Accurate and low-latency positioning is a key enabler for optical links with Low Earth Orbit (LEO) satellites, where millisecond-level beam alignment is required to maintain reliable high-data-rate communication. This paper presents a learning-driven dual-line laser scanning framework for fast and precise satellite positioning. Unlike conventional Gaussian-beam acquisition systems that rely on multiple sequential beams or mechanical steering, the proposed approach employs two orthogonal line-shaped laser beams to perform structured optical scanning over the ambiguity region without any moving parts. A physics-based model incorporating atmospheric attenuation, turbulence, and MRR-based reflection is developed, and a data-driven neural estimator is trained to map received optical energy patterns to the satellite's two-dimensional position. Simulation results demonstrate that the learning-driven method achieves near-MAP accuracy with typical errors of 7-10 m and deterministic scanning time of 1-2 ms, while conventional two-stage Gaussian-beam schemes exhibit comparable errors but random sensing durations of up to 5 ms. The proposed framework therefore offers a favorable trade-off between positioning accuracy, computational complexity, and sensing latency, making it a practical candidate for next-generation optical LEO tracking systems.

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

Accurate characterization of free-space optical (FSO) channels requires joint estimation of transmitter pointing errors, receiver angle-of-arrival (AoA) fluctuations, and turbulence-induced fading. However, existing literature addresses these impairments in isolation, since their multiplicative coupling in the received signal severely limits conventional estimators and prevents simultaneous recovery. In this paper, we introduce a novel multi-aperture FSO receiver architecture that leverages spatial diversity across a lens array to decouple these intertwined effects. Building on this hardware design, we propose a hierarchical deep learning framework that sequentially estimates AoA, transmitter pointing error, and turbulence coefficients. This decomposition significantly reduces learning complexity and enables robust inference even under strong atmospheric fading. Simulation results demonstrate that the proposed method achieves near-MAP accuracy with orders-of-magnitude lower computational cost, and substantially outperforms end-to-end learning baselines in terms of estimation accuracy and generalization. To the best of our knowledge, this is the first work to demonstrate practical joint estimation of these three key parameters, paving the way for reliable, turbulence-resilient multi-aperture FSO systems.

In this paper, we investigate the performance of wireless-powered communication (WPC) systems operating under the harvest-then-transmit protocol with a three-piecewise nonlinear energy harvesting (EH) model, which explicitly captures the sensitivity and saturation behavior of practical EH circuits. In parallel, we adopt a versatile fading characterization by assuming Nakagami-mchannels in the EH phase and generalized Gamma channels in the information transfer phase, the latter being of practical interest as it unifies several standard fading distributions and accurately approximates fading environments in next-generation networks. Based on this setting, we develop a novel analytical framework that yields, for the first time, exact analytical expressions for the average throughput in the delay-limited, delay-tolerant, and Quality of Service (QoS) delay-constrained transmission modes. We then apply the developed framework to study the throughput of WPC-enabled reconfigurable intelligent surface-assisted systems as a direct application example. We conduct extensive Monte-Carlo simulations under various system configurations to confirm the accuracy of the analytical results. Our results show that nonlinear EH effects yield a more realistic benchmark for the average throughput compared to the conventional linear EH model, particularly under stringent sensitivity and saturation constraints.

This paper investigates a movable-antenna-assisted dual-hop space-air-ground non-terrestrial network (NTN) operating under an underlay spectrum sharing paradigm. A satellite communicates with multiple unmanned aerial vehicles (UAVs) over free-space optical (FSO) links, while each UAV simultaneously serves a cluster of ground users over radio-frequency (RF) channels subject to interference constraints imposed by a primary receiver. To efficiently exploit the complementary advantages of FSO and RF transmission and the additional spatial degrees of freedom offered by movable antennas, a joint optimization framework is developed to maximize the system sum rate. The proposed framework jointly optimizes satellite and UAV power allocation, multi-antenna precoding at the UAVs, and the positions of multiple movable antennas mounted on each UAV. An alternating-optimization algorithm is employed, where minimum mean square error (MMSE) based precoding accounts for both communication and interference channels, power allocation is convexified using auxiliary rate variables and successive convex approximation, and antenna locations are optimized via Taylor-based convex surrogates. Simulation results demonstrate that the proposed approach significantly outperforms benchmark schemes with fixed antenna locations, heuristic optimization and reinforcement learning methods, while providing robust performance across different system parameters.

Thermal noise modulation (TNM) encodes information in thevarianceof Johnson noise, enabling ultra-low/zeropower operation with intrinsic physical-layer security. While recent demonstrations have focused on binary TNM, its twolevel alphabet fundamentally limits spectral efficiency. This paper develops anM-ary TNM (M-TNM) framework that maps symbols to multiple variance levels, thereby conveying (log2M) bits per channel use. We first establish a structural property of the optimal detector: for zero-mean Gaussian classes with ordered variances, the maximum-likelihood (ML) decision regions are contiguous in the energy statistic and each symbol’s boundaries depend only on its two adjacent variance levels. Leveraging this result, we derive closed-form expressions for theoptimalML detection thresholds and the associatedexactaverage symbol error rate (SER) forM-TNM. We then formulatevariance-constellation optimization—the selection and spacing of symbol variances—as a SER-minimization problem under practical constraints. Analytical results are validated via extensive Monte Carlo simulations, which show near-perfect agreement with theory and demonstrate substantial BES reductions for optimized constellations compared with naïve (e.g., uniformly spaced) variance levels. The proposedM-TNM framework thus improves spectral efficiency while preserving TNM’s energy and security promise, positioning variance-domain modulation as a viable physical-layer technique for bandwidth-limited, large-scale Internet of Things (IoT) deployments.

This paper presents an efficient analytical framework for evaluating the performance of inter-satellite communication systems utilizing orbital angular momentum (OAM) beams under pointing errors. An accurate analytical model is first developed to characterize intermodal crosstalk caused by beam misalignment in OAM-based inter-satellite links. Building upon this model, we derive efficient expressions to analyze and optimize system performance in terms of bit error rate (BER). Unlike traditional Monte Carlo-based methods that are computationally intensive, the proposed approach offers accurate performance predictions. This enables a substantial decrease in computation time while maintaining high accuracy, thanks to the use of analytical expressions for both crosstalk and BER. This fast and accurate evaluation capability is particularly critical for dynamic low Earth orbit (LEO) satellite constellations, where network topology and channel conditions change rapidly, requiring real-time link adaptation. Furthermore, we systematically design and evaluate asymmetric OAM mode sets, which significantly outperform symmetric configurations in the presence of pointing errors. Our results also reveal key insights into the interaction between beam divergence, tracking accuracy, and link distance, demonstrating that the proposed framework enables real-time optimization of system parameters with high fidelity. The analytical findings are rigorously validated against extensive Monte Carlo simulations, confirming their practical applicability for high-mobility optical wireless systems such as LEO satellite networks.

We propose a novel clustered random beamforming (RBF) scheme for multi-user multiple-input multiple-output (MIMO) downlink systems. The objective of this scheme is to maximize the number of served users while ensuring that each user meets a minimum quality of service (QoS) requirement. The system model comprises a base station (BS) equipped with multiple transmit antennas, serving multiple users. Hierarchical modulation (HM) is employed for each beam to improve spectral efficiency. To analyze the performance of the proposed scheme, we develop an analytical model by mapping the symbols of all users within a cluster onto different layers of a HM constellation. We demonstrate that the proposed scheme not only increases the number of served users but also enhances the sum-rate capacity of the system. Additionally, we discuss the selection of the optimal HM parameter based on predefined performance requirements, illustrated through a sample case. Simulation results indicate that the proposed hierarchical random beamforming (HRBF) scheme is highly promising for multi-user massive MIMO downlink systems, particularly in the context of 5G New Radio (NR) and future 6G networks and beyond.

This paper proposes an efficient path-planning scheme for unmanned aerial vehicles (UAVs) aimed at addressing coverage holes in wireless networks. Coverage holes can undermine the quality of service (QoS) of terrestrial cellular networks where they cause outage times longer than a threshold value dictated by the different application requirements. The proposed approach leverages the self-organizing map (SOM), an unsupervised machine learning technique, to design a UAV trajectory that minimizes the flight path length, while ensuring a coverage hole-free cell or guaranteeing a maximum outage time across the existing holes. The designed path also satisfies constraints on minimum and maximum UAV velocity. Simulation results show that for realistic scenarios, we can practically eliminate all coverage holes when one UAV travels over the designed path. For more extreme scenarios, we show that we need to deploy multiple UAVs to satisfy the QoS requirements where each UAV covers a partition of the holes. To achieve optimal partitioning, we utilize the ant colony algorithm.

Non-terrestrial networks (NTNs) using unmanned aerial vehicles (UAVs) as temporary base stations enable flexible coverage in disaster-stricken areas and congested regions. This paper presents a beamforming design for rate splitting multiple access (RSMA) enabled UAV underlay networks that addresses untrusted users (potential eavesdroppers), enforces max–min fairness in achievable user rates, accommodates imperfect CSI, and limits interference to a licensed primary receiver. We transform the non-convex optimization problem via auxiliary-variable-based reformulation and successive convex approximation (SCA), obtaining a tractable convex semi-definite program (SDP). Simulations demonstrate that the proposed framework achieves perfect rate fairness (Jain’s fairness index = 1.0) across all users while delivering robust sum-rate performance under varying numbers of users, UAV transmit antennas, and secrecy requirements.

This work investigates the bit error probability (BEP) of thermal noise modulation (TNM) in Rayleigh fading channels. TNM has emerged as a promising ultra-low-power modulation scheme, where information is conveyed through variations in thermal noise variance. Existing studies provide only approximate BEP expressions and assume perfect channel state information (CSI) at the receiver. Furthermore, current detection strategies require per-symbol threshold adjustments, which, along with CSI estimation, introduce significant computational overhead for power-constrained devices. To address these limitations, we propose a fixed-threshold detection strategy that eliminates the need for channel estimation and threshold adaptation. Additionally, we derive an exact closed-form BEP expression for TNM under Rayleigh fading and formulate the problem of optimal threshold selection as an optimization task, solvable using efficient line-search techniques such as gradient descent. The accuracy of our analytical results is validated through Monte Carlo simulations, demonstrating strong agreement with theoretical predictions. These contributions provide a more precise characterization of TNM performance in fading environments, paving the way for its practical implementation in energy-efficient wireless systems.

Due to the expected exploitation of high frequency bands to meet increasing demands on high data rates, multi-hop links are being under investigation as a solution for limited transmission distances over these bands. As such, many of recent transmission schemes are analyzed over multi-hop scenarios. To this end, in this paper, we design a dual-hop system of Index Modulation based Orthogonal Frequency Division Multiple Access (IM-OFDMA) with the help of decode-and-forward relays. Two different scenarios are considered, with single and multiple DF relays. Moreover, the average error rate is analyzed for the proposed system considering the two different scenarios. Our results indicate that the overall performance in the case of a single DF relay is governed by the performance of the second hop. Also, results reveal that a significant gain can be attained if a single relay is assigned to each user rather than one relay for all users.

This paper investigates the performance of satellite-based quantum key distribution (QKD) under realistic channel conditions, emphasizing the adverse effects of photon loss introduced by pointing errors and the presence of background photons. By leveraging a stochastic framework, we model the probability distribution of the number of photons detected at the receiver, considering Gaussian beam propagation, finite receiver aperture, and Poisson-distributed background noise. We then evaluate essential QKD performance metrics—such as the quantum key rate, quantum bit error rate (QBER), and outage probability—across a wide range of beam waist sizes and tracking accuracies. Numerical simulations demonstrate how subtle increases in the pointing error variance can drastically reduce the secure key generation rate, particularly for tightly focused beams. These findings highlight the importance of precise pointing systems and careful selection of optical parameters to ensure an acceptable QBER and a sufficiently high secret key rate in inter-satellite QKD links.

The next generation of wireless communication networks must offer robust connectivity to serve users in remote or disaster-stricken regions where terrestrial infrastructure is unavailable or compromised. High-altitude platforms (HAPs), functioning as non-terrestrial network (NTN) nodes, can rapidly restore coverage and extend service reach by transmitting directly to ground users. To further enhance communication performance, simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) can be deployed alongside HAPs, intelligently shaping the wireless channel to improve channel reliability. In this work, we investigate a HAP-assisted NTN in which a STAR-RIS aids downlink transmission to multiple ground users under a rate-splitting multiple access (RSMA) protocol with imperfect successive interference cancellation (SIC). The joint design of power allocation at the HAP and STAR-RIS beamforming presents a challenging non-convex problem because of the coupled rate expressions and rank-one constraints on the STAR-RIS matrices. To address this, we introduce auxiliary variables and apply successive convex approximation (SCA) to convexify rate functions, while employing a difference-of-convex (DC) programming approach to handle the rank-one requirement. An alternating optimization framework is then developed to iteratively solve a convex power allocation subproblem and a penalized semi-definite program for STAR-RIS beamforming. Simulation results show the efficacy of the proposed framework, showing excellent performance even under imperfect SIC and with discretized phase shifts at the STAR-RIS.

Combining free-space optical (FSO) technology with uncrewed aerial vehicles (UAVs) introduces dynamic, rapidly deployable relay systems, overcoming line-of-sight (LoS) constraints and extending high-speed communication networks’ reach, albeit with critical considerations of weight and power consumption. There are two main types of optical communication technologies for relays: conventional optical relays, such as amplify-and-forward (AF)/decode-and-forward (DF) systems, and intelligent reflecting surface (IRS)-based relays, each facing significant challenges. To this end, this paper introduces a novel hybrid two-way free-space optical (FSO) relay system utilizing modulating retro-reflector (MRR) technology to address specific challenges in UAV-based optical communication. Unlike traditional amplify-and-forward (AF) and decode-and-forward (DF) relay systems, which struggle with power consumption and IRS-based systems that suffer from sensitivity to angular fluctuations, the proposed MRR-based approach offers a strategic compromise. The proposed system combines MRR technology with conventional lens-based systems and network coding to enhance stability against UAV’s angular movements. Performance evaluations reveal that while the proposed MRR-based relay system offers significant advantages under conditions of high angular instability, it achieves comparable or superior performance relative to AF/DF and IRS-based systems within certain operational parameter ranges. This study thus advances the discussion on UAV-based relay solutions by offering an in-depth analysis of the proposed system’s application potential and its specific constraints.

In this paper, rate splitting (RS) has been applied in a mixed dual-hop radio frequency/free space optical (RF/FSO) relay system. In particular, assuming fixed-gain amplify-and-forward (AF) and decode-and-forward (DF) relay protocols, the performance of both heterodyne detection and intensity-modulated direct detection (IM-DD) has been analytically evaluated. In this framework, it is assumed that the RF channel suffers from Rayleigh fading, while the FSO channel is affected by the atmospheric turbulence-induced fading withMálaga($\mathcal {M}$) distribution and pointing errors. For the considered system, we derive expressions for the outage probability, the average bit error rate (BER), and the average channel capacity. In addition, the asymptotic performance of the system at high signal-to-noise ratios regime is analyzed, special cases are discussed, and useful insights have been extracted. Finally, numerical results are presented to verify the accuracy of the analytical and asymptotic expressions.

This paper focuses on the optimal design of a modulated retroreflector (MRR) laser link to establish a high-speed downlink for cube satellites (CubeSats), taking into account the weight and power limitations commonly encountered by these tiny satellites. To this end, first, a comprehensive channel modeling is conducted considering key real channel parameters including mechanical gimbal error, fast steering mirror angle error, laser beamwidth, MRR area, atmospheric turbulence, and channel coherence time. Accordingly, a closed-form expression for the distribution of the received signal is derived and utilized to propose a maximum likelihood based method to sense and estimate the initial position of the satellite. Subsequently, the distribution of the distance estimation error during the sensing phase is formulated as a function of the laser beamwidth and the gimbal error, which enables us to fine-tune the optimal laser beamwidth to minimize sensing time. Moreover, using the sensing and initial satellite distance estimation, two positioning algorithms are proposed. To compare the performance of the proposed positioning method, we obtain the lower bound of the positioning error as a benchmark. Finally, by providing comprehensive simulations, we evaluate the effect of different parameters on the performance of the considered MRR-based system in both the sensing and positioning phases.

Given the growing significance of CubeSats for real-time Earth monitoring and space networking, there is an increasing demand for high-speed links for CubeSats facing constraints related to the weight, dimensions, and power consumption of telecommunication equipment. This article addresses such a need by designing a modulating retroreflector (MRR)-based optical downlink system tailored for fast-moving CubeSats, highlighting joint tracking and communication operations using a single transmitter for high data rates. Key contributions encompass precise system characterization, the design of a dual-transmitter system to maximize channel capacity, derivation of channel capacity as a function of 2M + 4 random variables, and the development of an MRR-based CubeSat downlink system. Subsequently, leveraging the obtained analysis and results, we design a system with a single transmitter, enabling simultaneous tracking and communication operations through optimal adjustments of beam timing and placement in the satellite’s vicinity. Pertinent analyses demonstrate optimal laser beam adjustments to achieve maximum capacity while maintaining tracking accuracy. Monte Carlo simulations are used to validate a closed-form expression for efficient optimization of system parameters. Comprehensive simulations assess the effect of different parameters, offering crucial insights for optimal system design.

Low Earth orbit (LEO) satellite constellations are emerging as a key enabler of next-generation communications, offering global coverage and significantly lower latency compared to traditional terrestrial networks and geostationary satellites. However, further latency reduction is essential for time-critical applications such as real-time sensing, autonomous systems, and interactive services. One critical bottleneck is the optical-to-electrical (O/E) and electrical-to-optical (E/O) conversions at intermediate nodes in multi-hop links, which introduce unwanted processing delays. To address this, we investigate an all-optical relay system based on Optical Hard Limiters (OHL), which operate purely in the optical domain to suppress noise and restore signal quality without requiring O/E conversions. First, we present a rigorous analysis of inter-satellite multi-relay communication under the OHL relaying architecture, comparing it against conventional Amplify-and-Forward (AF) and Decode-and-Forward (DF) schemes. Through this comparison, we highlight both the advantages and limitations of OHL relays, including their particular sensitivity to parameter choices such as the threshold setting and divergence angle at the transmitter. Recognizing that a LEO constellation is inherently time-varying—satellites move relative to one another, causing continuous changes in link distances and tracking errors—we propose a joint optimization strategy. This scheme adaptively tunes the OHL decision threshold and beam divergence in real time to maintain optimal performance, ultimately lowering error rates and latency. Extensive simulations in a large-scale LEO network demonstrate the viability of our method and offer insights into practical implementation for next-generation inter-satellite communication systems.

Coverage holes directly affect the quality of service (QoS) and reliability of wireless networks and should be avoided as much as possible. In this paper we address this issue through the deployment of unmanned aerial vehicles (UAVs) as mobile base stations and we propose proper UAV path planning. While most of the works in the literature define holes based on statistical sense, i.e., when the coverage probability for a point on cell is below a certain threshold, e.g., 95%, in this paper, we target applications that allow only for short time disconnections, and define a location that is not covered for a certain amount of time to be in a coverage hole. To minimize such holes, we use optimal UAV path planning based on the two families of trajectories, namely, spiral and oval curves. We show that the proposed oval curves will result in a better performance in addressing the coverage holes and can guarantee a minimum signal to noise ratio over the whole coverage area, and over a guaranteed amount of time.

Terahertz (THz) band is widely nominated to be exploited in next wireless networks to meet the high demand on data rates. However, unlike lower frequency bands, several challenges come up while designing a transmission system over the THz band. Among these challenges is the hardware impairments at the transmitter front-end. It is well known that hardware impairments limit the error performance of any transmission system, where a lower bound on the error rate is usually noticed. Such an impact becomes more severe as the operating frequency increases, which significantly degrades the transmission over THz band. To this end, this paper proposes an adaptive modulation scheme that is able to overcome this problem. Moreover, the optimal detector for the proposed adaptive modulation scheme is presented. Mathematical modeling and analysis of the performance of the conventional QAM and the proposed modulation scheme are included along with simulation results that verify the analytical findings. Results indicate that the error performance of the proposed scheme is much better than conventional QAM where no error floor is noticed.

Involving Unmanned Aerial Vehicles (UAVs) in wire-less networks has been widely investigated in the literature considering different scenarios such as emergency scenarios in which UAV(s) can play a significant role by compensating the damaged/lost network's components. For example, Sensor Networks (SNs) in emergency scenarios may lose some sensor nodes or the Central Entity (CE) itself, which requires fast, prompt and efficient alternative to replace them. Therefore, UAVs are widely nominated to such a role due to their flexibility and maneuverability. However, compared to ground-based entities, UAVs suffer from the continuous position fluctuations, which directly affects the antenna's orientation. Such an effect becomes a serious challenge in high frequency links such as millimeter wave (mmWave) links which are very sensitive to antennas misalignment. In this paper, the performance of UAV-based SNs is addressed by analyzing the impact of UAV's vibration on the performance metrics including detection and false-alarm probabilities. To this end, two network models are adopted, namely, Ground-based Hybrid SN (G-HSN) and Aerial-based Hybrid SN (A-HSN), depending on whether the CE is ground or aerial node. A mathematical framework is followed to express performance metrics in closed form expressions considering practical conditions including channel fading, orientation fluctuations of directional mmWave antennas and path loss. Simulation results validate the accuracy of the derived mathematical expressions and depict the impact of different operational parameters.

This paper presents a novel approach for joint Terahertz (THz) communication and three-dimensional (3D) map reconstruction using an Unmanned Aerial Vehicle (UAV). Due to the need for precise mapping to provide Line-of-Sight (LoS) THz communication services, our approach leverages the UAV's trajectory to initiate the reconstruction and updating of the 3D environment in real-time. The UAV starts without any prior knowledge of the target area, establishing THz communications with users scattered among 3D obstacles. A comprehensive system model is developed, incorporating realistic antenna patterns, UAV oscillations, and received signal power modeling for accurate analysis. We establish an adaptive methodology for 3D environment modeling, dynamically constructing and refining a grid-based obstacle map. A dynamic algorithm for real-time 3D map reconstruction is implemented, utilizing received signal strengths to update the environmental model. Detailed simulations validate the effectiveness of our proposed methods, demonstrating significant improvements in computational efficiency and real-time decision-making in UAV-assisted communication networks. Our results highlight the potential of this joint approach in enhancing operational efficiency and communication reliability under varying environmental conditions.

Exploiting the high-frequency bands, such as millimeter wave band (mmWave), has become a pressing need due to the increasing demand on the high data rates. However, two main challenges are still hindering the usage of mmWave band, which are represented by the short transmission distance and strict Line-of-Sight (LoS) requirements. To this end, relaying schemes have been widely nominated to address these two challenges, where the transmission distance can be extended and the LoS can be attained by the aid of a well-positioned relay. One of the promising spectral-efficient relaying schemes is the well known Two-Way Relaying (TWR). In this paper, the performance of the TWR scheme is investigated for aerial links operating over the mmWave band. Specifically, the bit error rate (BER) is analyzed for a dual-hop system in which source, relay and destination are represented by hovering unmanned aerial vehicles. A closed form expression of the average BER is derived considering the impact of the antennas' fluctuations, modulation order, transmission distance, beamwidth, path loss and channel fading. Simulation results are explored to investigate the impact of all operational parameters.

In this paper, we study the performance of a cooperative underwater acoustic communication/free-space optical communication (UAC-FSO) transmission system supported by a fixed-gain amplify-and-forward (AF) relay, where an underwater source transmits signals to a satellite through the help of a buoy functioning as a relay located on the sea surface. In particular,Kand κ - μ shadowed fading distribution models are used to characterize the UAC link, while the FSO channel follows a unified Málaga distribution existing pointing errors. Based on the above considerations, we calculate the cumulative distribution function (CDF) and probability density function (PDF) of the end-to-end (e2e) signal-to-noise ratio (SNR). To assess the system performance, expressions for the outage probability (OP) and average bit-error rate (BER) are further obtained in closed-form. Furthermore, driven by a desire for more explicit insights, the high SNR analyses of the OP and average BER, and upper and lower bounds on the average capacity are presented. Finally, through Monte Carlo simulations, the correctness of the theoretical analysis is verified.

The main subject of this work is to make a detailed comparison between the performance of free-space optical (FSO) and terahertz (THz) links under different conditions. To this end, we first perform a detailed modeling of the FSO and THz channels, taking into account all real channel parameters. Then we compare the performance of THz and FSO links using two important wireless communication performance metrics, i.e., channel capacity and outage probability. We mathematically show that, unlike FSO links, the pointing errors of THz links are not function of linklength. Finally, by examining the strengths and weaknesses of both technologies, we show that increasing the linklength has a more destructive effect on FSO links compared to the THz links. For shorter linklength, FSO links have better performance, however, with the increase in linklength, THz has the ability to reach higher capacity than FSO links.

As an alternative solution for quick disaster recovery of backhaul/fronthaul links, in this paper, a dynamic unmanned aerial vehicles (UAV)-assisted heterogeneous (HetNet) network equipped with directional terahertz (THz) antennas is studied to solve the problem of transferring traffic of distributed small cells. To this end, we first characterize a detailed three-dimensional modeling of the dynamic UAV-assisted HetNet, and then, we formulate the problem for UAV trajectory to minimize the maximum outage probability of directional THz links. Then, using deep reinforcement learning (DRL) method, we propose an efficient algorithm to learn the optimal trajectory. Finally, using simulations, we investigate the performance of the proposed DRL-based trajectory method.

In this study, the instantaneous channel capacity of orthogonal frequency division multiplexing (OFDM) transmission with low carrier frequency offset (CFO) over additive white Gaussian noise (AWGN) channels is obtained by taking advantage of the relationship between average bit error rate (ABER) and average channel capacity (ACC) performance metrics. In other words, the channel capacity of OFDM is investigated using bit error rate performance with a novel measurement method. Moreover, the results are extended to the exact ACC analysis over generalized flat fading channels and the cases of Rayleigh and Nakagami-m flat fading environments are investigated. The proposed closed-form expressions are compared with the well-known Gaussian approximation (GA) method to show their advantage and then verified with numerical results.

We propose a fixed-wing unmanned aerial vehicles (UAV)-based millimeter wave (mmWave) backhaul architecture that is offered as a cost effective and easy to deploy solution, to connect a disaster or remote area to the nearest core network. First, we fully characterize the single relay fixed-wing UAV-based communication system by taking into account the effects of realistic physical parameters, such as the UAV’s circular path, critical points of the flight path, heights and positions of obstacles, flight altitude, tracking error, the severity of UAV’s vibrations, the real 3D antenna pattern, mmWave atmospheric channel loss, temperature and air pressure. Second, we derive the distribution of the signal-to-noise ratio (SNR) metric, which is based on the sum of a series of Dirac delta functions. Using the SNR distribution, we derive analytical expressions for the outage probability and the ergodic capacity of the considered system as a function of all system parameters. To provide an acceptable quality of service for longer link lengths, we extend the analytical expressions to a multi-relay system. The accuracy of the analytical expressions are verified by Monte-Carlo simulations. Finally, by providing sufficient simulation results, we investigate the effects of key channel parameters such as antenna pattern gain and flight path on the performance of the considered system; and we carefully analyze the relationships between those parameters in order to maximize the average channel capacity.

The main contribution of this paper is to analyze a long networked flying platform (NFP)-based millimeter wave (mmWave) backhaul link that is offered as a cost effective and easy to deploy solution to connect a disaster or remote area to the nearest core network. The same network model can be considered for interlinking Low Earth Orbit (LEO) satellite constellations with different geometry and channel characteristics. For this aim, we characterize the backhaul channel as a function of realistic physical parameters such as heights and distances of obstacles along the route, flight altitude and the intensity of NFPs' vibrations, the actual 3D antenna pattern, etc. For the characterized channel, we derive an analytical closed-form expression for the outage probability. Finally, using the obtained results, we provide a fast algorithm for the optimal parameter design of the considered system that minimizes the cost.

In this work, we study the performance of reconfigurable intelligent surface (RIS)-aided communication systems over the sum of cascaded Rician fading channels. To facilitate the performance analysis of the practical scenario, we consider the case that imperfect channel state information (CSI) is available at the RIS. We derive the closed-form expressions of several performance metrics in terms of the exact outage probability, ergodic capacity and average bit error rate (BER). Through analytical and numerical results, we examine the effect of the number of reflecting elements at the RIS and different system parameters on the overall system performance.

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.

Massive connectivity and spectral efficiency are among the main defining terms of the future communications era. Therefore, revolutionary technologies are required to keep up with the connectivity and spectrum demands. Non-orthogonal multiple access (NOMA) and Index modulation (IM) techniques have emerged as promising candidates to satisfy the spectral efficiency and capacity demands. In this paper, a novel downlink multiple access scheme, called IM-NOMA, is proposed reaping the advantages of both NOMA and IM. In IM-NOMA, the bit block to be sent to each user is divided into two sub-blocks. While the first sub-block is modulated, the second sub-block determines a subset from the available set of channels to carry the modulated symbol. As in power-domain NOMA, the base station (BS) superimposes the modulated symbols carried by the same frequency carrier after a proper power allocation. In such, spectral efficiency is boosted by the extra bits carried by the indexes of the used carriers and the error performance is improved by spreading the users' signals in power domain as well as diversifying the symbols of each user over multiple channels. An upper bound on the average BER is derived in a closed form expression under maximum likelihood (ML) detection method and Rayleigh fading channel. Simulation results show a significant improvement in the BER of the proposed IM-NOMA as compared to the conventional NOMA scheme.

Employing reconfigurable intelligent surfaces (RIS) is emerging as a game-changer candidate, thanks to their unique capabilities in improving the power efficiency and supporting the ubiquity of future wireless communication systems. Conventionally, a wireless network design has been limited to the communicating end points, i.e., the transmitter and the receiver. In general, we take advantage of the imposed channel state knowledge to manipulate the transmitted signal and to improve the detection quality at the receiver. With the aid of RISs, and to some extent, the propagation channel has become a part of the design problem. In this paper, we consider a single-input single-output cooperative network and investigate the effect of using RISs in enhancing the physical layer security of the system. Specifically, we formulate an optimization problem to study the effectiveness of the RIS in improving the system secrecy by introducing a weighted variant of the secrecy capacity definition. Numerical simulations are provided to show the design trade-offs and to present the superiority of RIS-assisted networks over the conventional ones in terms of the system’s secrecy performance.

With the expected explosive growth in the user density in wireless networks, recent works have investigated the integration of index modulation (IM) with multiple access schemes, namely the non-orthogonal multiple access (NOMA) scheme. IM-based NOMA schemes have shown appealing advantages in terms of error performance and power efficiency. However, the advantages come with higher detection complexity as an inherited disadvantage of applying IM. In this paper, we propose a constellation rotation-based approach to reduce the detection complexity of the successive interference cancellation (SIC) algorithm. Specifically, we assume two orthogonal constellations to be utilized for transmission. Hence, the introduced rotated-constellations result in orthogonality in the users' signals space which reduces the inter-user interference domains. Therefore the SIC detection complexity is reduced as well. The proposed approach is shown to have a 50% average reduced complexity as compared to classical SIC. Numerical simulations show a significant improvement in terms of error performance as compared to IM-NOMA scheme.

In this letter, we study the interacting effects of the exclusion zone and eavesdropper colluding strategy on the secrecy performance of multiple-antenna transmission system. In particular, a base station (BS) applies random beamforming and user scheduling to enhance the legitimate user channel while multiple eavesdroppers try, either individually or jointly, to overhear the transmitted signal. We derive the closed-form expressions of secrecy outage probability (SOP) of the system with and without exclusion zone under different colluding strategies. Through selected numerical examples, we develop the design guideline on the size of the exclusion zone for a target security level subject to eavesdroppers' density and colluding strategy.

This paper presents two generic charging and transmission schemes to find the trade-off between the altitude of an Unmanned aerial vehicle (UAV) base station and the acquired harvested energy for maximum coverage area. The first scheme seeks the optimal balance between the coverage area and the harvested energy using simultaneous wireless charging and information transmission (SWCIT). The second scheme proposes time allocation to increase the UAV flying period where multiplexing information transmission and energy harvesting is optimized. Finally, the two schemes are implemented on a real UAV system to verify whether equipping it with energy harvesting technology is practical or not.

Advances in the design of Full-Duplex (FD) transceivers with low residual Self-Interference (SI) levels has led to their deployment in various wireless communication applications. One promising field that FD transceivers could play a major role in reshaping its dynamics is physical layer security. This paper investigates the performance of FD transceivers in the context of Military FD Radios (MFDR). Particularly, the adopted system model builds on recent investigations regarding the feasibility of deploying an MFDR in a scenario where it simultaneously jams a receiver node while intercepting the signal of a transmitter node, thus simultaneously disrupting and intercepting a tactical communication link of an opponent team. The secrecy performance of the MFDR is theoretically quantified by deriving its outage in interception probability expression. The results reveal that a well-performing SI cancellation scheme can increase the secrecy performance of an MFDR.

Although small cells (SC) densification approach plays a prominent role in achieving the data rate and coverage requirements in 5G networks, it poses serious challenges concerning the flexible and cost efficient backhauling solutions. The traditional terrestrial backhauling hubs are subject to limited line of sight probabilities in such dense networks. Having this challenge in hand, and recognizing the increasing interest in the unmanned aerial vehicle (UAV) enabled communication systems, we address the problem of wireless multihop backhauling of SCs using UAV hubs. We present two linear optimization programs that optimize the SC-UAV association and the SC-SC formation in order to maximize the total backhaul flow. Some practical constraints are considered, such as backhaul reliability, association criteria, SC relaying capacity and the number of available links at each SC. Numerical simulations show that the approach allowing partial demand fulfillment of the SCs outperforms the binary one in terms of accumulated rate and the percentage of associated SCs, even at low number of maximum hops and links allowed.

Network densification is promising to achieve higher data rates and higher network capacity, while causing new backhauling challenges especially when the network is highly dense. The flexible and cost efficient backhauling solutions are among the most concerning issues. Due to the high density of the next generation networks, the terrestrial wired backhauling approaches are not cost efficient. Having this challenge in hand, and recognizing the advantages of the networked flying platform (NFP)-enabled communications, we address in this paper the problem of wireless multi-hop backhauling of small cells (SC) using NFP hubs. We propose an efficient iterative heuristic algorithm that jointly optimize the SCNFP association and the SC-SC formation in order to maximize the total backhaul flow, with practical constraints in consideration, such as backhaul reliability, hardware and processing limitations in the SCs and the NFPs. Numerical simulations show that the proposed algorithm can achieve close to optimal solution at a much faster convergence rate.

Random unitary beamforming (RUB) can reduce the implementation complexity of massive multiple- input singleoutput (MIMO) downlink transmission by only requiring partial channel state information (CSI) at the transmitter. In this paper, we investigate the secrecy performance of a single- cell massive MIMO system with RUB in the presence of multiple noncolluding eavesdroppers. Specifically, we derive the closed-form expressions of the secrecy outage probability (SOP) for a massive MIMO transmission and its single legitimate user case. Numerical results are presented to illustrate the performance-complexity tradeoff among RUB based, zero-forcing (ZF) beamforming based, and maximum ratio transmission (MRT) based massive MIMO transmission schemes. We show that RUB based scheme can enhance secrecy performance of massive MIMO transmission with lower implementation complexity.

The evolution of Internet of Things (IoT) as a new application in wireless networks mandates the utilization of wireless cooperative relaying to overcome the energy limitations of IoT devices. Multi-hop relaying is a communication scheme, where packets are forwarded from source to destination through intermediate relay nodes. All these relay nodes are assumed to have buffers for temporarily storing their received packets. During each time-slot, one node can be selected among all nodes to transmit and forward a single packet to the consequent relay node towards the final destination. Based on the nature of the data and its sensitivity to the delay, different schemes can be used to control the movement of packets in the multi-hop networks. This paper presents a framework for the delay analysis of buffered multi-hop networks based on a recently proposed packet-forwarding scheme that uses the best hop for transmission. Based on the channel, the best hop, having the highest signal-to-noise ratio (SNR), is selected. This hop selection procedure produces selection diversity, which minimizes the error and outage probability. The network delay is studied analytically, based on a finite-state Markov chain model. Also, we derive analytical closed form expressions for the average queue length for each relay buffer in the network and the end-to-end network delay. Finally, we compare the delay and outage of the best hop scheme with the conventional multi-hop transmission scheme. The results show how the number of intermediate relays and the buffer size of each one can affect the network delay.

Conventional beamforming transmission techniques can enhance physical layer secrecy performance while requiring the full channel state information (CSI) of legitimate users and even that of eavesdroppers at the transmitter. However, providing full CSI of legitimate users at the transmitter can be challenging in practice. Thus, it is of considerable interest to enhance secrecy performance with partial CSI of legitimate users at the transmitter. Random unitary beamforming (RUB) is a low‐complexity multiple antennas transmission scheme requiring limited CSI. In this study, the authors investigate the secrecy performance of RUB transmission over multiple‐input single‐output single‐eavesdropper and multiuser multiple‐input multiple‐output single‐eavesdropper channels. They also propose a novel RUB‐based artificial noise (AN) method for multiple antennas communication system. They derive the closed‐form expressions of the exact and the asymptotic ergodic secrecy rate and the secrecy outage probability for these transmission scenarios. Numerical results are presented to illustrate the trade‐off between performance and complexity of the resulting physical layer security design. They show that the deployment of RUB and RUB‐based AN offers an attractive solution for enhancing the security of wireless transmission systems.

In this study, the authors present and evaluate the performance of two mode selection schemes for device‐to‐device (D2D) enabled unmanned aerial vehicle‐based wireless networks. The proposed schemes are based on a threshold received signal strength and an average threshold D2D distance to select the D2D mode. The focus of the two schemes is either to enhance the quality of the signal or the connectivity in case of emergency situations. To evaluate the performances of the schemes, they derive the corresponding expressions of the probability of using D2D mode and ergodic capacity. Numerical results show the advantage of the presented schemes in off‐loading traffic from aerial platforms and shed lights on the effect of the environment on the performance of D2D enabled aerial networks.

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

Epileptic seizure detection could be detected through investigating the electroencephalography (EEG), which is deemed to be very important for IoT wearable sensor-based health systems. EEG-based classification is crucial for a wide-range of applications to analyze real-time vital signs using features concerning predefined set of data classes. The aim of this paper is to conduct a comparative study for several classification techniques and demonstrate the effect of uncertainty in the EEG data on the classification accuracy. We define a model for decomposing the EEG using various transformation such as discrete cosine transform, discrete wavelet transform into several sub-bands. After feature extraction, a comparative study to assess the classification algorithms' performance is conducted. In addition, we evaluate their overall accuracy and complexity as performance measures. For this purpose, we use the support vector machine (SVM) and the Artificial Neural Network (ANN). These are chosen as classifier models to study the performance of the obtained features. The discussion will include the evaluation of the classifiers' performance using the EEG-based epileptic seizure data in two categories, noiseless and noisy. In addition, there are some statistical features extracted to characterize the complete EEG data feeding to these two classifiers. A publically available EEG dataset is employed for both normal and epileptic seizure for automatic epileptic seizure detection as a benchmark.

In this paper, we study important physical layer security metrics over wiretap channels with Rician fading. We derive the average secrecy outage rate (ASOR) expression to quantify the average secrecy zero level crossing rate, and the expression of the average secrecy outage duration (ASOD), which measures (in second) how long in average the system remains in the secrecy outage status. Simulation results are conducted to confirm and discuss the theoretical derived expressions. The results show that maximum Doppler frequency shift and signal to noise ratio (SNR) at the eavesdropper are the main factors that affect the ASOR and ASOD.

Free space optical communications (FSO) have witnessed a growing interest in the last few decades due to its various advantages. However, this technology suffers from its sensitivity to the atmospheric conditions such as fog, dust storms, and high temperature, etc. In order to overcome these issues, many techniques have been proposed in literature. In this paper, we study the effectiveness of using hybrid automatic re-transmissions request (HARQ) technique to overcome FSO channels impairments. Two HARQ protocols are considered: ALOHA (ALO) and incremental redundancy (IR). For each protocol, we derive the outage probability expressions, throughput and the average number of the re-transmissions. In addition, we study the HARQ performance in terms of delay and packet sojourn. The results show the effectiveness of using HARQ-IR over ALO protocol.

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.

Random unitary beamforming (RUB) is a multiple antenna transmission scheme requiring limited channel state information (CSI) with low computational complexity. In this paper, we investigate the security performance of RUB over a multiple-input single-output single-eavesdropper (MISOSE) broadcast channel. We derive the closed-form expressions of ergodic secrecy rate and the secrecy outage probability (SOP) of the resulting design. Meanwhile, the effect of artificial noise (AN) on physical layer (PHY) security performance of RUB-based MISOSE transmission is also investigated. Numerical results are presented to illustrate the tradeoff between performance and complexity of the proposed PHY security design. We show that the deployment of RUB offers an attractive solution for enhancing the security of wireless transmission systems.

In this paper, we investigate the communication security in aerial wiretap channels, where a network flying platform (NFP) wishes to send information to a legitimate destination, in the presence of multiple eavesdroppers. The stochastic geometry is used to describe the eavesdroppers' locations, with a given density, and in a specific environment. Based on that, we derive the secrecy outage probability expression for a standard aerial communication network with a single eavesdropper. Then, we evaluate the physical layer security of a standard and a beamforming based aerial communication systems in the presence of multiple eavesdroppers. Monte Carlo simulations are conducted to confirm and discuss the analytical results. These results show that the physical layer security in aerial wiretap channels is essentially affected by the altitude of the NFP, the eavesdroppers' density, and the type of environment.

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.

This paper presents an analytical methodology for the evaluation of two important physical layer security metrics in wiretap channels. Specifically, we first introduce the concept and the expression of average secrecy outage rate (ASOR) to quantify the average secrecy level crossing rate at a predefined secrecy threshold level. Then, we derive the expression of a new metric, namely, average secrecy outage duration (ASOD), which is a measure (in seconds) that describes how long on average the system remains in the secrecy outage status. The results are quite general and account for diversity-based systems operating over independent and identically distributed (i.i.d.) Nakagami-m fading channels. Monte Carlo simulations are conducted to confirm and discuss the analytical results. These results show that the ASOR and the ASOD are essentially affected by the diversity order, and the maximum Doppler frequency shift. In particular, and unlike the ASOD, the ASOR has a maximum value that should be considered in designing systems that are sensitive to secrecy level drops, even for short periods of time. In addition, the proposed new metric of ASOD might have large values even if the currently used metric of average fade duration shows low values on the communication link under consideration.

In this paper, a new mode selection scheme for device-to-device (D2D)-enabled cellular communications with mobility is proposed and evaluated. The new scheme is based on an average threshold D2D distance between two given users to trigger the D2D mode. Besides, a tractable analytical framework considering relative movement for mode selection in moving D2D scenarios is presented. Under the proposed model, the expressions of the average threshold D2D distance, the probability of using D2D mode, the successful transmission probability, and the minimum D2D mode residence time are derived, which are the key parameters to evaluate the performance of D2D-enabled networks. Monte Carlo simulations are conducted to confirm the analytical results and the advantages of the proposed scheme over previously proposed ones. They show also the importance of studying the dynamic performance of the system in terms of user mobility as articulated in the calculated minimum D2D residence time.

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

The densification of small cell base stations in a 5G architecture is a promising approach to enhance the coverage area and facilitate the ever increasing capacity demand of end users. However, the bottleneck is an intelligent management of a backhaul/fronthaul network for these small cell base stations. This involves efficient association and placement of the backhaul hubs that connects these small-cells with the core network. Terrestrial hubs suffer from an inefficient non line of sight link limitations and unavailability of a proper infrastructure in an urban area. Realizing the popularity of flying platforms, we employ here an idea of using networked flying platform (NFP) such as unmanned aerial vehicles (UAVs), drones, unmanned balloons flying at different altitudes, as aerial backhaul hubs. The association problem of these NFP-hubs and small- cell base stations is formulated considering backhaul link and NFP related limitations such as maximum number of supported links and bandwidth. We then present an efficient and distributed solution of the designed problem, which performs a greedy search in order to maximize the sum rate of the overall network. A favorable performance is observed via a numerical comparison of our proposed method with optimal exhaustive search algorithm in terms of sum rate and run-time speed.

5G+ systems expect enhancement in data rate and coverage area under limited power constraint. Such requirements can be fulfilled by the densification of small cells (SCs). However, a major challenge is the management of fronthaul links connected to an ultra dense network of SCs. A cost effective and scalable idea of using network flying platforms (NFPs) is employed here, where the NFPs are used as fronthaul hubs that connect the SCs to the core network. The association problem of NFPs and SCs is formulated considering a number of practical constraints such as backhaul data rate limit, maximum supported links and bandwidth by NFPs and quality of service requirement of the system. The network centric case of the system is considered that aims to maximize the number of associated SCs without any biasing, i.e., no preference for high priority SCs. Then, two new efficient greedy algorithms are designed to solve the presented association problem. Numerical results show a favorable performance of our proposed methods in comparison to exhaustive search.

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.

This paper studies cooperative transmission for securing a two‐hop network where the eavesdropper can wiretap the relay channels. With outdated channel state information (CSI), we propose an opportunistic relaying with destination‐based jamming scheme, where one relay is chosen to forward the confidential signal and the destination sends jamming signal to confuse the eavesdropper. To reveal the capability of the proposed scheme in improving the system secrecy, we investigate the two common performance metrics under different assumptions on the eavesdropper CSI. First, lower bound on the ergodic secrecy capacity (ESC) is derived assuming perfect knowledge of the eavesdropper channel gains. We, then, extend our results to the scenario where only the legitimate channel gain is known. Here, lower bound on the secrecy outage probability (SOP) is obtained. To further improve the utility of the destination, under total power constraint, a power allocation between data signal and jamming noise is considered. Both the analysis and simulation results indicate that the system performance is significantly improved in terms of both ESC and SOP when the jamming power at the destination is adjusted according to the selected relay position. We verify that, with outdated CSI, the secrecy performance of opportunistic relaying is severely degraded.

A mixed free-space optical (FSO) communication scheme is considered with the help of a fixed gain relay. We assume that the links from the source to the relay are radio frequency links with multiuser diversity operating in η-μ fading channels, while the link between the relay and the destination is a FSO link with M-distribution. We first derive expressions for the cumulative distribution function of the end-to-end signal-tonoise ratio for the considered system with pointing errors, and then use them to derive some exact closed-form expressions for the outage probability, bit-error rate performance, and ergodic capacity. To investigate the diversity order and effects of different system and channel parameters on the system performance, we further present the asymptotic performance expressions. On the other hand, we also present the performance analysis for a mixed RF/FSO system with variable-gain relaying and multiuser diversity. Results show that the system diversity orders for both fixed and variable gains systems are min{2Kμ, g2/2, α/2, k/2}, where K is the number of users, g represents the pointing error, and α and k are the channel parameters. Finally, some selected numerical results are given to corroborate the derived analytical results.

Device-to-device (D2D) communications are now considered an integral part of future 5G networks, which will enable direct communication between user equipments and achieve higher throughputs than conventional cellular networks, but with the increased potential for co-channel interference. The physical channels, which constitute D2D communications, can be expected to be complex in nature, experiencing both line-ofsight (LOS) and non-LOS conditions across closely located D2D pairs. In addition to this, given the diverse range of operating environments, they may also be subject to clustering of the scattered multipath contribution, i.e., propagation characteristics which are quite dissimilar to conventional Rayleigh fading environments. To address these challenges, we consider two recently proposed generalized fading models, namely κ-μ and η-μ, to characterize the fading behavior in D2D communications. Together, these models encompass many of the most widely utilized fading models in the literature such as Rayleigh, Rice (Nakagami-n), Nakagami-m, Hoyt (Nakagami-q), and One-sided Gaussian. Using stochastic geometry, we evaluate the spectral efficiency and outage probability of D2D networks under generalized fading conditions and present new insights into the tradeoffs between the reliability, rate, and mode selection. Through numerical evaluations, we also investigate the performance gains of D2D networks and demonstrate their superiority over traditional cellular networks.

In this paper, a general 3-D model for K-tier heterogeneous networks (HetNets) is proposed to investigate the performances of device-to-device (D2D) communications in highrise centers. The new model is based on stochastic geometry, where a 3-D Mat'ern hard- core process (MHCP) is used to describe the different cells positions in the network with realistic constraints. We derive the expressions of average successful transmission probability (STP) for D2D and standard cellular (SC) communications with different well known interference management techniques. Monte Carlo simulations are conducted to evaluate the analytical results. These results confirm the accuracy of the proposed model which presents an efficient description and tractable model to evaluate the performance of dense small cells deployment in HetNets.

Cellular networks are usually described by two-dimensional (2-D) models. These models are appropriate for rural or suburban areas but are not suitable for dense urban environments, where a large number of heterogeneous small cells are deployed to satisfy the rapid increase in mobile subscribers and communication service demands. In this paper, a new general 3-D model for heterogeneous cellular networks is proposed. The 3-D Poisson point process (PPP) is used to describe the positions of the picocells and femtocells in the network. We derive the average coverage probability expressions of downlink heterogeneous cellular networks for a given set of parameters. Monte Carlo simulations are conducted to evaluate the analytical results and the advantage of the proposed 3-D model when compared to the traditional 2-D model. The results confirm the accuracy of the proposed model which presents an efficient description and tractable model of dense small cells deployment in heterogeneous cellular networks.

For wireless devices, which are commonly powered using batteries, energy becomes a severe bottleneck. Many efforts have been spent on the efficient use of battery energy. Recently, an alternative has been explored for prolonging the lifetime of energy constrained wireless networks and low-power wireless sensor networks. This alternative operates by availing the ambient radio-frequency (RF) signal, where energy harvesting (EH) and information transmission processes can simultaneously take place. EH has emerged as a promising technique for green communications, as it can power communication systems with renewable energy. In this paper, we investigate the performance of overlaid wireless sensor transmission powered from existing wireless system by RF-EH. More specifically, we first propose a new transmission strategy, and we derive the exact closed-form expression for the distribution function of harvested energy over one coherence time over Rayleigh fading channels. Second, we derive the expressions of average packet loss, average packet delay and average consumed power of sensor transmission subject to interference from existing system. Finally, numerical results are used to confirm the analytical expression, and to validate the performances of the proposed strategy when compared to existent transmission strategies.

This work proposes a method to optimize the resource block allocation in the cloud radio access network for a long term evolution system. This approach consists in three steps. First, the baseband unit collects the required information about users. Then, it optimizes the resource block allocation by carrying out a genetic algorithm. After that, it sends the solution to the remote heads. The simulations results show that the performance of the proposed system is significantly higher with respect to universal frequency reuse.

According to 3GPP standards, the Single Carrier Frequency Division Multiple Access (SC-FDMA) was adopted for Uplink transmission in LTE networks. As multiple mobiles may have access to neighboring base stations simultaneously, the inter-cell interferences are inevitable with this technique and the system throughput is impacted. To mitigate the inter-cell interference and improve, therefore, the system performance, severe schemes have been proposed. However, these schemes target either adapting the power control in mobile stations (e.g. Fractional Power Control technique) or improving the Resource Block (RB) allocation (e.g. Universal Frequency Reuse technique). In this paper, we propose two new techniques to reduce the inter-cell interferences namely: (i) Dynamic Fractional Power Control and (ii) a Resource Block Allocation based on Genetic Algorithm. In addition, we advocate that, by combining these technique, the system can achieve a better throughput.

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.

In this paper, we present an optimization of the effective area spectral efficiency (EASE) metric for point-to-point transmission systems, and decode-and-forward (DF) relaying communications networks under Nakagami-m fading channels. For each transmission mode, we derive a closed-form expression for the maximum transmission range and use it to derive the average affected area, and the average ergodic capacity. We then introduce the EASE expression to quantify the spatial spectral utilization efficiency. For DF relaying, the EASE metric is based on a newly introduced index, namely, useful relaying index (URndx), which is used to validate the communication possibility between a source and a relay for given transmission parameters in a given environment, and provides information about the necessity of using relaying communications. Based on the expression of EASE, we derive the optimal transmission powers that maximize the EASE for each mode. Through mathematical analysis and numerical examples, we show that the EASE metric provides a new perspective on the design of wireless transmissions, especially the transmission power optimization process.

We consider a cooperative relaying system with any number of source terminals, one shared relay, and a common destination. We assume a slotted time division multiple access (TDMA) framework in which each source terminal is allocated a fraction of the time. We propose a novel hybrid cooperative scheme for the described network. In contrast to former works which assume that the relay only transmits in the idle time slots, we assume that the relay can, simultaneously, transmit with the source terminals via multi-access channel (MAC). In hybrid cooperative scheme, the relay operates in two modes each with a certain probability; the TDMA mode and the MAC mode. We derive expressions for the stability conditions and the average delay for all the queues in the network. We design the probability of each relaying mode such that the stable throughput is maximized while the network queues are stable. The problem is formulated as a non-convex quadratic constrained quadratic programming (QCQP) optimization problem. Numerical results reveal that the hybrid cooperative scheme significantly enhances the performance of the network in terms of stability region, average delay, and spectral efficiency.

Cooperative spectrum sensing (CSS) has been proposed as a solution for radio spectrum resources scarcity problem. Clustering technique is introduced to increase the performance of CSS and overcome the shadowing and fading effects, also to reduce the control channel overhead for big number of cooperative users. However, Clustering is facing two major issues which are the data falsification caused by the malicious users and the increasing delay and energy consumption especially when the cluster heads is far away from the fusion center. In this paper, we propose a cluster and forward based on the trust CSS. By dividing all the secondary users into clusters and using of the trust threshold for selecting the most trusted CHs that can send their sensing decisions to the fusion center which can save more energy consumption and delay transmission, and improve the spectrum sensing performance.

In this paper, a cooperative communications scheme with interference management is proposed for Full-Duplex (FD) multi-user, decode-and-forward (DF) relay networks. The scheme is based on a relay selection method that maximizes the received signal-to-noise ratio (SNR). We evaluate the performance of the system using two approaches for interference management, namely, the constellation real parts (CRP) of the modulated signals, and a power adjustment technique. We derive expressions for the average outage probability of the up-link (UL) and downlink (DL) for the proposed scheme, as well as for standard halfduplex (HD), and standard FD. Numerical results are provided to validate the analysis and the performance of the proposed scheme.

We consider a cognitive radio system that consists of primary user, secondary user, and their destinations. The secondary user has a relaying capability, i.e., it transmits the relayed packets from the primary user. Unlike most of the previous works that restrict the secondary user to transmit only in the idle time slots, we assume that the secondary user interferes on the primary user with certain probability that is optimized to maximize the stable throughput of the secondary network under certain level of quality of service constraints for the primary one. We show how significantly our proposed scheme improves the performance of the secondary user and increases the maximum stable throughput of the primary user over the traditional cooperative policies that restrict the secondary user to exploit only the periods of silence of the primary user.

In this study, the authors investigate the downlink area spectral efficiency (ASE) of infrastructure relay enhanced cellular systems. ASE for cellular systems is defined as the maximum achievable data rate per unit bandwidth per unit area supported by a base station. In relay enhanced systems, ASE can serve as an attractive performance metric by capturing the benefit of the relatively smaller spatial footprint of relay transmission and the resulting lower co‐channel interference. In this study, by applying a moment generating function based approach, the authors first derive the general closed‐form statistics of the total interference from dominant co‐channel cells over Rayleigh fading channels. These interference statistics are readily applied to the calculation of the resulting ASE. The authors also propose a novel in‐cell frequency reuse scheme to further exploit the smaller transmission power of relay stations (RSs). Through selected numerical examples, the authors show that in relay enhanced systems, the cell size, transmission power and RS positions should be properly selected to gain better ASE performance than the conventional systems without relays.

We investigate the spectrum sharing possibility as an incentive to enhancing the physical layer security. The concept behind this is that a legitimate source-destination pair, communicating in the presence of a passive eavesdropper, stimulates the help of another source-destination node looking for transmission opportunities, referred to as secondary network (SN). We thus propose a cooperative scheme, whereby the secondary transmitter will act as a friendly jammer to confound the eavesdropper and be granted to share the spectrum of the legitimate pair, referred to as primary network (PN), as a reward. To evaluate the performance of our cooperative scheme, we derive the closed-form expression of the secrecy outage probability (SOP). However, to assess its usefulness, we introduce the mutual outage probability (MOP) and we carry out analysis to derive its expression. We show that a satisfactory SOP is a necessary condition for triggering the cooperation, whereas a satisfactory MOP is a sufficient condition to guarantee that the PN-SN collaboration leads to a win-win situation. Results have been presented considering outdated channel state information, thus giving more insights for practical setups.

Abstract Cooperative communication is a promising technique for future wireless networks. It can be used in improving communication reliability and enhancing spectrum efficiency by using the broadcast nature of radio communication and exploiting cooperative diversity. However, its performance gain degrades in the presence of co‐channel interference, which makes it essential to propose interference mitigation schemes. In this paper, we introduce three cooperative communication schemes with interference management for multi‐user cooperative wireless networks. The first scheme (best relay selection) is used as a performance benchmark because it completely avoids the interference problem by using the Frequency‐Division Multiple Access technique. The second scheme (best available relay selection) maximizes the received signal‐to‐noise ratio while keeping the interference levels below a certain threshold, and the third scheme (General Order Relay and User Selection) is based on iterative resource allocation algorithm. We derive exact closed‐form expressions of average bit error probability, outage probability, and average consumed power for the proposed schemes. Simulations are used to validate the analytical results. The results confirm the advantage of the proposed cooperation schemes in enhancing the system performance and improving the interference management. Copyright © 2015 John Wiley & Sons, Ltd.

In this paper, we investigate the spectrum sharing incentive to enhance the physical layer security. The concept behind is that a legitimate source- destination pair, communicating in the presence of a passive eavesdropper, stimulates the help of another source-destination nodes looking for transmission opportunities, referred to as secondary network (SN). We thus propose a cooperative scheme, whereby the secondary transmitter will act as a friendly jammer to confound the eavesdropper and be granted to share the spectrum of the legitimate pair, referred to as primary network (PN), as a reward. To evaluate the performance of our cooperative scheme, we derive the closed form expression of the secrecy outage probability (SOP). However, to assess its usefulness, we introduce the mutual outage probability (MOP) and we carry out analysis to derive its expression. We show that a satisfactory SOP is a necessary condition for triggering the cooperation, whereas a satisfactory MOP is a sufficient condition to guaranty that the PN-SN collaboration leads to a win-win situation.

In this paper, we introduce a new metric, namely: effective area spectral efficiency (EASE), to quantify the spectral efficiency as well as the spatial properties of decoding and forward (DF) relaying wireless communications networks with interference management. The EASE metric is based on the average affected area, the average ergodic capacity, and a new introduced index, namely: source relay communication index (SRCndx). We derive a closed-form expression for the maximum transmission range under Rayleigh fading environment. Based on the maximum transmission range, we define and derive the average affected area and the average ergodic capacity for DF relaying communications system. The SRCndx is used to validate the communication possibility between a source and a relay for given transmission parameters in a given environment, and provides information about the necessity of using relaying communications. We then introduce the EASE expression to quantify the spatial spectral utilization efficiency. Through mathematical analysis and numerical examples, we show that the EASE metric provides a new perspective on the design and optimization of wireless transmissions, especially the transmission power selection process.

In a spectrum sharing relay-aided secondary network, the design objective shifts from minimizing the violation of the primary system constraints when using the outdated channel state information (CSI), to making the best use of the available (imperfect) channel gain while maintaining the primary system Quality of Service (QoS). To address this trend, we propose a new power allocation based on a linear regression model and we carry out the outage analysis as well as the asymptotical analysis where we deduce the diversity order of the secondary network. The performance results show that the proposed power allocation strategy outperforms the existing schemes in the literature and provides an accurate tight approximation of the ideal case under perfect CSI while respecting the primary system QoS and using outdated CSI.

In this paper, we introduce a new power spectral efficiency (PSE) metric, to quantify the spectral efficiency and the total required power to cover a certain area with a predefined received signal quality. This metric offers the possibility to find the optimal transmission power in each base station to cover the total given area with the minimum total consumed power. We start by deriving the closed-form expression of the maximum transmission range under Rayleigh fading environment. Based on the maximum transmission range, we define and derive the average affected area, the total consumed power and the average ergodic capacity. Then, we introduce the PSE expression, to quantify the power spectral utilization efficiency. After that, we evaluate the new metric for cellular network with 3-cell frequency reuse cluster (3-CFRC), without inter-cell interference (ICI), and the network with 1-cell frequency reuse cluster (1-CFRC) and ICI. Through mathematical analysis and numerical examples, we show that the PSE metric provides a new perspective on the design and optimization of wireless transmissions, especially on the transmission power selection.

Future mobile networks are converging toward heterogeneous multitier networks, where macro-, pico-, and femtocells 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.

In this paper, we consider a free-space optical (FSO) communication scheme with the help of a variable gain relay where the links from the source to the relay are radio-frequency (RF) links while the links between the relay and the destination are FSO links with M-distribution. To mitigate the effects of multipath fading and atmospheric turbulence, we apply transmit diversity at the source and selection combining (SC) at the destination, while the relay is only equipped with single RF receive antenna for receiving signal and one aperture for relaying the information. With this setup, we first derive expressions for the outage probability, bit-error rate, and the average capacity. We further investigate the effect of pointing errors on the system performance.

In this paper, we consider an amplify-and-forward hybrid satellite-terrestrial co-operative network in the presence of cochannel interference (CCI). More specifically, we assume that both the satellite-destination and the satellite-relay links undergo the shadowed-Rician fading and the relay-destination link follows the Nakagami-m fading. By applying the amplify-and-forward (AF) protocol at the terrestrial relay, both the relay and the destination are corrupted by cochannel interference from the terrestrial network. Based on this setup, we first derive the approximate statistical distributions signal-to-interference-plus noise ratio (SINR) and then analyze the system performance. To obtain more insights into the system performance, some asymptotical bit-error rate (BER) results are provided. Moreover, we extend our analysis to a multiple-relay network and analyze the asymptotic performance. Finally, some numerical results are provided to verify our analysis.

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 10-2as compared to conventional fixed network coding schemes.

In this paper, two cooperative communications schemes with inter-relay interference (IRI) management are proposed for wireless multi-user decode-and-forward (DF) relay networks. The schemes are based on a DF half-duplex (HD) relaying protocol and a relay selection method which maximizes the signal-to-noise ratio (SNR) of the second hop. To minimize the IRI, the first scheme [constellation real part (CRP)] uses a new transmission scheme based on the constellation real parts of the modulated signals, and the second scheme [previous message buffering (PMB)] uses a buffering technique at the relays. To assess the performance, we derive the expressions of the average bit error rate (BER) for the proposed schemes. Numerical results are given to confirm the analytical expressions and the advantage of the proposed schemes in enhancing interference management for wireless cooperative networks.

Nurturing design skills in students is in the heart of all engineering curricula. While academic courses and capstone activities inject the design process in students, involvement in practical design activities is believed to have higher impact on student preparation for engineering professions. Capitalizing on this fact, this paper aims to study the impact of Shell Echo-Marathon (SEM), an international engineering design competition, on engineering student learning at Qatar University (QU). The paper elaborates on the involvement of QU students in the competition and the experience accumulated over the years. In addition, impact measure methodology of the empirical investigation is highlighted together with the process of instruments development. This is followed by details on the investigation findings. The paper concludes the existence of positive impact of SEM on 21stcentury engineering competencies development, as well as, a shift in perceptions, confidence, and attitudes of engineering profession, engineering nature, and personal skills.

In this paper, we investigate relay-sharing in cognitive radio (CR) networks to improve the secondary user's (SU's) performance in the presence of interference. It is assumed that the relay is shared simultaneously between both the primary user (PU) and the SU by using hierarchical modulation at the relay node while mapping the PU's and the SU's symbols onto two different layers of the hierarchical constellation. The proposed scheme relies on power adjusting, where the SU power adjustment is based on channel side information (CSI) and tolerable interference limit at the PU. Theoretical analyses along with simulation results are provided to evaluate the performance of the proposed dynamic spectrum sharing scheme. The results demonstrate that the proposed relay-sharing scheme may result in about 7dB and 15dB signal to noise ratio (SNR) improvement for the SU in low-SNR and high-SNR regimes, respectively.

In this paper, we study the problem of multiple antenna spectrum sensing by using cyclostationary features of Primary Users (PUs) signals in Cognitive Radios (CRs). We consider the general case of multiple antenna sensing in the presence of spatially and temporally correlated noise when the PU signal has more than one cyclic frequency. We model and formulate the multiple antenna sensing problem as a composite hypothesis testing problem and use the Generalized Likelihood Ratio Test (GLRT) to derive a detector for the general model mentioned above. Then, we also propose the GLRT-based detectors for the two special cases of: 1) spatially uncorrelated but colored noise; 2) spatially white noise. Moreover, in order to calculate the decision threshold, the asymptotic performance of the proposed detectors under the null hypothesis is given. The provided simulation results show the superiority of the performance of the proposed detectors compared to the recently-proposed cyclostationary-based detectors.

In this paper, we study the performance of hybrid cognitive radio (CR) system in the presence of channel estimation error (CER) in terms of bit-error-probability (BEP) and outage probability. We assume simultaneous switching between the underlay and overlay modes of CR where in, the secondary user (SU) probabilistically accesses the primary user's (PU) channel in both underlay and overlay modes to utilize the benefits of hybrid CR system. The PU uses two-layer 2/4 — ASK constellation with unequal level of bit protection against the interference imposed by the SU's transmitter. The SU wishes to opportunistically use the PU's channel to transmit its own data while maintaining the performance of the PU's layer with the worst level of protection at a desired value. We derive the SU's maximum allowable transmit power in the overlay mode to guarantee the PU reliable communication then, we discuss about the optimum value of the hybrid switching rate that minimizes the outage probability of the SU along with, deriving the exact BEP and outage probability expressions for the secondary system as well as that of the first and the second layers of the PU system.

We consider in this paper relay assignment for cooperative systems with mutiple source-destination pairs. The objective here is to assign the relays to the source-destination pairs in a such way that all pairs would achieve the maximum diversity. Normally, for a network with mutiple source-destination pairs, none of the destinations can acquire the channel state information (CSI) of the entire network without feedback. To this end, we design a practical limited feedback strategy in conjunction with two relay assignment schemes, i.e., fullset selection and subset selection, which are based on maximizing the minimum end to end (E2E) signal to noise ratio (SNR) among all pairs. In this strategy, each destination acquires its signal-to-noise ratios (SNRs), quantizes them, and feeds them back to the relays. The relays construct the end-to-end (E2E) SNR matrix and select the relay assignment choice from all possible relay assignment permutations or only a subset of these permutations. We analyze the performance of these schemes over independent Rayleigh fading channels in terms of the worst E2E SNR. We investigate the asymptotic performance of the proposed schemes at high SNR. We show that relay assignment with quantized CSI can achieves the same first-order diversity as that of full CSI, but there is a second-order diversity loss. We also demonstrate that increasing the quantization levels yields performance that is close to that of having full knowledge of the CSI.

Privacy and security have an increasingly important role in wireless networks. A secure communication enables a legitimate destination to successfully retrieve information sent by a source, while it disables the eavesdropper (illegitimate destination) to interpret the intercepted information. Physical (PHY) layer security approaches for wireless communications can prevent eavesdropping without encryption. It exploits the physical characteristics of the wireless channel in order to transmit messages securely. They are typically feasible when the source-destination channel is better than the source-eavesdropper channel. Cooperative schemes are a means to improve the performance of secure wireless communications. We propose an improved relay selection scheme, based on source-eavesdropper channel SNR restriction, that will guarantee the best secrecy rate at the destination under QoS condition. Performance has been studied in terms of secrecy rate, outage probability and average error probability. Simulations shows that the proposed scheme outperforms in terms of the secrecy rate at the destination when compared to techniques in the literature.

In this paper, two cooperative communications schemes with inter-relay interference (IRI) management are proposed for multi-user wireless networks. The first scheme (Interference management at relay station (IMR)) uses the decode-and-forward (DF) relaying protocol and treats the IRI at relay station. The second scheme (Interference management at mobile station (IMM)) uses the amplify-and-forward (AF) relaying protocol and treats the IRI at mobile station. Numerical results are given to confirm the advantage of the proposed schemes in enhancing interference management for wireless cooperative networks.

Future mobile networks are converging towards being heterogeneous, owing to the co-existence of multi-tier networks within the same geographical area, including macro, pico- and femto-cells. The deployment of such networks is generally based on user demand, which is irregular and random, implying that the deployment of base stations (BSs) is random as well. As a result, analyzing the communication protocols over heterogeneous networks (HetNets) is very challenging. A popular approach is to use stochastic geometry and treat the location of the BSs as points distributed according to a spatial Point Process. Most of the related work on the interference modeling normally assumes homogeneous Poisson point process (PPP). This assumption holds when the nodes are uniformly distributed in space, such as sensor networks or ad-hoc networks. Due to geographical factors, it may be the case for mobile users to cluster around highly populated cities and the PPP assumption does not provide an accurate model for the interference in these conditions. This motivates us to find better ways to characterize the aggregate interference when the transmitting nodes are clustered following a Poisson Cluster Process (PCP). Furthermore, the BSs belonging to different tiers may differ in terms of the transmit power, the node densities, and their link reliabilities. To this end, we consider K-tier HetNets, where, by using the Laplace transform approach, we characterize the aggregate interference at a given destination as a heavy-tailed distribution. Using the derived distribution, we investigate the probability of outage and coverage for such networks. Due to some difficulty in obtaining closed-form expressions for these measures, we derive tight bounds and verify that through numerical examples. We also compare the performance of HetNets when the nodes are clustered and otherwise. We observe that using the PPP results in larger success probability, but using the clustered process results in a larger coverage probability. We also observe that there is an optimal intensity, i.e., number of nodes, that achieves the maximum coverage probability for the given SINR (signal-to-interference-plus-noise ratio) threshold.

The scarcity of radio resources coupled with the high data rate demands in the last few years, induced incremental relaying techniques one of the most promising technologies. This is simply because it allows reaping spatial diversity and saving the channel resources at the same time. In this contribution, we make use of the Decode and Forward incremental relaying technique in OFDM cooperative systems by proposing and analyzing two subcarrier mapping algorithms. Based on outage probability, diversity order and power gain analysis, we derive the optimal parameters for these algorithms to enhance the outage performance of DF cooperative system by reducing the effect of error propagation. Also, an incremental relay sharing scheme is proposed allowing multiple sources to share the same relay and results in an improved spectral efficiency, which is critical given the high cost and scarcity of RF spectrum driven by the growing big data.

In this contribution, we investigate the outage performance of OFDM-based Decode and Forward (DF), Amplify and Forward (AF) and Controlled DF (CDF) cooperative systems under IQ Imbalance (IQI). In particular, tractable and compact approximate outage probability expressions are derived and the effect of the different IQI parameters is analyzed for different SNR ranges. Furthermore, by localizing the error floor in terms of IQI and SNR for each technique, we demonstrate when it is more beneficial to invest in increasing the transmission power or in compensating the imbalance. Moreover, we prove that the IQI compensation should be concentrated in the relay for some techniques and in the destination for some others. A comparative study between AF, DF, CDF and direct link transmission techniques is also conducted for different IQI parameters, SNR ranges, transmission rates and relay's position. Hence, this work may create a paradigm for future studies of more effective adaptive IQI compensation techniques that concentrate the compensation on the right IQI, SNR ranges, transceivers depending on the used transmission technique.

Area spectral efficiency (ASE) was introduced as a metric to quantify the spectral utilization efficiency of cellular systems. Unlike other performance metrics, ASE takes into account the spatial property of cellular systems. In this paper, we generalize the concept of ASE to study arbitrary wireless transmissions. Specifically, we introduce the notion of affected area to characterize the spatial property of arbitrary wireless transmissions. Based on the definition of affected area, we define the performance metric, generalized area spectral efficiency (GASE), to quantify the spatial spectral utilization efficiency as well as the greenness of wireless transmissions. After illustrating its evaluation for point-to-point transmission, we analyze the GASE performance of several different transmission scenarios, including dual-hop relay transmission, three-node cooperative relay transmission and underlay cognitive radio transmission. We derive closed-form expressions for the GASE metric of each transmission scenario under Rayleigh fading environment whenever possible. Through mathematical analysis and numerical examples, we show that the GASE metric provides a new perspective on the design and optimization of wireless transmissions, especially on the transmitting power selection. We also show that introducing relay nodes can greatly improve the spatial utilization efficiency of wireless systems. We illustrate that the GASE metric can help optimize the deployment of underlay cognitive radio systems.

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%.

We consider in this paper relay assignment for cooperative systems with mutiple source-destination pairs. The objective here is to assign relays to the source-destination pairs in such a way that all pairs achieve the maximum diversity. In networks with mutiple source-destination pairs, it is normally difficult for destinations to acquire the channel state information (CSI) of the entire network without feedback. To this end, we design a practical limited feedback strategy in conjunction with two relay assignment schemes, i.e., fullset selection and subset selection, which are based on maximizing the minimum end-to-end (E2E) signal to noise ratio (SNR) among all pairs. In this strategy, each destination acquires its SNR, quantizes it, and feeds it back to the relays. The relays then construct the E2E SNR table and select the relay assignment permutation from all possible relay assignment permutations or only a subset of these permutations. We analyze the performance of these schemes over independent Rayleigh fading channels in terms of the worst E2E SNR. We derive closed-form expressions for the E2E bit error rate (BER) and investigate the asymptotic performance at high SNR. We show that relay assignment with quantized CSI can achieve the same first-order diversity as that of the full CSI case, but there is a second-order diversity loss. We also demonstrate that increasing the quantization levels yields performance that is close to that of having full knowledge of the CSI.

The demand for qualified national engineers is high in Qatar; Currently, a wide gap in supply exists. Consequently, urgent and proactive interventions at K-12 level to promote engineering profession to school students are required to increase the number of Qatari students who choose to study engineering. Since 2009, the College of Engineering at Qatar University is running the Life is Engineering Program (LIEP) as an outreach springboard program targeting the country high schools. This paper presents a reflection on the LIEP project, its organization, structure, and an assessment of its impact on high school students.

This paper provides the findings of a study on investigating the required contextual engineering skills in Qatar in light of the global engineering skills. A set of 20 attributes were identified in the literature, and surveys were implemented to measure the importance of these skills in Qatar. The targeted groups spanned from students, practicing engineers, senior industrial engineers, and academicians. The basic logic behind surveying various engineering groups was mainly to evaluate the capabilities of the current engineering labor force as well as evaluating the potential of the future engineering supply (current students). The main findings indicated the consensus of participating groups on the importance of enhancing communication skills. High perceptual gaps were identified in communication skills, business and entrepreneurship and practical skills. Remedy actions that were proposed so that the future supply of engineers is featured by required and desired level of selected skills.

In this paper, two cooperative communications schemes with interference management are proposed for multiuser wireless networks. The first scheme (Interference Constraint based Relay Selection (ICRS)) maximizes the received SNR while keeping the interference levels below a certain threshold, and the second scheme (General Order Relay and User Selection (GORUS)) is using channel state informations (CSIs) based resource allocation algorithm to improve the system performance. We derive exact closed form expressions of outage probability, ergodic capacity, and average bit error probability for the proposed schemes. Simulations are used to validate the analytical expressions. The results confirm the advantage of the proposed schemes in enhancing interference management and link reliability.

Cooperative communication is a promising technique for future wireless networks. It can be used in improving communication reliability, and enhancing power and spectrum efficiency. However, its performance gain degrades in the presence of co-channel interference which makes it essential to propose interference mitigation schemes. In this paper, we introduce two efficient cooperative communication schemes with interference management for multi-user cooperative wireless networks that are based on best relay and user selection (BRUS) technique. BRUS maximizes the received signal to noise ratio (SNR) while minimizing the interference by an optimal time slot allocation for the users. The first introduced scheme is using BRUS technique only if the user demands cooperation, and the second scheme is always using BRUS technique to enhance the system performance. We derive exact closed form expressions for the outage probability, ergodic capacity, average consumed power, and average bit error probability for the introduced cooperative schemes. Simulations are used to validate the analytical results and an agreement is observed. The results confirm the advantage of the introduced cooperation schemes in enhancing the system performance and improving the interference management.

Area spectral efficiency (ASE) was introduced as a metric to quantify the spectral efficiency of cellular systems. Unlike other performance metrics, ASE takes into account the spatial property of cellular wireless system. In this paper, we generalize the ASE metric for arbitrary wireless transmission. Specifically, we introduce the concept of affected area, based on which we evaluate the spectral and power efficiency of three different transmission scenarios, namely single user point-to-point transmission, two-user X channel transmission and two-hop relay transmission. We derive the closed-form expressions of resulting generalized ASE (GASE) metric for each transmission scenario over Rayleigh fading environment. We show through mathematical analysis and numerical examples that there is an optimal choice of transmitting power value in terms of maximizing the area spectrum efficiency.

Cognitive Radio is a promising concept that offers solution to spectral crowding problem by introducing opportunistic usage of the unoccupied licensed frequency bands. Improving the network capacity and reliability of the communication has been recently studied and this through the introduction of cooperative technology in the CR networks. Furthermore, OFDM system is considered to be a good candidate to enhance the flexibility of the CR network, where individual carriers can be switched off for occupied frequencies by the primary user (PU) or for interfering Sub-channels. In this paper, an underlay cognitive network based on OFDM technique is proposed where secondary users (SUs) coexisting with a PU are adhering to stringent interference threshold constraint. Cooperative technology will help improving the CR network performances under PU limitation. This paper proposes new Sub-channel selection schemes which adapt either the transmission power or the sequential order of selection to enhance the system performances. We first provide the exact probability density function (PDF) of the received SNR at the secondary destination. Then, the PDFs are used to derive closed form expressions of the outage probability, the average BER and the ergodic capacity. Analytic results are validated by simulation for the proposed Sub-channels selection schemes.

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 10-3as compared to conventional fixed network coding schemes.

In this paper, two cooperative communications schemes with interference management are proposed for orthogonal frequency division multiple access (OFDMA) based wireless networks. The first scheme maximizes the received signal to noise ratio (SNR) while keeping the interference levels below a certain threshold, and the second scheme maximizes the received SNR with resource allocation algorithm based . We derive exact closed form expressions for the probability density function (PDF) of the SNR, outage probability, ergodic capacity, average bit error probability, and average consumed power for the proposed cooperative schemes. Simulations are used to validate the analytical results and an agreement is observed. The results confirm the advantage of the introduced schemes in enhancing interference management and in improving spectrum efficiency and link reliability.

Poisson point process has been widely adopted in the analysis of co-channel interference in dense frequency reuse systems. With the decaying power law, the aggregate co-channel interference in Poisson field can be viewed as the output of a shot noise process. In this paper, we first consider the wireless transmission of a single transmitter- receiver pair with a guard zone centered at the receiver. Other interfering transmitters are randomly distributed outside the guard zone according to a spatial Poisson process. We derive the generic closed-form moment generating function (MGF) of the aggregate interference over Rayleigh fading channels. Then we show that the ergodic capacity of wireless transmission in Poisson field over Rayleigh fading channels can be expressed as function of the MGF of the aggregate interference. Through numerical examples, we examine the accuracy of the Gaussian approximation on the aggregate interference in terms of ergodic capacity calculation. Finally we introduce the Matern point process to model the spatial distribution of multiple transmitter-receiver pairs with guard zones centered at respective receivers.

Cognitive radio is a promising technology to improve the spectrum utilization by allowing the unlicensed (secondary) user share a frequency bandwidth with the licensed (primary) owner under the condition that no harmful interference is imposed on the licensee. In this paper, we analyze the spatial spectrum utilization efficiency of underlay cognitive radio transmission. In particular, we apply the performance metric, generalized area spectral efficiency (GASE), to the performance evaluation and design of underlay cognitive radio transmission. Closed-form expressions are given on the ergodic capacity of the primary and secondary user in underlay cognitive radio transmission over Rayleigh fading channels, which are then utilized to evaluate GASE metric. Mathematical analysis and numerical examples show the asymptotic GASE performance of the underlay cognitive radio transmission with respect to the interference constraint on the primary receiver. We show that GASE performance metric provides a new perspective to the design of underlay cognitive radio transmission.

We address in this paper the challenge of coping with asymmetric data rates in two-way relay channels. We consider a relay network comprising two sources and one relay. The sources communicate at different rates through the relay. That is, we assume that one source uses M1-QAM (quadrature amplitude modulation) and the other uses M1/M2-QAM hierarchical modulation where M1≠ M2. For the underlying network, we consider two decode-and-forward (DF) relaying schemes. One scheme combines hierarchical zero padding and network coding (HZPNC) at the relay. The novelty of this scheme lies in the way the two signals (that have different lengths) are network-coded at the relay. The other scheme is referred to as opportunistic user selection (OUS) where the user with a better end-to-end channel quality is given priority for transmission. We analyze both schemes where we derive closedform expressions for the end-to-end (E2E) bit error rate (BER). Since the two schemes offer a trade-off between performance and throughput, we analyze and compare both schemes in terms of channel access probability and average throughput. We show that HZPNC offers better throughput and fairness for both users, whereas OUS offers better performance. We also compare the performance of HZPNC with existing schemes including the original zero padding, nesting constellation modulation and superposition modulation. We show through examples the superiority of the proposed HZPNC scheme in terms of performance and/or reduced complexity.

Relaying primary signals by cognitive base-stations (CBSs) can help the primary system and thus win CBSs a higher chance to transmit their own signals. For this purpose, conventional zero-forcing (CZF) beamforming is a straightforward solution where the primary and cognitive signals are transmitted from a multi-antenna CBS without causing interference to each other. However, with CZF, no priority is given to the primary user (PU), which is not consistent with the idea of cognitive radio. In this paper, we shall propose a prior ZF (PZF) scheme which gives priority to the PU by transmitting primary signals without considering their interference to the cognitive users (CUs), while cognitive signals are not allowed to generate interference to the PU. As a result, PZF provides a better channel for the CBS-PU link than CZF but the same channel gain for the CBS-CU links as CZF. We compare PZF and CZF by considering both the transmit power with given target rates and the outage performance with given transmit power, where closed-form conditions are derived to indicate their respective advantages. One of the important contributions of this paper is to prove that, with one CU, a target rate of 1 bit/s/Hz for the CU is the key point that differentiates PZF and CZF, which is independent of the number of CBS-antennas, the channel distributions, and the signal-to-noise power ratio (SNR).

ABSTRACT In this paper, we proposed several antenna selection schemes for cooperative diversity systems with adaptive transmission. The proposed schemes were based on dual‐hop relaying where a relay with multiple‐antenna capabilities at reception and transmission is deployed between the source and the destination nodes. We analyzed the performance of the proposed schemes by quantifying the average spectral efficiency and the outage probability. We also investigated the trade‐off of performance and complexity by comparing the average number of active antennas, path estimations, and signal‐to‐noise ratio comparisons of the different proposed schemes. Copyright © 2011 John Wiley & Sons, Ltd.

ABSTRACT In cooperative communications, multiple relays between a source and a destination can increase the diversity gain. Because all the nodes must use orthogonal channels, multiple‐relay cooperation becomes spectrally inefficient. Therefore, a bestrelay selection scheme was recently proposed. In this paper, we analyzed the performance of this scheme for a system with the relays operating in amplify‐and‐forward mode over identical Nakagami‐m channels using an exact source–relay–destination signal‐to‐noise ratio (SNR).We derived accurate closed‐form expressions for various system parameters including the probability density function of end‐to‐end SNR, the average output SNR, the bit error probability, and the channel capacity. The analytical results were verified through Monte Carlo simulations. Copyright © 2011 John Wiley & Sons, Ltd.

In this paper, performance of relay based MAC protocol for two way traffic is evaluated in non-saturated condition. In real life, non-saturation is more prevalent where nodes are mostly idle. The idea of Two Way Opportunistic-Medium Access Control (TWO-MAC) protocol is inspired from the current drive to "go green". TWO-MAC uses high data rate nodes to work as relays (by using network coding) for the low data rate nodes in two way communications. TWO-MAC results in higher throughput, lower delays and reduced energy consumption. This is due to the use of relay and network coding for two way transmission which lowers overall blocking time and contention due to faster transmission. Further, it lowers unnecessary overhead and overhearing which reduces the energy consumption. The contribution in this work is the throughput, energy and delay gains of using network coding at the relay in a realistic non-saturated network.

Cognitive networks in underlay settings operate simultaneously with the primary networks satisfying stringent interference limits. This condition forces them to operate with low transmission powers and confines their area of coverage. In an effort to reach remote destinations, underlay cognitive sources make use of relaying techniques. Selecting the best relay among those who are ready to cooperate is different in underlay settings than traditional non-cognitive networks. In this paper, we present a relay selection scheme which uses the quotient of the relay link signal to noise ratio (SNR) and the interference generated from the relay to the primary user to choose the best relay. The proposed scheme optimizes this quotient in a way to maximize the relay link SNR above a certain value whereas the interference is kept below a defined threshold. We derive closed expressions for the outage probability and bit error probability of the system incorporating this scheme. Simulation results confirm the validity of the analytical results and reveal that the relay selection in cognitive environment is feasible in low SNR regions.

Best relay selection is a bandwidth efficient technique for multiple relay environments without compromising the system performance. The problem of relay selection is more challenging in underlay cognitive networks due to strict interference constraints to the primary users. Generally, relay selection is done on the basis of maximum end-to-end signal to noise ratio (SNR). However, it requires large amounts of channel state information (CSI) at different network nodes. In this paper, we present and analyze a reactive relay selection scheme in underlay cognitive networks where the relays are operating with fixed gains near a primary user. The system model minimizes the amount of CSI required at different nodes and the destination selects the best relay on the basis of maximum relay to destination SNR. We derive close form expressions for the received SNR statistics, outage probability, bit error probability and average channel capacity of the system. Simulation results are also presented to confirm the validity of the derived expressions.

In underlay cognitive networks with regenerative relaying, secondary users operating with primary user are adhering to stringent interference constraint which limits their transmission power and coverage area. In order not to violate this interference limit, underlay network will make the use of relays to transmit signal over the secondary network. The retransmitting relay will be selected among the available secondary users; hence, relay selection becomes more challenging due to strict interference limits. Relay selection is based not only on interference limit, but also on threshold constraint which ensure the satisfactory reception of the signal by the relays and the destination and which is an important criterion since the relaying protocol used is Decode and Forward (DF). This paper proposes three new relay selection schemes based on relay to destination link and relay to primary link qualities. We derive closed form expressions of the probability density function (PDF) of the SNR at the secondary destination, the outage probability and the average bit error probability. Analytical results are validated by simulations and they provide comparative analysis of the different relay selection scheme proposed herein.

Selective OFDMA is a powerfull relaying technique with subcarrier based selection. This scheeme has a high potential to considerably improve the performance of cooperative OFDM systems. However, in practice, the implementation of this technique may suffer from multiple problems caused by the separation of the subcarriers from each other's. In this Contribution, we propose a new selection scheme denoted “Optimized Selective OFDMA” that approaches selective OFDMA performance in terms of outage probability while reducing its synchronization problems that may be caused by the subcarriers separation. The outage probability and diversity order of the proposed scheme are derived. The number of the used paths is also analyzed. We proved that for high SNR, the introduced scheme gives exactly the same performance of selective OFDMA with only one path for all the subcarriers.

Selective cooperation is a well investigated technique in non-cognitive networks for efficient spectrum utilization and performance improvement. However, it is still a nascent topic for underlay cognitive networks. Recently, it was investigated for underlay networks where the secondary nodes were able to adapt their transmit power to always satisfy the interference threshold to the primary users. This is a valid assumption for cellular networks but many non-cellular devices have fixed transmit powers. In this situation, selective cooperation poses a more challenging problem and performs entirely differently. In this paper, we extend our previous work of selective cooperation based on either hop's signal to noise ratio (SNR) with fixed gain and fixed transmit power relays in an underlay cognitive network. This work lacked in considering the primary interference over the cognitive network and presented a rather idealistic analysis. This paper deals with a more realistic system model and includes the effects of primary interference on the secondary transmission. We first derive end-to-end signal to interference and noise ratio (SINR) expression and the related statistics for a dual-hop relay link using asymptotic and approximate approaches. We then derive the statistics of the selected relay link based on maximum end-to-end SINR among the relays satisfying the interference threshold to the primary user. Using this statistics, we derive closed form asymptotic and approximate expressions for the outage probability of the system. Analytical results are verified through simulations. It is concluded that selective cooperation in underlay cognitive networks performs better only in low to medium SNR regions.

The performance of a joint Hierarchical Modulation (HM) and Network Coding (NC) scheme for two way relay networks is introduced and evaluated in this paper. The proposed scheme uses selective relaying based on a signal-to-noise (SNR) threshold at the relay. In particular, a two way cooperative network with two sources and one relay is considered. Two different protection classes are modulated by a hierarchical 4/16-Quadrature Amplitude Modulation (QAM) constellation at the source. Based on the instantaneous received SNR at the relay, the relay decides to retransmit both classes by using a hierarchical 4/16-QAM constellation, the more- protection class by using a Quadrature Phase Shift Keying (QPSK) constellation, or remains silent. These thresholds at the relay give rise to multiple transmission scenarios in a two way cooperative network. Simulation results verify our analysis and efficacy of the scheme.

In this contribution, we investigate Decode and Forward (DF) OFDM Ad-hoc Routing strategy with bottleneck maximization. We derive the exact outage probability and diversity order for the different scenarios: with and without subcarrier grouping and with and without joint selection, where joint selection refers to the selection of the last two hops together. When joint selection is performed, numerical results show that a power gain can be obtained when using subcarrier grouping technique.

In a communication system with multiple cooperative relays, selecting the best relay utilizes the available spectrum more efficiently. However, selective relaying poses a different problem in underlay cognitive networks compared to the traditional cooperative networks due to interference thresholds to the primary users. In most cases, a best relay is the one which provides the maximum end-to-end signal to noise ratio (SNR). This approach needs plenty of instantaneous channel state information (CSI). The CSI burden could be reduced by partial relay selection. In this paper, a partial relay selection scheme is presented and analyzed for an underlay cognitive network with fixed gain relays operating in the vicinity of a primary user. The system model is adopted in a way that each node needs minimal CSI to perform its task. The best relay is chosen on the basis of maximum source to relay link SNR which then forwards the message to the destination. We derive closed form expressions for the received SNR distributions, system outage, probability of bit error and average channel capacity of the system. The derived results are confirmed through simulations.

In this paper, we investigate the area spectral efficiency (ASE) in a three-node cooperative network with opportunistic relaying. On one hand, in conventional cellular network, ASE is defined to be the average data rate per unit bandwidth per unit area supported by a base station. On the other hand, in cooperative network, since there is no such a central base station, we redefine ASE to be the average data rate per unit bandwidth within a particular area, in which simultaneous transmissions are not allowed. In addition, to maximize the instantaneous capacity between the source and the destination, we apply a transmission mode selection criterion to determine whether the direct transmission or the multi-hop relay transmission is utilized. We derive the mathematical expression of ASE with the consideration of the relay node location and the impact of path-loss and fading effects. We show through selected numerical examples the effect of relay node position on the performance metric of ASE.

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.

The cooperative relaying process with multiple relays needs proper coordination among the communicating and the relaying nodes. This coordination and the required capabilities may not be available in some wireless systems where the nodes are equipped with very basic communication hardware. We consider a scenario where the source node transmits its signal to the destination through multiple relays in an uncoordinated fashion. The destination captures the multiple copies of the transmitted signal through a Rake receiver. We analyze a situation where the number of Rake fingers N is less than that of the relaying nodes L. In this case, the receiver can combine N strongest signals out of L. The remaining signals will be lost and act as interference to the desired signal components. To tackle this problem, we develop a novel signal combining technique based on channel shortening principles. This technique proposes a processing block before the Rake reception which compresses the energy of L signal components over N branches while keeping the noise level at its minimum. The proposed scheme saves the system resources and makes the received signal compatible to the available hardware. Simulation results show that it outperforms the selection combining scheme.

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.

Spread spectrum receivers with generalized selection combining (GSC) RAKE reception were proposed and have been studied as alternatives to the classical two fundamental schemes: maximal ratio combining and selection combining because the number of diversity paths increases with the transmission bandwidth. Previous work on performance analyses of GSC RAKE receivers based on the signal to noise ratio focused on the development of methodologies to derive exact closed-form expressions for various performance measures. However, some open problems related to the performance evaluation of GSC RAKE receivers still remain to be solved such as the exact performance analysis of the capture probability and an exact assessment of the impact of self-interference on GSC RAKE receivers. The major difficulty in these problems is to derive some joint statistics of ordered exponential variates. With this motivation in mind, we capitalize in this paper on some new order statistics results to derive exact closed-form expressions for the capture probability and outage probability of GSC RAKE receivers subject to self-interference over independent and identically distributed Rayleigh fading channels, and compare it to that of partial RAKE receivers.

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.

The anticipated increase in the density of the deployed wireless sensor networks calls for spectrum sharing through unlicensed access to licensed spectrum. The key technology for spectrum sharing in this scenario is cognitive radio networks. Cognitive relaying scenarios, where a cognitive (unlicensed) user provides relaying services to a licensed (primary) user, have been proposed before as a method to increase chances of spectrum white spaces. In this paper, we develop a wireless sensor network framework containing a cognitive user (sensor node), with delay sensitive data and limited power budget. The cognitive user offers relaying capability to the primary traffic when the primary connection fails to deliver. The cognitive user utilizes a scheduling mechanism that grants priority to relayed traffic over its own traffic. In our framework, the cognitive user is allowed to control the volume of the relayed traffic through an admission control parameter. The objective of the sensor node (cognitive user) is to minimize its traffic delay, subject to certain power budget allowed for relaying the primary traffic. Our key contributions in this work are the development of the aforementioned framework, the establishment of a mathematical formalization for this problem, the derivation of mathematical expressions for average power consumption and average packet delay, and the solution for the developed optimization problem by finding a value for the admission control parameter that minimizes delay while satisfying power budget constraints.

In a wireless sensor network with relaying capability, intermediate relay nodes are with limited energy budget. To maximize lifetime of relay nodes, selective relay strategies, requiring full channel state information (CSI), are used to utilizes the best relaying channel. Pilot overhead on relay nodes lifetime is reduced by assuming pilot transmission from destination instead of relay nodes. This is only valid if channel reciprocity is assumed. It is evident that channel reciprocity is not valid for low cost simple transceivers, such as sensor node transceivers. In this paper a novel sequential random (SR) selective cooperative relay strategy is proposed in amplify and forward (AF) relay networks for applications with non reciprocal links. The outage probability of SR strategy is derived and its average lifetime, average number of pilot signals per transmitted message, and average transmit energy per message are simulated. SR strategy uses less number of pilots than selective cooperative relay strategies (S-CRSs). A dynamic transmit power threshold is adopted for the proposed SR strategy to improve its performances. In contrast to S-CRSs, in which each relay knows its CSI to the destination before every transmission, SR acquires CSI on the need- to-know basis and significantly outperforms the lifetime of S-CRSs for large number of relays and relatively good channel conditions. The proposed SR selects relay nodes in a distributed manner which makes it suitable for heterogeneous ad-hoc sensor networks in which some relays have processing capabilities and less restricted energy supply, i.e. these relays can act as the intermediate destinations.

During the last few years, several authors have studied the performance of many different types of channel codes for cooperative communication systems. Some of these codes include Low Density Parity Check (LDPC), Turbo and convolutional codes. Published results show that these codes effectively improve both the data rate and system performance in cooperative schemes such as the decode and forward (DF). Reed-Solomon (RS) codes are one of the most widely used channel codes in wireless communication systems today. These codes have also been adopted by several 3G standards including several digital video broadcasting (DVB) standards and also by worldwide interoperability for microwave access (WiMAX) which is based on the IEEE 802.16 standard. However, published literature shows no specific study to investigate their performance for the cooperative communication environment. Using the component codes adopted in the IEEE 802.16 standard, this paper investigates the performance of the concatenated RS coding schemes for cooperative communication. This paper first analyses the standard RS-coded DF cooperative system. The results obtained for the standard RS-coded scheme show huge gains over the non-cooperative system. Some of the problems of the standard DF relay protocol is then discussed and an improved RS-coded DF cooperative scheme is proposed. The performance of the proposed scheme shows significant gains over the standard RS-coded DF system.

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.

For third generation systems and ultrawideband systems, RAKE receivers have been introduced due to the advantage of RAKE receivers which is their ability to combine different replicas of the transmitted signal arriving at different delays in a rich multipath environment. In principle, RAKE receivers combine all resolvable paths which gives the best performance in a rich diversity environment. However, this is usually costly in terms of hardware required as the number of RAKE fingers increases. Therefore, generalized selection combining (GSC) RAKE reception was proposed and has been studied by many researcher as an alternative to the classical two fundamental diversity schemes: maximal ratio combining and selection combining. Previous work on performance analyses of GSC RAKE receivers based on the signal to noise ratio focused on the development of methodologies to derive exact closed-form expressions for various performance measures. However, the remaining set of uncombined paths affect the overall performance both in terms of loss in power. Therefore, to have a full understanding of the performance of GSC RAKE receivers, we introduce in this paper the notion of capture probability, which is defined as the ratio of the captured power (essentially combined paths power) to that of the total available power. The major difficulty in these problems is to derive some joint statistics of ordered exponential variates. With this motivation in mind, we capitalize in this paper on some new order statistics results to derive exact closed-form expressions for the capture probability over independent and identically distributed Rayleigh fading channels.

The cooperative relaying process needs proper coordination among the communicating and the relaying nodes. This coordination and the required capabilities may not be available in some wireless systems, e.g. wireless sensor networks where the nodes are equipped with very basic communication hardware. In this paper, we consider a scenario where the source node transmits its signal to the destination through multiple relays in an uncoordinated fashion. The destination can capture the multiple copies of the transmitted signal through a Rake receiver. We analyze a situation where the number of Rake fingers N is less than that of the relaying nodes L. In this case, the receiver can combine N strongest signals out of L. The remaining signals will be lost and act as interference to the desired signal components. To tackle this problem, we develop a novel signal combining technique based on channel shortening. This technique proposes a processing block before the Rake reception which compresses the energy of L signal components over N branches while keeping the noise level at its minimum. The proposed scheme saves the system resources and makes the received signal compatible to the available hardware. Simulation results show that it outperforms the selection combining scheme.

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).

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 this paper, we propose several relay selection schemes for cooperative diversity systems with adaptive transmission. The proposed schemes are based on dual-hop relaying with multiple relays. We analyze the performance of the proposed schemes by quantifying the average spectral efficiency and the outage probability. We also investigate the tradeoff of performance and complexity among proposed schemes by evaluating the average number of active relays, path estimations, and signal- to-noise ratio comparisons. Our proposed schemes and analysis are also applicable to single relay systems with multiple antennas.

Mobile users with single antennas can still take advantage of spatial diversity through repetition based cooperative transmission. In this paper, we consider a scheme in which the relay chooses to cooperate only if the source-destination channel is of an unacceptable quality. In our study, we consider a regenerative relay in which the decision to cooperate is based on a signal-to-noise ratio (SNR) threshold and consider the effect of the possible erroneously detected and transmitted data at the relay. We derive an expression for the end-to-end bit-error rate (BER) of binary phase-shift keying (BPSK) modulation and look at the optimal strategy to minimize this end-to-end BER at the destination for high SNR. Some selected performance results show that computer simulations based results coincide with our analytical results.

Closed-form expressions for the average outage duration (AOD) of multihop regenerative communication systems over generalized fading channels are presented. Both noise-limited and interference-limited systems are studied. To show the usefulness of the presented expressions, some specific fading scenarios are considered. In addition, some numerical examples of interest comparing direct versus relayed transmission and studying the effect of increasing the number of hops and/or cochannel interferers are plotted and discussed.

Closed-form expressions for the statistics of the harmonic mean of two independent and identically distributed gamma variates are presented. The probability density function of the harmonic mean of two F variates is also derived. These statistical results are then applied to study the performance of wireless communication systems with nonregenerative relays over flat Nakagami fading channels. More specifically, outage probability formulas for noise-limited systems as well as systems affected by interference are obtained. Furthermore, general expressions for average bit-error rates are also derived. Finally, comparisons between regenerative and nonregenerative systems are presented. Numerical results show that the former systems clearly outperform the latter ones for low average signal-to-noise ratios (SNRs). They also show that the two systems have similar performance at high average SNRs.

This letter presents a study on the end-to-end performance of dual-hop wireless communication systems equipped with nonregenerative fixed gain relays and operating over flat Rayleigh-fading channels. More specifically, it first derives generic closed-form expressions for the outage probability and the average probability of error when the relays have arbitrary fixed gains. It then proposes a specific fixed gain relay that benefits from the knowledge of the first hop's average fading power and compares its performance with previously proposed relay gains that in contrast require knowledge of the instantaneous channel state information of the first hop. Finally, the letter investigates the effect of the relay saturation on the performance of the systems under consideration. Numerical results show that nonregenerative systems with fixed gain relays have a comparable performance to nonregenerative systems with variable gain, more complex, relays. These results also show that relay saturation of these systems results in a minimal loss in performance.

Relayed transmission is a way to attain broader coverage by splitting the communication link from the source to the destination into several shorter links/hops. One of the main advantages of this communication technique is that it distributes the use of power throughout the hops. This implies longer battery life and lower interference introduced to the rest of the network. In this context, this paper investigates the optimal allocation of power over these links/hops for a given power budget. All hops are assumed to be subject to independent Rayleigh fading. Outage probability which is the probability that the link quality from source to destination falls below a certain threshold is used as the optimization criterion. Numerical results show that optimizing the allocation of power enhances the system performance, especially if the links are highly unbalanced in terms of their average fading power or if the number of hops is large. Interestingly, they also show that nonregenerative systems with optimum power allocation can outperform regenerative systems with no power optimization.

The paper presents a study on the end-to-end performance of dual-hop wireless communication systems equipped with non-regenerative fixed gain relays and operating over flat Rayleigh fading channels. More specifically, it first derives generic closed-form expressions for the outage probability and the average probability of error when the relays have arbitrary fixed gains. It then proposes a specific fixed gain relay that benefits from the knowledge of the first hop's average fading power and compares its performance with previously proposed relay gains that, in contrast, require knowledge of the instantaneous channel state information of the first hop. Finally, the paper investigates the effect of the relay saturation on the performance of the systems under consideration. Numerical results show that non-regenerative systems with fixed gain relays have a comparable performance to non-regenerative systems with variable gain relays. These results also show that relay saturation of these systems results in a minimal loss in performance.

End-to-end performance of two-hops wireless communication systems with nonregenerative relays over flat Rayleigh-fading channels is presented. This is accomplished by deriving and applying some new closed-form expressions for the statistics of the harmonic mean of two independent exponential variates. It is shown that the presented results can either be exact or tight lower bounds on the performance of these systems depending on the choice of the relay gain. More specifically, average bit-error rate expressions for binary differential phase-shift keying, as well as outage probability formulas for noise limited systems are derived. Finally, comparisons between regenerative and nonregenerative systems are presented. Numerical results show that the former systems clearly outperform the latter ones for low average signal-to-noise-ratio (SNR). They also show that the two systems have similar performance at high average SNR.

This paper presents an analytical framework for the performance evaluation of cellular mobile radio systems equipped with smart antenna systems. In particular, the paper focuses on low-complexity systems which are able to successively suppress the strongest active interferers. The desired user fading statistics is assumed to be flat Rayleigh, Rician, or Nakagami, whereas the interfering signals are assumed to be independent and subject to slow flat Rayleigh fading. The paper starts by presenting generic closed-form expressions for the the carrier-to-interference ratio probability density function after interference cancellation. Based on that, exact closed-form expressions for the outage probability and average error rate formulas are derived. Finally, a comparison with a practical cancellation scheme and the impact of some practical considerations on the performance of successive interference cancellation are investigated. More specifically, the effect of traffic loading, the overall spectral efficiency gain, and the impact of time delay are studied.

Closed-form expressions for the statistics of the harmonic mean of two independent and identically distributed gamma variates are presented. The probability density function of the harmonic mean of two F variates is also derived. These statistical results are then applied to study the performance of wireless communication systems with non-regenerative relays over flat Nakagami (1960) fading channels. It is shown that these results can either be exact or tight lower bounds on the performance of these systems depending on the choice of the relay gain. More specifically, outage probability formulas for noise limited systems as well as systems affected by interference are obtained. Furthermore, average bit error rate and outage capacity expressions are derived. Finally, comparisons between regenerative and non-regenerative systems are presented. Numerical results show that the former systems clearly outperform the latter ones for low average signal-to-noise-ratio (SNR). They also show that the two systems have similar performance at high average SNR.

Optimum power loading algorithms for selective transmitter diversity systems over flat Nakagami fading channels are considered. Both power limited and interference limited systems are studied. It is shown that the diversity links statistics can be exploited to optimally allocate the power among the transmitter antennas. The performance criterion used in optimizing the power allocation is the outage probability which is the probability that the link quality falls below an acceptable level. As an illustration of the mathematical formalism some numerical results for particular cases of interests are plotted and discussed.

Soft handoff uses macroscopic diversity to improve the performance of wireless communication systems. In this context, this paper shows that the diversity link statistics can be exploited to optimally allocate the power available for the uplink of soft handoff schemes operating over correlated lognormal shadowing channels. The performance criterion used in optimizing the power allocation is the outage probability which is the probability that the link quality falls below an acceptable level. As an illustration of the mathematical formalism some numerical examples for particular cases of interests are plotted and discussed. These results show that the performance gain provided by these power optimized soft handoff systems increases as the diversity order and/or the correlation coefficient between diversity links increases.

This paper presents new exact closed-form expressions for the outage probability of cellular systems (without diversity reception) in which the desired user follows a Rician distribution whereas the interferers are subject to correlated Rayleigh fading. It then capitalizes on a numerical technique by D. Raphaeli (see IEEE Trans. Inf. Theory, vol.42, p.1002-7, 1996) dealing with the accurate evaluation of the cumulative distribution function of quadratic forms in complex Gaussian random variables to solve for the outage probability in the more general case of arbitrary fading statistics, average fading powers, diversity branches correlation, and/or correlation between the interferers. In some scenarios, the numerical approach reduces to finite-sum closed-form expressions.