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.

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, 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.

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.

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.

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, 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.

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.

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 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.

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 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.

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 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.

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 simulation plots are presented that are in accordance with the formulations developed here. As a consequence of the derivation of the required PDF, an interesting solution to a very general probability problem is proposed, namely, given the PDF of the modulus of a variate, find the PDF of the variate. Additionally, several new integral relations of the modified Bessel function are given.

A very general fading scenario is considered for which exact, closed-form formulas for the correlation coefficient of the instantaneous power at the output of a Maximal Ratio Combining receiver is derived. The model includes the main fading environments as very particular cases. It is valid for both identically and non-identically distributed multipath clusters, and for both stationary and nonstationary environments. Most interestingly, the model allows for an uncountable number of combinations of novel fading scenarios. A further generalization is achieved with the angle as well as the time of arrival of the waves permitted to vary throughout the multipath clusters, as opposed to known models for which these parameters are fixed. Several particular examples illustrate the applicability of the general formulations.

Indoor and outdoor field trial measurements are used to validate the autocorrelation function derived in an exact manner for the α-μ fading signal. In addition, accurate closedform approximations for both autocorrelation function and to the power spectrum of the α-μ envelope are obtained and also validated. Comparisons are performed and an excellent fit to the field measurements is found in both isotropic and anisotropic environments.

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.