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In this paper, we investigate the dynamic power allocation for a visible light communication (VLC) cellular system consisting of two coordinating attocells, each equipped with one access-point (AP). The coordinated multipoint (CoMP) between the two cells is introduced to assist users experiencing high inter-cell-interference (ICI). Specifically, the coordinated zero-forcing (ZF) precoding is used to cancel the ICI at the users located near the centers of the cells, whereas the joint transmission (JT) is employed to eliminate the ICI at the users located at the edge of both cells and to improve their receptions as well. Furthermore, two multiple access techniques are invoked within each cell, namely, non-orthogonal-multiple-access (NOMA) and cooperative non-orthogonal-multiple-access (C-NOMA). Hence, two multiple access techniques are proposed for the considered multi-user multi-cell system, namely, the CoMP-assisted NOMA scheme and the CoMP-assisted C-NOMA scheme. For each scheme, two power allocation frameworks are formulated each as an optimization problem, where the objective of the former is maximizing the network sum data rate while guaranteeing a certain quality-of-service (QoS) for each user, whereas the goal of the latter is to maximize the minimum data rate among all coexisting users. The formulated optimization problems are not convex, and hence, difficult to be solved directly unless using heuristic methods, which comes at the expense of high computational complexity.

This paper investigates the downlink transmission of reconfigurable intelligent surface (RIS)-aided cooperative non-orthogonal-multiple-access (C-NOMA), where both half-duplex (HD) and full-duplex (FD) relaying modes are considered. The system model consists of one base station (BS), two users and one RIS. The goal is to minimize the total transmit power at both the BS and at the user-cooperating relay for each relaying mode by jointly optimizing the power allocation coefficients at the BS, the transmit power coefficient at the relay user, and the passive beamforming at the RIS, subject to power budget constraints, the successive interference cancellation constraint and the minimum required quality-of-service at both cellular users. To address the high-coupled optimization variables, an efficient algorithm is proposed by invoking an alternating optimization approach that decomposes the original problem into a power allocation sub-problem and a passive beamforming sub-problem, which are solved alternately. For the power allocation sub-problem, the optimal closed-form expressions for the power allocation coefficients are derived. Meanwhile, with the aid of difference-of-convex rank-one representation and successive convex approximation, an efficient solution for the passive beamforming is obtained. The simulation results validate the accuracy of the derived power control closed-form expressions and demonstrate the gain in the total transmit power brought by integrating the RIS in C-NOMA networks.

Due to the dramatic increase in high data rate services and in order to meet the demands of the fifth-generation (5G) networks, researchers from both academia and industry are exploring advanced transmission techniques, new network architectures and new frequency spectrum such as the visible light and the millimeter wave (mmWave) spectra. Visible light communication (VLC) particularly is an emerging technology that has been introduced as a promising solution for 5G and beyond, owing to the large unexploited spectrum, which translates to significantly high data rates. Although VLC systems are more immune against interference and less susceptible to security vulnerabilities since light does not penetrate through walls, security issues arise naturally in VLC channels due to their open and broadcasting nature, compared to fiber-optic systems. In addition, since VLC is considered to be an enabling technology for 5G, and security is one of the 5G fundamental requirements, security issues should be carefully addressed and resolved in the VLC context. On the other hand, due to the success of physical layer security (PLS) in improving the security of radio-frequency (RF) wireless networks, extending such PLS techniques to VLC systems has been of great interest. Only two survey papers on security in VLC have been published in the literature. However, a comparative and unified survey on PLS for VLC from information theoretic and signal processing point of views is still missing. This paper covers almost all aspects of PLS for VLC, including different channel models, input distributions, network configurations, precoding/signaling strategies, and secrecy capacity and information rates. Furthermore, we propose a number of timely and open research directions for PLS-VLC systems, including the application of measurement-based indoor and outdoor channel models, incorporating user mobility and device orientation into the channel model, and combining VLC and RF systems to realize the potential of such technologies.

This paper investigates the performance of cooperative non-orthogonal multiple access (C-NOMA) in a cellular downlink system. The system model consists of a base station (BS) serving multiple users, where users with good channel quality can assist the transmissions between the BS and users with poor channel quality through either half-duplex (HD) or full-duplex (FD) device-to-device (D2D) communications. We formulate and solve a novel optimization problem that jointly determines the optimal D2D user pairing and the optimal power control scheme, where the objective is maximizing the achievable sum rate of the whole system while guaranteeing a certain quality of service (QoS) for all users. The formulated problem is a mixed-integer non-linear program (MINLP) which is generally NPhard. To overcome this issue, we reconstruct the original problem into a bi-level optimization problem that can be decomposed into two sub-problems to be solved independently. The outer problem is a linear assignment problem which can be efficiently handled by the well-known Hungarian method. The inner problem is still a non-convex optimization problem for which finding the optimal solution is challenging. However, we derive the optimal power control policies for both the HD and the FD schemes in closedform expressions, which makes the computational complexity of the inner problems polynomial for every possible pairing configurations. These findings solve ultimately the original MILNP in a timely manner that makes it suitable for real-time and low latency applications. Our simulation results show that the proposed framework outperforms a variety of proposed schemes in the literature and that it can obtain the optimal pairing and power control policies for a network with 100 users in a negligible computational time.

Unmanned aerial vehicles (UAVs) have recently been envisaged as an enabling technology of 5G. UAVs act as an intermediate relay node to facilitate uninterrupted, high quality communication between information sources and their destination. However, UAV energy management has been a major issue of consideration due to limited power supply, affecting flight duration. Thus, we introduce in this paper a unified energy management framework by resorting to wireless power transfer (WPT), simultaneous wireless information and power transfer (SWIPT) and self-interference (SI) energy harvesting (EH) schemes, in cooperative relay communications. In our new technique, UAVs are deployed as relays equipped with a decode and forward protocol and EH capability operating in a full-duplex (FD) mode. The UAV assists information transmission between a terrestrial base station and a user. The UAV's transmission capability is powered exclusively by the energy harvested from WPT, radio frequency signal transmitted from the source via time-switching SWIPT protocol and SI exploitation. We improve the overall system throughput by the use of FD based UAV-assisted cooperative system. In this proposed system, we formulate two optimization problems to minimize end-to-end outage probability, subject to UAV's power profile and trajectory for a DF relay scheme, respectively. The KKT conditions have been used to obtain closed-form solutions for the two formulated problems. Numerical simulation results validate all the theoretical results. We demonstrate that the performance of our proposed unified EH scheme outperforms that of existing techniques in the literature. Ministry of Human Resource Development Scopus

Among the challenges of realizing the full potential of light-fidelity (LiFi) cellular networks are user mobility, random device orientation and blockage. We study the impact of those challenges on the performance of LiFi in an indoor environment using measurement-based channel models. We adopt spatial modulation (SM), which has been shown to be energy efficient in many applications, including LiFi. We consider two configurations for placing the photodiodes (PDs) on the user equipment (UE). The first one is referred to as the screen receiver (SR) whereby all the PDs are located on one face of the UE, whereas the other one is a multi-directional receiver (MDR), in which the PDs are located on different sides of the UE. The latter configuration was motivated by the fact that SR exhibited poor performance in the presence of random device orientation and blockage. We show that MDR outperforms SR by over $10$ dB at BER of $3.8\times10^{-3}$. Moreover, an adaptive access point (AP) selection scheme for SM is considered where the number of APs are chosen adaptively in an effort to achieve the lowest energy requirement for a target BER and spectral efficiency. The user performance with random orientation and blockage in the whole room is evaluated for sitting and walking activities. For the latter, we invoke the orientation-based random waypoint (ORWP) mobility model. We also study the performance of the underlying system on the uplink channel where the same techniques are used for the downlink channel. Specifically, as the transmitted uplink power is constrained, the energy efficiency of SM is evaluated analytically. It is shown that the multi-directional transmitter (MDT) with adaptive SM is highly energy efficient. As a benchmark, we compare the performance of the proposed framework to that of the conventional spatial multiplexing system, and demonstrate the superiority of the proposed one., 30 pages, 14 figures, Journal paper

We study in this paper the secrecy performance of the multiple-input single-output (MISO) visible light communication (VLC) channel in the presence of randomly located eavesdroppers. We consider a system model comprising a transmitter, equipped with multiple fixtures of LEDs, one legitimate receiver equipped with a single photo-diode (PD) and a group of randomly located (and colluding) eavesdroppers, each equipped with a single PD. We derive a closed-form expression for the average achievable secrecy rate as a function of the beamforming vector, the input distribution, and the location density of the eavesdroppers using stochastic geometry. We then investigate the optimal beamformer that maximizes the average achievable secrecy rate. We corroborate the analytical results through Monte Carlo simulations and we demonstrate substantial improvements provided by the proposed scheme over existing ones. ? 2018 IEEE. This publication was made possible by the sponsorship agreement in support of research and collaboration by Ooredoo, Doha, Qatar, by Qatar National Research Fund under NPRP Grant NPRP8-052-2-029, by FQRNT and by Concordia University, Montreal, Canada. The statements made herein are solely the responsibility of the authors. Scopus

Cooperative relaying is often deployed to enhance the communication reliability (i.e., diversity order) and consequently the end-to-end achievable rate. However, this raises several security concerns when the relays are untrusted, since they may have access to the relayed message. In this paper, we study the achievable secrecy diversity order of cooperative networks with untrusted relays. In particular, we consider a network with an N -antenna transmitter (Alice), K single-antenna relays, and a single-antenna destination (Bob). We consider the general scenario, where there is no relation between N and K , and therefore, K can be larger than N. Alice and Bob are assumed to be far away from each other, and all communication is done through the relays, i.e., there is no direct link. Providing secure communication while enhancing the diversity order has been shown to be very challenging. In fact, it has been shown in the literature that the maximum achievable secrecy diversity order for the adopted system model is one (while using artificial noise jamming). In this paper, we adopt a nonlinear interference alignment scheme that we have proposed recently to transmit the signals from Alice to Bob. We analyze the proposed scheme in terms of the achievable secrecy rate and secrecy diversity order. Assuming Gaussian inputs, we derive an explicit expression for the achievable secrecy rate and show analytically that a secrecy diversity order of up to \min (N,K)-1 can be achieved using the proposed technique. We provide several numerical examples to validate the obtained analytical results and demonstrate the superiority of the proposed technique to its counterparts that exist in the literature. Qatar Foundation;Qatar National Research Fund;Natural Sciences and Engineering Research Council of Canada;Concordia University Scopus

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

We consider the problem of secrecy rate maximization in a multi-input multi-output multi-eavesdropper (MIMOME) wiretap channel and present an exact solution. A general system model with a multi-antenna eavesdropper and a multi-antenna full-duplex receiver is considered. In particular, we perform joint beamforming and artificial noise optimization in an effort to maximize the achievable secrecy rate. The optimization is performed in the presence of artificial noise generated by both transmitter and legitimate receiver. The resulting optimization problem is non-convex and difficult to solve. We develop a minorization-maximization algorithm to solve the problem. It is iterative in nature and guaranteed to converge to a locally optimal solution. The results can therefore be used to benchmark existing methods. Numerical results are presented to demonstrate the efficacy of the proposed approach. This work was made possible by NPRP grant NPRP 8-052-2-029 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. Scopus

In this paper, we address the problem of asymmetric data rates in two-way communication systems. In particular, we propose an adaptive transmission scheme that combines network coding (NC) and opportunistic user selection (OUS) with a threshold that determines which transmission mode to use. The underlying system model comprises two source nodes communicating with each other through a relay node. The source nodes are assumed to have different data rate requirements; therefore, they employ different modulation schemes. As per the proposed scheme, if the end-to-end (E2E) signal-to-noise ratio (SNR) of both users are above a specified threshold, both sources transmit over orthogonal channels, and the relay node uses hierarchical modulation and NC to relay the combined signals to both sources in the third time slot. Otherwise, the user with the better E2E SNR transmits, whereas the other user remains silent. The advantage of the proposed scheme is that it compromises between throughput and reliability. That is, when both users transmit, the throughput improves, whereas when the better user transmits, multiuser diversity is achieved. Assuming asymmetric channels, we derive exact closed-form expressions for the E2E bit error rate (BER), access probability, and throughput for this scheme and compare its performance with existing schemes. We also investigate the asymptotic performance of the proposed scheme at high SNRs where we derive the achievable diversity order for both users. We show through analytical and simulation results that the proposed scheme improves the overall system throughput, the fairness between the two users, and the transmission reliability. This all comes while achieving full spatial diversity for both users. Scopus

Future wireless networks are expected to achieve higher data rates and ubiquitous coverage by seamless cooperation among diverse network technologies. However, it also increases the risk of co-channel interference and introduces the possibility of correlation in the aggregated interference. To address this problem, we adopt a stochastic geometry based approach by assuming that the interfering nodes are randomly distributed according to a Poisson point process (PPP). Using this approach, we derive closed-form expressions for the successful transmission probability and local delay in relay networks with correlated interference. Additionally, we find the optimal transmission probability p that jointly maximizes the successful transmission probability and minimizes the local delay. Numerical results are provided to confirm that the proposed joint optimization strategy achieves significant performance gains compared to conventional schemes. This work was supported in part by the Engineering and Physical Sciences Research Council (EPSRC) under Grant References EP/H044191/1 and EP/L026074/1, and in part by the NPRP Grant 4-1119-2-427 from the Qatar National Research Fund (a member of Qatar Foundation). Scopus

In this paper we investigate the performance of distributed space-time block coding (D-STBC) orthogonal frequency division multiplexing (OFDM) over underwater acoustic (UWA) channels. In particular, we consider a relaying system consisting of one source, two relays, and one destination. The relays operate in amplify-and-forward (AF) mode. The underlying channels are assumed to be time-varying frequency selective channels, where the only source of time variation is the relative motion between transceivers. Alamouti D-STBC scheme is used in the second hop, and a two stage receiver is adopted at the destination: in the first stage, multiple resampling (MR) preprocessing of the received signals is performed to minimize the effect of intercarrier interference (ICI), and in the second stage ICI equalization is performed in the frequency domain to further reduce the effect of the residual ICI. To further boost the performance, successive interference cancellation (SIC) is used, where the estimates of the signals at the output of the ICI equalizer are used as tentative decisions. Compared to the single resampling (SR) front end preprocessing, simulation results show the superiority of MR front-end receiver. Also, SIC further boosts the performance, but still, there is a significant gap with respect to the ICI-free limit, when the receiver has perfect knowledge of ICI coefficients and eliminates them completely. Scopus

In this paper, we propose an adaptive network coding scheme for cognitive relay networks comprising multiple secondary sources communicating with a common destination in the presence of multiple primary users. Conventional network coding schemes developed for cognitive radio networks normally use global encoding kernels to achieve the minimum end-to-end outage probability. Finding the global kernel is computationally inefficient especially when the number of nodes in a network changes. To this end, we propose a network coding scheme that evenly groups the codewords into multiple subsets, linearly combines the network encoded codewords over the reduced subset, and dynamically adjusts the encoding set size to minimize the end-to-end outage probability. An advantage of the proposed network coding scheme is that it achieves lower end-to-end outage probability as compared to the conventional network coding scheme over the whole signal-to-noise ratio (SNR) range with a small additional overhead. We derive closed-form expressions for the link outage probability while taking the interference constraints into consideration. We also derive the exact end-to-end outage probability of the proposed scheme and compare its performance to that of conventional fixed network coding. We show that the proposed scheme provides a trade-off between the probability of relay cooperation and network coding gain. We demonstrate through numerical examples that the proposed adaptive network coding scheme achieves gains of more than 4 dB at a target outage probability of $10^{-2} $ as compared to conventional fixed network coding schemes.

This paper presents a concise yet comprehensive description of a multi-channel Vehicle-to-Infrastructure (V2I) communication system. Existing mathematical models for such a system overlook some of its essential behavioural characteristics such as the reneging, force-termination and ultimately blocking of service requests. A multi-server queueing model with First-In–First-Out (FIFO) service policy is proposed for the purpose of accurately capturing the dynamics of the above-mentioned communication system and evaluating its performance. Approximations were exploited as a mean to enhance this model’s mathematical tractability. Simulations are conducted in the context of a realistic scenario with the objective of validating the proposed approximate model, verifying its accuracy and characterizing the system’s performance in terms of several new metrics. The reported results indicate a cataclysmic access request blocking probability ranging from 66 % to 85 % . An Access Request Deadline-Aware (ARDA) service policy is then proposed to reduce the blocking probability and improve the system’s response time. Indeed ARDA achieved an improvement over FIFO of almost 70 % in terms of the overall blocking probability and 22 % to 66 % in terms of the system’s response time.

Future mobile networks are converging toward heterogeneous multitier networks, where macro-, pico-, and femto-cells are randomly deployed based on user demand. A popular approach for analyzing heterogeneous networks (HetNets) is to use stochastic geometry and treat the location of BSs as points distributed according to a homogeneous Poisson point process (PPP). However, a PPP model does not provide an accurate model for the interference when nodes are clustered around highly populated areas. This motivates us to find better ways to characterize the aggregate interference when transmitting nodes are clustered following a Poisson cluster process (PCP) while taking into consideration the fact that BSs belonging to different tiers may differ in terms of transmit power, node densities, and link reliabilities. To this end, we consider K-tier HetNets and investigate the outage probability, the coverage probability, and the average achievable rate for such networks. We compare the performance of HetNets when nodes are clustered and otherwise. By comparing these two types of networks, we conclude that the fundamental difference between a PPP and a PCP is that, for a PPP, the number of simultaneously covered mobiles and the network capacity linearly increase with K. However, for a PCP, the improvements in the coverage and the capacity diminish as K grows larger, where the curves saturate at some point. Based on these observations, we determine the scenarios that jointly maximize the average achievable rate and minimize the outage probability. Qatar National Research Fund Scopus

In this paper, we consider spectrum sharing between cognitive radio (CR) users and a licensed primary user (PU) to enhance the spectrum efficiency. In the considered scenario, the PU is not able to successfully transmit data to the intended receiver. In this case, the cognitive base station (CBS) in the vicinity of the primary network offers its help to transmit the primary data. In return, the CR system shares the spectrum with the primary system. In particular, we formulate an opportunistic spectrum sharing approach that determines the optimal beamforming weights at the CBS to maximize the overall worst throughput of the CRs while guaranteeing a certain quality of service (QoS) for the PU. Various formulations of beamforming are proposed, which consider different relaying scenarios, including robust designs that are applicable with imperfect channel state information (CSI) at the PU. The original optimization problems are nonconvex and do not have any closed-form solutions. However, using a convex optimization approach, we transform them into convex forms and find approximate solutions using semidefinite programming (SDP) along with randomization techniques. We consider different examples that demonstrate the efficiency of the proposed approach. Scopus

In this paper, we consider Doppler compensation for time varying underwater acoustic channels. The underlying system model comprises two sources, S1 and S2, communicating with each other through a relay, R. The transmission process goes through two phases. In the first phase, the two sources transmit simultaneously and the relay receives. In the second phase, the relay processes the received signals and broadcasts the combined signal to the destinations, i.e., sources. The underlying channels are considered to be time varying frequency selective channels, where the only source of time variation is the relative motion between the transceivers. Orthogonal frequency division multiplexing (OFDM) is used as a means to combat frequency selectivity. Two cases are considered, namely, when R uses multiple resampling (MR) preprocessing to reduce the effect of intercarrier interference (ICI) resulting from the time variation, and when R uses single resampling (SR) preprocessing. In both cases, in the second phase of transmission, each source performs MR preprocessing and after subtracting its own signal ICI equalization is performed to further reduce the effect of residual ICI. Simulation results show that performing MR at R outperforms the case when SR is used at R, however, this comes at the expense of more hardware complexity. Grant NPRP 09-126-2-054 from the Qatar National Research Fund (a member of Qatar Foundation). Scopus

This paper presents a concise yet comprehensive description of a multichannel vehicle-to-infrastructure communication system. Existing mathematical models for such a system overlook some of its essential behavioral characteristics such as the reneging, force termination, and, ultimately, blocking of service requests (SRs). Thus, the reported performance results obtained from these models seem to be unrealistically overoptimistic. Accordingly, in this paper, a multiserver queueing model is proposed for the purpose of accurately capturing the dynamics of the aforementioned communication system and evaluating its performance. The proposed model is renowned for its complexity and the nonexistence of closed-form analytical expressions that characterize its fundamental performance metrics. Hence, approximations were exploited as a means to enhance this model's mathematical tractability. Simulations are conducted in the context of a realistic scenario with the objective of validating the proposed approximate model, verifying its accuracy, and characterizing the system's performance in terms of several new metrics. The simulations' results indicate a cataclysmic SR blocking probability in the range of 65%-85%. Scopus

A video presenting this paper is available in YouTube here: http://www.youtube.com/watch?v=cNgCJK4oimM&feature=youtu.be; A key challenge of future mobile communications research is to strike an attractive compromise between wireless network's area spectral-efficiency and energy-efficiency. This necessitates a clean-slate approach to wireless system design, embracing the rich body of existing knowledge especially on Multiple-Input-Multiple-Output (MIMO) technologies. This motivates the proposal of an emerging wireless communications concept conceived for single-RF large-scale MIMO communications, which is termed as Spatial Modulation (SM). The concept of SM has established itself as a beneficial transmission paradigm, subsuming numerous members of the MIMO system-family. The research of SM has reached sufficient maturity to motivate its comparison to state-of-the-art MIMO communications, as well as to inspire its application to other emerging wireless systems such as relay-aided, cooperative, small-cell, optical wireless and power-efficient communications. Furthermore, it has received sufficient research attention to be implemented in testbeds, and it holds the promise of stimulating further vigorous inter-disciplinary research in the years to come. This tutorial paper is intended to offer a comprehensive state-of-the-art survey on SM-MIMO research, to provide a critical appraisal of its potential advantages, and to promote the discussion of its beneficial application areas and their research challenges leading to the analysis of the technological issues associated with the implementation of SM-MIMO. The paper is concluded with the description of the world's first experimental activities in this vibrant research field.

We consider network coded cooperation for cognitive relay networks. The primary system comprises multiple sources and multiple destinations, whereas the secondary system comprises multiple sources, multiple relays and a single destination. We derive a closed form expression for the end-to-end outage probability for the secondary system while assuming the presence of interference constraints between the two sub-systems. Based on the diversity order analysis, we propose a framework for adaptive network coding. The proposed scheme involves using a small encoding set size for low link quality and a large encoding set for good link quality. Having a small set size increases the probability of having relay cooperation, which comes at the expense of some loss in coding gain, whereas using a large encoding set size decreases the probability of having relay cooperation, but achieves some network coding gains. Therefore, there is a fundamental trade-off between the probability of relay cooperation and the achievable network coding gains. Using numerical results, we show that the proposed adaptive network coding achieves up to 5 dB gain at target outage 10^−3 as compared to conventional fixed network coding schemes. Qatar National Research Fund NPRP 09-126-2-054

We consider a spectrum sharing scenario between cognitive radio (CR) users and a licensed primary user (PU), in which the PU is not able to successfully transmit data to its destined receiver. The cognitive base station (CBS) in the vicinity of the primary network, offers its help to transmit the primary data. In return, the CR system is able to share the spectrum with the primary system. We formulate an opportunistic spectrum sharing approach that determines the transmit beamforming weights in order to maximize the overall throughput of the CRs while guaranteeing the quality-of-service (QoS) of the PU. By applying algorithms based on zero-forcing beamforming (ZFB) along with optimal power allocation, the proposed approach is able to achieve cognitive and primary systems requirements. Scopus

Due to copyright restrictions, the access to the full text of this article is only available via subscription. We consider a multi-carrier cooperative underwater acoustic communication (UWAC) system and investigate the Doppler scaling problem arising due to the motion of different nodes. Specifically, we assume an orthogonal frequency division multiplexing (OFDM) system with amplify and forward (AF) relaying. Our channel model is built on large-scale path loss along with the short-term frequency-selective fading. For Doppler scaling compensation, we use multi-resampling (MR) receiver designs both at the relay and destination nodes. We present an extensive Monte Carlo simulation study to evaluate the error rate performance of the proposed UWAC system. In simulations, we use the publicly available VirTEX software in conjunction with the ray-tracing based BELLHOP software to precisely reflect the characteristics of an underwater geographical location and the movement of the nodes. Qatar National Research Fund (QNRF)

In this paper, we address the relay assignment in cooperative networks based on order statistics. We consider a network comprising a cluster of transmitting sources, a cluster of relay nodes and a single destination. This is motivated by the fact that the existing relay assignment schemes don't achieve diversity for the network configuration under consideration. We assume that the cooperation is in the amplify-and-forward (AF) mode, but proposed scheme is applicable to the decode-and-forward (DF) mode. In the relay assignment process, the source nodes that have weaker source-destination links, have higher priority in relay selection. We analyze the probability density function (PDF) and the average bit error rate (BER) for the proposed scheme where we invoke important results that we first derive for simple two-hop networks. Specifically, we calculate the exact expression for the PDF of the end-to-end signal-to-noise ratio (SNR). Compared to other relay assignment schemes (like maximizing sum of SNR values or maximizing the minimum selected SNR), the proposed scheme has the advantages of simplicity, higher diversity order and fairness.

In this paper, we address the problem of joint optimal threshold-based relaying and maximum likelihood (ML) detection in network-coded cooperative systems. The purpose of using threshold-based relaying is to circumvent the impact of error propagation, which could lead to degrading the system diversity. For simplicity, we consider a simple network comprising two source nodes and one relay node. The relay operates in the decode-and-forward (DF) mode and employs binary network coding. The communication between the two source nodes is bidirectional, resulting in a two-way relay channel. We assume binary phase shift keying (BPSK) signaling. For a given log-likelihood ratio (LLR)-based threshold used at the relay, we derive the ML detector at the destination assuming that the destination has full knowledge of the locations of the blocked bits at the relay. We then derive an expression for the corresponding end-to-end (E2E) bit error rate (BER) performance, which is used to find the optimal threshold. We also derive two practical discontinuous transmission detectors at the destination for the purpose of identifying whether the relay is forwarding or not. The performance of one of the detectors is similar to that when the destination knows the locations of the blocked bits, whereas the performance of the other detector suffers some performance degradation. We present several numerical examples that illustrate the efficacy of the proposed scheme as compared to existing schemes. Qatar Foundation; Qatar National Research Fund Scopus

Due to copyright restrictions, the access to the full text of this article is only available via subscription. In this paper, we consider a multi-carrier and multi-relay underwater acoustic communication (UWAC) system and investigate the relay selection problem. Our channel model is built on an aggregation of both large-scale path loss and small-scale fading. For path loss modeling, we use the publicly available ray-tracing algorithm Bellhop software to precisely reflect the characteristics of an underwater geographical location (such as the sound speed profile, sound frequency, bathymetry, type of bottom sediments, depths of nodes, etc). For relay selection, we consider different selection criteria which rely either on the maximization of signal-to-noise ratio (SNR) or the minimization of probability of error (PoE). These are used in conjunction with so-called per-subcarrier, all-subcarriers, or subcarrier grouping approaches in which one or more relays are selected. We present an extensive Monte Carlo simulation study to evaluate the error rate performance of the UWAC system with different relay selection schemes under consideration and point out differences with terrestrial radio-frequency (RF) systems. Qatar National Research Fund

In this paper, we consider two relay assignment schemes for cooperative networks comprising multiple source-destination pairs. Both schemes are based on the max-min criterion and aim at achieving the maximum spatial diversity for all pairs. One scheme is used as a performance benchmark since it considers all possible relay assignment permutations and selects the best one. The other scheme, on the other hand, considers only a subset of those permutations and selects the best one. The advantages of the latter one is that it reduces the complexity of the assignment process, in addition to making the performance analysis tractable. We examine these schemes over asymmetric channels using M-ary phase shift keying signaling. We consider both amplify-and-forward (AF) and decode-and-forward (DF) relaying where we derive expressions for the end-to-end (E2E) symbol error rate (SER). In both cases, we show that the full spatial diversity is achieved. To account for error propagation in DF relaying, we adopt a threshold-based relaying scheme whereby the relays forward only bits that are deemed reliable and remain silent otherwise. We compare this scheme to Genie-aided relaying where the relays forward only correctly decoded bits. We analyze these two schemes and derive expressions for the E2E SER performance. We present several numerical examples that validate the analytical results. Qatar Foundation; Qatar National Research Fund Scopus

This paper deals with relay assignment in cooperative networks based on the max-min criterion. A simple algorithm is proposed in order to find the optimum relay assignment permutation to avoid brute-force prohibitive search method. It is assumed that there are N source-destination pairs and M relays in the network. The problem is how to assign each relay to a source-destination pair in order to achieve the highest diversity order. The proposed algorithm achieves the maximum spatial diversity for all of the nodes and leads us to simplify the analysis and simulation of the optimal answer. It is shown that the Probability Density Function (PDF) of the end-to-end signal-to-noise ratio (SNR) after relay assignment can be expressed as a weighted sum of the order statistics of the PDF of individual end-to-end links. Since the analytical calculation of the weighting coefficients in the mentioned weighting sum becomes difficult, we propose an approximation in order to calculate the mentioned weighting coefficients. The validity of this fit is shown through some simulations.

In cooperative communications, error propagation at the relay nodes degrades the diversity order of the system. To combat that effect, we present a novel technique to control error propagation at the relays, which is implemented in the context of a distributed turbo code. In the presented technique, the relay calculates the log-likelihood ratio (LLR) values for the bits sent from the source. These values are subjected to a threshold to distinguish the reliable bits from the unreliable bits. The relay then forwards the bits that are deemed reliable and discards the bits that are not, resulting in fewer errors propagating to the destination. The assumption here is that the destination does not know the location of the discarded bits. We develop upper bounds on the end-to-end bit error rate, enabling us to optimize the threshold in terms of the minimum end-to-end bit error rate. We compare our technique with existing techniques that have been proposed to control error propagation, including using only a cyclic redundancy code check at the relay, forwarding analog LLR values, and by employing no error control at the relay at all. We demonstrate, through several numerical examples, that the proposed scheme outperforms all existing schemes Scopus

We consider in this paper relay assignment for cooperative systems with multiple two-way relay channels. The nodes corresponding to one two-way relay channel (henceforth referred to as pair) communicate with each other through a relay. The relays use network coding to simultaneously transmit the signals corresponding to the pairs they are assigned to. We propose two relay assignment schemes. One scheme considers all possible relay assignment permutations and selects the one that yields the best performance, and the other one considers only a subset of these permutations and selects the best one. The advantage of the latter is that it results in a significant reduction in computational complexity, in addition to making the analysis more tractable. We analyze the performance of these schemes over asymmetric independent Rayleigh fading channels. We also consider semi-symmetric and symmetric channels as special cases. We derive closed-form expressions for the end-to-end bit error rate performance for all scenarios and show that the full diversity order is achieved, which is the number of available relays. We present several examples to verify the theoretical results. Qatar National Research Fund Scopus

In relay networks, error propagation at the relay nodes degrades the performance of the system. To combat that effect, it has been suggested to implement a reliability threshold at the relay to control error propagation. Specifically, the relay calculates log-likelihood ratio (LLR) values for the bits sent from the source. These values are subjected to a threshold to selectively forward bits that are most reliable and discard bits that are less so, resulting in less errors propagating to the destination. We investigate the application of this technique to a network-coded two-way relay channel where the relay is assisting two sources simultaneously. We first consider network-coded systems without channel coding, and then consider network-channel coded systems. We examine two modes of thresholding, one based on the individual bits, and the other based on the combined bits. We provide the full analysis for the bit-error rate (BER) performance of both thresholding modes and optimize the thresholds accordingly. We demonstrate that the optimum thresholds based on both modes give similar performances and are far better than the case of no thresholding. We also consider the performance of the proposed thresholding techniques for network-channel coded systems. We present several numerical examples that illustrate the efficacy of employing thresholding at the relay nodes (for networks with and without channel coding). Qatar National Research Fund (QNRF) Grant XP0034 and Qatar Telecom (Qtel) Grant XP0037 Scopus

We consider in this paper relay assignment for cooperative systems with multiple two-way relay channels. The nodes corresponding to one two-way relay channel (henceforth referred to as pair) communicate with each other through a relay. The relays use network coding to simultaneously transmit the signals corresponding to the pairs they are assigned to. We propose two relay assignment schemes. One scheme considers all possible relay assignment permutations and selects the one yields the best performance, and the other one considers only a subset of these permutations and selects the best one. The advantage of the latter is that it results in a significant reduction in computational complexity, in addition to making the analysis more tractable.We consider the cases when a relay is assigned to a single pair and the case when a relay is assigned (simultaneously) to multiple pairs. To achieve the latter, we use higher order modulation schemes at the relay nodes. We analyze the performance of these schemes over symmetric and asymmetric independent Rayleigh fading channels, and derive closed-form expressions for the end-to-end bit error rate performance. We present several examples to verify the theoretical results. Scopus

In cooperative communications, error propagation at relays degrades the diversity order of the system. To combat that effect, it has been suggested to implement a reliability threshold at the relay to control error propagation. The relay calculates log-likelihood ratio (LLR) values for the bits sent from the source. These values are subjected to a threshold to selectively forward bits that are most reliable and discard bits that are less so, resulting in less errors propagating to the destination. We investigate the application of this technique to a network- coded two-way relay channel, where the relay is assisting two sources simultaneously. We investigate two modes of thresholding at the relay: at the individual-bit level and at the combined- bit level. We analyze the bit-error rates of both thresholding modes and optimize the threshold for both. We show significant gains using thresholding over an unthresholded network-coded system. Based on system simulations, we conclude that utilizing separate thresholds yields better results than utilizing a combined threshold scheme.

In cooperative communications, error propagation at relays reduces the diversity order of the system. To combat that effect, we present a novel technique to control error propagation at the relays, which is presented in the context of a relay cooperating with a source to communicate with a destination using a turbo code. The relay calculates log-likelihood ratio (LLR) values for the bits sent from the source. These value are subjected to a threshold to selectively forward bits that are most reliable and discard bits that are less so, resulting in less errors propagating to the destination. We derive upper bounds on the bit-error rates for our proposed system and use them to optimize the threshold at the relay, showing that our system provides a better trade off between prevented errors and blocked correct bits. We compare our scheme with controlling error propagation using only a cyclic redundancy code(CRC) check at the relay, forwarding analog LLR values, and employing no error control at the relay at all. Based on system simulations, we show that our proposed scheme provides a significant diversity gain compared to other techniques.

In this paper we study a special type of quasi-cyclic (QC) codes called skew QC codes. This set of codes is constructed using a non-commutative ring called the skew polynomial rings $F[x;\theta ]$. After a brief description of the skew polynomial ring $F[x;\theta ]$ it is shown that skew QC codes are left submodules of the ring $R_{s}^{l}=(F[x;\theta ]/(x^{s}-1))^{l}.$ The notions of generator and parity-check polynomials are given. We also introduce the notion of similar polynomials in the ring $F[x;\theta ]$ and show that parity-check polynomials for skew QC codes are unique up to similarity. Our search results lead to the construction of several new codes with Hamming distances exceeding the Hamming distances of the previously best known linear codes with comparable parameters., Comment: 12 pages. submitted