We address comprehensive optimization of uplink spectral efficiency (SE) and resource allocation in multiple reconfigurable intelligent surfaces (RIS)-aided cell-free massive MIMO (CF-mMIMO) systems. While integrating CF-mMIMO with RISs enhances SE, existing solutions assume ideal conditions or separately optimize access point (AP) clustering, large-scale fading decoding (LSFD), RIS phase-shifts, and power allocation. To bridge these gaps, we propose a unified statistical channel state information (CSI)-based optimization (SCOP) framework that jointly optimizes AP clustering, LSFD, RIS phase-shift control, and uplink power allocation to maximize the minimum uplink SE. A closed-form SE expression is derived for maximum ratio (MR) combining, accounting for both direct and cascaded channels with spatially correlated Ricean fading. Leveraging statistical CSI significantly reduces real-time acquisition overhead while enabling robust and efficient uplink transmission design. SCOP is solved via a multi-step strategy: (i) for fixed power, an iterative algorithm computes joint optimization parameters (JOP) vector representing a combination of AP clustering, LSFD, and RIS phase-shift parameters; (ii) a closed-form solution updates power allocation; and (iii) an alternating optimization jointly refines both. We also introduce a novel method to extract the optimal system parameters from the JOP. Simulation results show that, in a representative 60 APs, 30 users, and 4 RISs scenario, the proposed SCOP framework lifts the median uplink SE from 2.4 bit/s/Hz to 3.9 bit/s/Hz (+65%) and more than doubles the bottom 5% rate, with similar 60–120% gains in other setups.

Transceiver hardware impairments (THI) significantly affect the performance of communication systems, necessitating their analysis for realistic performance evaluation and robust system design. Furthermore, integrating channel coding is imperative for enhancing error resilience under diverse fading environments. However, the joint impact of THI and channel coding on error performance in downlink rate-splitting multiple access (RSMA) systems remains unexplored. This paper investigates the impact of THI on the symbol error rate (SER) and explores the effectiveness of block codes in mitigating these impacts in RSMA systems. More precisely, a detailed analysis of the constellation-based SER is conducted for a multi-user RSMA setup, considering both transmitter and receiver hardware impairments with perfect and imperfect detection events of the common stream. Moreover, closed-form expressions for the coded average SER (ASER) are derived under both zero and non-zero THI for multi-user RSMA systems over Nakagami-mfading with varying shape parameters in single-input single-output (SISO) scenarios. This study also presents asymptotic ASER expressions with diversity order analysis under both zero and non-zero THI. The accuracy of these derived ASER expressions is verified through Monte Carlo simulations. Furthermore, the SISO analytical framework is extended to the multiple-input multiple-output scenario, with comparative evaluations against the SISO scenario to demonstrate the practical applicability of the proposed coded RSMA system.

In wireless security, the phrase ‘seeing is believing’ takes on a profound meaning as we explore the practical application of machine learning in enhancing the security of wireless networks. This paper focuses on a specific type of denial-of-service attacks, called as the reactive injection attack, for which a modified signalling strategy in non-coherent communications is proposed. This strategy harnesses the potential of active intelligent reflecting surface-based transmitter (AIRS-T), secret-key-based strategy, and deep learning-based techniques, i.e. convolutional neural networks (CNN). Based on the joint use of these schemes, the proposed strategy shows a significant covertness gain in terms of Kullback-Leibler divergence (KLD), followed by a rich pattern distribution of scatterplots for the decoded signals. The worst case (1-element AIRS-T) and the best case (N-element AIRS-T) are also evaluated, and it is found to be consistent with the achieved covertness results. The covertness gain translates into enhanced performance when CNN is used for classification, leading to faster model convergence and higher accuracy when compared to transfer learning using models like Resnet50, VGG16, and InceptionV3.

This paper proposes a hybrid user group (HUG) scheme for simultaneous transmitting and reflecting-reconfigurable intelligent surface (STAR-RIS)-aided millimeter wave (mmWave) cell-free massive multiple-input multiple-output (CF mMIMO) systems using rate-splitting multiple access (RSMA), where users in transmission and reflection zones are optimally paired. Nearby APs serve users, and multiple STAR-RISs enhance signals under imperfect SIC. We formulate a max-min spectral efficiency (SE) problem to jointly optimize power allocation, STAR-RIS phase shifts, and user grouping, leading to a mixed-integer non-convex problem. To solve it, we relax discrete variables and decompose the problem into sub-problems, using bisection search for phase shifts and a low-complexity iterative algorithm for power allocation. Simulations show the HUG scheme improves average SE by 14.03%, 18.79%, and 33.42% compared to random grouping, conventional beamforming, and HUG with conventional RIS, respectively.

Orthogonal time-frequency space (OTFS) modulation is a promising technique for next-generation wireless communication, particularly in high-mobility scenarios. However, the performance of the OTFS modulation is degraded by hardware impairments (HIs), particularly at high frequencies and mobility. Thus, efficient signal detection is essential for reliable OTFS-based communication. To overcome this, various detectors, including convolutional neural networks (CNN), minimum mean square error (MMSE), and message passing (MP), have been explored in the literature, with CNN-based detectors demonstrating superior performance over MMSE and MP. In this work, we propose a residual channel attention network (RCAN) to robustly detect OTFS signal in the presence of HIs. Using deep residual learning and channel attention mechanisms to improve detection accuracy under varying channel conditions. Extensive simulations, considering different OTFS frame sizes, user mobility levels, and HI parameters, confirm that the proposed RCAN consistently outperforms conventional detectors, including CNN, in all scenarios evaluated. This makes RCAN a highly effective solution for OTFS-based high-mobility wireless communication systems, both with and without HI.

Movable antenna (MA) is an innovative technology that can enhance channel condition by altering antenna position within a local area. Integrating MA can further improve the performance of multiuser communications in non-orthogonal multiple access (NOMA) systems. In this paper, we investigate MA enabled NOMA for multiuser downlink communication, where the base station (BS) is equipped with a fixed-position antenna (FPA) array to serve multiple MA enabled user terminals (UTs). An optimization problem is formulated to maximize the minimum achievable rate among all the UTs by jointly optimizing the positions of MAs of each UT, the precoding matrix at BS, and the successive interference cancellation (SIC) decoding indicator matrix at UTs, subject to the limited movement area of the MAs, the maximum transmit power of the BS, and the SIC decoding condition. To solve this non-convex problem, we combine the hippo optimization (HO) method with the alternating optimization (AO) method to obtain a suboptimal solution efficiently. Simulation results show that the proposed algorithm can significantly improve the rate performance of the NOMA system compared to the conventional FPA system as well as other benchmark schemes.

User-centric (UC) distributed massive multiple-input multiple-output (D-mMIMO), commonly called cell-free mMIMO, is an essential technology for ensuring more uniform coverage and higher spectral and energy efficiencies in next-generation communication systems. This paper investigates an alternative effective channel estimation method to address the issue of the lower channel hardening degree experienced by UC D-mMIMO systems. Specifically, this paper proposes a hybrid approach for effective channel estimation, allowing users to estimate their channels either through downlink pilot-based training, by using a blind algorithm, or by relying on statistical channel state information (CSI). The hybrid estimation method is compared with the case where each method is applied individually, as well as with the ideal case of perfect CSI. The analysis is conducted using various system parameter settings, such as the number of antennas and users, and it accounts for the presence of pilot contamination. Simulation results reveal that the proposed hybrid channel estimation algorithm is able to provide the best spectral efficiency performance in any network parameter configuration, while reducing the estimation normalized mean-square error compared to conventional statistical CSI.

The demand for high-precision localization services has surged significantly due to the rise of intelligent transportation and autonomous driving in large-scale indoor factories. To mitigate the challenge of reduced positioning accuracy resulting from line-of-sight (LOS) occlusion in indoor visible light positioning (VLP) technology, this paper proposes a positioning strategy that leverages optical intelligent reflecting surfaces (IRSs) within indoor Internet of Vehicles (IoVs) environments. This scheme utilizes the principle of range positioning based on the time difference of arrival (TDOA). A weighted least squares (WLS) method is initially derived as a benchmark positioning approach based on TDOA. Additionally, two high-accuracy positioning methods, namely, the Chan method and Taylor series expansion method, are proposed for different scenarios. The Chan method provides a closed-form solution suitable for low-computation cases, while the Taylor series expansion method can be iteratively exploited in complex situations. The detailed procedures of these three positioning approaches are also presented. Furthermore, theoretical analyses of the computational complexity of the WLS method, Chan method, and Taylor series expansion method are provided. Additionally, the positioning performance of the Cramér-Rao lower bound (CRLB) is analyzed and derived for the considered system model. The simulation results illustrate that the system model and positioning methods outlined can asymptotically achieve the derived CRLB, thereby validating the efficacy of the proposed positioning scheme and methods. These advancements hold significant potential for industrial automation, logistics operations, and safety-critical autonomous guided vehicles (AGVs) in smart factories, where robust centimeter-level positioning is essential for collision avoidance and task coordination under dynamic occlusion conditions.

This work proposes a novel semi-non-orthogonal multiple access (NOMA) and data transmission technique, called Semi-NOMA, to enhance the spectrum utilization of aerial simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided symbiotic networks without using successive interference cancellation approaches as classical NOMA. In particular, the proposed scheme is investigated with active and passive STAR-RIS models combined with infinite blocklength (IBL) and finite blocklength (FBL) regimes under discrete phase-shift alignments. For IBL scenarios, the ergodic capacity and outage probability are derived under both approximation and asymptotic frameworks. Besides, a joint optimization problem of the power allocation factor and energy splitting coefficient is also formulated to maximize the ergodic sum capacity (ESC), where closed-form solutions are derived for both active and passive STAR-RIS models. For FBL scenarios, not only the approximation and asymptotic frameworks are derived for the average achievable rate and block-error rate, but also an approximated convex form is derived for a non-convexity optimization problem of min-max blocklength. Numerical results corroborate the efficacy of the proposed Semi-NOMA over the baseline schemes, the developed mathematical frameworks, and the solutions of the ESC maximization and min-max blocklength.

Terahertz (THz) hybrid analog-digital beamforming systems face significant performance degradation due to group delay mismatch in phase shifters, which distorts beam patterns and significantly reduces the array gain. This paper presents a comprehensive framework to model, analyze, and mitigate group delay mismatch in THz hybrid analog-digital beamforming systems, addressing a critical oversight in existing research. We first establish a theoretical model for group delay mismatch in THz hybrid analog-digital beamforming systems, revealing its frequency-dependent Gaussian distribution and its cascading impact on beam squint and capacity degradation. Our analysis explicitly incorporates spatial group delay gradients, ultra-wideband channel characteristics, and hardware non-idealities, deriving closed-form expressions for beam misalignment and capacity loss. These results demonstrate that group delay-induced degradation scales quadratically with center frequency and is insensitive to bandwidth or array size, redefining THz system design priorities. We then propose a robust frequency selective hybrid analog-digital beamforming algorithm that jointly optimizes analog and digital beamformers to counteract group delay distortions. The analog beamformer, designed via Riemannian manifold optimization, minimizes spatial group delay gradients under the constant modulus constraints, while the digital precoders employs minimax optimization to compensate residual phase errors across subcarriers. Simulations under realistic THz channel conditions validate the accuracy of our analysis and the effective of our proposed design. Specially, our algorithm achieves 33.91 bps/Hz spectral efficiency at 10 dB SNR, outperforming conventional methods by 31.5% without requiring additional expensive analog lines.

This paper comprehensively investigates the performance of downlink multi-user rate-splitting multiple access (RSMA) networks under Nakagami-m fading channels. We first develop the mathematical outage probability (OP) and ergodic capacity (EC) frameworks, deriving exact expressions for both, along with asymptotic analysis in high signal-to-noise ratio (SNR) and low-rate regions, which serve as the foundation for deducing the approximate and maximal energy-reliability and energy-spectral formulas. To enhance system performance, we tackle the non-convex problems of jointly optimizing common and private power allocation (PA) coefficients to minimize the maximal OP performance. Moreover, we also delve into optimizing PA coefficients and rate-splitting factors concurrently to maximize the ergodic sum capacity (ESC). Addressing the influence of modulation schemes on user error rates, we introduce mathematical frameworks for symbol error rate (SER) considering four combined modulation schemes based on binary phase-shift keying (BPSK) and quadrature-phase shift keying (QPSK), the whole cases are quantified in terms of exact and asymptotic manners. Furthermore, we present a straightforward approach to optimize the PA coefficients to minimize the maximal SER performance. Finally, Monte-Carlo simulations are presented to validate our developed frameworks and optimization solutions.

Irregularly spaced arrays are appearing in diverse ares, such as wearable devices, stealth aircrafts. This paper studies the two-dimensional (2D) direction-of-arrival (DOA) estimation issue for an irregularly spaced electromagnetic vector sensor (EMVS) array. An estimation method of signal parameters via rotational invariance technique (ESPRIT) approach is developed. Unlike existing ESPRIT-like algorithms, the proposed approach in this paper not only estimates the rough directional cosine waveform via the rotational invariance of the polarized response matrix, but also finds the refined directional cosine waveform via the rotational invariance of the spatial response matrix. This proposed algorithm is capable of offering closed-form analytics, thus greatly facilitating 2D-DOA estimation. Numerical results shown in this paper verify that the proposed approach outperforms existing ESPRIT-like algorithms at a sightly increased costs of computation. In addition, numerical results presented in this paper for the proposed 2D-DOA estimation approach also corroborate the theoretical derivations.

This paper proposes a millimeter-wave (mmWave) multiple-input multiple-output (MIMO) transmission scheme termed spatial scattering shift keying (SSSK), which exploits spatial scattering modulation (SSM) to encode information through the indices of channel scatterers rather than conventional symbol constellations. The proposed SSSK achieves superior reliability compared to amplitude-phase modulation (APM) schemes, while simultaneously reducing hardware complexity. Specifically, the scatterer-index-based signaling mechanism mitigates the detection complexity inherent in APM systems by avoiding explicit symbol-level demodulation. In addition, we illustrate the advantages of SSSK by investigating the interaction between SSSK and fading channels. We derive closed-form expressions for the average bit error probability (ABEP) tight upper bound of the proposed scheme using two different approaches based on the greedy detection algorithm. To gain more insights, we further derive the asymptotic ABEP expression and diversity gain. To characterize the performance, we rigorously derive tight upper bounds on the ABEP using two complementary approaches: union bound and pairwise error probability analysis under a greedy detection framework. Furthermore, asymptotic ABEP expressions are established to reveal the achievable diversity gain. Moreover, we design maximum likelihood (ML) detectors with serial and parallel architectures and corresponding ABEP upper bounds. Simulations validate the analytical derivations and demonstrate SSSK outperforms APM in ABEP at high signal-to-noise ratios. The proposed greedy detector reduces computational complexity compared to the serial ML detector while maintaining comparable ABEP performance.

Vehicle Edge Computing (VEC) assists vehicles in performing latency-sensitive tasks by deploying resources near the vehicle. Designing an incentive mechanism for vehicles and VEC is crucial for realizing an intelligent transmission system. Considering the rationality of resource allocation, we model the utility functions of the VEC and the vehicle, which are used as optimization objectives. Specifically, the VEC allocates resources through pricing to maximize revenue under resource-constrained conditions, and the vehicle weighs payments against energy consumption to determine offloading and resource allocation. Given the vehicle movement and the variable channel state, we use the Deep Reinforcement Learning (DRL) algorithm to solve these optimization problems. To reduce the learning difficulty of the DRL algorithm in complex VEC scenarios with multiple optimization variables, we propose a Pricing-Driven Resource Allocation (PDRA) algorithm that performs mobility-aware task offloading and calculates the optimal values of the optimization variables in the utility function of the vehicle to reduce the decision dimension. Furthermore, we also propose a DRL-based Pricing-Driven Dynamic Resource Allocation (DPDDRA) algorithm to achieve efficient resource allocation. Extensive experimental results show that the proposed algorithms can reduce the learning difficulty while maximizing VEC and vehicle revenue in complex VEC scenarios.

In this paper, we investigate the joint design of beamforming vectors and trajectory for unmanned aerial vehicles (UAVs) in integrated sensing and communications networks, aiming to maximize the achievable covert rate (ACR) for legitimate users against multiple passive wardens. Considering the worst-case scenario, where the wardens strategically select optimal decision thresholds, we derive the minimum detection error probability and incorporate covertness constraints within the beamforming scheme. Our approach entails formulating the design as a non-convex optimization problem for maximizing the average ACR along the UAV trajectory. The formulation takes into account various practical constraints, such as the maximum transmit power, UAV flight speed limitations, minimum beamforming gain towards sensing targets, and the detection probability threshold for wardens. To address this intricate problem, we propose a block coordinate descent-based optimization algorithm. This algorithm alternates between updating beamforming vectors and UAV trajectories, offering a high-quality suboptimal solution to the original problem. Theoretical analyses reveal that when the detection probability threshold is sufficiently small, a linear correlation emerges between the maximum relative variation ratio in the average received signal power at wardens under two hypotheses and the detection probability. Furthermore, to enhance covertness against the wardens, it is necessary to either decrease the projection of information beamforming covariance matrix or increase the projection of sensing beamforming covariance matrix onto the subspace spanned by the eavesdropping channel vectors. Finally, extensive simulations are presented to validate the covert performance enhancements of our proposed methodology, compared with various baseline schemes adopting existing approaches.

As one of the promising technologies, reconfigurable intelligent surface (RIS) and simultaneous transmitting and reflecting RIS (STAR-RIS) have attracted great interest. However, the existing RISs offer broadband tuning capability without filtering function due to the absence of radio frequency (RF) units, which easily leads to the unexpected tuning of the RIS undesired signals, especially in large-scale deployments. For the target network, it is difficult to obtain the parameter settings of RISs to serve other networks, which causes the unpredictability of the wireless environment. In this paper, we consider the covert communication in a STAR-RIS assisted random wireless network with randomly distributed blockages. We investigate the impact of STAR-RIS large-scale deployment on covert communication and leverage its inherent unpredictability for improving the covertness. We derive the average detection error probability for warden within the random wireless networks. Furthermore, we optimize the passive beamforming of STAR-RIS to maximize the covert communication rate, considering both direct and indirect line-of-sight (LoS) links. To address this, we employ an alternating optimization (AO) algorithm based on the semi-definite programming (SDP) method. Finally, numerical results demonstrate significant enhancements and increase covert capability achieved through the large-scale deployment of STAR-RIS.

As a recently proposed reconfigurable intelligent surface (RIS) architecture, active RIS has drawn considerable interest. The important feature of the active RIS is its ability to strengthen the impinging signals to mitigate the multiplicative fading effect inherent in passive RIS-aided systems. Herein, we explore the performance of an active RIS-aided uplink multi-user massive multiple-input multiple-output (MIMO) system, considering the phase noise at the RIS. Furthermore, a two-timescale scheme is utilized, where the base station (BS) beamforming is designed based on the instantaneous aggregated channel state information (CSI), while the statistical CSI is used for designing the phase shifts of the active RIS. In addition, the linear minimum mean square error (LMMSE) estimator is adopted to estimate the aggregated channel, which combines both the cascaded and direct channels. According to the estimated channel, a closedform expression for the lower bound of achievable rate is derived. Based on the theoretical expressions, the power scaling laws for the considered system are also investigated. Specifically, when each user’s transmit power is proportionally reduced by the quantity of BS antennasMor RIS elementsN, we find that the amplified thermal noise causes the lower bound of the achievable rate to approach zero asMorNtends to infinity. Moreover, an optimization approach based on a genetic algorithm (GA) is introduced to obtain the optimal phase shifts for maximizing the achievable rate. Numerical results reveal that the active RIS can greatly enhance the performance of the massive MIMO system compared to its passive counterpart.

The additional degree of freedom (DoF) in the distance domain of near-field communication offers new opportunities for physical layer security (PLS) design. However, existing works mainly consider static eavesdroppers, and the related study with mobile eavesdroppers is still in its infancy due to the difficulty in obtaining the channel state information (CSI) of the eavesdropper. To this end, we propose to leverage the sensing capability of integrated sensing and communication (ISAC) systems to assist PLS design. To comprehensively study the dynamic behaviors of the system, we propose a Pareto optimization framework, where a multi-objective optimization problem (MOOP) is formulated to simultaneously optimize three key performance metrics: power consumption, number of securely served users, and tracking performance, while guaranteeing the achievable rate of the users with a given leakage rate constraint. A globally optimal design based on the generalized Bender’s decomposition (GBD) method is proposed to achieve the Pareto optimal solutions. To reduce the computational complexity, we further design a low-complexity algorithm based on zero-forcing (ZF) beamforming and successive convex approximation (SCA). Simulation results validate the effectiveness of the proposed algorithms and reveal the intrinsic trade-offs between the three performance metrics. It is observed that near-field communication offers a favorable beam diffraction effect for PLS, where the energy of the information signal is nulled around the eavesdropper and focused on the users.

In this paper, we propose a best node and friendly jammer (bN-fJ) scheme for secure short-packet multi-hop communications in Internet-of-Things (IoT) networks. Under imperfect channel state information (CSI) conditions, the best IoT node is chosen for data transmission and a friendly jammer is chosen later to confuse the received signals at a multi-antenna eavesdropper. Approximate and asymptotic closed-form expressions for the secrecy throughput of the bN-fJ scheme are obtained, offering valuable insights into the system designs. Numerical results show that the proposed bN-fJ scheme achieves much better performance than benchmarking schemes, especially with 1 more bit/channel use and 9.4 dB enhancement in communication reliability measure. Moreover, under imperfect CSI and large antennas at the eavesdropper, the secrecy throughput and communication reliability of the system can be improved by employing the proposed node selection, e.g., a 12.5 dB improvement at 10 antennas at Eve and the imperfect CSI of 0.8. Finally, the secrecy throughput is presented as a concave curve for the number of hops and blocklengths, which enables us to identify the optimal hops and blocklength for secure multi-hop short-packet transmissions.

The IEEE 802.11ad and IEEE 802.11ay standards define the medium access control (MAC) operation for millimeter-wave communications. During the beamforming training (BFT) period, the stations compete for the channel adopting a modified version of slotted aloha, where stations mandatorily transmit and can retry in a finite number of slots. This change makes the probability of a station accessing a given slot dependent on the previous ones. Additionally, stations halt the competition for BFT, if the number of consecutive collisions reaches a predefined collision threshold (CT) value. In this paper, we characterize the probability of a station interrupting the competition for BFT due to consecutive collisions. To address the characterization of the interruption for the BFT competition, we model the individual collision distribution and the distribution of a station reaching the CT over multiple BFT periods. The proposed model is validated through simulation and its performance is compared to other approximated IEEE 802.11ad models, confirming the accuracy of the proposed approach.

The objective of this paper is to address the following question: Are shadowing, multi-path, and line-of-sight (LoS) effects friends or foes? To this end, we examine shadowed k-μ fading channels as they characterize shadowing, multi-path and LoS components. Furthermore, we explore the impact of various types of shadowing. To answer the main question of this paper, we consider different wireless communication systems, including selection combining, maximal-ratio combining, equal-gain combining, and space-shift keying multiple-input multiple-output modulation. Extensive Monte-Carlo simulation results reveal that there is no single definitive answer. In other words, the aforementioned phenomena can either be advantageous or disad-vantageous depending on the type of the wireless communication system. However, it is important to note that these insights apply to both perfect channel state information (CSI) and imperfect CSI.

Intelligent reconfigurable surface (IRS) is an innovative and promising technology to achieve intelligent reconfigurable wireless environment, and thus, enables cost-effective and energy-efficient wireless communications. Due to the broadcasting nature of the wireless signals, the reflected signal in IRS-assisted wireless communications networks might suffer from eavesdropping. Thus, it is essential to tackle the secrecy aware problems in IRS-assisted wireless communications networks. In this article, we consider an aerial IRS (AIRS) assisted wireless relay network scenario, where IRS is mounted on the aerial platform. The artificial noise is added to interrupt the eavesdropping. A secrecy rate maximization problem is formulated subject to the total transmit power and reflecting phase shift constraints. To solve this problem, we first divide the secrecy maximization problem into three subproblems, i.e., transmit power allocation, AIRS trajectory design, and reflecting phase shift optimization. These three subproblems are solved alternately until convergence to maximize the secrecy rate. Especially, for the AIRS trajectory design and reflecting phase shift optimization, we employ the successive convex approximation (SCA) and positive semidefinite relaxation (SDR) technologies to convert the nonconvex optimization problems into convex problems, respectively. The intercept probability of the proposed optimal schemes is derived and the theoretical analyses show that the intercept probability can be reduced by increasing the numbers of IRS elements. Simulation results show that the joint optimization of transmit power, AIRS trajectory and reflecting phase shift can effectively improve the secrecy rate.

The dual-function radar communication (DFRC) system utilizes a hardware platform to achieve both radar and communication functions. In comparison to independently exploiting radar and communication systems, the DFRC system can significantly reduce system redundancy, volume, weight, and energy consumption. This makes DFRC an important area of research with practical value in advanced internet of things (IoT) techniques. This paper explores an exemplary DFRC system based on the orthogonal chirp division multiplexing (OCDM) methodology. In this system, OCDM achieves chirp multiplexing through the Fresnel transform and is identified as a potential replacement for OFDM in high-speed communication systems. In the proposed system, we integrate index modulation (IM) into the communication subsystem and utilize the subchirp index of OCDM to convey additional communication information, thereby significantly enhancing the communication rate of the DFRC system. Furthermore, a radar processing algorithm utilizing the OCDM signal is developed. This algorithm integrates the sparsity-aided compressed sensing (CS) algorithm into the radar subsystem to enhance estimation precision and reduce the sampling rate and hardware complexity of the radar receiver. Based on the proposed OCDM-assisted DFRC scheme, the communication rate of the DFRC system and the ambiguity function of OCDM are evaluated. The feasibility and effectiveness of the proposed DFRC system are confirmed through numerical calculations and simulation results.

A millimeter wave (mmWave) line-of-sight (LOS) multi-input-multioutput (MIMO) system is studied, where spatial multiplexing is achieved through LOS transmissions in near-field areas. A comprehensive analysis of the achievable rate in mmWave LOS MIMO system with respect to the rotation of uniform linear arrays (ULAs) is provided for both full-rank orthogonal and rank-deficient LOS MIMO channels. 1) For full-rank orthogonal scenarios, the necessary and sufficient condition for the LOS MIMO system to have the maximum achievable rate is established. 2) For rank-deficient scenarios, a closed-form expression for the Degrees of Freedom (DoF) of mmWave LOS MIMO channels is derived. Subsequently, the alternating phase approximation-based (APA-based) hybrid precoding algorithm is proposed, where the number of radio frequency (RF) chains is determined by the DoF of mmWave LOS MIMO channels and the initial phase of iterative optimizations is determined according to the optimal digital precoder. Our numerical results confirm the effectiveness of our analysis and proposed algorithm. It is also unveiled that 1) the DoF can be maximized through a specific angle design and 2) compared to existing hybrid precoding algorithms, our proposed APA-based hybrid precoding algorithm exhibits superior robustness against the angular rotation of ULAs.

This paper proposes a reconfigurable intelligent surface-aided two-way full-duplex communication with non-orthogonal multiple access transmission schemes to improve spectrum utilization. Besides, the joint impact of error phase-shift quantization, imperfect successive interference cancellation, and residual loop interference on the system performance have also been investigated. Under Nakagami-m fading channels, approximate closed-form expressions for the outage performance and the ergodic rate are derived. Through asymptotic analyses, some insights are achieved, including the diversity order, the coding gain, and the ergodic slope. Moreover, three adaptive power allocation optimization problems are formulated aiming to: 1) minimize outage probability, 2) achieve max-min rate fairness, and 3) maximize the user’s sum rate subject to the quality-of-service constraint. Simulation results not only validate the theoretical analyses and the optimal/sub-optimal solutions but also reveal three following observations. First, the considered system outperforms the orthogonal multiple access baseline. Second, increasing the number of control bits for the error phase-shift quantization and/or the number of RIS’s elements can significantly reduce the impact of imperfect successive interference cancellations. Third, employing one of three adaptive power allocation solutions improves the system’s performance significantly.

This paper investigates rate-splitting multiple access (RSMA) networks with multiusers assisted by aerial intelligent reflecting surfaces (AIRS). To improve the sum-rate of the system, the UAV’s trajectory and phase-shift vectors are optimized, in which the mobility scenarios with static and dynamic users are explored. In particular, long short-term memory (LSTM)-based frameworks for predicting the UAV’s trajectory and the phase-shift of the reflecting elements of AIRS are proposed. For more insight, a third model is created by combining information from the static and dynamic scenarios. Furthermore, to improve the transmit beamforming at the BS, an algorithm based on alternating optimization (AO) under the assumptions of imperfect successive interference cancelation (SIC) is presented. Training progress and testing results are provided to demonstrate the efficiency of the proposed models. In addition, numerical simulations are presented to verify the performance gains in terms of sum-rate. The simulation results show that the UAV performs better in trajectory prediction and phase-shift when different investigated scenarios are not combined.

In this paper, an effective simultaneous channel estimation and sensing algorithm is proposed for millimeter wave (mmWave) multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithm consists of a two-stage channel estimation scheme and a reliable sensing scheme, which enables high-quality channel estimation and precise sensing in three-dimensional (3D) space. Specifically, the proposed algorithm first puts forward an improved simultaneous orthogonal matching pursuit algorithm that utilizes structural relation between the sparse basis and indexes to implement coarse estimation of multiple parameters. Subsequently, taking the obtained coarse parameters as initial values, optimization of channel parameters is achieved using the idea of maximum likelihood and gradient descent algorithm. Finally, we develop a reliable sensing scheme to realize user localization and mapping of scattering environment in various scenarios. Cramér-Rao bounds (CRBs) of the parameters and positions are also derived and used as a benchmark in simulations. Simulation results demonstrate that compared with the existing algorithms, the proposed algorithm has better channel estimation and sensing performance and is closer to CRBs. Moreover, even in challenging environment with unknown user orientation and clock bias, the proposed algorithm can achieve precise user localization and mapping of scattering environment.

This paper investigates the performance of scalable user-centric (UC) distributed massive multiple-input multiple-output (D-mMIMO) systems with multiple central processing units (CPUs), commonly called cell-free mMIMO. Specifically, a framework incorporating processing capacity and inter-CPU communication constraints is proposed. Two methods are presented for limiting the number of radio units (RUs) serving each user equipment (UE). The first method is performed by the CPUs, while the second one is implemented at the UEs and RUs. Both methods prevent the computational complexity (CC) for channel estimation and precoding signals from increasing with the number of RUs. The backhaul signaling demands are presented and modeled, and it is considered that each CPU can serve only a restricted number of UEs managed by other CPUs to mitigate inter-CPU communication. Two strategies to adjust the RU clusters according to the network implementations are also proposed. We compare the proposed approaches with a traditional scalable UC system. Simulation results reveal that the proposed techniques allow UC systems to keep their spectral efficiency (SE) under minor degradation while reducing the CC by 98% and improving energy efficiency (EE). Besides, managing inter-CPU communication controls backhaul traffic effectively, and RU cluster adjustments further reduce CC.

This paper studies an intelligent reflecting surface (IRS)-aided multi-antenna simultaneous wireless information and power transfer (SWIPT) system where anM-antenna access point (AP) servesKsingle-antenna information users (IUs) andJsingle-antenna energy users (EUs) with the aid of an IRS with phase errors. We explicitly concentrate on overloaded scenarios whereK+J>MandK≥M. Our goal is to maximize the minimum throughput among all the IUs by optimizing the allocation of resources (including time, transmit beamforming at the AP, and reflect beamforming at the IRS), while guaranteeing the minimum amount of harvested energy at each EU. Towards this goal, we propose two user grouping (UG) schemes, namely, the non-overlapping UG scheme and the overlapping UG scheme, where the difference lies in whether identical IUs can exist in multiple groups. Different IU groups are served in orthogonal time dimensions, while the IUs in the same group are served simultaneously with all the EUs via spatial multiplexing. The two problems corresponding to the two UG schemes are mixed-integer non-convex optimization problems and difficult to solve optimally. We first provide a method to check the feasibility of these two problems, and then propose efficient algorithms for them based on the big-M formulation, the penalty method, the block coordinate descent, and the successive convex approximation. Simulation results show that: 1) the non-robust counterparts of the proposed robust designs are unsuitable for practical IRS-aided SWIPT systems with phase errors since the energy harvesting constraints cannot be satisfied; 2) the proposed UG strategies can significantly improve the max-min throughput over the benchmark schemes without UG or adopting random UG; 3) the overlapping UG scheme performs much better than its non-overlapping counterpart when the absolute difference betweenKandMis small and the EH constraints are not stringent.

This paper investigates the performance of downlink rate-splitting multiple access (RSMA)-aided unmanned aerial vehicle (UAV) communication systems, wherein a multi-antenna UAV exploits RSMA to serve multiple ground users. Considering nonline-of-sight environments, double-shadowed scattering channel modeling is adopted to generically characterize the impacts of mobility and shadowing on UAV-to-ground communications, assuming imperfect successive interference cancellation (SIC). Besides, a unified precoder design is proposed to fully capture the benefits of multi-antenna paradigms. Closed-form expressions for the users' outage probability (OP) and ergodic capacity are derived. In addition, asymptotic analysis is carried out to get further insights into the system design, such as the diversity gain and ergodic slope. Numerical results are presented, and it is shown that: 1) the effects of double-shadowed scattering on the system outage performance can be significantly reduced by increasing the number of antennas installed at the UAV; 2) the imperfect SIC error can be minimized by properly optimizing the power allocation of the common stream; and 3) RSMA provides superior users' ergodic capacity compared to its orthogonal and non-orthogonal multiple access counterparts.

This paper proposes an effective algorithm for gain-phase error (GPE) and angle estimation in multiple-input multiple-output (MIMO) radar. First, the received signals are constructed into a third-order tensor model containing information such as GPE in transmitting arrays (Tx) and receiving arrays (Rx), directions-of-departures (DODs) and directions-of-arrivals (DOAs). Then, a tensor-based two-stage estimation algorithm to estimate GPE and angles is developed. In the first stage, GPE and angle information are obtained by the least squares Khatri-Rao factorization (LS-KRF) separation. In the second stage, the GPE in Tx and Rx is estimated by a two-step GPE estimation scheme, while the DODs and DOAs are estimated by a non-iterative angle estimation scheme based on the spatial smoothing preprocessing. Simulation results show the superiority of the proposed GPE and angle estimation algorithm.

This paper investigates the performance of scalable user-centric (UC) distributed massive multiple-input multiple-output (D-mMIMO) systems, widely known in the literature as cell-free mMIMO, with limited processing capacity. Specifically, it is assumed that the computational complexity (CC) of performing channel estimation and precoding signals does not increase with the number of access points (APs). In this regard, it is considered that each user equipment (UE) can only be associated with a finite number of APs. Moreover, a method is proposed for adjusting the AP clusters according to the network implementation, i.e., centralized or distributed. We compare the proposed approaches with a scalable UC system that does not perform AP cluster adjustment and does not prevent the processing demands from growing with the number of APs. Simulation results reveal that UC systems can keep the spectral efficiency (SE) under minor degradation even if the processing capacity is limited, reducing the CC by up to 96%. Besides, the proposed method for adjusting the AP cluster leads to further reductions in CC.

Dual-polarized rate-splitting multiple access (RSMA) with polarization multiplexing has appeared recently as an attractive downlink transmission technique for dual-polarized massive multiple-input multiple-output (MIMO) systems. Dual-polarized RSMA does not require successive interference cancellation (SIC), which avoids practical issues of imperfect SIC decoding. Nevertheless, depolarization phenomena can still degrade the performance of this featured technique. In this work, we exploit the advanced capabilities of a dual-polarized intelligent reflecting surface (IRS) for unleashing an enhanced RSMA polarization multiplexing. To optimize the IRSs, we develop a multi-objective Frank-Wolfe-based algorithm for jointly mitigating cross-polar interference and improving the reception of common and private data streams at multiple users. Representative simulation examples demonstrate that dual-polarized IRS-MIMO-RSMA can efficiently mitigate detrimental depolarization phenomena and outperform conventional systems.

In this paper, we consider an Aerial Intelligent Reconfigurable Surface (AIRS) assisted wireless relay network, where IRS is implemented on the aerial platform, and the artificial noise is added to interrupt the eavesdropping. Based on the proposed system setup, it is formulated the secrecy rate maximization problem subject to the total transmit power and reflecting phase shift constraints. To solve this problem, we divide it into three sub-problems, i.e., transmit power allocation, AIRS trajectory optimization, and reflecting phase shift optimization. Alternately optimizing these three sub-problems, security aware joint optimal resource allocation schemes are proposed. Numerical simulations show that the joint optimal power allocation, AIRS trajectory, and reflecting phase shift schemes can effectively guarantee security performance for AIRS assisted wireless relay networks.

Since Facebook officially changed its name to Meta in Oct. 2021, the metaverse has become a new norm of social networks and three-dimensional (3D) virtual worlds. The metaverse aims to bring 3D immersive and personalized experiences to users by leveraging many pertinent technologies. Despite great attention and benefits, a natural question in the metaverse is how to secure its users' digital content and data. In this regard, blockchain is a promising solution owing to its distinct features of decentralization, immutability, and transparency. To better understand the role of blockchain in the metaverse, we aim to provide an extensive survey on the applications of blockchain for the metaverse. We first present a preliminary to blockchain and the metaverse and highlight the motivations behind the use of blockchain for the metaverse. Next, we extensively discuss blockchain-based methods for the metaverse from technical perspectives, such as data acquisition, data storage, data sharing, data interoperability, and data privacy preservation. For each perspective, we first discuss the technical challenges of the metaverse and then highlight how blockchain can help. Moreover, we investigate the impact of blockchain on key-enabling technologies in the metaverse, including Internet-of-Things, digital twins, multi-sensory and immersive applications, artificial intelligence, and big data. We also present some major projects to showcase the role of blockchain in metaverse applications and services. Finally, we present some promising directions to drive further research innovations and developments toward the use of blockchain in the metaverse in the future.

Unmanned aerial vehicle (UAV) communication systems usually operate in harsh scenarios, which require accurate information about the topology and wireless channel to achieve the desired transmission performance. Therefore, when millimeter-wave (mmWave) communication with its intrinsic Line-of-Sight (LoS) condition is adopted, accurate target localization is essential to determine the spatial relationship between the UAV and the grounded receivers (Rxs). In this article, a computer-vision (CV)-aided jointly optimization scheme of flight trajectory and power allocation is designed for mmWave UAV communication systems by utilizing the visual information captured via cameras equipped at the UAV. Compared with traditional schemes, the implementation cost and overhead can be greatly saved as no radio frequency transmissions are required in the proposed localization scheme. In addition, the transmit power at the UAV is jointly optimized with its flight trajectory in two different cases. Finally, simulation results are presented to demonstrate the efficiency of the proposed schemes.

Average-consensus protocol is one of the ways to develop distributed time-synchronization algorithms in Internet of Things (IoT) networks. However, the large number of iteration leads to a common time notion issue in nodes. This poses a critical challenge in the convergence of the time-synchronization algorithm and resulting asymptotic convergence in the average consensus protocol. In this article, a parameter-sharing-based average-consensus time-synchronization (PACTS) algorithm is proposed. For fast convergence, the proposed PACTS quickly forwards the time information to multihop nodes and employs multihop average-consensus instead of single-hop average consensus. Specifically, a node asynchronously and periodically broadcasts the relative clock offset estimation of neighbors with its local time information. Meanwhile, the relative clock offset estimation of the multihop node is calculated and used to estimate the average value. Consequently, an average consensus among local multihop nodes is obtained. As a result, the iteration number and convergence time are significantly reduced over the network. Finally, the experimental results indicate that the proposed PACTS algorithm has low complexity, high accuracy, and quick convergence.

In this paper, we study short-packet communications in multi-hop networks with wireless energy transfer, where relay nodes harvest energy from power beacons to transmit short packets to multiple destinations. It is proposed a novel cooperative beamforming relay selection (CRS) scheme which incorporates partial relay selection and distributed multiuser beamforming to achieve a high-reliable transmission in two consecutive hops. A closed-form expression for the average block error rate (BLER) of the CRS scheme is derived, based on which an asymptotic analysis is also carried out. To achieve optimal channel uses allocation, we formulate a fairness end-to-end throughput maximization problem which is generally NP-hard due to the non-concavity of the objective function and mixed-integer constraints. To solve this challenging problem efficiently, we first relax channel uses to be continuous and transform the relaxed problem into an equivalent non-convex one, but with a more tractable form. We then develop a low-complexity iterative algorithm relying on inner approximation framework to convexify non-convex parts that converges to at least a locally optimal solution. Towards real-time settings, we design an efficient deep convolutional neural network (CNN) with multiscale-accumulation connections to achieve the sub-optimal solution of the relaxed problem via real-time inference processes. Numerical results are presented to verify the analytical derivations and to demonstrate performance improvements of the CRS scheme over the benchmark ones in terms of BLER, reliability, latency, and throughput in various settings. Moreover, the designed CNN provides the lowest root-mean-square error compared to the state-of-the-art deep learning approaches while the CNN-aided optimization framework estimates accurately the optimal channel uses allocation with low execution time.

The polarization domain provides an extra degree of freedom (DoF) for improving the performance of multiple-input multiple-output (MIMO) systems. This paper takes advantage of this additional DoF to alleviate practical issues of successive interference cancellation (SIC) in rate-splitting multiple access (RSMA) schemes. Specifically, we propose three dual-polarized downlink transmission approaches for a massive MIMO-RSMA network under the effects of polarization interference and residual errors of imperfect SIC. The first approach implements polarization multiplexing for transmitting the users’ data messages, which removes the need to execute SIC in the reception. The second approach transmits replicas of users’ messages in the two polarizations, which enables users to exploit diversity through the polarization domain. The third approach, in its turn, employs the original SIC-based RSMA technique per polarization, and this allows the BS to transmit two independent superimposed data streams simultaneously. An in-depth theoretical analysis is carried out, in which we derive tight closed-form approximations for the outage probabilities of the three proposed approaches. Accurate approximations for the ergodic sum-rates of the two first schemes are also derived. Simulation results validate the theoretical analysis and confirm the effectiveness of the proposed schemes. For instance, under low to moderate cross-polar interference, the results show that, even under high levels of residual SIC error, our dual-polarized MIMO-RSMA strategies outperform the conventional single-polarized MIMO-RSMA counterpart. It is also shown that the performance of all RSMA schemes is impressively higher than that of single and dual-polarized massive MIMO systems employing non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) techniques.

Data collection and manipulation from real environments are two tasks that, in a well-organized way, can support the next generation of wireless networks (NGWNs) in resource allocation-related tasks such as spectrum frequency and transmission power. Due to the complexity of NGWNs, several metaheuristics have been proposed to help in these operations. However, a large number of these studies present solutions using only one algorithm or procedure, i.e., the research uses just one metaheuristic to resolve the radio resource allocation problem. This fact tends to lead to a loss of performance because, in this way, there is no considerable variability in the search strategies for better solutions. To tackle this problem, we propose a framework that maximizes the total throughput of an NGWN that implements the multi-carrier cell-less non-orthogonal multiple access (MC-CL-NOMA) architecture. The framework executes this activity using scenario information to feed an ensemble metaheuristic method. The empirical results show that the framework presents better performance than utilizing one metaheuristic individually. Moreover, the proposed method outperforms algorithms applicable in future 6G NOMA scenarios.

Aiming at overcoming practical issues of successive interference cancellation (SIC), this paper proposes a dual-polarized rate-splitting multiple access (RSMA) technique for a downlink massive multiple-input multiple-output (MIMO) net-work. By modeling the effects of polarization interference, an in-depth theoretical analysis is carried out, in which we derive tight closed-form approximations for the outage probabilities and ergodic sum-rates. Simulation results validate the accuracy of the theoretical analysis and confirm the effectiveness of the proposed approach. For instance, under low to moderate cross-polar interference, our results show that the proposed dual-polarized MIMO-RSMA strategy outperforms the single-polarized MIMO-RSMA counterpart for all considered levels of residual SIC error.

This paper investigates the performance of downlink (DL) pilot-based training to estimate the effective channel in user-centric cell-free massive multiple-input multiple-output (MIMO) networks. An algorithm for DL pilot assignment is proposed based on the level of interference between each user equipment (UE). It is proposed a refinement method for access point (AP) selection that controls the maximum AP cluster size of UEs. The strategy aims to control the maximum number of APs serving each UE to reduce the disparities among the AP cluster sizes. DL pilot-based training is compared with the blind, perfect and statistical channel state information (CSI) methods, assuming different precoding techniques, AP selection schemes, and the presence of pilot contamination. Our results demonstrate the following: (i) the proposed DL pilot assignment algorithm outperforms the baseline solutions; (ii) the proposed AP selection refinement method can improve the energy efficiency up to 86.6% without compromising the spectral efficiency; and (iii) DL pilot-based estimation reduces the normalized mean-square error significantly compared with blind and statistical CSI methods.

In this paper, we design an efficient deep convolutional neural network (CNN) to improve and predict the performance of energy harvesting (EH) short-packet communications in multi-hop cognitive Internet-of-Things (IoT) networks. Specifically, we propose a Sum-EH scheme that allows IoT nodes to harvest energy from either a power beacon or primary transmitters to improve not only packet transmissions but also energy harvesting capabilities. We then build a novel deep CNN framework with feature enhancement-collection blocks based on the proposed Sum-EH scheme to simultaneously estimate the block error rate (BLER) and throughput with high accuracy and low execution time. Simulation results show that the proposed CNN framework achieves almost exactly the BLER and throughput of Sum-EH one, while it considerably reduces computational complexity, suggesting a real-time setting for IoT systems under complex scenarios. Moreover, the designed CNN model achieves the root-mean-square-error (RMSE) of 1.33 × 10-2on the considered dataset, which exhibits the lowest RMSE compared to the deep neural network and state-of-the-art machine learning approaches.

In this paper, we propose a deep learning (DL)-based method to automatically identify the modulations of orthogonal frequency-division multiplexing (OFDM) signals in wireless communication systems. In particular, a cost-efficient OFDM modulation classification convolutional neural network (COM-ConvNet) is principally designed with grouped convolutional layers to reduce computing complexity significantly. Remarkably, reconstructing the high-dimensional data array of OFDM signals allows our deep network to learn the underlying sample correlations within every symbol and among different symbols sufficiently. We leverage residual connection and attention connection with element-wise addition and element-wise multiplication layers in specific-designed processing blocks to enhance the pattern learning efficiency. For performance evaluation, we test the proposed method on a synthetic six-modulation OFDM signal dataset under impaired channel conditions and conduct diverse simulations, such as ablation study, parameter investigation, and complexity analysis. COM-ConvNet achieves cost efficiency (i.e., small network size and low computational cost) while maintaining an acceptable accuracy when compared with other DL models.

Information theoretic learning (ITL) criteria have emerged useful for mitigating degradations caused by unknown non-Gaussian noise processes in future wireless communication systems. Specifically, the reproducing kernel Hilbert space (RKHS) based approaches relying on ITL based learning criteria are envisioned to provide near-optimal mitigation of unknown hardware impairments and non-Gaussian noises. Among several ITL criteria, the recent works find the minimum error entropy with fiducial points (MEEF) promising due to its guarantee of unbiased estimation and generalization over generic noise distributions. However, MEEF based learning approaches are known to depend on an accurate kernel-width initialization. Also, the optimal value of this kernel-width is well-known to vary temporally and across deployment scenarios. To remove the dependency on kernel-width, a hyperparameter-free MEEF based adaptive algorithm is derived using random-Fourier features with sampled kernel widths (RFF-SKW). In addition, a detailed convergence analysis is presented for the proposed hyperparameter-free MEEF, which promises a near-optimal error-floor independent of step-size and guarantees convergence for a wide range of step sizes. The promised hyperparameter-independence and improved convergence for the proposed hyperparameter-free MEEF are validated by computer simulations considering different case studies.

This paper investigates reconfigurable intelligent surface (RIS)-aided cognitive non-orthogonal multiple access (NOMA) systems, where an RIS is deployed to serve two users under multi-primary users’ constraints. Our analysis assumes imperfect channel state information and successive interference cancellation under scenarios with and without line-of-sight (LoS) link between source and users. We derive exact closed-form expressions for the outage probability, throughput, and an upper bound for the ergodic capacity (EC). To provide further insights, an asymptotic analysis is carried out by considering two power settings at the source. It is also determined the optimal data rate factors of all users that maximize the system throughput. In addition, a deep learning framework (DLF) for EC prediction is designed. Numerical results show that: i) compared to the system without LoS link, the performance of the proposed system with LoS link can significantly improve when the number of reflecting elements at the RIS increases, and ii) the proposed system has superior performance compared to its orthogonal multiple access counterpart. Furthermore, our proposed DLF exhibits the lowest root-mean-square error and low execution-time among other approaches, verifying the effectiveness of this method for future analysis.

This paper presents a deep learning-based method to automatically recognize low probability of intercept (LPI) radar waveforms against diversified jamming attacks. Concretely, an efficient convolutional neural network (CNN) architecture, namely Densely-Accumulated Network (DANet), is introduced to learn the time-frequency representation transformed by the Wigner-Ville distribution. Such an architecture has several novel densely-accumulated connection modules specified by various symmetric and asymmetric convolutional layers to enrich diversified features at multiple representational maps. Besides, the skip-connection and dense-connection are leveraged to improve feature learning efficiency and prevent the vanishing gradient when the network goes deeper. Some image processing techniques (e.g., global thresholding and digital filtering) are adopted to enhance the quality of time-frequency image. Relying on simulations, we benchmark the proposed method on a synthetic 13-waveform dataset and also investigate the influence of hyper-parameters (such as image size, number of modules, training data size) on the overall recognition performance. Remarkably, with average accuracy of 98.2% at 0 dB signal-to-noise ratio (SNR), DANet outperforms several backbone CNNs and state-of-the-art networks of LPI waveform recognition while keeping a cost-efficient model.

In this paper, we study short-packet communications (SPCs) in multi-hop wireless-powered Internet-of-Things networks (WPINs), where IoT devices transmit short packets to multiple destination nodes by harvesting energy from multiple power beacons. To improve system block error rate (BLER) and throughput, we propose a best relay-best user (bR-bU) selection scheme with an accumulated energy harvesting mechanism. Closed-form expressions for the BLER and throughput of the proposed scheme over Rayleigh fading channels are derived and the respective asymptotic analysis is also carried out. To support real-time settings, we design a deep neural network (DNN) framework to predict the system throughput under different channel settings. Numerical results demonstrate that the proposed bR-bU selection scheme outperforms several baseline ones in terms of the BLER and throughput, showing to be an efficient strategy for multi-hop SPCs. The resulting DNN can estimate accurately the throughput with low execution time. The effects of message size on reliability and latency are also evaluated and discussed.

Wireless communications empowered by Reconfigurable Intelligent Surfaces (RISs) are recently gaining remarkable research attention due to the increased system design flexibility offered by RISs for diverse functionalities. In this paper, we consider a Multiple Input Multiple Output (MIMO) physical layer security system with multiple data streams including one legitimate and one eavesdropping passive RISs, with the former being transparent to the eavesdropper and the latter’s presence being unknown at the legitimate link. We first focus on the eavesdropping subsystem and present a joint design framework for the eavesdropper’s combining matrix and the reflection coefficients of the eavesdropping RIS. Then, focusing on the secrecy rate maximization, we propose a physical layer security scheme that jointly designs the legitimate precoding matrix and the Artificial Noise (AN) covariance matrix, as well as the legitimate combining matrix and the reflection coefficients of the legitimate RIS. Our simulation results reveal that, in the absence of a legitimate RIS, transceiver spatial filtering and AN are incapable of offering nonzero secrecy rates, even for eavesdropping RISs with small numbers of elements. However, when an L-element legitimate RIS is deployed, confidential communication can be safeguarded against cases with even more than a 5L-element eavesdropping RIS.

In this paper, we propose and investigate an aerial reconfigurable intelligent surface (aerial-RIS)-aided wireless communication system. Specifically, considering practical composite fading channels, we characterize the air-to-ground (A2G) links by Namkagami-m small-scale fading and inverse-Gamma large-scale shadowing. To investigate the delay-limited performance of the proposed system, we derive a tight approximate closed-form expression for the end-to-end outage probability (OP). Next, considering a mobile environment, where performance analysis is intractable, we rely on machine learning-based performance prediction to evaluate the performance of the mobile aerial-RIS-aided system. Specifically, taking into account the three-dimensional (3D) spatial movement of the aerial-RIS, we build a deep neural network (DNN) to accurately predict the OP. We show that: (i) fading and shadowing conditions have strong impact on the OP, (ii) as the number of reflecting elements increases, aerial-RIS achieves higher energy efficiency (EE), and (iii) the aerial-RIS-aided system outperforms conventional relaying systems.

This paper derives a model that computes the optimal carrier sensing range adopted by transmitters of a Carrier Sense Multiple Access (CSMA) network when multiple wireless networks coexist in the same band. To investigate the optimal carrier sensing range, we define a utility function that considers the impact of the carrier sensing threshold in the medium access probability and the transport capacity of a CSMA transmitter. Consequently, the definition of the carrier sensing range is a tradeoff between the avoidance of low channel access probability due to low carrier sensing thresholds and the avoidance of a high number of concurrent transmissions (collisions) per unit area due to high carrier sensing thresholds. Considering the aggregate power sensed by a CSMA node, we derive a method to compute the optimal carrier sensing range. The proposed solution is adopted in two different scenarios, considering the cases when the coexisting networks are spatially overlapped or non-overlapped. Extensive simulation results indicate that the optimal carrier sensing range is accurately approximated for the two scenarios, and higher optimal carrier sensing ranges are obtained for the overlapped scenario. The proposed method is a useful tool to find the optimal carrier sensing range when the spatial distribution of the coexisting networks is known.

Assuming a cooperative non-orthogonal multiple access (NOMA) system with rate fairness in a scenario with multiple users and arbitrary relays, this paper investigates adaptive power allocation (PA), relay selection (RS), and user pairing (UP) policies. Specifically, two adaptive PA optimization problems are formulated, one at the base station (BS) and another at the selected relays. Closed-form expressions for the power allocation factors are derived as well as an algorithm that provides the optimal solution at the BS. In order to show the superiority of the proposed study, our results are compared with other benchmark schemes in terms of outage probability, Jain's fairness index, and average sum rate.

In this article, we study the physical-layer security (PHY-security) improvement method for a downlink nonorthogonal multiple access (NOMA) system in the presence of an active eavesdropper. To this end, we propose a full-duplex (FD)-enabled NOMA system and a promising scheme, called the minimal transmitter selection (MTS) scheme, to support secure transmission. Specifically, the cell-center and cell-edge users act simultaneously as both receivers and jammers to degrade the eavesdropper channel condition. Additionally, the proposed MTS scheme opportunistically selects the transmitter to minimize the maximum eavesdropper channel capacity. To estimate the secrecy performance of the proposed methods, we derive an approximated closed-form expression for secrecy outage probability (SOP) and build a deep neural network (DNN) model for SOP evaluation. Numerical results reveal that the proposed NOMA system and MTS scheme improve not only the SOP but also the secrecy sum throughput. Furthermore, the estimated SOP through the DNN model is shown to be tightly close to other approaches, i.e., the Monte-Carlo method and analytical expressions. The advantages and drawbacks of the proposed transmitter selection scheme are highlighted, along with insightful discussions.

In this paper, we study the statistical characterization and modeling of distributed multi-reconfigurable intelligent surface (RIS)-aided wireless systems. Specifically, we consider a practical system model where the RISs with different geometric sizes are distributively deployed, and wireless channels associated to different RISs are assumed to be independent but not identically distributed (i.n.i.d.). We propose two purpose-oriented multi-RIS-aided schemes, namely, the exhaustive RIS-aided (ERA) and opportunistic RIS-aided (ORA) schemes. A mathematical framework, which relies on the method of moments, is proposed to statistically characterize the end-to-end (e2e) channels of these schemes. It is shown that either a Gamma distribution or a Log-Normal distribution can be used to approximate the distribution of the magnitude of the e2e channel coefficients in both schemes. With these findings, we evaluate the performance of the two schemes in terms of outage probability (OP) and ergodic capacity (EC), where tight approximate closed-form expressions for the OP and EC are derived. Representative results show that the ERA scheme outperforms the ORA scheme in terms of OP and EC. In addition, under i.n.i.d. fading channels, the reflecting element settings and location settings of RISs have a significant impact on the system performance of both the ERA or ORA schemes.

In this paper, we investigate the performance of a mixed radio-frequency-underwater wireless optical communication (RF-UWOC) system where an unmanned aerial vehicle (UAV), as a low-altitude mobile aerial base station, transmits information to an autonomous underwater vehicle (AUV) through a fixed-gain amplify-and-forward (AF) or decode-and-forward (DF) relay. The analysis accounts for the main factors that affect the system performance, such as the UAV height, air bubbles, temperature gradient, water salinity variations, detection techniques, and pointing errors. Employing the fixed-gain AF and DF relays, expressions for some key performance metrics are derived, e.g., outage probability (OP), average bit error rate (ABER), and average channel capacity. Moreover, in order to get further insights, asymptotic analyses for the OP and ABER are also carried out. Additionally, for the two relaying systems, we derive analytical expressions for the optimal UAV altitude that minimizes the OP. Simulation results show that the UAV altitude influences the system performance and there is an optimal altitude which ensures a minimum OP. Furthermore, it is demonstrated that the diversity order of the fixed-gain AF relaying depends on the RF link and pointing errors, while the diversity order of the DF relaying depends on the detection techniques and pointing errors.

In this paper, we investigate an underlay cognitive non-orthogonal multiple access (NOMA)-based coordinated direct and relay transmission network with imperfect successive interference cancellation, imperfect channel state information, and co-channel interference caused by a multi-antenna primary transmitter. In the secondary network, a source communicates with a near user via direct link and with a far user through the assistance of multiple relays subject to transmit power constraints. Four relay selection schemes are proposed to enhance the performance of NOMA users and the overall system throughput. In our analysis, exact closed-form expressions for the outage probability (OP) of NOMA users and for the overall system throughput are derived. To provide further insights, a performance floor analysis is carried out considering two power-setting scenarios: (i) the transmit powers at the secondary source and relays go to infinity and (ii) the peak interference constraint goes to infinity. Towards real-time configurations, we also design a deep learning (DL) framework for the OP and system throughput prediction. Our results show that the deep neural network exhibits the lowest run-time prediction and root-mean-square error among the proposed DL models. Furthermore, the predicted results based on DL framework match with those of the analysis and simulation.

In this paper, sub-channel scheduling, task assignment and power allocation are investigated for orthogonal multiple access (OMA)-based and non-orthogonal multiple access (NOMA)-based mobile edge computing (MEC) systems. Based on different channel conditions and computational capacities, computational tasks are partially offloaded to the MEC server via OMA or NOMA protocols. In order to minimize the total energy consumption, an optimization problem under the task execution latency constraint is formulated and divided into two sub-problems. By utilizing matching theory, the formulated sub-channel allocation problem is solved by a proposed low-complexity algorithm, where the joint optimization of task assignment and power allocation is performed at each iteration. Based on the delay constraint, some insights are obtained, and the closed-form solutions of task assignment coefficients and transmit power are derived. Furthermore, the offloading strategy in both OMA and NOMA schemes is analyzed, which shows that the optimal task assignment coefficient is decided by the energy consumption efficiency (ECE). Simulation results indicate that: i) the proposed sub-channel allocation algorithm and derived closed-form solutions can significantly improve the MEC system in terms of the energy consumption; ii) the provided offloading strategy can be dynamically and efficiently employed with different channel conditions and computational capacities.

Ensuring physical layer security is a crucial task in conventional and emerging communication systems, which are typically characterized by stringent quality of service and security requirements. This also accounts for wireless technologies in the context of the Internet of Things paradigm, which are expected to exhibit considerably increased computational complexity. Based on this, the present contribution investigates the secrecy outage performance of a dual-hop decode-and-forward (DF) mixed radio-frequency/ underwater optical wireless communication (RF/UOWC) system. Such wireless network configurations are particularly useful in efficient and demanding scenarios, such as military communications. Therefore, our analysis considers one single-antenna source node (S) communicating with one legitimate destination node (D) via a DF relay node (R) equipped with multiple antennas for reception. Particularly, the relay receives the incoming signal from S via an RF link, applies selection-combining (SC) technique, fully decodes it, re-encodes it, and then forwards it to the destination via a UOWC link. The communication is performed under the eavesdropper's attempt to intercept the S - R hop (RF side). In this context, a closed-form expression for the secrecy outage probability is derived along with a thorough asymptotic analysis in the high SNR regime, based on which the achievable diversity order is provided. The offered results provide useful insights on the impact of some key system and channel parameters on the secrecy outage performance, such as the number of eavesdroppers, the number of relay antennas, fading severity parameters of RF links, and water turbulence severity of the UOWC link. The conducted analysis shows that the secrecy outage probability is dominated only by the R-D link in the high SNR regime, regardless of the S-R parameters, such as the number of relay antennas and the average SNR at the relay branches. The offered analytic results are corroborated with respective results from computer simulations. Since these parameters are closely related with the computational complexity at the involved terminals, the offered insights are useful for the design and computationally sustainable operation of such systems.

The present work investigates the physical layer security of wireless communication systems over non-homogeneous fading environments, i.e., η-μ and λ-μ fading models, which are typically encountered in realistic wireless transmission scenarios in the context of conventional and emerging communication systems. This study considers a single-input multiple-output system that consists of a single-antenna transmitter, a multi-antenna legitimate receiver, and an active multi-antenna eavesdropper. To this end, novel exact analytical expressions are derived for the corresponding average secrecy capacity and secrecy outage probability, which are corroborated by respective results from computer simulations. Capitalizing on the offered results, the physical layer security is quantified in terms of different parameters, which leads to useful insights on the impact of non-homogeneous fading environment and the number of employed antennas on the achieved physical layer security levels of the underlying system configuration. The offered results and insights are useful for the design of such systems as well as for the computational requirements and sustainability relating to such systems, since emerging communications are largely characterized by stringent quality of service and complexity requirements.

In this paper, a backscatter cooperation (BC) scheme is proposed for non-orthogonal multiple access (NOMA) downlink transmission. The key idea is to backscatter the surplus power of the received downlink signals at one user to enhance the reception of the user who cannot recover its information. To evaluate the performance of the proposed BC-NOMA scheme, three benchmark schemes are introduced, which include the non-cooperation (NC)-NOMA scheme, the conventional relaying (CR)-NOMA scheme, and the incremental relaying (IR)-NOMA scheme. For all these schemes, the outage performance, the expected rate, and the diversity-multiplexing trade-off (DMT) performance are analyzed. Additionally, the optimal power allocation to maximize the expected rate of the BC-NOMA scheme in the high signal-to-noise ratio region is derived. The theoretical results show that the proposed BC-NOMA can enhance both transmission reliability (outage performance) and transmission effectiveness (expected rate) compared with NC-NOMA. Furthermore, representative numerical results are presented to validate the theoretical results. It is shown that unlike the two relaying-based cooperative schemes (CR/IR-NOMA), which improve the outage performance at the cost of transmission rate, the proposed BC-NOMA scheme can enhance the transmission reliability without impairing the spectrum efficiency, which makes backscattering an appealing solution to cooperative NOMA downlinks.

Intelligent reflecting surface (IRS), which is composed of a large number of low-cost passive elements, has the ability to reflect the incident signal independently with an adjustable phase and amplitude shifts, has been regarded as a key and emerging technology for achieving the cost-effectively spectrum and addressing energy issues in the next generation of wireless networks. In this paper, we propose a general wideband non-stationary channel model for IRS-assisted multiple-input multiple-output (MIMO) communication scenarios, which aims at capturing the underlying propagation characteristics of IRS-assisted communication systems. By properly adjusting the key system parameters, the proposed channel model can be used to describe various IRS-assisted communication scenarios. Furthermore, we separate the channel between the mobile transmitter (MT) and mobile receiver (MR) into the subchannel between the MT and IRS, the subchannel between the IRS and MR, and the subchannel between the MT and MR. The physical properties of each subchannel are investigated accordingly; then, we develop an equivalent channel model to study the key characteristics of the proposed IRS-assisted channel model, such as the time-varying spatial-temporal (ST) cross-correlation functions (CCFs), temporal auto-correlation functions (ACFs), and frequency CCFs. Finally, numerical results demonstrate that the proposed channel model is practical for describing the IRS-assisted MIMO wireless communication scenarios.

A dual-polarized intelligent reflecting surface (IRS) can contribute to a better multiplexing of interfering wireless users. In this paper, we use this feature to improve the performance of dual-polarized massive multiple-input multiple-output (MIMO) with non-orthogonal multiple access (NOMA) under imperfect successive interference cancellation (SIC). By considering the downlink of a multi-cluster scenario, the IRSs assist the base station (BS) to multiplex subsets of users in the polarization domain. Our novel strategy alleviates the impact of imperfect SIC and enables users to exploit polarization diversity with near-zero inter-subset interference. To this end, the IRSs are optimized to mitigate transmissions originated at the BS from the interfering polarization. The formulated optimization is transformed into quadratically constrained quadratic sub-problems, which makes it possible to obtain the optimal solution via interior-points methods. We also derive analytically a closed-form expression for the users’ ergodic rates by considering large numbers of reflecting elements. This is followed by representative simulation examples and comprehensive discussions. The results show that when the IRSs are large enough, the proposed scheme always outperforms conventional massive MIMO-NOMA and MIMO-OMA systems even if SIC error propagation is present. It is also confirmed that dual-polarized IRSs can make cross-polar transmissions beneficial to the users, allowing them to improve their performance through diversity.

In this paper, we consider an overlay cognitive satellite-terrestrial relay network comprising a primary satellite source-receiver pair and a secondary transmitter-receiver pair on the ground while taking into account the practical relay transceiver hardware impairments. Herein, a primary satellite source communicates with its terrestrial user by utilizing both direct and relay links, using an amplify-and-forward based cooperative communication technique. Considering distortion noises in relay signal processing, we obtain new closed-form expressions for the outage probability of primary satellite network and secondary terrestrial network and explicitly derive the asymptotic outage behaviour for primary network by adopting pertinent heterogeneous fading models. Both primary and secondary networks satisfy their quality-of-service criteria for the low data rate requirements only. However, for the high data rate requirements, a ceiling effect is shown to occur which eventually makes the spectrum sharing infeasible.

Multiple-mode orthogonal frequency division multiplexing with index modulation (MM-OFDM-IM) is a promising IM technique for OFDM systems to improve the spectral efficiency. In this paper, a secure index and data symbol modulation (DSM) scheme based on MM-OFDM-IM systems is proposed by employing the notion of the channel reciprocity assuming time division duplexing mode. The channel state information of the legitimate link is adopted as a secret key for protecting data transmission. We investigate randomized mapping rules for both IM and DSM, based on which even though an eavesdropper has estimated the permutations of modes and their carrying symbols, he cannot decode the message correctly. Particularly, the randomized mapping rules for IM are designed by exploiting the redundancy of mode permutations. Both numerical simulations and theoretical analysis for achievable secure rate and bit error rate are processed, whose results demonstrate that the proposed scheme can enhance the physical-layer security of the considered communication system.

Most physical-layer authentication techniques use channel information to prevent spoofing attacks. In such techniques, one must estimate the channel information for each authentication procedure. However, when the number of pilots decreases, authentication accuracy also decreases due to low channel estimation quality. This paper proposes a novel signal relation-based authentication method that relies on the detection of received signal symbols and does not require the estimation of channel information in the testing stage. It is noteworthy that the authentication performance of the proposed scheme remains in a good level. We develop two different solutions for the detection of received signal symbols, namely, minimum mean-square error and long short-term memory. Extensive simulation results show the main insights of the proposed signal relation-based authentication method compared to conventional channel-based authentication method.

This paper investigates the achievable sum rate and the outage capacity of generalized frequency division multiplexing systems (GFDMs) with minimum mean-square error (MMSE) receivers over frequency-selective Rayleigh fading channels. To this end, a Gamma-based approximation approach for the probability density function of the signal-to-interference-plus-noise ratio is presented, based on which accurate analytical formulations for the achievable sum rate and outage capacity are proposed. The accuracy of our analysis is corroborated through Monte Carlo simulation assuming different GFDM parameters. Illustrative numerical results are depicted in order to reveal the impact of the key system parameters, such as the number of subcarriers, number of subsymbols, and roll-off factors, on the overall system performance.

In this paper, we propose a novel successive sub-array activation (SSAA) diversity scheme for a massive multiple-input multiple-output (MIMO) system in combination with non-orthogonal multiple access (NOMA). A single-cell multi-cluster downlink scenario is considered, where the base station (BS) sends redundant symbols through multiple transmit sub-arrays to multi-antenna receivers. An in-depth analytical analysis is carried out, in which an exact closed-form expression for the outage probability is derived. Also, a high signal-to-noise ratio (SNR) outage approximation is obtained and the system diversity order is determined. Our results show that the proposed scheme outperforms conventional full array massive MIMO setups.

A backscatter cooperation (BC) mechanism is pro-posed for power-domain non-orthogonal multiple access (NOMA) downlink transmission. The key idea is to enable one user to split and then backscatter part of its received signals to improve the reception at another user. To evaluate its performance, three benchmark schemes are introduced. They are non-cooperation (NC)-NOMA, conventional relaying (CR)-NOMA, and incremental relaying (IR)-NOMA. For all these schemes, the analytical expressions of the minimum total power to avoid information outage are derived, based on which their respective outage performance, expected rates, and diversity-multiplexing trade-off (DMT) are investigated. Analytical results show that the proposed BC-NOMA strictly outperforms NC-NOMA in terms of all the three metrics. Furthermore, theoretical analyses are validated via Monte-Carlo simulations. It is shown that compared with CR/IR-NOMA, only the proposed BC-NOMA scheme can enhance the transmission reliability without impairing the transmission rate.

The identification of spectrum opportunities is a pivotal requirement for efficient spectrum utilization in cognitive radio systems. Spectrum prediction offers a convenient means for revealing such opportunities based on the previously obtained occupancies. As spectrum occupancy states are correlated over time, spectrum prediction is often cast as a predictable time-series process using classical or deep learning-based models. However, this variety of methods exploits time-domain correlation and overlooks the existing correlation over frequency. In this paper, differently from previous works, we investigate a more realistic scenario by exploiting correlation over time and frequency through a 2D-long short-term memory (LSTM) model. Extensive experimental results show a performance improvement over conventional spectrum prediction methods in terms of accuracy and computational complexity. These observations are validated over the real-world spectrum measurements, assuming a frequency range between 832-862 MHz where most of the telecom operators in Turkey have private uplink bands.

In this paper, the performance of a digital communication system subject to $\kappa-\mu$ Extreme fading conditions is investigated. Specifically, exact and novel analytical expressions for the normalized average channel capacity and average error rate are derived assuming arbitrary fading parameters. An asymptotic error analysis is also performed in order to get further insights for the system performance. Finally, our analytical results are corroborated through Monte Carlo simulations. Although the derived expressions of the exact performance metrics include an infinite series, it is worth noting that they all converge rapidly and steadily after a few terms. In light of this, a truncation error analysis on one of the underlying performance metrics under investigation is provided and it is shown that the relative error is less or equal to 0.001% for as many as 10 terms.

We consider a multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) network in which a source node, Alice, communicates with an energy-harvesting destination node, Bob, in the presence of a passive eavesdropper. To secure the wireless transmission, Alice generates a hybrid artificial noise (AN) in both frequency and time domains. Moreover, in order to collect more energy, Bob splits the received signal power of the cyclic prefix of each OFDM block. We then propose two non-convex optimization problems to balance both the need for security and the need for harvesting energy at Bob. While one considers maximizing the secrecy rate, the other approach aims at maximizing the harvested energy. Path-following algorithms of low computational complexity are developed and evaluated. Our numerical results show the gain of our proposed scheme and the effectiveness of our proposed algorithms.

In this paper, we propose a novel hybrid user pairing (HUP) scheme in multiuser multiple-input single-output non-orthogonal multiple access networks with simultaneous wireless information and power transfer. In this system, two information users with distinct channel conditions are optimally paired while energy users perform energy harvesting (EH) under non-linearity of the EH circuits. We consider the problem of jointly optimizing user pairing and power allocation to maximize the overall spectral efficiency (SE) and energy efficiency (EE) subject to user-specific quality-of-service and harvested power requirements. A new paradigm for the EE-EH trade-off is then proposed to achieve a good balance of network power consumption. Such design problems are formulated as the maximization of non-concave functions subject to the class of mixed-integer non-convex constraints, which are very challenging to solve optimally. To address these challenges, we first relax binary pairing variables to be continuous and transform the design problems into equivalent non-convex ones, but with more tractable forms. We then develop low-complexity iterative algorithms to improve the objectives and converge to a local optimum by means of the inner approximation framework. Simulation results show the convergence of proposed algorithms and the SE and EE improvements of the proposed HUP scheme over state-of-the-art designs. In addition, the effects of key parameters such as the number of antennas and dynamic power at the BS, target data rates, and energy threshold, on the system performance are evaluated to show the effectiveness of the proposed schemes in balancing resource utilization.

In this paper, an impartial simultaneous wireless information and power transfer (SWIPT)-assisted cooperation mechanism is proposed for a non-orthogonal multiple access (NOMA) downlink scenario. The key idea of the proposed impartial user cooperation mechanism (IUCM) is that not only the cell-center user but also the cell-edge user applies the power-splitting architecture and utilizes the harvested energy to forward the other user's information. Analytical results show that the proposed IUCM outperforms the traditional partial cooperation mechanism in terms of outage probability, diversity order and diversity-multiplexing trade-off (DMT). For comparison, the proposed IUCM is further incorporated into an orthogonal frequency-division multiple access (OFDMA) framework, which is shown to preserve the same diversity order, while has a worse but more flexible DMT performance compared with the IUCM in the NOMA framework. Furthermore, a classic non-linear energy harvesting model is introduced, based on which the outage probability, diversity order and DMT performance of the proposed IUCM and the conventional mechanism are analyzed. Finally, representative numerical results are given to validate our theoretical analysis.

In this paper, we propose a novel successive sub-array activation (SSAA) diversity scheme for a massive multiple-input multiple-output (MIMO) system in combination with non-orthogonal multiple access (NOMA). Considering a single-cell multi-cluster downlink scenario, where the base station (BS) sends redundant symbols through multiple transmit sub-arrays to multi-antenna receivers, a low-complexity two-stage beamformer, that is constructed based only on the long-term channel statistical information, is proposed. An in-depth analytical analysis is carried out, in which an exact closed-form expression for the outage probability is derived. A high signal-to-noise ratio (SNR) outage approximation is obtained and the system diversity order is determined. The ergodic sum-rate is also investigated, in which a closed-form solution is evaluated considering a particular case. Numerical and simulation results are provided to validate the analytical analysis and to demonstrate the performance superiority of the proposed SSAA scheme. For example, our results show that the proposed system operating with SSAA outperforms conventional full array massive MIMO setups.

This paper addresses multi-user multi-cluster massive multiple-input-multiple-output (MIMO) systems with non-orthogonal multiple access (NOMA). Assuming the downlink mode, and taking into consideration the impact of imperfect successive interference cancellation (SIC), an in-depth analytical analysis is carried out, in which closed-form expressions for the outage probability and ergodic rates are derived. Subsequently, the power allocation coefficients of users within each sub-group are optimized to maximize fairness. The considered power optimization is simplified to a convex problem, which makes it possible to obtain the optimal solution via Karush-Kuhn-Tucker (KKT) conditions. Based on the achieved solution, we propose an iterative algorithm to provide fairness also among different sub-groups. Simulation results alongside with insightful discussions are provided to investigate the impact of imperfect SIC and demonstrate the fairness superiority of the proposed dynamic power allocation policies. For example, our results show that if the residual error propagation levels are high, the employment of orthogonal multiple access (OMA) is always preferable than NOMA. It is also shown that the proposed power allocation outperforms conventional massive MIMO-NOMA setups operating with fixed power allocation strategies in terms of outage probability.

In this paper, the problem of coexistence interference caused by unlicensed networks is addressed. An analytical framework is developed to characterize the aggregate interference caused to a fixed licensed user when the coexisting interferers belong to different unlicensed networks. The distribution of the interference is analyzed considering the stochastic nature of the path loss, as well as fast fading and shadowing effects. The unlicensed users are deployed according to a homogeneous Poisson Process, and it is shown that the distribution of aggregate interference can be approximated by an α-μ distribution. Our results show the impact of node densities, transmitted power levels, and access channel probabilities on the distribution of the aggregate interference. Numerical and simulated results are compared to assess the accuracy of the proposed analysis.

This paper studies the performance of multi-hop cognitive wireless powered communication networks (WPCNs), where secondary nodes can harvest energy from multiple dedicated power beacons and access spectrum of primary receivers (PRs) to support data transmission. Specifically, we consider a practical scenario of cognitive WPCNs, where the channel state information (CSI) of interference links is assumed to be imperfect. To improve the network performance, we propose dual-hop scheduling scheme (DHS) for multi-hop network, where two best relays in two consecutive clusters are selected for data transmission. We then analyze the performance of the proposed scheme in terms of outage probability and outage floor. Numerical results show that DHS scheme outperforms random scheduling scheme, arising as an efficient scheme for multi-hop transmission. Furthermore, the effects of the number of hops, number of power beacons, and time switching ratio on multi-hop cognitive WPCNs are evaluated and discussed.

In this paper, we study the problem of jointly optimizing user pairing and beamforming design in multiuser multiple-input single-output (MU-MISO) non-orthogonal multiple access (NOMA) downlink systems with simultaneous wireless information and power transfer (SWIPT). Aiming at maximizing the achievable sum throughput subject to energy harvesting (EH) constraints, we propose a hybrid user pairing beamforming scheme (HBS), where two users with distinct channel conditions are optimally selected to perform user pairing. Moreover, we adopt a non-linear EH model for energy users to reveal their practical circuit characteristics. The sum throughput problem is formulated as a class of mixed-integer nonconvex optimization programming which is computationally prohibitive. To solve this challenging problem, we propose a low-complexity iterative algorithm, yet efficient, based on sequential convex approximation method to arrive at least the local optima. Numerical results are provided to demonstrate the performance improvement of the proposed HBS scheme over the multiuser beamforming one without user pairing, revealing to be an effective scheme for MU-MISO-NOMA downlink systems.

We introduce the α-κ-μ shadowed (α-KMS) fading distribution as a natural generalization of the versatile α-κ-μ and α-η-μ distributions. The α-KMS fading distribution unifies a wide set of fading distributions, as it includes the α-κ-μ, αη-μ, α-μ, Weibull, κ-μ shadowed, Rician shadowed, κ-μ and ημ distributions as special cases, together with classical models like Rice, Nakagami-m, Hoyt, Rayleigh and one-sided Gaussian. Notably, the α-KMS distribution reduces to a finite mixture of α-μ distributions when the fading parameters μ and m take positive integer values, so that performance analysis over αKMS fading channels can be tackled by leveraging previous (existing) results in the literature for the simpler α-μ case. As application examples, important performance metrics like the outage probability and average channel capacity are analyzed.

This paper investigates the performance of multi- cluster multi-user massive multiple-input multiple- output (MIMO) networks with non-orthogonal multiple access (NOMA) and a dual-polarized antenna array. Considering the downlink mode in which a single base station communicates with multiple users, a precoder design is proposed with the aim to maximize the number of user groups that are simultaneously served within a cluster. A closed- form expression for the outage probability is derived, based on which an asymptotic analysis is carried out and the diversity gain is determined. Moreover, the outage sum-rate is also examined. Representative numerical examples are presented along with insightful discussions. Our results show that the proposed dual-polarized MIMO-NOMA design outperforms conventional single-polarized systems, even for high cross-polar interference.

In this paper, we consider a wireless-powered communication network with transmit antenna selection (TAS), where an energy-limited multi-antenna information source, powered by a dedicated power beacon, communicates with a mobile user (MU). The MU is equipped with a single-antenna and its mobility is characterized by the well-known random waypoint mobility model. Differently from previous works which considered only static scenarios, this paper aims to investigate wireless power and information transfer in the scenario with a random mobile user under Nakagami-m fading. To this end, exact analytical expressions for the outage probability, average delay-limited throughput, ergodic capacity, and average delay-tolerant throughput are derived. The analytical results are compared with Monte-Carlo simulations in order to validate the analysis and provide useful insights on the impact of different parameters on the system performance.

The present contribution investigates the physical layer security in a cognitive radio network (CRN). To this end, we consider an underlay uplink CRN consisting of multiple secondary sources, a single-antenna secondary base station, and multiple eavesdroppers. In addition, we assume that the secondary sources transmit their data sequentially and that a jammer is randomly chosen from the remaining source nodes to send a jamming signal to the eavesdroppers. However, in an uplink underlay CRN, a friendly jammer is not always allowed to use its maximal transmit power as the secondary users are required to continuously adapt their power in order to avoid causing interference to the primary users. As a consequence, enhancing the system security using a jammer with low transmit power in the presence of numerous eavesdroppers turns out to be questionable. In this regard, we derive novel analytic expressions that assist in quantifying the achievable security levels and the corresponding limitations. This leads to the development of useful insights on the impact of network parameters on the performance of the system's security. The offered analytic results are corroborated through Monte Carlo simulation. It is shown, that for a low transmit power of the friendly jammer, the system's security can only be enhanced for a small number of eavesdroppers.

This paper studies the end-to-end performance of dual-hop amplify-and-forward (AF) relaying systems, where the relays have no constant power supplies but can harvest energy from both the desired signal and nearby co-channel interferences (CCIs). A hybrid energy harvesting (EH) protocol, which is a combination of the existing time switching (TS) and power splitting (PS) protocols, is adopted at the relays. To enhance system performance, an opportunistic relay selection is exploited and the extreme value theory is applied to analyze the asymptotic throughput of the system. Our analysis reveals that when the number of relays (N) is sufficiently large, the system through- put scales with ln ln N. Moreover, the hybrid EH protocol is demonstrated to outperform both TS and PS protocols while it is equivalent to PS protocol in the high signal-to-noise-ratio (SNR) region.

In this paper, the error performance of binary and quaternary phase-shift keying signals with imperfect carrier phase recovery is investigated assuming a flat slow α-μ fading channel and the presence of thermal noise. The phase noise is considered to be Tikhonov and Gaussian distributed. More specifically, an analytical expression for the exact average bit error rate is obtained, as well as its corresponding asymptotic expression for large signal-to-noise ratio (SNR) values. The derived expressions are valid for arbitrary values of the fading parameters, namely α and μ, and they are validated through Monte Carlo simulations. Several important conclusions concerning the system performance as a function of the channel parameters, namely, non-linearity, clustering, and loop SNR, are drawn.

This paper investigates the secrecy performance of a multiple-input multiple-output (MIMO) system with transmit antenna selection (TAS) and maximal-ratio combining (MRC) over generalized α-μ fading. In the underlying wireless communication system, a single antenna is selected to transmit confidential messages to a multi-antenna receiver in the presence of a passive multi-antenna eavesdropper. In our analysis, new closed-form expressions are obtained for the secrecy outage probability (SOP) and the probability of strictly positive secrecy capacity (SPSC), and are subsequently corroborated through Monte Carlo simulations to assess their validity. Moreover, asymptotic studies of the secrecy outage probability are derived and explicitly show the effects of various system parameters on the secrecy diversity gain.

This paper investigates the secrecy performance of multiple-input multiple-output systems under generalized α - μ fading conditions. To this end, we focus on two distinct scenarios: 1) the transmitter has knowledge of the channel state information (CSI) of the eavesdropper channel and 2) the transmitter is not aware of the CSI of the wiretap link. By considering transmit antenna selection in the underlying system, we develop closedform analytical expressions for the lower bound of the secrecy outage probability (SOP) and the probability of the strictly positive secrecy capacity. Furthermore, two novel approaches are proposed to derive an analytical expression of the average secrecy capacity (ASC). First, the ASC is expressed as a function of the average capacity of the desired link and an interaction term regarded as the ASC loss. Second, the ASC is expressed in terms of the average capacities of the desired and eavesdropper links, plus an interaction term that can be regarded as some sort of ASC gain due to the statistical independence between the desired and eavesdropper links. In addition, asymptotic studies of the SOP and the ASC at high signal-to-noise ratio are carried out, which precisely reveal the secrecy diversity and array gains.

In this paper, the upper bound and asymptotical approximation for the bit error rate (BER) as well as for the frame error rate of linear block codes under maximum-likelihood decoding operating over exponentially correlated (EC) generalized-fading channels are proposed. Particularly, our analysis takes the convolutional codes as an example and three different fading scenarios are considered, namely Rice, Nakagami-m, and Weibull fading. Such scenarios have a common property that is spherically invariant random process (SIRP). Our analysis relies on the derivation of new lower bounds per each eigenvalue since the BER as well as the FER expressions are written in terms of the pairwise error probability, which by its turn depends on the eigenvalues of the channel covariance matrix. Based on that, the upper bounds and asymptotic expressions of the considered metrics subject to the three above fading models are derived, which show to be tighter than the previous results published elsewhere in the literature, especially for the high signal-to-noise ratio regions.

This paper aims to design and evaluate the performance of multi-cluster multi-user dual-polarized massive multiple-input multiple-output (MIMO) systems with non-orthogonal multiple access (NOMA). Assuming the downlink mode in which a single base station communicates with multiple users, with all terminals being equipped with multiple co-located dual-polarized antennas, two precoder designs are proposed: (i) the first one aims to maximize the number of user groups that are simultaneously served within a cluster; and (ii) the second approach aims to provide further improvements compared to the first one by exploring polarization diversity. Closed-form expressions for the outage probability are derived for both approaches, based on which the respective asymptotic studies are carried out and the diversity gains are determined. The ergodic sum-rates are also derived. Representative numerical examples are presented along with insightful discussions. For instance, our results show that the proposed dual-polarized MIMO-NOMA designs outperform conventional single-polarized systems, even for high cross-polar interference. Simulation results are plotted to corroborate the analytical framework and analysis.

Mobile-to-mobile communications are offering clear benefits including spectrum and energy efficiency and low latency. However, these systems have to deal with various constraints with the most notable being related to the power consumption as well as the various peculiarities of the time varying wireless medium. In order to improve these systems' performance, antenna selection (AS) techniques have been proposed as an efficient approach to satisfy the hardware/power limitations that exist. However, these systems' performance is directly affected by the fast varying wireless medium, since AS is frequently performed using outdated estimates of the signal-to-noise ratio. In this paper, we propose a transmit AS scheme, whose operation is based on the shadowing side information (SSI) that is acquired in every stationarity region. By adopting SSI as an AS criterion, the negative consequences of the outdated channel state information (CSI) can be alleviated, since large-scale fading varies more slowly as compared to small-scale fading. The performance of this scheme is analyzed using the measures of the outage probability and the average bit error probability. It is shown that the proposed scheme outperforms the corresponding one that is based on (outdated) CSI, especially in scenarios with good channel conditions.

This paper investigates the upper and lower bounds on achievable secrecy rate of multiple-input multiple-output (MIMO) wiretap channels by employing zero-forcing (ZF) detectors at, both the legitimate and malicious receivers' sides. Particularly, both large (i.e., gamma distribution) and small scale (i.e., Rayleigh distribution) fadings are taken into considerationwith the purpose of better simulating some practical scenarios. Motivated by previous works, bounds of achievable secrecy rate for MIMO wiretap channels over uncorrelated and min semi-correlated Rayleigh fading are derived with closed-form expressions. Furthermore, an asymptotic scenario in which the number of antennas tends to infinity is evaluated. Finally, thecloseness of our analytical expressions is corroborated through simulation results.

In this paper, we analyze the performance of simultaneous wireless information and power transfer (SWIPT) in two-way relay systems with an active direct link under Nakagami-m fading environment. We consider a three-node based cooperative relaying system consisting of two source nodes and a battery-enabled relay node. Herein, one round of information exchange between source nodes can be accomplished in three time phases. Relay node relies on amplify-and-forward (AF) operation to broadcast the signals, while the destination nodes utilize maximal-ratio combining (MRC) technique to make use of both the direct and relay links. Moreover, the relay node harvests energy from the radio-frequency (RF) signals in the first two phases using power-splitting (PS) approach and then utilizes the harvested energy for information processing and broadcasting the received signals in the third phase. For the considered setup, we proficiently derive accurate expressions for the outage probability (OP), system throughput, and energy efficiency. Numerical results enlighten the importance of considering direct link with MRC in the SWIPT-enabled two-way relay systems.

In this paper, a tighter upper bound for the bit error rate (BER) of convolutional codes operating under exponentially correlated Nakagami- m fading is proposed. Our analysis relies on the derivation of new lower bounds per each eigenvalue since the BER expression is written in terms of the pairwise error probability, which by its turn depends on the eigenvalues of the channel covariance matrix. In comparison to previous results published elsewhere in the literature, the proposed bounds are shown to be tighter, specially for high signal-to-noise ratio regions, being therefore of great importance and usefulness for the forthcoming researches in the field. Moreover, this result can be generalized to all linear block codes as well as other fading models as the proposed eigenvalues' upperbounds are the main key of this work.

In this paper, we propose an energy harvesting-based overlay hybrid satellite-terrestrial spectrum sharing system which exploits both direct and relay links from a primary satellite source to terrestrial user with the coexistence of a secondary transmitter-receiver pair on the ground. It is assumed that an energy-constrained secondary transmitter deploys the time switching-based relaying protocol to harvest energy from the primary satellite's signal and then performs an amplify-and-forward based relay cooperation with the primary satellite network to explore an opportunity for spectrum access. By adopting Shadowed-Rician fading for satellite links and Nakagami-m fading for terrestrial links, we derive analytical expressions for the outage probability of primary satellite network and secondary terrestrial network. In addition, to get useful insights, expressions for throughput and energy efficiency of primary satellite network are also provided assuming a delay-limited scenario. Moreover, our results illustrate the impact of power splitting factor and energy harvesting time fraction on the system performance.

The Fisher-SnedecorFdistribution was recently proposed as an accurate and tractable composite fading model in the context of device-to-device communications. The present work derives the product of the Fisher-SnedecorFcomposite fading model, which is useful in characterizing fading effects in numerous realistic communication scenarios. To this end, novel analytic expressions are first derived for the probability density function, the cumulative distribution function and the moment of the product ofNstatistically independent, but not necessarily identically distributed, Fisher-SnedecorFrandom variables. Capitalizing on these expressions, we derive tractable closed-form expressions for channel quality estimation of the proposed model as well as the corresponding outage probability and average bit error probability for binary modulations. The offered results are corroborated by extensive Monte-Carlo simulation results, which verify the validity of the derived expressions. It is shown that the number of cascaded channels affects considerably the corresponding performance, as a variation of over an order of magnitude is observed across all signal-to-noise ratio regimes.

In this study, the authors consider an energy‐constrained secondary user which harvests energy from the primary signal and forwards this latter with the guarantee of spectrum access. Two key protocols are proposed, namely time‐splitting cooperative spectrum sharing (TS‐CSS) and power‐sharing cooperative spectrum sharing (PS‐CSS), based on time splitting and power sharing at the relay, respectively. Assuming a Nakagami‐ m fading model, exact closed‐form expressions for the outage probabilities of the primary and secondary users are derived in decode‐and‐forward and amplify‐and‐forward relaying modes. From the obtained results, it is shown that the secondary user can carry its own transmission without any adverse impact on the performance of the primary user and that the PS‐CSS protocol outperforms the TS‐PSS protocol in terms of outage probability over a wide range of signal‐to‐noise ratio. Furthermore, the effect of various system parameters, such as splitting ratio, distance between nodes and harvesting efficiency, on the system outage performance on employing the proposed protocols is investigated and several insights are drawn.

In this study, a cooperative non‐orthogonal multiple access (NOMA) scheme is investigated in an underlay cognitive radio network. With this aim, a number of secondary users are concerned in the cooperative NOMA, in which a user with strong channel gains is properly selected (to act as a relay) by a multi‐antenna base station ( ) for assisting another user with poor channel gains in the presence of a primary network. Closed‐form expressions for the outage probability of the secondary network are derived, based on which the asymptotic outage behaviours of each secondary user assuming various realistic cases are discussed. Our analytical results clearly reveal that an outage floor may exist in the outage probability of the secondary users, being determined by the interference constraint and the number of antennas at the . The impact of the multiple antennas and the number of cooperative NOMA users on the system performance is examined, and it shows that the cell‐edge user under poor channel gains can benefit from both the cooperative NOMA and opportunistic relay transmission.

Important statistical properties of the double-generalized Gamma (dGG) distribution are studied in this paper. The dGG distribution is suitable for modeling non-homogeneous double-scattering radio propagation fading conditions, which can be frequently observed in vehicle-to-vehicle (V2V) communications. In this context, important statistical metrics for the bivariate dGG distribution, such as the joint probability density function, cumulative distribution function, and the moments, are derived for the first time, while simplified expressions for the corresponding marginal statistical metrics are presented. Moreover, the second-order statistics of this distribution are also analytically studied. The derived analytical framework has been employed to analyze the performance of a transmit antenna selection system operating in V2V communication channels modeled by the dGG distribution. In this scenario, the impact of outdated channel state information (CSI) to the system's performance is investigated in terms of various metrics, including the level crossing rate and the average fade duration. Furthermore, simplified asymptotic closed-form expressions for the outage probability have been derived to examine the achievable diversity and coding gains. Based on our analysis, insightful discussions are provided. It is shown that the diversity gain is independent from the number of transmit antennas when the available CSI becomes outdated.

In this paper, we study the performance improvement methods for a cell-edge user of two-user non-orthogonal multiple access (NOMA) systems in downlink scenarios. To this end, we propose two cooperative relaying schemes, namely ON/OFF-full-duplex relaying (ON/OFF-FDR) and ON/OFF-half-duplex relaying (ON/OFF-HDR) schemes. More specifically, in order to improve the performance of the cell-edge user, we consider a cell-center user as a relay, where either FDR or HDR can be employed to assist the direct NOMA transmission from a base station (BS) to the cell-edge user. An ON/OFF mechanism is proposed to decide whether the cooperative relaying transmission is necessary or not. The ON/OFF relaying decision is made based on the quality of the direct and relaying links from the BS to the cell-edge user. The performance of the two proposed schemes is investigated in terms of outage probability and sum throughput. Numerical results reveal that the proposed schemes not only provide essential outage performance improvements for the cell-edge user, but also are able to improve the sum throughput of the two-user NOMA systems. The advantages and drawbacks of each proposed scheme are highlighted and insightful discussions are provided.

In this paper, we investigate the performance of a spectrum sharing decode-and-forward multiple-relay network (SSDMN) with direct link under the joint impact of two practical and detrimental imperfections, called transceiver hardware impairments (HIs) and channel estimation errors (CEEs). Here, we consider the hardware distortions induced by all the non-ideal secondary nodes and residual interferences originating from imperfect channel estimations. By taking these imperfections into account, we derive an exact closed-form expression for the outage probability of the considered SSDMN employing selection cooperation over independent and non-identical Rayleigh fading channels. We identify three important ceiling effects invoked by HIs, namely relay cooperation ceiling (RCC), direct link ceiling (DLC), and overall system ceiling. Specifically, we highlight the impact of these prevailing effects on the system's performance and provide some useful insights about the tolerable level of HIs while designing the practical systems for a given rate requirement. We also illustrate that a potential direct link and a relaying link can, respectively, partially compensate for the incurred performance losses due to RCC and DLC effects. Moreover, we investigate the asymptotic behavior of outage probability to highlight the impact of HIs and CEEs on the achievable diversity and coding gains.

Wireless energy harvesting has emerged as an efficient solution to prolong the lifetime of wireless networks composed of energy‐constrained nodes. In this paper, we consider a multipoint‐to‐multipoint relay network, where multiple source nodes communicate with their respective destination nodes via intermediate energy‐constrained decode‐and‐forward (DF) relay. The performance of two different transmission modes, namely, delay tolerant and delay nontolerant, is studied. Based on power‐splitting relaying protocol (PSR), optimal energy harvesting and distribution schemes for both transmission modes are provided. In addition, for more realistic and practical analysis, we consider a nonlinear energy conversion model for energy harvesting at the relay node. Our numerical results provide useful insights into different system parameters of a nonlinear energy harvesting‐based multipair DF relay network.

In this paper, important statistical properties of the double- generalized Gamma (dGG) distribution, a generic modelthat has been used for modeling the double-scattering radio propagation fading conditions, are obtained and investigated. In this context, useful statistical metrics, such as the probability density function, cumulative distribution function, and the moments, are derived. Moreover, the second-order statistics of this distribution are also analytically studied for the first time. Based on them, exact expressions for the outage probability, the average symbol error probability, and the capacity of a transmit antenna selection system operating in a vehicle-to-vehicle (V2V) communication environment are obtained. In addition, the level crossing rate and the average fade duration are studied. Simplified asymptotic closed-form expressions have been also obtained and used to investigate the diversity and coding gains.

In this paper, the interference statistics in a clustered device-to-device network is studied and the cluster centers are modeled as a Poisson point process. For such, each active receiver device chooses its transmitter randomly following the uniform distribution. We first characterize the statistics of the number of intra/inter-cluster interferers and develop the simplified expressions for several special cases. With the aid of these derived probability distributions of the number of interferers, the Laplace transforms of intra/inter-cluster interference are formulated and simplified approximations for the special cases are also attained. Finally, several applications of the derived interference statistics are shown to illustrate the performance difference between the proposed interference model and the assumed one in the literature.

In this paper, we study performance and reliability improvement for a cell-edge user in downlink scenarios of two-user non-orthogonal multiple access (NOMA) networks. To this end, we propose a full-duplex (FD) cooperative scheme, in which a near user acts as a FD relay to forward source's signals to a far user, while the far user employs distributed switch-and-stay combining (DSSC) technique to process the incoming signals. We then investigate the performance of the far user in terms of outage probability (OP). In particular, we obtain a closed-form expression for the OP of the far user as well as its asymptotic OP. The developed analysis is corroborated through Monte-Carlo simulation. Numerical results reveal that the proposed scheme achieves better outage performance in comparison with conventional NOMA systems. It is also showed that the choice of the switching threshold used in DSSC technique and/or the target data rate of the system sensitively affects the outage performance of the proposed scheme.

In this paper, we analyze the performance of a cognitive amplify-and-forward multi-relay network (CAFMRN) with direct link under the joint impact of transceiver hardware impairments (HIs) and channel estimation errors (CEEs). Herein, we consider hardware distortions induced by all the non-ideal secondary nodes and residual interferences originating from imperfect channel estimations. Taking these imperfections into account, we derive a new closed-form expression for the outage probability of the considered CAFMRN employing selection cooperation over independent and non-identical Rayleigh fading channels. Furthermore, we derive an asymptotic outage behavior to assess achievable diversity order. We illustrate that HIs may invoke ceiling effects into the system, namely relay cooperation ceiling (RCC) and overall system ceiling (OSC). Specifically, we showcase that the RCC ceases the full cooperation, whereas, the OSC may further aggravate the system outage performance under a high-rate requirement. By modeling the CEEs as decreasing function of signal-to-noise-ratio, we demonstrate that CEEs pose critical impact on the coding gain only and not on the diversity gain of the system. Finally, numerical and simulation results are presented to validate our theoretical findings.

Transmit antenna selection (TAS) schemes have been adopted in various communication systems to provide improved performance with relatively reduced complexity. The quality-of-service (QoS) in these systems depends on the availability of perfect channel state information (CSI). In practice, it is difficult to acquire perfect CSI, since it may get outdated due to feedback process. In this paper, we investigate the impact of outdated CSI in a TAS system that operates in a vehicle-to-vehicle (V2V) communication environment. The V2V channel is modeled by the double-Weibull (DW) distribution, a widely adopted distribution for modelling scenarios where both the transmitter and the receiver are in motion. In this context, we introduce and derive important statistical metrics for the bivariate DW distribution, such as the probability density function, the cumulative distribution function, and the moments. Based on them, exact expressions for the outage probability and the average bit error probability of the TAS scheme are derived under the assumption of outdated CSI. Moreover, simplified asymptotic closed-form expressions have been obtained. These expressions are then used to study the performance of the system under consideration.

In this paper, we investigate outage performance and diversity gain of transmit antenna selection (TAS) schemes in two-user multiple-input single-output non-orthogonal multiple access (MISO-NOMA) cooperative downlink transmissions. To this end, two TAS criteria, namely Criterion I and Criterion II, are proposed, which select an antenna that experiences the best fading condition of the channel from the source to the far user and to the near user, respectively. Additionally, considering the near user as a relay to help improve the reliability of the far user, hybrid simultaneous wireless information and power transfer (SWIPT) architecture is adopted to power the near user's relaying operation. Tight closed-form approximate expressions for the outage probability (OP) of both users are derived. Numerical results reveal that Criterion I and II achieve, respectively, the diversity order of K + 1 and 2 at the far user, and 1 and K at the near user, where K denotes the number of transmit antennas at the base station.

We consider a hybrid satellite-terrestrial spectrum sharing system wherein multiple terrestrial secondary networks coexist with a primary satellite network. We propose an amplify-and-forward based cooperative spectrum sharing protocol by employing an opportunistic secondary network selection (OSNS) scheme. The best secondary network is selected to minimize the outage probability of primary satellite system and to achieve spectrum access. With overlay approach, the selected secondary network allocates its partial power to relay the satellite signal and utilizes the remaining power to transmit its own signal. We derive closed-form expressions of outage probability for both primary and secondary networks and examine their achievable diversity order over hybrid fading channels. Numerical and simulation results validate our analysis and highlight the performance gains of the considered system with OSNS scheme.

In this paper, the joint effect of multiple jamming signals and noise at the eavesdropper in wiretap channels is studied in terms of the secrecy outage probability, where all nodes are deployed with multiple antennas. Specifically, the transmitter employs a transmit antenna selection (TAS) technique; the legitimate receiver uses either a selection combining (SC) or a maximal-ratio combining (MRC) scheme; and the eavesdropper adopts the MRC technique. Exact closed-form expressions for the secrecy outage probability are derived for both SC and MRC schemes at the legitimate receiver, which are applicable for arbitrary number of power distributed jamming signals. An asymptotic secrecy outage analysis is conducted, which shows that the diversity gains for all combining schemes equal to the product of the number of antennas at the transmitter and the legitimate receiver. Representative numerical examples are presented to illustrate the effects of the key system parameters on the secrecy outage performance. Finally, the proposed analysis is validated through Monte Carlo simulation results.

In this paper, general, easy to compute, exact expressions for the probability density function, cumulative distribution function and average value of the ratio of independent non-identically distributed κ-μ random variables, with arbitrary parameters, are derived. As an application example, capacity analysis of spectrum sharing systems undergoing κ-μ fading is provided. Specifically, expressions for the outage capacity with peak interference power constraint, the delay-limited capacity, and the ergodic capacity with average interference power constraint are derived. Numerical results are shown in order to investigate the effect of the fading parameters on the system capacity.

Non‐orthogonal multiple access (NOMA) in amplify‐and‐forward relay systems with partial relay selection (PRS) is investigated. More specifically, new exact closed‐form expressions for the outage probabilities at two users are derived, based on which an asymptotic analysis at high signal‐to‐noise ratio (SNR) is carried out. Additionally, to investigate the performance gap between the NOMA and orthogonal multiple access (OMA) schemes, a closed‐form approximate expression at high SNR for the sum rate is derived. Furthermore, relying on the results, the impact of the PRS on the sum rate and outage probability of the proposed NOMA scheme is examined. In particular, the derived asymptotic expressions show that the proposed scheme can improve over the traditional OMA not only the sum rate but also the user fairness. Finally, simulation results are presented to corroborate the analytical results.

Consider an underlay cognitive radio network where an eavesdropper (Eve) targets to intercept the information exchanging between the primary nodes. The secondary system is allowed to access the licensed spectrum as long as it does not violate the target quality‐of‐service (QoS) of the primary network. In return, the secondary network also assists the primary network against the malicious attack of the Eve. The authors aim at designing a resource allocation algorithm maximising the secrecy rate of the primary system while also satisfying the QoS requirement of the secondary system. To be more precise, a jamming noise accompanied by the information signal to degrade the Eve's channel and the information beamforming vector at the secondary transmitter is jointly optimised. The problem of interest is formulated as a non‐convex optimisation problem. For the case in which global channel state information (CSI) is available, the authors propose a path‐following algorithm which aims at locating a Karush‐Kuhn–Tucker solution to the original non‐convex program. By novel transformations and approximations, the authors arrive at only a simple convex problem of moderate dimension. For the case in which only statistics of the Eve's CSI are available, the authors reformulate the considered problem by replacing a non‐convex probabilistic constraint with a set of convex constraints. A worst‐case scenario for a secure communication, where an optimal linear decoder is used at the Eve, is also considered. The superior performance of the proposed design is revealed by numerically comparing it with other known solutions.

Considering a dual-hop energy-harvesting (EH) relaying system, this paper advocates novel relaying protocols based on adaptive time-switching (TS) for amplify-and-forward and decode-and-forward modes, respectively. The optimal TS factor is first studied, which is adaptively adjusted based on the dual-hop channel state information (CSI), accumulated energy, and threshold signal-to-noise ratio (SNR), to achieve the maximum throughput efficiency per block. To reduce the CSI overhead at the EH relay, a low-complexity TS factor design is presented, which only needs single-hop CSI to determine the TS factor. Theoretical results show that, in comparison with the conventional solutions, the proposed optimal/low-complexity TS factor can achieve higher limiting throughput efficiency for sufficiently small threshold SNR. As the threshold SNR approaches infinity, the throughput efficiency of the proposed optimal/low-complexity TS factor tends to zero in a much slower pace than that of the conventional solutions. Simulation results are presented to corroborate the proposed methodology.

In order to enhance secrecy performance of protecting scenarios, understanding the illegitimate side is crucial. In this paper, from the perspective of the illegitimate side, the security attack from a full-duplex cyber-weapon equipped with massive antenna arrays is considered. To evaluate the behavior of the proposed cyber-weapon, we develop a closed-form, a tight approximation, and asymptotic expressions of the achievable ergodic secrecy rate with taking into consideration imperfect channel estimation at the cyber-weapon. The results show that even under some disadvantage conditions, i.e., imperfect channel estimation and self-interference, the full-duplex massive array cyber-weapon can disable traditional physical layer protecting schemes, i.e., increasing the transmit power and the number of antennas at the legitimate transmitter. In addition, when a transmit power optimization scheme for maximizing the difference between the eavesdropping rate and the legitimate rate is applied at the full-duplex cyber-weapon, the malicious attack is even more dangerous. The results also reveal that when the legitimate side faces an advance adversary, it is essential to prevent important information in the training phases exposing to the illegitimate side.

In this paper, a dual-hop decode-to-forward cooperative system is considered where multiple relays are with finite energy storage and can harvest energy from the destination. In our analysis, the relays are spatially randomly located with invoking stochastic geometry. In an effort to improve spectral efficiency, an optimal source-relay link scheme is employed. Assuming Nakagami-m fading, two different scenarios are considered: 1) the single-antenna source with perfect channel state information (CSI) and 2) the multiple-antenna source with transmit antenna selection and imperfect CSI. In both scenarios, the destination node is equipped with a single transmit antenna to forward power via frequency radio signal to the relay candidates. For improving the system performance, multiple antennas at the destination are considered to process the multiple copies of the received signal from the best relay. For characterizing the performance of the proposed scenarios, exact closed-form analytical expressions for the outage probability are derived. To obtain further insights, we carry out diversity gain analysis by adopting asymptotic relative diversity. We also derive the exact closed-form analytical expression for the system throughput. Finally, simulation results are presented to corroborate the proposed analysis and to show that: 1) the system performance is improved by enlarging the area of the circle and the density of the relays and 2) the energy storage size has impacts on the performance of considered networks, which determines the maximal transmit power at relays.

This paper investigates the interference statistics in a clustered device-to-device network, where the cluster centers form a Poisson point process and each active receiver device selects the transmitter randomly following the uniform distribution. The statistics of the number of intra/inter-cluster interferers is first analyzed for both finite and infinite number of receivers per cluster. In addition, for the special cases where the numbers of transmitters and receivers differ significantly, simplified expressions are given. Then, using these derived probability distributions of the number of interferers, the Laplace transforms of intra/inter-cluster interference are established and simple approximations for the special cases are obtained as well. Our results show the following: 1) with the concurrent uniform transmitter selection, the distribution of the number of intra/inter-cluster interfering devices differs greatly from the truncated Poisson distribution assumed in the literature; 2) the model assumed in the literature underestimates the coverage probability specially for a small number of transmitters/receivers; and 3) unlike the model assumed in the literature, the ASE of the proposed model increases with the number of transmitters/receivers when the average number of active receivers is relatively small, while it does not monotonically vary with the number of transmitters/receivers when the average number of active receivers is large.

In this paper, we analyze the downlink outage performance of opportunistic scheduling in dual-hop cooperative networks consisting of one source, multiple radio-frequency energy harvesting relays, and multiple destinations. To this end, two low-complexity, suboptimal, yet efficient, relay-destination selection schemes are proposed, namely direct links plus opportunistic channel state information (CSI)-based selection (DOS) scheme and direct links plus partial CSI-based selection (DPS) scheme. Considering three relaying strategies, i.e., decode-and-forward (DF), variable-gain amplify-and-forward (VG-AF), and fixed-gain amplify-and-forward (FG-AF), the performance analysis in terms of outage probability (OP) is carried out for each selection scheme. For the DF and VG-AF strategies, exact analytical expressions and tight closed-form approximate expressions for the OP are derived. For the FG-AF strategy, an exact closed-form expression for the OP is provided. Additionally, we propose a gradient-based search method to find the optimal values of the power-splitting ratio that minimizes the attained OPs. The developed analysis is corroborated through Monte Carlo simulation. Comparisons with the optimal joint selection scheme are performed and it is shown that the proposed schemes significantly reduce the amount of channel estimations while achieving comparable outage performance. In addition, regardless of relaying strategy used, numerical results show that the DOS scheme achieves full diversity gain, i.e., M + K, and the DPS scheme achieves the diversity gain of M+1, where M and K are the numbers of destinations and relays, respectively.

In this paper, we study a hybrid satellite-terrestrial spectrum sharing system (HSTSSS) in which multiple terrestrial secondary networks cooperate with a primary satellite network for dynamic spectrum access. For complexity-aware HSTSSS design, we propose an amplify-and-forward-based overlay spectrum sharing protocol using partial and opportunistic secondary network selection schemes. The secondary network selection aims to minimize the outage probability of the primary satellite system and, thereby, to explore spectrum sharing opportunities. With the overlay approach, the selected secondary network allocates part of its power to relay the satellite signal and utilizes the remaining power to transmit its own signal. Considering Shadowed-Rician fading for satellite links, and Nakagami-m as well as Rician fading for terrestrial links, we derive closedform expressions for the outage probability of both primary and secondary networks, and examine their achievable diversity orders. Numerical and simulation results validate our analysis and highlight the performance gains of the proposed schemes for an HSTSSS with and without a direct satellite primary communication link.

In this paper, the outage performance of nonorthogonal multiple access (NOMA) schemes in amplify-and-forward (AF) relay systems with partial relay selection is investigated. To this end, closed-form expressions for the outage probabilities at two users are derived, based on which an asymptotic analysis at high signal-to-noise ratio (SNR) is carried out to provide additional insights into the system performance. In particular, the diversity order at each user is explicitly obtained. Numerical results illustrate the impact of relay selection on the performance of the considered NOMA scheme. Also, computer simulation results are presented to validate the accuracy of the attained analytical results.

In this paper, considering a mixed Rician and Rayleigh fading environment, we investigate the impact of line-of-sight (LOS) path component on the outage performance of dual-hop energy harvesting (EH) amplify-and-forward (AF) relay systems. To this end, a tight asymptotic outage lower bound expression is derived, which reveals that the system diversity order is limited to one and the outage behavior decays as log(SNR)/SNR, with SNR denoting the transmit signal-to-noise ratio (SNR) at the source. In contrast, when the LOS path component in the first-hop link becomes very strong (i.e., a large Rician K factor), our analytical results show that the easing-off factor log(SNR) can be effectively eliminated, becoming the decaying rate of outage curves steeper in this case and, consequently, improving the end-to-end transmission robustness since the outage curves scale as 1/SNR at high SNR regions.

In this study, the authors analyse the outage performance of multirelay decode‐and‐forward cooperative networks subject to two joint practical issues of wireless communications, namely, energy constraints and transceiver hardware impairment (HI). To deal with energy constraints at relay nodes, radio‐frequency (RF) energy harvesting (EH) technique is adopted. Considering the joint impacts of RF EH technique and HI, two relay selection schemes, namely, harvested energy‐based relay selection (HEbS) scheme and channel quality‐base relay selection (CQbS) scheme are proposed. Tight closed‐form approximate expressions for the outage probability of each scheme are derived and corroborated through Monte Carlo simulations. Some representative performance comparisons are carried out and show that the diversity order achieved by the HEbS scheme is 1 and is independent of the number of relays, K , whereas that achieved by the CQbS scheme is K , which is full diversity.

In this paper, a three-node energy-harvesting (EH) cooperative system with ROle SElection (ROSE) mechanism, where the cooperative role of each node is dynamically adjusted based on the surrounding wireless fading environment is considered. We first study a benchmark scenario with the fixed role configuration, from which a tight lower bound for the asymptotic outage behavior is derived. For this case, the achievable diversity is limited to two and the system outage behavior at high signal-to-noise ratio (SNR) is affected by the easing-off factor log c0together with log(log c0) denoting the source transmit SNR. After that, a dynamic role selection policy is incorporated and the asymptotic analysis shows that the ROSE mechanism can enhance the system diversity order to three, and the decaying law of outage behavior is no longer affected by the c0and (log c0) factors. In addition, the power-splitting receiver achieves the same asymptotic outage performance as that of the ideal EH receiver. Our results reveal that the line-of-sight component is crucial to improve the performance of the EH-based ROSE cooperative system, and the role decision with imperfect channel information has a detrimental effect on the system's diversity gain.

In this paper, we investigate the effects of line-of-sight (LoS) path and opportunistic scheduling (OS) on dualhop energy harvesting (EH) relay systems, where the LoS path exists between source and EH relay. We first study a single-destination case and show that the system diversity order is one and the outage behavior decays as log(SNR)/SNR, with SNR denoting the transmit signal-to-noise ratio (SNR) at the source. In the presence of a strong LoS path, asymptotic results reveal that the easing-off factor log(SNR) can be eliminated and a faster decay rate, 1/SNR, is achieved. Afterward, the OS of the second hops (N destinations) is considered, and its separate as well as joint effects on the system outage behavior are examined. In addition, the choice of power-splitting factor is discussed and the impacts of EH constraint on the performance limit compared with conventional relay system are investigated, showing that under the same transmit-power consumption from the fixed energy supply, the EH relay system could achieve the same performance with that of conventional relay system if the wireless power transfer in the first hop is lossless and the energy conversion efficiency at EH relay tends to be unity.

Although transmit-antenna selection (TAS) is an alternative low-cost strategy to capture the advantages of multi-antenna systems, its application may require a large amount of feedback transmissions. Owing to this fact and in order to reduce such requirement, herein we analyze the outage performance of two distributed TAS (DAS) schemes. One scheme employs maximal-ratio combining (DAS/MRC) at the destination, whereas the other employs selection combining (DAS/SC). We consider a dual-hop fixed-gain amplify-and-forward relaying network, equipped with multi-antenna source/relay nodes and a single-antenna destination. A lower bound expression is derived for the outage probability of each investigated scheme. Importantly, the derived bounds prove to be very tight approximations to the exact outage performance. Also, capitalizing on a strikingly interesting property of Stirling numbers of the second kind, we provide closed-form asymptotic expressions for the obtained bounds. Our results show that the diversity order of the proposed distributed schemes is identical to that of their optimal centralized counterparts, namely Nt+ min(Nrr, Nrt), with Nt, Nrr, and Nrtdenoting the number of transmit antennas at the source, the number of receive antennas at the relay, and the number of transmit antennas at the relay, respectively. In addition, as the relay approaches the destination, the outage performance of the proposed schemes approaches the optimal one.

This paper investigates the feasibility of deploying dynamic role selection (ROSE) mechanism in three-node energy harvesting (EH) cooperative relay systems. A benchmark case of fixed role configuration is first studied which shows that full diversity order can be achieved and the system outage probability at high signal-to-noise ratio (SNR) is determined by the scaling laws of log c0/c0^2 together with log(log c0)/c0^2, with c0 denoting the transmit SNR at the source. Then, the ROSE mechanism is merged into the EH cooperative system and it is analytically shown that the ROSE mechanism can enhance the system diversity order to three, and the high-SNR outage behavior is no longer affected by the log c0 and log(log c0) factors. Interestingly different from the conventional EH relay system with fixed role configuration, in the high SNR regime our results show that the energy conversion efficiency and the power-splitting factor at the relay have negligible effect on the outage behavior of ROSE cooperation.

This paper investigates the performance of dual-hop full-duplex relaying schemes subject to co-channel interference (CCI) and noise. In our analysis, two main scenarios are considered: in the first scenario, the link between the source and destination is seen as interference and, in the second one, it is seen as useful information. In both schemes, the effect of self-interference at the relay is taken into account. Accurate, closed- form expressions for the outage probability are derived for the general case, in which CCI and noise are assumed at both the relay and destination. The derived expressions allow for independent non-identically distributed Nakagami-m fading. Based on these expressions, special cases (but, yet new) assuming CCI only at the relay and assuming CCI only at the destination are examined. It is shown that CCI at the relay is more harmful for the system performance than CCI at the destination.

We assume a full-duplex (FD) cooperative network subject to hostile attacks and undergoing composite fading channels. We focus on two scenarios: a) the transmitter has full CSI, for which we derive closed-form expressions for the average secrecy rate; and b) the transmitter only knows the CSI of the legitimate nodes, for which we obtain closed-form expressions for the secrecy outage probability. We show that secure FD relaying is feasible, even under strong self-interference and in the presence of sophisticated multiple antenna eavesdropper.

This paper advocates a distributed role selection strategy to coordinate two-way relaying transmissions among three cooperative nodes. For such, the local channel state information comparison and decision feedback mechanism are merged into classical analog network coding (ANC) and time division broadcast (TDBC) protocols such that the cooperative role of each node can be designated in a distributed fashion. We refer to this distributed role selection rule as d-ROSE. In both ANC-based and TDBC-based two-way relaying scenarios, strict proof for the equivalence of d-ROSE and optimal ROSE is given, which indicates that albeit the different form, their final role decision is essentially the same. Outage analysis for the d-ROSE strategy is carried out and the scaling law of the system outage behavior at high signal-to-noise ratio (SNR) is characterized, which manifests that d-ROSE can enhance the system diversity gain to one-order higher relative to the ANC and TDBC protocols. It is also shown that d-ROSE can reduce the signaling overhead upto 60% to perform the outage-optimal role selection. Finally, the impacts of node placement on the outage performance as well as the average signaling overhead of d-ROSE are numerically evaluated and some useful conclusions are drawn.

In this paper, we investigate the end-to-end performance of dual-hop proactive decode-and-forward relaying networks with Nth best relay selection in the presence of two practical deleterious effects: i) hardware impairment and ii) co-channel interference. In particular, we derive new exact and asymptotic closed-form expressions for the outage probability and average channel capacity of Nth best partial and opportunistic relay selection schemes over Rayleigh fading channels. Insightful discussions are provided. It is shown that, when the system cannot select the best relay for cooperation, the partial relay selection scheme outperforms the opportunistic method under the impact of the same co-channel interference (CCI) . In addition, without CCI but under the effect of hardware impairment, it is shown that both selection strategies have the same asymptotic channel capacity. Monte Carlo simulations are presented to corroborate our analysis.

In this paper we investigate the performance of a cooperative network in the presence of an eavesdropper (Eve). Alice and Bob communicate with the help of a relay, which can operate either in half-duplex (HD) or full-duplex (FD) mode. We account for the self interference at the relay when operating under FD mode. Our analysis focus in the case that the CSI of Eve is not available at Alice. Thus, we derive closed-form expressions for secrecy outage probability. Our results allow us to compare the performance of FD and HD cooperative scenarios under secrecy constraints and, despite the additional interference at the relay, show the advantages of FD relaying over HD. In addition, we also show that a cooperative network is more vulnerable if Eve is closer to Alice than to the relay.

Orthogonal frequency division multiplexing (OFDM) is considered as a key technology for future wireless communication systems. One of the main disadvantages of OFDM systems is the high peak‐to‐average power ratio (PAPR) of the transmitted signals. When a non‐linear power amplifier (PA) is present, the high PAPR may cause the introduction of non‐linear distortions. On the other hand, cooperative communications have emerged as a promising technology for wireless networks. A theoretical analysis of an amplify‐and‐forward OFDM system taking into consideration the non‐linear distortions introduced by a PA is done. More specifically, approximate analytical expressions for the outage probability and average symbol error rate (SER) are derived and studied under several special situations. The performance analysis sheds light on the influence of PA non‐linearity on the outage probability and the average SER of an OFDM system. Particularly, it is shown that the non‐linear PA has a more significant effect over the average SER for medium and high signal‐to‐noise ratio (SNR) values and for small‐order constellations. The analytical results are validated by means of computer simulations for different system configurations and SNR levels.

In this study, the outage performance of a multi‐relay cooperative network operating in a spectrum sharing environment is analysed. Focusing on the secondary communication process, a relay selection procedure is performed prior to the communication process, where the selected relay is the one maximising the instantaneous end‐to‐end signal‐to‐noise ratio (SNR) and, at the same time, satisfying the power restrictions imposed by the primary receivers. The secondary destination employs a selection combining strategy, selecting the best path (direct link or relaying link) with the highest instantaneous SNR. Two kinds of relaying protocols are considered: decode‐and‐forward and amplify‐and‐forward. For the former, an exact closed‐form expression for the outage probability (OP) is derived, whereas for the latter an accurate approximation for the OP is obtained. Then, an asymptotic analysis is carried out and reveals that the system diversity order is N + 1 for both relaying protocols, with N denoting the number of relays. Monte Carlo simulations assess the accuracy of the authors’ results.

We design and analyze two distributed suboptimal schemes for transmit-antenna selection and link selection in a dual-hop, fixed-gain, amplify-and-forward relaying system, composed by one multi-antenna source, one single-antenna destination, and one single-antenna relay. The proposed schemes share the same antenna-selection policy, but differ from each other in the way the direct or relaying link is selected for communication. In a first scheme, the link is selected after transmission, at the destination; in a second scheme, it is selected before transmission, at the source. A great advantage of the proposed schemes over the optimal centralized solution is their low and constant delay/feedback overhead, regardless of the number of transmit antennas. In addition, the second scheme brings an improved spectral efficiency, once it saves one time slot when selecting the direct link. We derive analytical lower and upper bounds for the outage probability of the first scheme, in single-fold integral form, and exact closed-form expressions for the outage probability and mean spectral efficiency of the second scheme. We also perform an asymptotic analysis, showing that the first scheme achieves full diversity order, whereas the second scheme achieves full diversity order minus one, as a penalty for its improved spectral efficiency.

We analyze the outage performance of two spectrally efficient schemes for exploiting multiuser diversity in cognitive relaying networks. We consider a secondary network composed by one source node communicating with the best one out of $L$ available destinations, selected according to the channel quality of the direct links. If the transmission through the selected direct link drops below a given threshold, then a half-duplex decode-and-forward relaying transmission is invoked. At this stage, two schemes are considered. In the first scheme, the previously selected secondary destination employs maximal-ratio combining to handle the signals received from the secondary source and relay. In the second scheme, maximal-ratio combining is employed as well, but at a possibly different secondary destination, selected so as to maximize the signal-to-noise ratio at the combiner output. In both schemes, we consider an underlay spectrum-sharing mechanism. We derive exact analytical expressions for the outage probability of the proposed schemes and validate them by Monte Carlo simulations. An asymptotic analysis reveals that the proposed schemes can achieve full diversity order, equal to $L+1$. Importantly, we show that, at high signal-to-noise ratio, the mean spectral efficiency of the proposed schemes approximates that one attained by direct transmission.

Cooperative spectrum sharing systems (CSSSs) have recently received considerable attention from the wireless community due to their performance gains and spectrum utilization improvement when compared to traditional communication systems. Owing to this fact, we investigate the outage performance of CSSSs in the presence of multiple primary users (PUs). The secondary user (SU) network is composed by one source node, N decode-and-forward (DF) relays, and one destination. A best relay selection strategy is performed where the selected relay is that which maximizes the end-to-end signal-to-noise ratio (SNR) and, simultaneously, satisfies the interference constraint imposed by the MPU receivers. The communication between the SU source and SU destination is carried out through the help of one out of N DF relays and also via direct link. Afterwards, the SU destination selects the best path between the direct and the relaying path by using selection combining technique. A closed-form expression for the outage probability (OP) is derived, and an asymptotic analysis is carried out which reveals that the diversity order of the considered system equals N +1, showing that it is not affected neither by the number of PU receivers nor by the interference threshold. The presented analytical expressions are corroborated by means of Monte Carlo simulations and insightful discussions are provided.

We examine the impact of multiple primary receivers on cognitive multiple-input multiple-output (MIMO) relay networks with underlay spectrum sharing. For such a network, we propose transmit antenna selection with receive maximal-ratio combining (TAS/MRC) as an interference-aware design to satisfy the power constraints in the primary and secondary networks. To demonstrate this, we derive new closed-form expressions for the exact and asymptotic outage probability with TAS/MRC and decode-and-forward (DF) relaying over independent Nakagami-m fading channels in the primary and secondary networks. Several important design insights are reached. We find that the TAS/MRC strategy achieves a full diversity gain when the transmit power in the secondary network is proportional to the peak interference power in the primary network. Furthermore, we highlight that the diversity-multiplexing tradeoff (DMT) of TAS/MRC is independent of the primary network and entirely dependent on the secondary network.

In this work, we investigate the performance of cognitive multihop regenerative relaying systems in the “underlay” spectrum sharing scenario. The multiple relays perform “detect-and-forward” relaying strategy to convey a message with an order m quadrature amplitude modulation (m-QAM) from the source to the destination over independent but not necessarily identical Nakagami-m fading channels. We adopt a closed-form analysis framework based on univariate and bivariate Meijer G-functions to derive the end-to-end error performance (in terms of bit and symbol error rates), the outage probability, and the ergodic capacity. Various numerical examples are presented to illustrate the results with a large combination of system and fading parameters, and simulation results confirm the accuracy of our closed-form analysis.

This paper investigates the secrecy performance of multiple-input single-output (MISO) wiretap channels with transmit antenna selection (TAS) and subject to an interference-limited eavesdropper. By considering NAantennas at the transmitter, a single antenna at the legitimate receiver and eavesdropper, and multiple arbitrary co-channel interferers M at the eavesdropper, an exact closed-form expression for the secrecy outage probability is derived. The exact expression is simplified for two, yet novel, special cases. Based on these analytical expressions, an asymptotic secrecy outage analysis is carried out and it shows that the diversity order of the considered system equals to min(NA, M). The proposed analysis is corroborated through Monte Carlo simulation results. Illustrative numerical examples are depicted and insightful discussions are drawn. It is shown that the secrecy outage performance is largely limited by the number of interference channels and the number of transmit antennas due to diversity gain.

In this paper, a novel transmit antenna selection scheme for multiple input multiple output (MIMO) wiretap channels is proposed. This new scheme achieves higher secrecy performance by exploiting eavesdropper's channel state information. The key idea behind our proposal is to perform the antenna selection aiming to maximize the overall secrecy rate instead of maximizing the instantaneous signal-to-noise ratio of the main channel. In our analysis, we assume that the legitimate receiver and the eavesdropper employ a maximal-ratio combining technique to combine the received signals from the transmitter. Considering Nakagami-m fading channels, a closed-form expression for the secrecy outage probability is derived, which can be used as a quality of service metric. Further, an asymptotic analysis is carried out and the diversity/array gains are obtained. The ergodic secrecy rate is also investigated for the case of multiple input single output wiretap channels. Representative numerical examples are plotted and validated through Monte Carlo simulations. Insightful discussions are drawn from the proposed analysis.

This paper investigates the outage performance of a spectrally efficient scheme for multiuser cognitive relaying networks under spectrum sharing constraints. In the proposed setup, we consider a secondary network composed by one source node communicating with the best one out of L available destinations, selected according to the channel quality of the direct links. If the selected direct link drops below a given threshold, a half-duplex decode-and-forward relaying transmission strategy is invoked, and then the selected secondary destination employs a maximal-ratio combining technique to combine the received signals from the secondary source and relay. In our analysis, the overall transmit power at the secondary network is considered to be limited by the maximum tolerable interference power at the primary user receiver, as well as by the maximum transmit power available at the secondary nodes. A closed-form expression for the outage probability is obtained and validated by Monte Carlo simulations. Moreover, an asymptotic analysis is carried out and reveals that the achievable diversity order equals L+1.

A distributed and spectrally efficient link selection scheme for multiuser multirelay cooperative networks with transmit beamforming is proposed and analyzed in terms of outage probability and mean spectral efficiency. More specifically, we consider a network composed by a base station equipped with M antennas which is communicating with one out of L single-antenna mobile stations under the aid of one out of N single-antenna, variable-gain amplify-and-forward relay stations. Either the direct link or the dual-hop relaying link is selected for each communication process by following a distributed link selection scheme. Single-integral expressions for upper and lower bounds of the outage probability and mean spectral efficiency are obtained and validated by Monte Carlo simulations. Furthermore, closed-form expressions derived from the asymptotic analysis of the outage probability reveal that the proposed system achieves full diversity order, which is equal to ML+N. Importantly, it is shown that the mean spectral efficiency is always higher than half of that one attained by direct transmission.

In this paper, an optimal and centralized role selection scheme for two-user amplify-and-forward relaying systems is proposed, where two users compete to transmit their own information to a common destination. Relying on the instantaneous channel conditions, either of the two users can opportunely play the role of an information source and the other user will serve as an amplify-and-forward relay. The proposed framework aims to maximize the received signal-to-noise ratio (SNR) at the destination, which enhances the transmission reliability. In order to reveal the inherent impacts of different link statistics on the system outage behavior, a closed-form lower-bound expression for the outage probability is derived. Moreover, an asymptotic outage analysis is carried out and points out that the high-SNR outage behavior of the proposed scheme is dominated by the user-destination links irrespective of the inter-user link. Our analytical results are corroborated by means of Monte Carlo simulations.

We consider the uplink of a cooperative DS-CDMA communication system, where each user communicates with the base station through a direct link and with the help of relays. A multiuser detection approach for the joint estimation of spatial signatures, channel gains and transmitted symbols of all the users is proposed. Such an approach is derived from a trilinear tensor model for the received signal that combines the source-destination and relay-destination links. The proposed multiuser receiver provides an extension of the traditional trilinear DS-CDMA receiver to cooperative relaying systems, by including the users' relay-assisted links into the trilinear model. Moreover, in contrast to a previous work by the authors, where the relays of a cluster transmit over orthogonal channels, the proposed receiver operates in a more attractive scenario where all the relays simultaneously transmit towards the base station without requiring orthogonal codes. Simulation results illustrate the performance of the proposed receiver in comparison with alternative approaches.

Multihop relaying is an efficient strategy to improve the connectivity and extend the coverage area of secondary networks in underlay cognitive systems. In this work, we provide a comprehensive performance study of cognitive multihop regenerative relaying systems in an underlay spectrum sharing scenario with the presence of multiple primary receivers. Both interference power and peak power constraints are taken into account. In our analysis, all the links are subject to independent, non-identically distributed Nakagami-m fading. We derive closed-form expressions for the outage probability, high-order amount of fading, bit error rate, symbol error rate, and ergodic capacity. Different scenarios are presented to illustrate the obtained results and Monte Carlo simulations confirm the accuracy of our analytical derivations.

This paper investigates the outage performance of dual-hop decode-and-forward (DF) cooperative spectrum sharing systems in the presence of multiple primary user (PU) receivers and multiple secondary user (SU) destinations. Our analysis allows for a general Nakagami-m fading environment where distinct fading parameters as well as unequal average fading powers between the interference and relaying links are assumed. Focusing on the cooperation process among the SU nodes and making use of the underlay cognitive approach, an exact closed-form expression for the outage probability is derived. Our analysis employs an opportunistic scheduling algorithm for selecting one out of L SU destinations available. Additional interference constraints are also considered due to the presence of multiple PU receivers. The effects of fading severity, SU relay placement, and number of PU receivers, and SU destinations on the end-to-end system performance are examined through some representative numerical plots. Monte Carlo simulation results are presented to corroborate the proposed analysis.

We examine transmit antenna selection with receiver maximal-ratio combining (TAS/MRC) and transmit antenna selection with receiver selection combining (TAS/SC) in multiple-input multiple-output (MIMO) relay networks. Amongst L two-hop relay links, a single relay offering the highest end-to-end signal-to-noise ratio (SNR) is activated. Assuming independent non-identically distributed Weibull fading between the hops, new closed-form asymptotic expressions for the outage probability and the symbol error rate are derived considering NS, NR, and NDantennas at the source, the relays, and the destination, respectively. Based on such expressions, the diversity order and the array gain for M-ary phase shift keying and M-ary quadrature amplitude modulation are analyzed. We highlight that the diversity order of TAS/MRC is the same as TAS/SC. As such, we explicitly characterize the SNR gap between TAS/MRC and TAS/SC as the ratio of their respective array gains. An interesting observation is reached that for equal per-hop SNRs, the SNR gap between the two protocols is independent of L.

This paper investigates the outage performance of dual-hop amplify-and-forward (AF) two-way relaying systems with multiple co-channel interferers at the AF relay and two noisy end-sources. Assuming a Nakagami-m fading environment, a highly precise, easy-to-compute closed-form approximate expression for the outage probability is derived. Our analysis allows for general operating scenarios with distinct Nakagami-m fading parameters and unequal average fading powers between the hops. Monte Carlo simulation results are presented to corroborate the tightness of the proposed approximations.

In this letter, we present new blind receivers for uplink multiuser cooperative diversity systems. Considering amplify-and-forward (AF), fixed decode-and-forward (FDF), and selective decode-and-forward (SDF) relaying protocols, the proposed receivers exploits a unified formulation of the received signal as a CANDECOMP/PARAFAC (CP) model with dimensions$receive\ antenna\times cooperative\ branch\times symbol\ period$. Under the assumption that channel state information (CSI) is not available neither at the relays nor at the base station, the proposed receiver jointly and blindly estimates the transmitted symbols and channel parameters. In addition to avoiding the use of pilots symbols, the CP-based receiver can operate with less base station antennas than users or, alternatively, with a single relay per user.

This paper investigates the ergodic capacity and outage probability performance of multihop relaying networks subject to independent non-identically distributed Nakagami-m fading. Particularly, we exploit a typical amplify-and-forward relaying system with an arbitrary number of cooperative intermediate relays and no direct link between the source and destination nodes. In our analysis, channel state information is assumed to be known only at the receiving nodes and the cooperative links may have distinct fading parameters and distinct average signal-to-noise ratio (SNR) levels. In this context, a tight closed-form upper bound expression for the ergodic capacity is derived. For this, firstly the moment generating function (MGF) of the inverse of the end-to-end SNR is obtained in closed-form. Then, making use of this expression, an upper bound for the ergodic capacity is attained. Thereafter, we investigate the end-to-end outage probability performance of the multihop relaying channels in Nakagami-m fading by making use of the aforementioned MGF expression. Finally, Monte-Carlo simulation results are provided and show the tightness of the proposed bounds.

This Letter proposes a mechanism to combine multiuser diversity (MUD) and incremental relaying in a multiuser two-way relaying system, where one end-source exchanges messages with one out of N available end-sources by using a half-duplex amplify-and-forward (AF) relay. Outage analysis for the proposed scheme is carried out and, compared with the time division broadcast (TDBC) protocol, reveals that it not only realizes full diversity order (N+1) but also achieves improved expected spectral efficiency. Also, it is indicated that the expected spectral efficiency of the proposed scheme approaches that of the analog network coding (ANC) protocol in high signal-to-noise ratio (SNR) regime. Furthermore, the finite-SNR diversity-multiplexing tradeoff (DMT) is examined, from which the superiority of the proposed scheme to TDBC is confirmed as well. Other insightful comparisons in terms of the outage performance are presented that show the practical usefulness of our multiuser two-way relaying framework.

In this paper, we present a practical adaptive multiple-input multiple-output (MIMO) transmission approach for broadband wireless multiuser systems. The proposed scheme adaptively switches between open-loop and closed-loop MIMO modes, depending on the users' channel conditions and required rates, as a means to enhance the system throughput and performance. Each mode is a combination of a MIMO transmission technique (space-time block coding and/or spatial multiplexing or beamforming) and a modulation/coding scheme. We first evaluate the spectral efficiency of each mode and provide a simple way to select adaptively the best mode among the open-loop and closed-loop available transmission modes in a Rayleigh/composite fading environment. Based on these results, we propose a simple and practical switching criterion that is shown to yield significant spectral efficiency improvements over independent adaptive open-loop or closed-loop schemes for typical multiuser system scenarios. Simulations in an IEEE 802.16e framework illustrate significant improvements in throughput performance.

In this letter, we study the performance of two promising relay selection techniques, namely selection cooperation (SC) and opportunistic relaying (OPR), under a clustered fixed-gain relay setting. Such relay configuration finds applicability in practical ad-hoc and sensor networks, although it considers independent identically distributed channels among the links of each hop. Assuming Rayleigh fading and that all nodes are single-antenna devices, a comparative analysis between the selection strategies is performed in terms of the outage probability and average bit error rate. With this aim, closed-form expressions for the probability density function, cumulative distribution function, and moment generating function of the end-to-end signal-to-noise ratio (SNR) are derived. Our theoretical results are validated by means of Monte Carlo simulations, and show that, irrespective on the metric analyzed, OPR always yields higher performance than SC. Such conclusions differ from recent results reported in the open literature for decode-and-forward relays.

The performance of beamforming in dual-hop cooperative networks with fixed-gain relays is investigated. These kinds of relays offer low complexity and ease of deployment when compared with variable-gain relays. In our analysis, the source and destination nodes are equipped with multiple antennas, whereas the relay is assumed to be a single-antenna device. Closed-form expressions for the outage probability (OP), probability density function (PDF), moment generating function (MGF), and generalized moments of the end-to-end signal-to- noise ratio (SNR) are obtained. It is shown that when the same antenna configurations are considered at the source and destination sides, the power imbalance between the hops may be either beneficial or detrimental for the overall system performance. In addition, depending on whether the average SNR of the second hop is equal, lower, or higher than that of the first hop, an increase of the number of antennas may not necessarily result in a substantial improvement in performance.

By enhancing diversity, cooperation in wireless networks allows increasing the transmission reliability and extending the radio coverage, without the need of implementing multiple antennas at the terminals. In this context, this paper proposes a general approach for analyzing the performance of dual-hop cooperative diversity networks with nonregenerative variable-gain relays. Such a strategy allows for the use of both equal-gain combining (EGC) and maximal-ratio combining (MRC) techniques at the destination. In our analysis, closed-form approximations for the outage probability (OP) and average symbol error rate (SER) of linear modulations are presented. These expressions yield results instantaneously, regardless the number of relays used, in contrast with exact solutions which are rather intricate and difficult to evaluate, especially when the number of relays increases. The inherent simplicity of our methodology makes it attractive to serve as a benchmark in the design and performance evaluation of cooperative networks with variable-gain relays and employing either EGC or MRC. Numerical results are provided and compared with Monte Carlo simulations to illustrate the accuracy of the proposed expressions.

In this paper, making use of the relaying concept, which enables single-antenna devices to benefit from spatial diversity, a statistical analysis of dual-hop cooperative links using semi-blind (fixed gain) relays and partial relay selection, is provided. The selection scheme considers that the source monitors the connectivity among the nodes (relays) of the first hop only; an interesting assumption which finds applicability in practical ad-hoc and sensor networks. In our analysis, compact closed-form expressions are obtained for the outage probability (OP), probability density function (PDF), moment generating functions (MGFs), and generalized moments of the end-to-end signal-to-noise ratio (SNR). Furthermore, the influence of the relay selection on the system performance is analyzed and discussed. The analysis is also sustained by numerical results and comparisons. In particular, it is shown that the power imbalance between the hops may have positive or negative effects on the overall system performance, irrespective of the number of selected relays.

Accurate closed-form approximations to the sum of independent identically distributed eta-mu and kappa-mu random variables are provided. The proposed approximations turn out to be simple, precise, and useful for obtaining important performance metrics of communications systems where sums of variates arise. In particular, average bit error rate and level crossing rate of multibranch equal-gain combining receivers are attained to illustrate the applicability of the approximations. Some sample examples show that the intricate exact solution and the simple approximate expressions yield results that are almost indistinguishable from each other.

In this paper, we investigate the end-to-end performance of dual-hop relaying systems with beamforming over Nakagami-m fading channels. Our analysis considers semiblind (fixed-gain) relays with single antennas, and source and destination nodes equipped with multiple antennas. Closed-form expressions for the outage probability (OP), moment generating function (MGF), and generalized moments of the end-to-end signal-to-noise ratio (SNR) are derived. The proposed expressions apply to general operating scenarios with distinct Nakagamim fading parameters and average SNRs between the hops. The influence of the power imbalance, fading parameters, and antenna configurations on the overall system performance are analyzed and discussed through representative numerical examples. Furthermore, the exactness of our formulations is validated by means of Monte Carlo simulations.

The end-to-end performance of dual-hop cooperative links using semi-blind (fixed gain) relays and partial relay selection is investigated. The selection scheme considers that the source monitors the connectivity among the nodes (relays) of the first hop only. This scheme is interesting in practical ad-hoc and sensor networks. In our analysis, compact closed-form expressions are obtained for the outage probability, probability density function, moment generating functions, and generalized moments of the end-to-end signal-to-noise ratio(SNR), from which other relevant statistics that well-describe the distribution of the end-to-end SNR, such as mean, variance, kurtosis, skewness, and amount of fading, can also be deduced. Furthermore, the dynamic behavior of the end-to-end envelope is investigated, and the corresponding level crossing rate and average fade duration are obtained in an exact manner. Also, tight lower and upper bounds for these second-order statistics are presented in closed-form. Numerical results illustrating the system's performance in terms of the above metrics are provided, and the influence of the relay selection on performance is analyzed and discussed. For instance, it is shown that the power imbalance between the hops may have positive or negative effects on the overall system performance irrespective of the number of selected relays.

General exact expressions for the average level crossing rate (LCR) of multibranch selection (SC), equal-gain (EGC), and maximal-ratio (MRC) combiners operating over non-identical correlated Weibull fading channels are provided. They are given in terms of finite-range multifold integrals. Our exact formulations generalize those obtained recently in the literature for the dual-branch case. The exact solution requirements are then loosened to allow for simpler accurate approximate expressions. They appear in a fairly simple closed-form fashion, for the SC, and still as multiple integrals, for the EGC and MRC, but with much simpler integrands, which drastically reduces the calculation time. Sample numerical results are discussed by specializing the general expressions to a space-diversity system using three collinear horizontally spaced omnidirectional antennas at the mobile station.

This paper derives an exact closed-form expression for the phase-envelope joint distribution of the κ-μ fading environment, a general fading model that includes the Rice and the Nakagami-m models as special cases. The derived joint statistics are obtained so that compatibility with both Rice as well as Nakagami-m cases are kept. Monte Carlo simulations are used to validade the formulations.

Accurate closed-form approximations to the sum of independent identically distributed eta-mu and kappa-mu random variables are provided. The proposed approximations turn out to be simple, precise and find applicability in obtaining important performance metrics of communications systems where sums of variates arise. In particular, outage probability and average bit error rate of some modulation schemes of multibranch equal-gain combining receivers are attained to illustrate the usefulness of the approximations. In passing, based on the fading models of the corresponding fading scenarios, exact expressions for these metrics for multibranch maximal-ratio combining receivers are attained that present the same functional form of the corresponding expressions for a single branch.

Sums of fading envelopes arise in several wireless communications applications, such as equal-gain combining, outage probability, signal detection, etc. In this paper, based on the intricate task of evaluating the exact statistics of such sums, accurate approximations for the outage probability of equal-gain receivers subject to arbitrary independent cochannel interferers are proposed. In our analysis, an M-branch_N-interferer system subject to an alpha-mu fading scenario is considered. The results are obtained in terms of a singlefold integral, which replaces the M times N nested integrals required in the exact solution. The accuracy of our formulation is checked by comparing it with Monte Carlo simulation results. It is shown that approximate and simulated curves are practically indistinguishable from each other.

In this work, a number of new closed-form expressions for the eta-mu fading channels involving the joint statistics of the envelope, phase, and their time derivatives are obtained. Level crossing rate (LCR), average fade duration (AFD), and phase crossing rate (PCR) are also derived. The expressions are thoroughly validated by reducing them to some particular known cases and, more generally, by means of Monte Carlo simulation. We then provide alternative (i) singlefold integral exact formulations and (ii) highly-accurate approximations to the level-crossing statistics of multibranch maximal-ratio combining (MRC) and equal-gain combining (EGC) systems, respectively, operating over independent Hoyt fading channels, for which the exact solutions appear in the literature in multifold integral forms.

Sums of fading envelopes occur in several wireless communications applications, such as equal-gain combining, signal detection, outage probability, intersymbol interference, etc. The exact evaluation of the sum statistics is known to be very intricate. One of the purposes of this Letter is to provide highly accurate closed-form approximations to the probability density function and cumulative distribution function of the sum of independent identically distributed (i.i.d.) alpha-mu (generalized gamma) variates. Based on and as an extension of such an approach, simple precise approximations for the performance metrics of equal-gain combining and maximal-ratio combining receivers operating on i.i.d. alpha-mu fading channels are proposed. Samples examples are given to illustrate that, for practical purposes, exact and approximate solutions are indistinguishable from each other.

This paper derives a simple and closed-form expression for the phase-envelope joint distribution of the η-μ distribution, a general fading model that includes the Hoyt and the Nakagami-m models as special cases. In the same way, the marginal distribution of the phase is also obtained. These analytical expressions reduce in an exact manner to those of the particular cases comprised by the η-μ distribution. Some plots illustrate the behavior of the phase for several fading conditions.

This paper derives exact expressions for the level crossing rate and average fade duration of multibranch pure-selection, equal-gain, and maximal-ratio combiners operating over independent non-identical α-μ (also called generalized Γ or Stacy) fading channels. The derived expressions are in closed form for pure-selection combining and in integral form for equal-gain and maximal-ratio combining. For the two latter schemes, accurate closed-form approximations are then provided. The analytical results are validated by reducing the general expressions to known particular cases and, more generally, by means of simulation. Numerical examples are also given to illustrate the high accuracy of the proposed closed-form approximations.