavelengths enable the non-line-of-sight (NLOS) outdoor wireless connectivity which is attractive in many applications due to covertness and security. In the current literature, atmospheric turbulence effects are typically ignored under the assumption of short link distances and clear weather conditions. In this paper, we investigate the performance of multiple-input multiple-output (MIMO) UV systems over turbulence channels. Specifically, we derive bit error rate (BER) expressions over an NLOS UV channel which is modeled as an overlap of two log-normal LOS links. Our results demonstrate that fading variance is scaled by the number of transmitters and receivers and MIMO deployment with proper configurations results in significant BER improvements. Simulation results are further provided to confirm the analytical findings.

Visible light communication (VLC) involves the dual use of illumination infrastructure for high speed wireless access. Designing such optical based communication systems, realistic indoor optical channel modeling becomes an important issue to be handled. In this paper, first we obtain new realistic indoor VL channel characterizations and models, in a multiple-input multiple-output (MIMO) transmission scenario, using non-sequential ray tracing approach for the channel impulse responses (CIRs). Practical issues such as number of light emitting diode (LED) chips per luminary, spacing between LED chips, objects inside the room and cabling topology are also investigated. On the other hand, since indoor optical channels exhibit frequency selectivity, multi-carrier communication systems, particularly orthogonal frequency division multiplexing (OFDM) is used to handle the resulting inter-symbol interference in VLC systems. Hence, we propose a new MIMO-OFDM based VLC system, called MIMO enhanced unipolar OFDM (MIMO-eU-OFDM) by combining MIMO transmission techniques with the recently proposed eU-OFDM scheme. The bit error rate (BER) performance of the proposed system is investigated in the presence of the 2 × 2 and 4 × 4 realistic MIMO VLC channels and its BER performance is compared with the reference optical MIMO-OFDM systems.

In this study, the authors investigate the information theoretical limits on the performance of single‐carrier cooperative underwater acoustic communication systems in the presence of intersymbol interference. The authors assume decode‐and‐forward relaying and consider orthogonal half‐duplex cooperation. Under the assumptions of sparse and frequency‐selective Rician fading channel and non‐white correlated Gaussian ambient noise, the authors derive expressions for bounds on the individual and achievable rates for both cases where channel state information is available at the transmitter or not. Using these expressions, the authors optimise input signalling and relay location to maximise average achievable rates.

Although there has been a growing interest on the study of diversity-multiplexing tradeoff (DMT), the existing works are mostly restricted to the outcomes reported for Rayleigh, Rician, and Nakagami fading channels. In this paper, we investigate the optimal tradeoff in the presence of log-normal fading, which provides an accurate model for indoor wireless, optical wireless, and ultra-wideband channels. We consider a multiple-input multiple-output (MIMO) log-normal channel, derive the outage probability expression, and then present the asymptotical DMT expression. It is shown that the asymptotical maximum diversity gain tends to infinity with logarithm of signal-to-noise ratio (SNR). To have further insight, we normalize the DMT with that of single-input single-output case and express the relative asymptotical DMT of the MIMO log-normal channel as a function of the number of transmit and receive antennas. We further study DMT for finite SNRs and demonstrate convergence to the asymptotical case. Extensive numerical results are provided to corroborate the analytical derivation.

In this paper, we consider a cascaded radio frequency (RF) and hybrid RF/free space optical (FSO) system where several mobile users transmit their data over an RF link to a decode-and-forward relay node (e.g., a small cell base station) and the relay forwards the information to a destination (e.g., a macro-cell base station) over a hybrid RF/FSO backhaul link. The relay and the destination employ multiple antennas for transmission and reception over the RF links while each mobile user has a single antenna. The RF links are orthogonal to the FSO link but half-duplex with respect to each other, i.e., either the user-relay RF link or the relay-destination RF link is active. For this communication setup, we derive the optimal fixed and adaptive link allocation policies for sharing the transmission time between the RF links based on the statistical and instantaneous channel state information (CSI) of the RF and FSO links, respectively. Thereby, we consider the following two scenarios depending on the delay requirements: 1) delay-limited transmission where the relay has to immediately forward the packets received from the users to the destination, and 2) delay-tolerant transmission where the relay is allowed to store the packets received from the users in its buffer and forward them to the destination when the quality of the relay-destination RF link is favorable. Our numerical results illustrate the effectiveness of the proposed communication architecture and link allocation policies, and their superiority compared to existing schemes, which employ only one type of backhaul link.

Free space optical (FSO) links provide an efficient point-to-point wireless transmission solution. They are particularly useful for filling the connectivity gap between the radio frequency (RF) access network and the fiber optic-based backbone network. To address this combined use, there has been a growing interest in so-called mixed RF/FSO systems where RF transmission is used at one hop and FSO transmission at the other in a dual-hop configuration. In this paper, we investigate the performance of mixed RF/FSO systems along with a direct RF link where the FSO link and RF links, respectively, experience double generalized gamma turbulence with pointing error and Nakagami-m fading. Moreover, the effect of cochannel interference is considered at both relay and destination. Under the assumption of amplify-and-forward relaying and selection combining at the destination, we derive closed form expressions for outage probability and bit error probability in terms of Meijer's-G and two-variate Fox-H functions. Our analysis provides a generalized framework for several existing results, since our adopted turbulence and fading statistical models are the most general ones. Based on the high signal-to-noise-ratio analysis, we further provide a diversity gain analysis and discuss the bottlenecks in the mixed RF/FSO systems.

Network coded cooperation (NCC) is an effective method to improve the throughput efficiency in cooperative wireless networks. In an effort to have further gains over the initial works, which build upon the assumption of time division multiple access, the combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been proposed in the literature. In this paper, we consider orthogonal frequency division multiple access (OFDMA), an extension of the OFDM to a multiuser system where subsets of carriers are assigned to different users. We derive a closed-form expression for the outage probability of the system under consideration and present the diversity-multiplexing tradeoff (DMT) analysis. Our results demonstrate that NCC-OFDMA system is able to fully exploit both frequency and spatial diversity. Simulation results are presented to verify our theoretical analysis.

A major performance degrading factor in free space optical communication (FSO) systems is atmospheric turbulence. Spatial diversity techniques provide a promising approach to mitigate turbulence-induced fading. In this paper, we study the error rate performance of FSO links with spatial diversity over atmospheric turbulence channels described by the Double Generalized Gamma distribution which is a new generic statistical model covering all turbulence conditions. We assume intensity modulation/direct detection with on-off keying and present the BER performance of single-input multiple-output (SIMO), multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) FSO systems over this new channel model.

Optical wireless communication (OWC) systems provide many advantages over radio frequency (RF) wireless technologies in some scenarios, including significantly higher data rates and a large amount of available license-free frequency spectrum. Recently, OWC has also been proposed in fifth generation 5G standard as a tool to augment capacity due to RF spectrum crunching challenges. Various forms of OWC can be used to augment RF capacity at both access and backhaul/fronthaul levels. Despite the major advantages and various application areas, the widespread deployment of OWC is delayed by several challenges, such as the demand to maintain strict line-of-sight alignment between transmitter and receiver in long range outdoor applications; the need to combat attenuation due to adverse weather conditions such as fog, cloud, and turbulence; to modulate light emitting diodes at high frequencies without distortion at indoor visible light communication (VLC) applications and retaining power levels within the eye safety limits for laser transmitters and comfortable illumination levels for LED transmitters.

New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such technology that could help address the problem would be optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC, and optical scattering communications.

In this paper, we investigate the information theoretic performance of parallel relaying in free-space optical (FSO) communications over Gamma-Gamma fading channels. We consider an intensity modulation/direct-detection (IM/DD) FSO system with a single relay and a line-of-sight link between the source and the destination. Considering both amplify-and-forward and decode-and-forward relaying, we develop performance characterizations of each relaying mode in terms of outage probability, diversity gain, coding gain, and diversity-multiplexing tradeoff (DMT). Our results demonstrate that parallel relaying improves the performance of FSO systems by bringing diversity advantages. In addition, both relaying modes achieve the same DMT and diversity gain while their coding gains can be different depending on underlying channels' characteristics.

In this study, we investigate a relay-assisted visible light communication (VLC) system where an intermediate light source cooperates with the main light source. Specifically, we consider two light sources in an office space: one is the information source employed on the ceiling and the other one is a task light. Our system uses dc biased optical orthogonal frequency division multiplexing (DCO-OFDM) where the task light performs relaying to assist the communication and operates in half-duplex mode for both amplify-and-forward and decode-and-forward relaying. We investigate the bit error rate (BER) performance of the relay-assisted VLC system and optimize the performance through optimal ac/dc power allocation under illumination constraints. The dc power allocation is determined by sharing the number of LED chips between the terminals to satisfy a desired luminance ratio. The ac power allocation decides the fraction of the information signal power to be consumed at the terminals in order to minimize the BER. Numerical results reveal that cooperation brings significant performance gains over direct transmission. We further provide comparisons of two relaying techniques under consideration and discuss the effect of clipping noise on the error rate performance.

This paper presents a comprehensive study on the problem of decoding complexity in a Multi-Input Multi-Output (MIMO) Hybrid Automatic Repeat reQuest (HARQ) system based on Space-Time Block Codes (STBCs). We show that there exist two classes of fast-decodable MIMO HARQ systems: independent and dependent STBC structures. For the independent class, two types of protocols namely, fixed and adaptive threshold-based are presented and their effectiveness in both computational complexity reduction and spectral efficiency preservation are discussed. For the dependent class, a fast Sphere Decoder (SD) algorithm with a low computational complexity is proposed for decoding process of HARQ rounds. Two new concepts are introduced to leverage the fast-decodable notion in MIMO HARQ systems. Simulation results show that the proposed fast-decodable MIMO HARQ protocols in both classes provide a significant reduction in the decoding complexity as compared with the original MIMO HARQ method.

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

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

In this study, the authors investigate the information theoretical limits on the performance of point‐to‐point single‐carrier acoustic systems over frequency‐selective underwater channels with intersymbol interference. Under the assumptions of sparse and frequency‐selective Rician fading channel and non‐white correlated Gaussian ambient noise, the authors derive an expression for channel capacity and demonstrate the dependency on channel parameters such as the number, location and power delay profile of significant taps, as well as environmental parameters such as distance, temperature, salinity, pressure and depth. Then, the authors use this expression to determine the optimal carrier frequency, input signalling and bandwidth for capacity maximisation.

In this paper, we revisit the concept of fractionally spaced equalization (FSE) for broadband single-carrier amplify-and-forward (AaF) cooperative systems. Particularly, we investigate fractionally spaced frequency domain equalization (FS-FDE) for cooperative multi-relay systems. Our motivation stems from the elegant properties reported for the FSE in the point-to-point communication systems (i.e., its robustness to sampling phases and potential in achieving the optimum performance) and the scalability of the FDEs. In particular, we propose a TS/2-spaced equalizer that transforms the temporal sample sequence of the received signal to the frequency domain, applies linear/decision-feedback equalization, and returns the resulting signal back to the time domain for detection. The vital importance of using FS-FDE method in cooperative systems is disclosed in practical scenarios where the transmitted signals have nonzero roll-off components and sampling phase error may occur in relay(s) and destination terminals. Our results demonstrate that, under specific channel realizations and sampling errors, the cooperative systems with symbol spaced FDE (SS-FDE) fail to harvest the available cooperative diversity and the performance approaches to that of no relay scenario. On the other hand, the performance of cooperative system with FS-FDE method becomes independent of sampling phase errors and full benefit of cooperation is retained.

In this paper, we study network-coded cooperative diversity (NCCD) systems comprising multiple sources, one relay, and one destination, where the relay detects the packets received from all sources and performs Galois field (GF) network coding over rm GF(2m) before forwarding a single packet to the destination. Assuming independent Rayleigh fading for all links of the network, we derive simple and accurate closed-form approximations for the asymptotic symbol and bit error rates of NCCD systems. The derived error rate expressions are valid for arbitrary numbers of sources, arbitrary modulation schemes, and arbitrary constellation mappings and provide significant insight into the impact of various system and channel parameters on performance. Moreover, these expressions can be exploited for optimization of the constellation mapping as well as for formulation of various NCCD system optimization problems including optimal power allocation, relay selection, and relay placement.

In this paper, we investigate spectrum trading via auction approach for exclusive usage spectrum access model in cognitive radio networks. We consider a realistic valuation function which depends on channel capacity, delay sensitive and delay-insensitive data traffics of secondary users (SUs), and propose an efficient concurrent Vickrey-Clarke-Grove (VCG) mechanism for non-identical channel allocation among r-minded bidders in two different cases. In the first case, the SUs are r-minded but they can submit bid only for single channels. In the second case, the SUs are r-minded and they can submit bid for bundles of channels. We show that the first case is solvable in polynomial time but in the other one, the problem of determining auction outcomes is NP-hard. We propose two sub-optimal methods for solving this problem, namely greedy algorithm and randomized rounding linear programming (LP) relaxation algorithm. Due to the sub-optimal nature of solutions in the second case, VCG mechanism is not truthful anymore and the SUs can lie to maximize their utilities. To address this, we propose an auction mechanism with limited truthfulness property based on an iterative greedy algorithm.

In this paper, we consider a multi-hop vehicular network in which the transmitted signal propagates through sequential vehicle hops to arrive to the destination vehicle. We assume doubly-selective channels and employ precoding to take advantage of the underlying temporal and multipath diversities. Our performance analysis through pairwise error probability derivation shows that, via proper precoding, the precoded system is able to extract maximum available diversity in time (through Doppler diversity) and frequency (through multipath diversity) dimensions. Numerical results are further provided to confirm the analytical derivations and provide insight into the error rate performance.

This paper is concerned with a challenging problem of channel estimation for amplify-and-forward cooperative relay based orthogonal frequency division multiplexing (OFDM) systems in the presence of sparse underwater acoustic channels and of the correlative non-Gaussian noise. We exploit the sparse structure of the channel impulse response to improve the performance of the channel estimation algorithm, due to the reduced number of taps to be estimated. The resulting novel algorithm initially estimates the overall sparse channel taps from the source to the destination as well as their locations using the matching pursuit (MP) approach. The correlated non-Gaussian effective noise is modeled as a Gaussian mixture. Based on the Gaussian mixture model, an efficient and low complexity algorithm is developed based on the combinations of the MP and the space-alternating generalized expectation-maximization (SAGE) technique, to improve the estimates of the channel taps and their location as well as the noise distribution parameters in an iterative way. The proposed SAGE algorithm is designed in such a way that, by choosing the admissible hidden data properly on which the SAGE algorithm relies, a subset of parameters is updated for analytical tractability and the remaining parameters for faster convergence Computer simulations show that underwater acoustic (UWA) channel is estimated very effectively and the proposed algorithm has excellent symbol error rate and channel estimation performance.

In this paper, we address the pricing problem in open-access femtocell networks. We use economic and game theoretic approaches such as market equilibrium and non-cooperative game to propose novel pricing schemes. In our proposed solutions, the per unit price of spectrum can be determined dynamically and mobile service providers can gain more revenue than the fixed pricing scheme. Furthermore, in our solutions, femtocell owners have more incentives and satisfaction to justify their participation in open-access use.

In this paper, we investigate the performance of cooperative diversity transmission in free-space optical (FSO) communications. We consider an FSO cooperative diversity system with a single decode-and-forward relay and a line-of sight link between the source and the destination. Focusing on high signal-to-noise ratio (SNR) regime, we develop performance characterizations in terms of outage probability, outage diversity and coding gains, and diversity-multiplexing tradeoff (DMT). Our results demonstrate that cooperative diversity improves the performance of FSO communications by bringing diversity advantages.

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 consider the problem of wideband spectrum sensing by using the correlation among the observation samples in different subbands. The Primary User (PU) signal samples in occupied subbands are assumed to be zero-mean correlated Gaussian random variables and additive noise is modeled as colored zero-mean Gaussian random variables independent of the PU signal. It is also assumed that there is at least a minimum given number of subbands that are vacant of PU signals. First we derive the optimal detector and the Generalized Likelihood Ratio (GLR) detector for the case that the covariance matrix of PUs signal samples is unknown and the noise variance in the different subbands is known. Then, we propose an iterative algorithm for GLR test when both the covariance matrix of the PUs signal samples and the noise variances in the different subbands, are unknown. For analytical performance evaluation, we derive some closed-form expressions for detection and false alarm probabilities of the proposed detectors in low Signal to Noise Ratio (SNR) regime. The simulation results are further presented to compare the performance of the proposed detectors.

In this study, the authors introduce an adaptive bit loading scheme for an amplify-and-forward orthogonal frequency division multiple cooperative system with relay selection. The proposed bit loading scheme maximises the throughput for a target error rate taking into account the quality of imperfect channel estimation. It relies on estimated channel state information (CSI) at the destination node and partial CSI (which involves the number of loaded bits) at the source node. The authors provide extensive Monte Carlo simulation results to demonstrate the throughput and the no-transmission probability performance of the proposed scheme and discuss the effect of various system and channel parameters on the performance.

Cooperative communication has been recently applied to vehicular networks to enable coverage extension and enhance link reliability through distributed spatial diversity. In this study, the authors investigate the performance of a cooperative vehicular network over a doubly selective (i.e. frequency-selective and time-selective) fading channel. Under the assumption of amplify-and-forward relaying with orthogonal cooperation protocol, the authors derive a pairwise error probability (PEP) expression and demonstrate the achievable diversity gains. The authors’ results demonstrate that, through proper linear constellation precoding, the cooperative vehicular scheme is able to extract the maximum available diversity in frequency (through multipath diversity), time (through Doppler diversity) and spatial (through cooperative diversity) dimensions. The effect of imperfect channel state information is also studied through an asymptotical PEP analysis. The authors further conduct Monte-Carlo simulations to confirm the analytical derivations and present the error rate performance of the vehicular scheme under various mobility conditions and scenarios.

Automatic retransmission request (ARQ) is a feedback-based data link layer technique which enhances the reliability of communication in fading channels. In this paper, we investigate the performance of hybrid-ARQ (H-ARQ) techniques in coherent free-space optical (FSO) communication systems over atmospheric turbulence-induced fading channels. Under the assumption of a Gamma-Gamma statistical fading channel model, we derive outage probability and throughput expressions for three H-ARQ protocols. We further characterize the outage performance at high values of signal-to-noise-ratio (SNR) through diversity and coding gains. Our results provide insight into the performance mechanisms of different H-ARQ protocols in coherent FSO systems and demonstrate that significant performance gains can be achieved through the deployment of H-ARQ particularly in the strong turbulence regime.

In this paper, we propose an adaptive power loading algorithm for an OFDM (orthogonal frequency division multiplexing)-based multi-relay regenerative network with relay selection. The proposed scheme is based on selecting the "best" relay(s) according to different strategies and distributing the power adaptively among the subcarriers of source and selected relay(s) as to minimize an upper bound on the bit error rate (BER). Our Monte Carlo simulation results demonstrate superior performance gains through power loading with respect to conventional equal power loading schemes. The effects of imperfect channel estimation and relay location on the BER performance are further discussed.

Atmospheric turbulence is a major degrading factor in free-space optical (FSO) systems. Since turbulence-induced fading results in a very slowly-varying channel, reliable feedback is possible allowing the implementation of adaptive transmission in practical FSO systems. In this paper, we propose adaptive transmission algorithms in which transmit power and/or modulation size are varied according to the channel conditions. The design of adaptive algorithms is formulated as optimization problems of spectral efficiency and average power consumption at a targeted value of outage probability under peak power constraints. The proposed adaptive schemes bring significant improvements over their non-adaptive counterparts.

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.

In this letter, we propose a novel enhanced maximal-ratio combining (E-MRC) scheme for multi-branch decode-and-forward (DF) cooperative diversity systems consisting of a source, a destination, and multiple relays. This scheme employs adjustable parameters at the relays and the destination which enable this scheme to appropriately adapt to variations in the source-relay and relay-destination channel gains. For the proposed E-MRC scheme, we analyze the asymptotic error rate performance for high signal-to-noise ratios and Rayleigh fading. Based on the obtained analytical expressions, we find the optimal parameters that lead to minimization of the asymptotic error rate. Furthermore, we compare the error rate performance as well as the feedback overhead of E-MRC with those of important DF schemes, namely, cooperative MRC (C-MRC) and two DF schemes based on the link adaptive regeneration (LAR) concept, namely, LAR-αinstand LAR-α. We show that although the feedback requirements of E-MRC are similar to those of LAR-α, E-MRC achieves a performance similar to that of LAR-αinst, which has a substantially higher feedback overhead.

In this paper, the performance of multiple-bit delay detection (MBDD) systems in optical differential phase shift keying (DPSK) is investigated. Assuming that the dominant noise in the system is originated by the optical amplifier and using Gaussian approximation, we present a closed form equation which predicts the overall bit-error-rate (BER) performance of the system as a function of the number of delay segments, optical signal-to-noise ratio (OSNR) at the receiver, and bandwidth of the optical bandpass filter. It is shown that with a large number of delay segments, the performance of MBDD system approaches to the performance of homodyne coherent detection.

Initial works on cooperative vehicular communications build upon the assumption of frequency-flat and quasi-static fading channels. This can be justified only for narrowband systems in very slow traffic flows such as in rush-hours. In this paper, we consider doubly-selective (i.e., time- and frequency-selective) vehicular channels and investigate multipath-Doppler diversity for a cooperative vehicular system to extract the underlying rich diversity. Our performance analysis through pairwise error probability derivation shows that, through proper precoding, the proposed system is able to extract maximum available diversity in time, frequency and space. Monte Carlo simulations are further presented to confirm the analytical derivations and corroborate on the analytical results.

In this paper, we investigate the performance of automatic retransmission request (ARQ) protocols in coherent free-space optical (FSO) communications. Specifically, we study the outage performance of three different Hybrid-ARQ (H-ARQ) protocols and characterize their performance at large values of signal-to-noise ratio (SNR) through diversity and coding gains. Our results demonstrate that significant performance gains can be achieved by the deployment of H-ARQ in coherent FSO communications especially in strong turbulence regime. Our asymptotic outage analysis further demonstrates that all protocols provide the same diversity order (which is the product of the maximum number of ARQ rounds and the minimum of the channel parameters) while they provide different coding gains.

Cooperative communication is considered as a major enabling technique to meet the demanding performance requirements of future underwater acoustic (UWA) communication systems. In this paper, we investigate the information theoretic performance of a cooperative UWA system to provide insight into the fundamental performance limits imposed by underwater acoustic channels. We first propose an approximate statistical model for the non-stationary ambient noise. The proposed model allows mathematical tractability and is a good fit for most operating frequencies in practical systems. Based on this ambient noise model, we investigate the outage performance of a precoded cooperative multicarrier UWA system with amplify-and-forward (AF) relaying. Our numerical results for cooperative UWA systems demonstrate significant performance gains over point-to-point systems. The impact of several system and environmental parameters on the performance is further demonstrated.

In this paper, we consider infrastructure-to-vehicle cooperative communications in which roadside access points use vehicles as relaying terminals. This can be particularly useful in suburban or remote areas where the frequent deployment of roadside access points is not either possible or cost-effective. For the doubly-selective vehicular channel under consideration, we employ a precoded cooperative transmission technique to extract the underlying rich multipath-Doppler-spatial diversity. Under the assumption of decode-and-forward relaying, we derive a pairwise error probability expression and demonstrate the achievable diversity gains. We further provide Monte Carlo simulations to confirm the analytical derivations and provide insight into the error rate performance of infrastructure-to-vehicle cooperative communications.

High-frequency (HF) radio communication has been recognized as the primary means for long-range wireless communications since the advent of radio. With its unique features, HF communication continues to be used for a wide range of civilian, government and military applications as a powerful alternative to a myriad of more sophisticated communication systems. However demanding requirements of high-speed data communications impose new requirements on the HF system design and innovative approaches are required. In this paper, building upon the promising concept of cooperative transmission, we investigate the performance of a multicarrier coded HF system with amplify-and-forward relaying. Specifically, we consider orthogonal frequency division multiplexing (OFDM) with bit-interleaved coded modulation (BICM) and demonstrate the achievable diversity through the derivation of pairwise error probability. We also conduct Monte Carlo simulations to confirm the analytical derivations and present performance comparisons.

In this paper, we investigate spectrum trading via auction approach for cognitive radio networks. We consider a realistic valuation function in terms of different parameters for secondary users (SUs), and propose an efficient concurrent Vickrey-Clarke-Grove mechanism for non-identical channel allocation in two different scenarios. In the first scenario, SUs can bid for a single channel while in the other one, SUs can bid for a bundle of two channels. Our numerical results demonstrate significant revenue increase for the auctioneer and bidders in comparison with the conventional auction mechanisms.

In this study, the authors investigate the error rate performance of amplify-and-forward relaying over N*Nakagami fading channels. This is a recently introduced channel model that involves the product of N Nakagami-m-distributed random variables. Employing the moment generating function approach, the authors derive symbol error rate expressions for a single-relay system under instantaneous power scaling (IPS) and average power scaling (APS) factors at the relay node, that is, variable and fixed gains. The results achieved by the authors demonstrate that the achievable diversity order is a function of Nakagami fading parameter (m) and degree of cascading (N). An identical diversity order is obtained under both scaling factors when the relay is close to the destination. When the relay is close to the source, IPS becomes advantageous over APS. Monte–Carlo simulations are further provided to confirm the analytical results.

In this letter, we investigate cooperative diversity with relay selection over cascaded Rayleigh fading channels. In particular, we analyze the performance of a relay selection scheme for cooperative vehicular networks with the decode-and-forward (DF) protocol. Only the “best” relay, which satisfies an index of merit, is selected. We ignore the direct transmission between the source (S) and its destination (D), and assume that the destination has perfect knowledge of theS→RandR→Dchannel gains. We study the performance of the underlying scheme in terms of outage probability and investigate its achievable diversity order.

In this paper, we investigate multi-hop relaying as an efficient fading mitigation tool for coherent free-space optical (FSO) systems over atmospheric turbulence channels. We consider an FSO relaying system with decode-and-forward relay nodes and multiple heterodyne receivers with modal compensation. Based on a recently introduced statistical characterization for the combined effects of log-normal turbulence fading and modal compensation, we derive the outage probability and quantify the potential performance improvements through the derivation of diversity-multiplexing tradeoff (DMT) and diversity gain. Our outage analysis yields impressive power savings for multi-hop relaying even with a single-relay. In addition, the DMT analysis in practical signal-to-noise ratio (SNR) ranges demonstrates that multi-hop transmission scheme improves finite-SNR diversity gain throughout the range of the multiplexing gain.

In this paper, we investigate bit and power allocation strategies for an orthogonal frequency division multiplexing (OFDM) cooperative network over frequency-selective fading channels. We assume amplify-and-forward relaying and consider the bit error rate (BER) performance as our performance measure. Aiming to optimize the BER under total power constraint and for a given average data rate, we propose three adaptive algorithms; optimal power loading (OPL), optimal bit loading (OBL), and optimal joint bit and power loading (OBPL). Our Monte Carlo simulation results demonstrate performance gains through adaptive bit and power loading over conventional non-adaptive systems as well as currently available adaptive cooperative scheme in the literature. The impact of practical issues on the performance of proposed adaptive schemes such as imperfect channel estimation and limited feedback is further discussed.

In this paper, we propose high-rate distributed space-time-frequency codes (DSTFCs) to exploit maximum achievable diversity gains over frequency-selective fading channels. The proposed designs achieve full-rate for any number of cooperative nodes, and allow channel variations over multiple OFDM blocks within one DSTFC codeword. We analyze diversity gains of DSTFCs through both conditional and average pairwise error probability (PEP), and we proposes better design criteria based on one-side channel conditional PEP. We show that the difference between the frequency-selective channel orders of source-to-relay and relay-to-destination links may provide extra diversity advantages, thus additional performance gains. Through Monte-Carlo simulations, we demonstrate that proposed high-rate DSTFCs provide notable diversity advantages over existing designs.

In this paper, we study network-coded cooperative diversity (NCCD) systems comprising multiple sources, one relay, and one destination. The relay detects the packets received from all sources and performs Galois field network coding. We propose a simple cooperative maximum-ratio combining scheme for the destination which is shown to achieve the maximum diversity gain of the system. Furthermore, we provide a mathematical framework for the asymptotic analysis of NCCD systems with M-ary modulation for high signal-to-noise ratios. Based on this framework, we derive simple and elegant closed- form expressions for the asymptotic symbol and bit error rates which provide significant insight into the impact of various system and channel parameters on performance and can be exploited for performance optimization. Simulation results confirm the accuracy of the presented analysis and show that large performance gains are possible by optimizing the power allocation in NCCD systems based on the developed analytical results.

In this paper, we investigate the performance of a cooperative network with adaptive decode-and-forward (DF) relaying over time-selective frequency-flat Rayleigh fading channels. In adaptive DF relaying, only a subset of the available relays with "good" channels are allowed to participate in the relaying phase. The destination combines the direct and the relayed signals using maximal ratio combining technique. Pilot-symbol-assisted modulation (PSAM) is used for the estimation of time-varying fading channel coefficients. For the system under consideration, we derive an exact closed-form expression for the average bit error rate. Such closed form solutions are highly desirable because they allow for rapid and efficient evaluation of system performance. We further present computer simulations to validate our analytical results.

In this paper, we investigate the effect of channel estimation error on the error rate performance of a multi-relay system with amplify-and-forward relaying. We consider orthogonal relaying and study both conventional cooperative systems (i.e., all relays participate in the relaying phase) and opportunistic cooperative systems (i.e., only the best relay participates in the relaying phase). Based on the derivation of effective signal-to-noise ratio at the receiver taking into account channel estimation error, we obtain closed-form expressions for bit error rate. Monte-Carlo simulations are further provided to confirm the analytical results.

In this paper, we investigate the performance of constant-power adaptive L-ary quadrature modulation (L-QAM) for an opportunistic decode-and-forward (DF) relaying scheme. Specifically, we adopt adaptive five-mode L-QAM with fixed and optimum switching and derive expressions for the error rate, outage probability and spectral efficiency over both independent and identical (i.i.d.) and independent and non-identical (i.n.d.) Rayleigh fading channels.

The authors investigate the performance of amplify-and-forward relaying with two different pilot-symbol-assisted channel estimation methods. In the first estimation method, the cascaded channel consisting of source-to-relay and relay-to-destination links is estimated at the destination terminal. No channel estimator is required at the relay terminal. In the second estimation method, the estimation of cascaded channel is disintegrated into separate estimations of source-to-relay and relay-to-destination links which are carried out at the relay and the destination terminals, respectively. The latter method involves feed-forwarding a quantised version of the source-to-relay channel estimate to the destination terminal. Through the derivation of mean squared error (MSE) diversity gain analysis and Monte-Carlo simulations, the authors investigate the performance of amplify-and-forward relaying with these channel estimation methods. Our results demonstrate that full diversity can be achieved in the presence of channel estimation. The authors further show that cascaded channel estimator outperforms its competitor with a small number of quantisation bits. As the number of employed quantisation bits increase, disintegrated channel estimator approaches to its competitor eventually outperforming it.

We investigate detection methods for on-off keying (OOK) photon-counting Free Space Optical (FSO) systems in the presence of turbulence-induced fading, assuming no channel state information at the receiver. To recover the performance loss which is associated with symbol-by-symbol detection in such a scenario, we consider sequence detection techniques, exploiting the temporal correlation of the FSO channel. Due to its high complexity in the calculation of its metric, optimal maximum likelihood sequence detection (MLSD) is infeasible for most practical purposes. Hence, we propose a suboptimal low-complexity detection rule, which is based on the generalized maximum-likelihood sequence estimation. The proposed scheme allows the detection of sequence lengths that are prohibitive for conventional MLSD, without using any kind of channel knowledge. Monte Carlo simulation results show its performance to be very close to the optimum for large sequence lengths and various fading models.

We consider power allocation (PA) and relay positioning in a dual-hop decode-and-forward relaying system over Nakagami-m fading channels. We investigate adaptive power allocation (PA) with fixed relay location, optimal relay location with fixed power allocation and joint optimization of the PA and relay location under transmit power constraint in order to minimize outage probability and average error probability upper bound. Results show that the analyzed adaptive algorithms outperform uniform algorithms and direct transmission. It is also found that as the source destination channel is getting better, the less power will be needed to allocate to the relay node in order to satisfy the optimization criteria. Numerical results are presented to verify our analysis.

In this paper, we investigate amplify-and-forward (AF) incremental-best-relay cooperative diversity to make efficient use of the channel spectrum by exploiting a limited feedback from the destination terminal, e.g., a single bit indicating the success or failure of the direct transmission. If the destination provides a negative acknowledgment via feedback; in this case only, the best relay among M available relays retransmits the source signal in an attempt to exploit spatial diversity by combining the signals received at the destination from the source and the best relay. Closed-form expressions for the bit error rate, the outage probability and average channel capacity are determined over independent non-identical Rayleigh fading channels. Results show that the AF incremental-best-relay cooperative diversity can achieve the maximum possible diversity order, compared with the conventional cooperative-diversity networks, with higher channel utilization. In particular, the AF incremental-best-relay technique can achieve M + 1 diversity order at low signal-to noise ratio (SNR) and and exhibits a 20 to 30 dB gain relative to direct transmission, assuming single-antenna terminals at high SNR.

We investigate the incremental relaying protocol in conjunction with relay-selection cooperative diversity networks with an aim to make efficient use of the degrees of freedom of the channels by exploiting a limited feedback signal from the destination. In particular, whenever the direct link from the source to the destination is not favorable to decoding, the destination will request the help from the best relay. The performance of the incremental-relay-selection decode-and-forward technique over independent non-identical Rayleigh fading channels is derived in terms of average bit error probability, outage probability and average channel capacity. The analytic results show that the system assisted by the selection relaying can achieve full diversity at low SNR regime and exhibits a 20 to 30 dB gain relative to direct transmission, assuming single-antenna terminals.

A major performance-limiting factor in terrestrial optical wireless (OW) systems is turbulence-induced fading. Exploiting the additional degrees of freedom in the spatial dimension, multiple laser transmitters combined with multiple receiver apertures provide an effective solution for fading mitigation. Although MIMO (multiple-input multiple-output) OW systems have been extensively studied in recent years, most of these works are mainly limited to symbol-by-symbol decoding. Maximum likelihood sequence detection (MLSD) exploits the temporal correlation of turbulence-induced fading and promises further performance gains. In this paper, we investigate MLSD for IM/DD (intensity-modulation/direct-detection) MIMO OW systems over log-normal atmospheric turbulence channels. Even with a low-order modulation scheme such as on-off keying which is typically used in OW systems, the complexity of MLSD might be prohibitive. We therefore present an iterative sequence detector based on the expectation-maximization (EM) algorithm. The complexity of the proposed algorithm is much smaller than a direct evaluation of the log-likelihood function. The Monte-Carlo simulation results demonstrate that the EM-based algorithm outperforms the symbol-by-symbol decoder and achieves a performance which lies within 0.5 dB of that of the optimal MLSD.

In this paper, we investigate distributed differential space-time block coding (STBC) for cooperative communications over frequency-selective fading channels. We carefully exploit the unitary structure of the orthogonal STBC to design a low complexity differential STBC receiver for multi-carrier broadband cooperative networks. We consider amplify-and-forward relaying and assume a single-relay scenario. Under the assumption of perfect power control for the relay terminal and high signal-to- noise ratio for the underlying links, our performance analysis demonstrates that the considered scheme is able to exploit fully the spatial diversity. We further present Monte Carlo simulation results to confirm our analytical observations.

Although there already exists a rich literature on cooperative diversity, current results are mainly restricted to the conventional assumption of additive white Gaussian noise (AWGN). AWGN model realistically represents the thermal noise at the receiver, but ignores the impulsive nature of atmospheric noise, electromagnetic interference, or man-made noise which might be dominant in many practical applications. In this paper, we investigate the performance of cooperative communication over Rayleigh fading channels in the presence of impulsive noise modeled by Middleton Class A noise. Specifically, we consider a multi-relay network with amplify-and-forward relaying. Through the derivations of the pairwise error probability, we quantify the diversity advantages. Based on the minimization of a union bound on the error rate performance, we formulate optimal power allocation schemes and demonstrate significant performance gains over their counterparts with equal power allocation. An extensive Monte Carlo simulation is also presented to illustrate the performance of cooperative schemes in various impulsive environments.

In this paper, we investigate the performance of a single-relay cooperative scenario where the source, relay, and destination terminals are equipped with multiple transmit/receive antennas. We particularly focus on the so-called blind amplify- and-forward relaying in which the availability of channel state information at the relay terminal is not required. Through the derivation of pairwise error probability, we quantify analytically the impact of multiple antenna deployment assuming various scenarios which involve relay location and power allocation assumptions imposed on the cooperating nodes.

Over the last two decades, wireless communications has gained enormous popularity, offering attractive options for many personal and organizational communication needs due to major intrinsic characteristics such as flexibility, cost effectiveness, and mobility.

Although there already exists a rich literature on cooperative diversity, current results are mainly restricted to the conventional assumption of additive white Gaussian noise (AWGN). AWGN model realistically represents the thermal noise at the receiver, but ignores the impulsive nature of atmospheric noise, electromagnetic interference, or man-made noise which might be dominant in many practical applications. In this paper, we investigate the performance of cooperative communication over Rayleigh fading channels in the presence of impulsive noise modeled by Middleton Class A noise. Specifically, we consider a multi-relay network with amplify-and-forward relaying. Through the derivations of pairwise error probability, we quantify the diversity advantages. Based on the minimization of a union bound on the error rate performance, we formulate optimal power allocation schemes and demonstrate significant performance gains over their counterparts with equal power allocation. An extensive Monte Carlo simulation is also presented to illustrate the performance of cooperative schemes in various impulsive environments.

In this paper, we investigate a cooperative system with multiple relays and amplify-and-forward relaying over frequency-selective channels. To extract the available multipath diversity, we employ orthogonal frequency division multiplexing (OFDM) with precoding. Through the derivation of pairwise error probability (PEP), we demonstrate that PEP is not a simple exponential function of the signal-to-noise ratio (SNR), but it includes a term that involves some power of logarithm of the SNR. If that term is ignored, the diversity order is given by the summation of the channel length in the direct link and the minimum of channel lengths in each relaying link confirmed by simulation results. Based on the PEP expression, we also propose two relay selection strategies; one is on a per-subcarrier basis and the other is on an all-subcarrier basis. Our simulation results indicate that both strategies result in performance improvements although the per-subcarrier method performs better.

In this letter, we consider a multi-relay network operating in decode-and-forward mode. We propose a novel relay selection method with a low implementation complexity. Unlike the competing schemes, it requires neither error detection methods at relay nodes nor feedback information at the source. We derive a closed-form symbol error rate (SER) expression for multi-relay network under consideration and demonstrate that the proposed selection method is able to extract the full diversity. Extensive Monte Carlo simulations are also presented to confirm the derived SER expressions and to compare the performance of the proposed scheme with its competitors.

Cooperative communication techniques promise the advantages of multi-input multi-output (MIMO) communications for wireless scenarios with single-antenna terminals. A main assumption in majority of the research work on cooperative communications is the availability of channel state information at the receiver. In practice, knowledge of the channel is obtained by sending known training (pilot) symbols to the receiver. In this paper, we study the effect of training on the performance of an amplify-and-forward cooperative relaying system with pilot-assisted channel estimator over quasi-static Rayleigh fading channels. We consider average received signal-to-noise ratio at the destination node as the objective function and formulate optimization problems for a single-relay scenario under total network power (TNP) and individual node power (INP) constraints. We aim to answer the following fundamental questions: Q1) How should overall transmit power be shared between training and data transmission periods?; Q2) How should training power be allocated to broadcasting and relaying phases?; Q3) How should data power be allocated to broadcasting and relaying phases? Our simulation results demonstrate that optimized schemes significantly outperform the original schemes with equal power allocation. Depending on the relay location, performance gains up to 5.5 dB and 2.8 dB are observed, respectively, under TNP and INP constraints.

In this paper, we investigate the error rate performance of amplify-and-forward (AF) relaying with imperfect channel estimation. We consider a single-relay scenario with orthogonal and non-orthogonal AF (OAF and NAF) cooperative protocols. Two pilot-symbol-assisted receiver architectures are studied: in the mismatched-coherent receiver, the complex fading channel coefficients (i.e., both phase and amplitude) are estimated based on a linear minimum-mean-squared-error estimation approach and fed to a coherent sub-optimal maximum likelihood decoder as if the channels were perfectly known. In the partially-coherent receiver, channel amplitude is ignored and phase is estimated by a phase locked loop. For both receiver types, we analyze the achievable diversity orders for cooperative protocols under consideration and quantify the impact of channel estimation through the derivation of pairwise error probability. Our performance analysis reveals that a second order diversity order is obtained for the considered single-relay scenario indicating that full diversity is extracted. Our simulation results demonstrate that the performance degradation due to channel estimation with respect to the genie bound (i.e., perfect channel state information) is as small as 1.1 dB based on the employed detector. Performance results further show that partially-coherent receiver presents a similar performance to mismatched-receiver for sufficiently large loop SNRs although channel amplitude is completely ignored.

In this letter, we investigate the effect of imperfect channel estimation on the performance of distributed space-time block codes (DSTBCs) with amplify-and-forward relaying. Exploiting the orthogonality of the underlying code, we derive a maximum likelihood metric conditioned on the channel estimate acquired through the insertion of pilot symbols. For a large number of pilot symbols, we demonstrate that the proposed decoding rule coincides with the so-called mismatched receiver. On the other hand, as the number of pilot symbols decreases, the proposed decoder converges to a non-coherent detector. Through Monte-Carlo simulations, we further demonstrate that the performance of the proposed scheme lies within 0.8 dB of the genie receiver performance bound.

Optical wireless, also known as free-space optics, has received much attention in recent years as a cost-effective, license-free and wide-bandwidth access technique for high data rates applications. The performance of free-space optical (FSO) communication, however, severely suffers from turbulence-induced fading caused by atmospheric conditions. Multiple laser transmitters and/or receivers can be placed at both ends to mitigate the turbulence fading and exploit the advantages of spatial diversity. Spatial diversity is particularly crucial for strong turbulence channels in which single-input single-output (SISO) link performs extremely poor. Atmospheric-induced strong turbulence fading in outdoor FSO systems can be modeled as a multiplicative random process which follows the K distribution. In this paper, we investigate the error rate performance of FSO systems for K-distributed atmospheric turbulence channels and discuss potential advantages of spatial diversity deployments at the transmitter and/or receiver. We further present efficient approximated closed-form expressions for the average bit-error rate (BER) of single-input multiple-output (SIMO) FSO systems. These analytical tools are reliable alternatives to time-consuming Monte Carlo simulation of FSO systems where BER targets as low as 10-9are typically aimed to achieve.

Free-space optical (FSO) communication has received much attention in recent years as a cost-effective, license-free and wide-bandwidth access technique for high data rates applications. The performance of FSO communication, however, severely suffers from turbulence-induced fading caused by atmospheric conditions. Multiple laser transmitters and/or receivers can be placed at both ends to mitigate the turbulence fading and exploit the advantages of spatial diversity. Spatial diversity is particularly crucial for strong turbulence channels in which single-input single-output (SISO) link performs extremely poor. Atmospheric-induced strong turbulence fading in outdoor FSO systems can be modeled as a multiplicative random process which follows the K distribution. In this paper, we investigate the error rate performance of FSO systems for K-distributed atmospheric turbulence channels and potential advantages of spatial diversity deployments at the transmitter and/or receiver. Our results demonstrate significant diversity gains of multiple transmitter/receivers deployment in FSO channels. We further present efficient approximated closed-form expressions for the average bit-error rate (BER) of multiple-input single-output (MISO) and single-input multiple-output (SIMO) FSO systems. These analytical tools are reliable alternatives to time-consuming Monte Carlo simulation of FSO systems where BER targets as low as 10-9are typically aimed to achieve.

Cooperative communication techniques promise the advantages of MIMO (multi-input multi-output) communications for wireless scenarios with single-antenna terminals. A main assumption in majority of the existing literature on cooperative communications is the availability of channel state information at the receiver. In practice, knowledge of the channel is obtained by sending known training (pilot) symbols to the receiver. In this paper, we study the effect of training on the system performance for an amplify-and-forward relaying cooperative system with pilot-assisted channel estimator over quasi-static Rayleigh fading channels. Considering average received signal- to-noise ratio at the destination node as the objective function, we formulate an optimization problem for a single-relay scenario to answer the following fundamental questions: 1) How should the overall transmit power be shared between training and data transmission periods?; 2) How should training power be allocated to broadcasting and relaying phases?; 3) How should data power be allocated to broadcasting and relaying phases? Our simulation results demonstrate that optimized scheme significantly outperforms the original scheme with equal power allocation. Depending on the relay location, performance gains up to 5.5 dB are reported.

Cooperative communication techniques promise the advantages of MIMO (multi-input multi-output) communications for wireless scenarios with single-antenna terminals. In this paper, we investigate the performance of a vehicle-to-vehicle cooperative scheme where another vehicle in the vicinity of the source vehicle acts as a relay. The underlying source-to-relay, relay-to-destination, and source-to-destination links are modeled as cascaded (double) Rayleigh fading. This statistical model provides a realistic description of inter-vehicular channel where two or more independent Rayleigh fading processes are assumed to be generated by independent groups of scatterers around the two mobile terminals. We derive a pairwise error probability (PEP) expression for the inter-vehicular cooperative scheme under consideration and show that the full distributed spatial diversity is extracted. Based on the derived PEP expressions, we obtain union bounds on the bit error rate performance which are then minimized to optimally allocate power between broadcasting and relaying phases. Optimum power allocation brings performance gains up to 3dB depending on the relay location and deployed modulation scheme.

Although there has been a growing literature on cooperative diversity, the current literature is mainly limited to Rayleigh fading channel model which typically assumes a wireless communication scenario with a stationary base station antenna above roof-top level and a mobile station at street level. In this paper, we investigate cooperative diversity for inter-vehicular communication based on cascaded Rayleigh fading. This channel model provides a realistic description of inter-vehicular channel where two or more independent Rayleigh fading processes are assumed to be generated by independent groups of scatterers around the two mobile terminals. We investigate the performance of amplify-and-forward relaying for an inter-vehicular cooperative scheme assisted by a road-side access point which acts as a relay. Our diversity analysis reveals that the cooperative scheme is able to extract the full distributed spatial diversity. We further formulate a power allocation problem for the considered scheme to optimize the power allocated to broadcasting and relaying phases. Performance gains up to 2.8 dB are observed through optimum power allocation depending on the relay location.

In this paper, we investigate the error rate performance of amplify-and-forward (AF) relaying with imperfect channel estimation. We consider a single-relay scenario with orthogonal and non-orthogonal AF cooperative protocols assuming mismatched-coherent and partially-coherent receivers. For both receiver types, we analyze the achievable diversity orders for cooperative protocols under consideration and quantify the impact of channel estimation through the derivation of pairwise error probability. Extensive Monte Carlo simulations are further presented to collaborate on the analytical results.

The authors present and analyse a predictive closed-loop power control (CLPC) scheme, which employs a comb-type sample arrangement to effectively compensate multiple power control group delays over mobile fading channels. The authors consider both least squares and recursive least squares filters in our CLPC scheme. The effects of channel estimation error, prediction filter error and power control bit transmission error on the performance of the proposed CLPC method along with competing non-predictive and predictive CLPC schemes are thoroughly investigated. The results clearly indicate the superiority of the proposed scheme with its improved robustness under non-ideal conditions. Furthermore, the authors carry out a Monte-Carlo simulation study of a 5×5 square grid cellular network and evaluate the user capacity. Capacity improvements up to 90% are observed for a typical cellular network scenario.

Optimum power allocation is a key technique to realize the full potentials of relay-assisted transmission promised by the recent information-theoretic results. In this paper, we present a comprehensive framework for power allocation problem in a single-relay scenario taking into account the effect of relay location. In particular, we aim to answer the two fundamental questions: (Q1) How should the overall transmit power be shared between broadcasting and relaying phases?; (Q2) In the relaying phase, how much power should be allocated to relay-to- destination and source-to-destination links? The power allocation problem is formulated to minimize a union bound on the bit error rate (BER) performance assuming amplify-and-forward relaying. We consider both orthogonal and non-orthogonal cooperation protocols. Optimized protocols demonstrate significant performance gains over their original versions which assume equal sharing of overall transmit power between the source and relay terminals as well as between broadcasting and relaying phases. It is observed that optimized virtual (distributed) antenna configurations are able to demonstrate a BER performance as close as 0.4 dB within their counterpart co-located antenna configurations.

In this paper, we investigate the performance of a single-relay cooperative scenario where the source, relay and destination terminals are equipped with multiple transmit/receive antennas. We assume that conventional space-time block codes are employed in the underlying source-to-destination (SrarrD), source-to-relay (S rarr R) and relay-to-destination (R rarr D) links, and consider both decode-and-forward (DaF) and amplify - and-forward (AaF) relaying techniques. For the latter one, we consider two variants based on the availability of channel state information (CSI); namely, blind AaF and CSI-assisted AaF. Through the derivation of pairwise error probability, we quantify analytically the impact of multiple antenna deployment for each relaying technique under various scenarios which involve relay location and power control assumptions imposed on cooperating nodes. Our transmission model assumes that the source and destination terminals are equipped with MStransmit and N receive antennas, respectively, and the relay terminal is equipped with MRreceive and MTtransmit antennas. For a scenario where R rarr D and S rarr D links are balanced and S rarr R link experiences sufficiently large SNR, our performance analysis demonstrates that the maximum achievable diversity order is MTmin(MS, N)+MSN for blind AaF scheme and N(MT+MS) for both CSI-assisted AaF and DaF schemes. For another scenario where R rarr D link has a sufficiently large SNR and S rarr R and S rarr D links are balanced, CSI-assisted AaF, blind AaF and DaF schemes achieve diversity orders of MS(N + MR), MS(N + MT), and MSN, respectively. Other scenarios involving the availability of non-fading R rarr D link and poor inter-user channel quality are further investigated. An extensive Monte Carlo simulation study is also presented to corroborate the analytical results and to provide detailed performance comparisons among the three relaying techniques under consideration.

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.

Optimum power allocation is a key technique to realize the full potentials of relay-assisted transmission promised by the recent information-theoretic results. In this paper, we present a comprehensive framework for power allocation problem in a single-relay scenario taking into account the effect of relay location. In particular, we aim to answer the two fundamental questions: Q1) How should the overall transmit power be shared between broadcasting and relaying phases?; Q2) In the relaying phase, how much power should be allocated to relay-to-destination and source-to-destination links? The power allocation problem is formulated to minimize a union bound on the bit error rate (BER) performance assuming amplify-and-forward (AaF) relaying. We 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. Optimized protocols demonstrate significant performance gains over their original versions which assume equal sharing of overall transmit power between the source and relay terminals as well as between broadcasting and relaying phases. It is observed that optimized virtual (distributed) antenna configurations are able to demonstrate a BER performance as close as 0.4 dB within their counterpart co-located antenna configurations.

Cooperative diversity is a transmission technique where multiple nodes in a network cooperate to form a virtual antenna array realizing the benefits of spatial diversity in a distributed fashion. The coherent scenario considered in most existing work on cooperative diversity assumes the availability of perfect channel state information at the relay and destination terminals and is highly unrealistic in practical applications. In this paper, we investigate non-coherent and mismatched-coherent receivers for a cooperative diversity scheme assuming both quasi-static and time-varying fading channels for the underlying cooperative links. Specifically, we consider a distributed space- time block coded (STBC) system in a single-relay scenario operating in the amplify-and-forward relaying mode. Exploiting the orthogonal structure of distributed STBC, we first derive a non-coherent decoding rule which can be implemented in practice by a Viterbi-type algorithm. Although this decoding rule has been derived assuming quasi-static channels,- its inherent channel tracking capability allows its deployment over time- varying channels with a promising performance as a sub-optimal solution. As a possible alternative to non-coherent detection, we investigate the performance of mismatched-coherent receiver (i.e., coherent detection with imperfect channel estimation) within the considered relay-assisted transmission scenario. We further compare the performance of non-coherent and mismatched- coherent receivers to reveal their robustness under various mobility scenarios.

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel.

In this paper, we investigate time-domain equalization for distributed space-time codes in a relay-assisted transmission scenario over frequency-selective fading channels. Specifically, we consider the so-called time-reversal space-time block coding (TR-STBC) technique which has been of particular interest with its low computational complexity. We assume the special case of a single-relay where the source-to-relay (S → R), relay-to-destination (R → D), and source-to-destination (S → D) links experience possibly different channel delay spreads. Under the assumption of perfect power control for the relay terminal and high signal-to-noise ratio for the underlying links, our performace analysis demonstrates that distributed TR-STBC is able to achieve a maximum diversity order of min (L1, L3) +L2 + 2 where L1, L2, and L3 are the channel memory lengths for S → R, S → D, and R → D links, respectively. This illustrates that the smaller of the multipath diversity orders experienced in S → R and R → D links becomes the performance bottleneck for the relaying path. An extensive Monte Carlo simulation study is presented to corroborate the analytical results and to provide detailed performance comparisons with competing schemes.

In this paper, we investigate orthogonal frequency division multiplexing (OFDM) technique for distributed space-time block coding (D-STBC) in a relay-assisted transmission scenario over frequency-selective fading channels. We consider the special case of a single-relay where the source-to-relay (S - R), relay-to-destination (R - D) and source-to-destination (S - D) links experience possibly different channel delay spreads. Our analysis demonstrates that uncoded distributed OFDM-STBC (D-OFDM-STBC) scheme achieves a maximum diversity order of two for the considered single-relay scenario since it is able to exploit only the spatial diversity, but not the available rich multipath diversity. We further consider a combination of D-OFDM-STBC and Trellis Coded Modulation (TCM) with frequency-interleaving. Under the assumption of perfect power control for the relay terminal and high signal-to-noise ratio for the underlying links, our performance analysis demonstrates that D-OFDM-STBC scheme with judiciously designed outer TCM codes, i.e. sufficiently large effective code length (ECL), is able to achieve a maximum diversity order of min(L1, L3) + L2 + 2 where L1, L2, and L3 are the channel memory lengths for S - R, S - D and R - D links, respectively. This illustrates that the smaller of the multipath diversity orders experienced in S - R and R - D links becomes the performance bottleneck for the relaying path. An extensive Monte Carlo simulation study is presented to corroborate the analytical results and to provide further insights into the performance.

In this paper, we investigate single carrier frequency-domain equalization (SC-FDE) for distributed space-time block codes (D-STBC) in a relay-assisted transmission scenario over frequency-selective fading channels. We assume the special case of a single-relay where the source-to-relay (S rarr R), relay-to-destination (R rarr D), and source-to-destination (S rarr D) links experience possibly different channel delay spreads. Assuming perfect power control between R rarr D and S rarr D links and high signal-to-noise ratio for all underlying links, our performance analysis demonstrates that SC-FDE for D-STBC is able to achieve a maximum diversity order of min(L1, L3)+L2+2 where L1, L2and L3are the channel memory lengths for S rarr R, S rarr D, and R rarr D links, respectively. This illustrates that the smaller of the multipath diversity orders experienced in S rarr R and R rarr D links becomes the performance bottleneck for the relaying path. For the special case of a non-fading relaying path where line-of-sight propagation is possible in either one of these underlying links, we demonstrate that the maximum diversity orders of L1+L2+2 and L3+L2+2 are achievable assuming nonfading R rarr D and S rarr R links, respectively. An extensive Monte Carlo simulation study is presented to corroborate the analytical results and to provide detailed performance comparisons among the competing schemes

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. In this paper, we derive error performance bounds for coded FSO communication systems operating over temporally correlated lognormal turbulence fading channels. Our analytical derivations build upon an approximation to the sum of correlated lognormal random variables. The derived bounds are tighter than the previously published results and are valid for a wider range of turbulence conditions. Simulation results are further demonstrated to confirm the analytical results

Cascaded Rayleigh distribution is used to model multipath fading in mobile-to-mobile communication scenarios and provides a better fit to experimental data in such scenarios compared to the conventional Rayleigh channel model. In this letter, we derive an exact expression for the pairwise error probability (PEP) of space-time trellis codes over the cascaded Rayleigh fading channel, which is in the form of a simple single finite-range integral. Through the derived PEP expression, we present the maximum diversity order achievable over such channels and demonstrate the performance degradation in comparison to conventional Rayleigh channels. Monte-Carlo simulations are further demonstrated to confirm the analytical results

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. In this paper, we derive error performance bounds for coded FSO communication systems operating over atmospheric turbulence channels, considering the recently introduced gamma-gamma turbulence model. We derive a pairwise error probability (PEP) expression and then apply the transfer function technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to confirm the analytical results.

In this paper, we derive closed-form expressions for the bit error rate (BER) for a distributed space-time block coded system, considering both perfect and imperfect channel estimation. For the special case of single relay, under the assumption of an appropriate power control, we show that the diversity order of two is obtained, achieving the maximum diversity order. Our analysis also demonstrates the existence of an error floor due to channel estimation errors.

We study the asymptotic behavior of a cooperative scheme operating in amplify-and-forward (AF) mode when the number of relay terminals tends to infinity. An optimal code design rule is derived through the investigation of a pairwise error probability (PEP) expression. The design rule is based on an Euclidean-distance-type metric including some scaling terms due to different path loss and shadowing effects associated with underlying relay links. Since the design criterion is channel-dependent, it does not allow a practical code design. However, we further demonstrate that "classical" Euclidean distance can be used as a sub-optimal solution for realizing cooperative diversity advantages which we further verify through our simulation study. Our simulation results also indicate that distributed space-time codes based on Euclidean distance yield good results even for a small number of relays.

We investigate time- and frequency-domain equalization techniques for quasi-orthogonal space-time block codes. The proposed schemes extend the original symbol-level scheme developed for flat-fading channels to a block-level implementation either in the time or frequency domain. Receiver structures which exploit the embedded quasi-orthogonality, therefore keeping the equalizer complexity at a manageable level, are presented. The considered schemes are applied to the EDGE TDMA system and their performances are compared under both perfect and estimated channel conditions.

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. In this paper, error performance bounds for coded FSO communication systems operating over atmospheric turbulence channels is derived, considering the recently introduced gamma-gamma turbulence model. Also a pairwise error probability (PEP) expression is derived and then applies the transfer function technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to confirm the analytical results.

We analyze the error rate performance of coded wireless optical links operating over atmospheric channels, where the turbulence-induced fading is modeled by the negative exponential distribution, K distribution and I-K distribution. First, we derive bounds on the pairwise error probability (PEP) for each fading model and then apply the transfer function technique in conjunction with derived PEP bounds to obtain bit error rate performance. Simulation results are also included to confirm the analytical results.

We analyze the performance of space-time trellis codes over shadowed Rician fading channels. The shadowed Rician channel is a generalization of the Rician model, where the line-of-sight path is subjected to a lognormal transformation due to foliage attenuation, also referred to as shadowing. Using the moment generating function method, we derive an exact expression for the pairwise error probability (PEP) of space-time trellis coded systems operating over this channel. The asymptotic analysis of PEP shows that the design criteria of space-time trellis codes proposed for Rayleigh fading still hold when used over shadowed Rician channels. We also present simulation results for bit-error rate performance under various degrees of shadowing.

We present analytical performance results for space-time trellis codes over spatially correlated Rayleigh fading channels. Bit-error-probability estimates are obtained based on the derivation of an exact pairwise error probability expression using a residue technique combined with a characteristic function approach. We investigate both quasi-static and interleaved channels and demonstrate how the spatial fading correlation affects the performance of space-time codes over these two different channel models. Simulation results are also included to confirm the accuracy of analytical estimates.

This work investigates the achievable information rate with the state-of-the-art turbo coding and intensity modulation/direct detection for outdoor long-distance free-space optic (FSO) communications. The channel under weak atmospheric turbulence is modeled as a log-normal intensity fading channel where on-off keying makes it look asymmetric. While no effort is made to spectrally match the code to the asymmetry of the channel, the decoding strategy is optimally adjusted to match to the channel response. In addition to fixed rate turbo coding, a family of variable rate turbo codes are constructed and discussed. Shannon capacity is also briefly visited to denote the theoretic limit. It is shown that under low turbulence a single long turbo code is sufficient to get within 1 dB from the capacity, but when the turbulence gets strong, adaptive coding is necessary to close the gap. We expect these results to be useful for current and immediate future systems.

We investigate maximum-likelihood (ML) sequence estimation for space-time block coded systems without assuming channel knowledge. The quadratic form of the ML receiver in this case does not readily lend itself to efficient implementation. However, under quasi-static channel conditions, the likelihood function reduces to a simple form similar to the classical correlation receiver in matrix notation. It also allows the development of a recursive expression that can be easily implemented by a Viterbi-type algorithm with a reasonable complexity. Although the receiver is suboptimum for the nonstatic case, its performance is close to the optimum for a range of signal-to-noise ratios.

We introduce a space-time block-coded orthogonal frequency-division multiplexing (MC-OFDM) scheme for frequency-selective fading channels which does not require channel knowledge either at the transmitter or at the receiver. The decoding algorithm is based on generalized maximum-likelihood sequence estimation. Due to the assumed orthogonality structure of STBC, the decoding rule reduces to a single step. Its form also allows the derivation of a recursive expression, which can be easily implemented by a Viterbi-type algorithm. We investigate the performance of the proposed scheme over two-tap Rayleigh fading channels. Simulation results show the performance of the proposed recursive-type receiver to be near optimum.

In this paper, analytical performance results for space-time trellis codes over Rayleigh fading channels are presented. Bit error probability estimates are obtained, based on the derivation of an exact pairwise error probability expression through a residue technique combined with characteristic function approach. We investigate both quasi-static and interleaved channels as well as the effect of spatial fading correlation on the performance of space-time codes in both channels. Simulation results are also included to confirm the accuracy of analytical estimates.

Most current space-time block codes are designed based on an orthogonality principle. By relaxing the orthogonality requirement, it is possible to construct new codes with higher throughput efficiency. In this paper, new non-orthogonal space-time block codes for three transmit antennas are proposed, achieving throughput rates larger than those of currently known orthogonal designs. The proposed codes are found through a code search based on the rank criterion, determinant criterion and rank distribution. Performance results through Monte Carlo simulations demonstrate that there is a trade-off between the error-rate, diversity order and throughput rate.

Three space-time block-coding schemes with two transmit antennas for frequency-selective fading channels are described and compared. The three schemes implement the symbol-level Alamouti scheme (see Alamouti, S., IEEE Journal on Selected Areas in Communications, p.1451-8, 1998) developed for flat-fading channels at a block level either in the time or frequency domains. Receiver structures that aim at exploiting the spatial diversity offered by the two transmit antennas and the temporal diversity offered by the channel frequency selectivity are presented. The three schemes are applied to the EDGE TDMA system and their performance compared on a typical urban channel under both perfect and estimated channel conditions at the receiver.