This paper investigates a near-field covert communication system enhanced by an absorptive reconfigurable intelligent surface (ARIS) and a fluid antenna system (FAS), enabling covert transmission to arbitrary receiver locations. By generating near-field spherical waves via large-scale antenna arrays at Alice and ARIS, covert transmission to Bob is enabled while evading detection by Willie. We jointly optimize Alice’s hybrid precoding, ARIS reflection coefficients, and Bob’s active port selection to maximize the worst-case covert transmission rate. We begin by evaluating ARIS’s suitability versus conventional RIS. We demonstrate the asymptotic orthogonality of near-field beam-focusing vectors in the 3D domain for uniform planar arrays, and characterize the beam-focusing behavior in cascaded ARIS-enabled covert transmissions. Additionally, we reveal the channel gain improvement owing to FAS over traditional antenna systems. To solve the coupled non-convex problem, we propose a low-complexity block coordinate descent algorithm. It incorporates Fibonacci search for hybrid precoding, three complexity-performance trade-off strategies for reflection coefficients optimization, and both exhaustive search and linear conic relaxation for active port selection. Finally, we recover precoding via an alternating minimization scheme. Numerical results show that (i) significant improvement of covert transmission is achieved only with both ARIS and FAS, when Bob and Willie are co-located; (ii) the proposed algorithm outperforms near-field and far-field beam alignment schemes without ARIS, as well as beam focusing of full-map zeroing with ARIS, when Bob and Willie share the same reception direction.

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

The application of integrated sensing and communications (ISACs) technology in air-ground networks, which include unmanned aerial vehicles (UAVs), offers unique opportunities for improving both sensing and communication performances. However, this type of networks are also sensitive to the peculiar characteristics of the aerial communications environment, which include shadowing and scattering caused by man-made structures. This paper investigates an aerial ISAC network and proposes a UAV-selection strategy for improved communication performance. We first derive analytical expressions for the received signal-to-interference ratio for both communication and sensing functions. These expressions are then used to analyze the outage probability of the communication part and the ergodic radar estimation information rate of the sensing part. The presented results reveal the impact of shadowing severity and interference on the system’s performance.

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

In this work, a mixed underwater acoustic/optical wireless transmission system under both amplify-and-forward (AF) and decode-and-forward (DF) relaying protocols is proposed. Assuming pointing errors and both heterodyne detection (HD) as well as intensity modulation/direct detection (IM/DD) techniques in the underwater optical link, we deduce the exact analytical formulas for the outage probability (OP), average bit error rate (ABER), and average capacity of the system under consideration. Also, we further derive the corresponding asymptotic expressions to gain intuitive physical insights about the system and channel models under consideration. Additionally, we extend the analysis to a more general multi-sensor system. Finally, we check the analytical results by Monte Carlo simulations.

In this paper, we derive analytical expressions for the outage probability and the ergodic capacity of a wireless powered communications (WPC) system operating in a Nakagami-m fading environment. The source node harnesses power from a dedicated power beacon (PB) with multiple antennas and uses the harvested power to transmit data to a destination node in the presence of co-channel interference. The analysis considers both fixed and random distance between the PB and the source node. The analytical results show the impact of link level parameters such as the path loss exponent, co-channel interference, number of antennas in the PB and density of PBs on the performance of the WPC system. Numerical and simulation results are presented to illustrate the analysis.

Unmanned aerial vehicle (UAV)-aided communications is a promising emerging technology that will be adopted in the next generation communication networks. In this paper, a number of advanced cooperative UAV-aided communication solutions is proposed and evaluated, which considerably improve the performance of traditional terrestrial networks. Specifically, two generalized multi-UAV-selection schemes are introduced, depending on whether a direct link between the source and the destination is available or not. Once the UAVs have been selected, they may retransmit the source message to the destination using distributed space-time coding, or, in the presence of channel state information, distributed beamforming. A number of metrics to evaluate the performance on a realistic channel model was adopted, including the end-to-end outage probability (OP), the so-called goodput, and the average number of path estimations, as well as the total UAV communications power consumption. Exact expressions for the OP were obtained in terms of finite sums. Moreover, convenient closed-form expressions for the asymptotic OP were derived, valid for large numbers of UAVs, which are also very accurate for few UAVs. All results were compared with numerical simulations, which clearly depict the performance improvement induced by the proposed schemes as well as the impact of various system and channel parameters to the performance.

In this paper, the performance of a mixed Terahertz/free-space optical (THz/FSO) wireless transmission system is studied, where the joint effects of channel fading and pointing errors are considered for both THz and FSO links. Employing the semi-blind amplify-and-forward protocol, we derive the cumulative distribution function and probability density function of the end-to-end signal-to-noise ratio. By applying the derived statistics, exact expressions for the outage probability (OP), average bit error rate (BER), and average channel capacity are obtained. In order to attain useful physical insights, asymptotic OP and average BER expressions are also presented. Based on them, the diversity gain is determined, which is shown to depend on channel fading and pointing errors of both links. In addition, by taking into account the hardware impairments, the OP of the non-ideal hardware system is derived. Moreover, the analysis is extended to multi-antenna scenarios and the asymptotic OP is obtained. Finally, illustrative numerical results are plotted and it can be observed that the path loss, channel fading, pointing errors and hardware impairments lead to a considerable degradation in system performance.

Unmanned aerial vehicle (UAV)-enabled communications have been proposed as a critical part of the beyond fifth-generation (5G) cellular networks. This type of communications is frequently characterized by line-of-sight (LoS) and dynamic propagation conditions. However, in various scenarios, the presence of large obstacles in the LoS path is unavoidable, resulting in shadowed fading environments. In this paper, a new channel model is proposed, in which the effects of mobility and shadowing are simultaneously considered. In particular, the performance of a UAV-based communication system operating in a shadowed double-scattering channel is analyzed. The new channel model is generic, since it models various fading/shadowing conditions, while it is in terms of easy-to-evaluate mathematical functions. Moreover, a low complexity UAV selection policy is proposed, which exploits shadowing-related information. The proposed scheme offers a reduction of the signal processing complexity, without any important degradation on the performance, as compared to alternatives approaches. In this context, a new analytical framework has been developed for investigating the performance of the new strategy. Finally, the main outcomes of this paper are also validated by empirical data, collected in an air-to-ground measurement campaign.

Double-scattering propagation models have been applied in various wireless communication environments, including vehicle-to-vehicle communications and cooperative amplify-and-forward relaying. In all these scenarios, when obstacles exist between the transmitter (Tx) and the receiver (Rx), the average received power varies in a random manner, resulting to the shadowing effect. The combination of double-scattering and shadowing represents a composite fading scenario, which models worse than Rayleigh fading conditions. In this paper, we propose a new shadowed double-scattering distribution that can model various fading and shadowing conditions. To this aim, the inverse-gamma (IG) distribution is employed for modeling the shadowing coefficient and the Nakagami- m for the multipath fading. For the resulting double-Nakagami-IG distribution, an analytical stochastic framework has been developed for presenting important statistical properties. Moreover, the theoretical results have been compared with empirical ones, based on a measurement campaign that has been performed in an unmanned aerial vehicle (UAV)-to-ground communication scenario. It is shown that the proposed new distribution can efficiently model such communication environments, since it provides a notable fit to the empirical data.

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.

Antenna selection (AS) is considered as an ideal approach for improving the performance of vehicle-to-vehicle (V2V) communications, while satisfying the hardware and power limitations that exist in vehicles. However, the promised gain over single antenna schemes is affected by the fast varying wireless channel, since AS is frequently performed using outdated versions of the signal-to-noise ratio. In this paper, we propose an AS scheme that exploits the stationarity of large scale fading. In particular, by adopting shadowing information 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 the proposed technique is analyzed using the criterion of outage probability. It is shown that the new scheme outperforms the corresponding one that is based on outdated CSI, especially in scenarios with mild channel conditions, in terms of fading/shadowing severity. Moreover, empirical results have been used to verify the effectiveness of the considered shadowing model.

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, 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.

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, 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, we investigate the performance of spatial modulation (SM) for vehicle-to- infrastructure (V2I) and vehicle-to-vehicle (V2V) communications over a generalized multiple scattering channel. This channel model provides a realistic statistical description of an intervehicular communication environment and includes as special cases the Rician, Rayleigh, and double-Rayleigh channels. We derive an accurate closed form expression for the pairwise error probability of SM, which is used to obtain the error rate performances of SM over various propagation scenarios. The performance analysis reveals the impact of the number of receive antennas on the error rate performance of SM transmission over multiple scattering channels. Various numerically evaluated results are presented to demonstrate the correctness of the proposed analysis.

In this paper, we study the influence of interference in a inter-vehicular communication system. The interfering links are modeled using the multiple scattering radio channel, which fits quite well with experimental data for mobile-to-mobile communications. For this scenario, we derive the signal-to-interference ratio statistics of a single input single output system. Moreover, the analytical results are generalized by taking into account a diversity reception scenario with two well-known schemes, namely selection diversity and maximal ratio combining. In addition, we analytically study the realistic scenario of outdated (imperfect) channel estimates at the receiver side. The outage probability (OP) is considered as the performance metric for all scenarios considered. Based on the numerical evaluation of the derived analytical expressions for the OP, we depict the impact of interference, diversity schemes and outdated estimates on the system's quality of service. Simulation results are also included to validate the analytical results.

Vehicle-to-vehicle communication systems are characterized by a highly time varying channel. For this reason, many relay selection schemes demand the continuous monitoring of all available channel links, resulting to an unnecessary increase of the system overhead processing. In this paper, an alternative (lower complexity) cooperative relaying selection scheme is adopted. For this new scheme, the influence of interference as well as the existence of outdated channel state information in a decode-and-forward relaying scenario is investigated. To this aim the bivariate double-Nakagami probability density function is introduced and used to model the correlation between the channel states during the selection and the data transmission instances. The performance is analysed using the criteria of outage probability and the average symbol error probability. It is shown that with the proposed approach a significant reduction in feedback load processing is achieved, with only a slight loss in performance.

In this paper, the correlated double-Rayleigh distribution is introduced and analyzed. This distribution has been found to provide an accurate statistical description of the vehicle-to- vehicle multipath propagation. Novel infinite series expressions for the joint probability density and the cumulative distribution functions are derived, while the joint moments are also obtained in closed form. Although the new statistical metrics can be employed in a wide range of applications, this paper focuses on the performance of dual-branch selection diversity receivers. In this context, the outage probability (OP) and the average bit error rate (ABER) of dual- branch selection diversity operating over double- Rayleigh fading channels are analytically evaluated. Moreover, an asymptotic analysis is also included, resulting to simpler closed-form expressions for the OP and ABER. Finally, the applicability of the analytical results were verified using the measurements based WINNER2 models.

Reconfigurable antenna arrays provide important advantages such as reduction on the number of the physical antennas or reduced hardware complexity in terms of receiving radio frequency chains. Both these properties are critical in vehicle-to-vehicle (V2V) communication systems, which are characterized by inflexible dimensions and cost constraints. In this paper, a new pattern reconfigurable antenna array selection scheme is proposed that aims to reduce the complexity of the traditional antenna selection techniques. For this scheme, an analytical statistical framework is developed for characterizing the output signal-to-noise ratio (SNR). The analysis is general enough to take into consideration the impact of correlation on the antenna patterns. Special cases are also studied assuming independence between the patterns, while the diversity and coding gains are also evaluated. The performance is analysed using the criteria of outage probability and the average bit error probability. Moreover, the new scheme is applied in a V2V communication scenario based on the IEEE 802.11p standard, while it is compared with other well-known receivers under different channel model conditions.

In this paper, we investigate the performance of space shift keying (SSK) transmission over a generalized multiple scattering channel. This channel model provides a realistic statistical description of an intervehicular communication environment and includes as special cases the Rician, Rayleigh, and double-Rayleigh channels. We derive an accurate closed-form expression for the pairwise error probability of SSK, which is used to obtain the uncoded and coded error rate performances of SSK over various propagation scenarios. An asymptotic analysis for high values of the signal-to-noise ratio is carried out to determine the diversity and coding gains of the proposed system. The performance analysis reveals the impact of the number of receive antennas and the use of soft and hard decision decoding on the error rate performance of the SSK transmission over multiple scattering channels. Various numerically evaluated results accompanied by Monte Carlo simulations are presented to demonstrate the correctness of the proposed analysis.

In several wireless communication systems, low complexity channel selection schemes are required, as power efficient mechanisms to improve performance. In this context, the best selection scheme, in terms of performance, comes at the cost of increased complexity, since it necessitates a continuous monitoring of all available channels. In this paper, a new threshold-based channel selection strategy is proposed, which decreases the system complexity, without considerably affecting the system performance. Assuming independent but nonidentically distributed channel conditions, a generic analytical framework is first presented, based on the Markov chain theory. Then, the proposed selection scheme is applied to three specific communication scenarios, namely multichannel reception, transmit antenna selection with diversity reception, and cooperative relay selection. In all three cases, closed-form results are obtained and used to analyze the performance of the systems under consideration. It is shown that based on the proposed scheme, computational complexity is reduced and thus important energy savings can be achieved, without a significant loss in performance.

Abstract A new user selection strategy is investigated and analyzed in a multiuser relaying environment in the presence of co‐channel interference. The proposed selection scheme aims at avoiding unnecessary feedback load processing, in cases where a target threshold, in the received instantaneous signal‐to‐noise ratio, is exceeded. Assuming that perfect channel state information is available, closed‐form lower bound expressions are derived for the cumulative distribution function of the output signal to interference plus noise ratio. Moreover, the impact of outdated channel state information on the system's performance is also investigated. In addition, under the assumption of high signal‐to‐noise ratio conditions, simplified approximated expressions are also provided for the cumulative distribution functions of the output signal to interference plus noise ratio, which are employed to study the outage probability and bit error probability performance of the system. It is shown that with the proposed approach, a significant reduction in feedback load processing is achieved, with only a slight loss in performance. Copyright © 2016 John Wiley & Sons, Ltd.

In this paper, the effects of interference on composite fading environments, where multipath fading coexists with shadowing, are investigated. Based on some mathematical convenient expressions for the sum of squared K-distributed random variables, which are derived for the first time, important statistical metrics for the signal to interference and noise ratio (SINR) are studied for various cases including non identical, identical and fully correlated statistics. Furthermore, our analysis is extended to multi-channel receivers and in particular to selection diversity (SD) receivers, investigating two distinct cases, namely, signal-to-noise ratio based and SINR-based SD. For all scenarios, simplified expressions are also provided for the interference limited case, where the influence of thermal noise is ignored. The derived expressions are used to analyze the performance, in terms of the average bit error probability (ABEP) and the outage probability (OP), of such systems operating over composite fading environments. A high SNR analysis is also presented for the OP and the ABEP, giving a clear physical insight of the system's performance in terms of the diversity and coding gains. The analysis is accompanied by numerical evaluated results, clearly demonstrating the usefulness of the proposed theoretical framework.

In this paper the performance of a cooperative communication system operating over a composite fading environment and is also subject to interference is investigated. More specifically, assuming a decode-and-forward (multi-relay) cooperative network with relay selection, exact and approximated expressions are derived for the cumulative distribution function of the output signal to interference plus noise ratio. Based on these expressions, the outage and the average bit error probabilities of the system under consideration are studied and used to obtain various numerical performance evaluation results, depicting in all cases the usefulness of the proposed approach.

Selection diversity (SD) represents an attractive solution in various wireless communication scenarios, since it offers relatively low complexity and improved performance. However, in many practical situations the continuously monitoring of all the available diversity paths, which is mandatory in SD schemes, adds unnecessary overhead to the systems. In this paper, in order to alleviate this complexity, a new threshold-based SD (t-SD) receiver is proposed. In particular, in t-SD, the current path is used as long as its signal to noise ratio is above a predefined threshold, otherwise the receiver becomes a pure SD. For this new concept we present an analytical framework for important statistical metrics accompanied by a complexity analysis. Various numerical evaluated results illustrate that the proposed scheme outperforms other well known schemes in terms of the performance and complexity trade off.

In this paper the outage probability of a cognitive decode-and-forward relay network operating over Nakagami-m fading channels is evaluated. Based on the underlay approach, secondary transmissions are allowed only in cases where interference constraints on the primary destination receivers are satisfied. Additionally, by taking into consideration the interfering effects coming from the multiple primary transmitters, the signal-to-interference-plus-noise-ratio statistic of the secondary nodes is investigated. The derived results include exact expressions as well as approximated ones for high values of the maximum allowed transmitted power at the cognitive network and/or the interference limited case. We present numerical performance evaluation results for various channel conditions and communication scenarios. These results are complemented by equivalent computer simulated ones, which validate the accuracy of the proposed analysis.

In this paper we study the statistics of the sum of not necessarily identically distributed kappa, that is, K, random variables (RV)s. Assuming half-integer values for the shaping parameters, novel closed-form expressions for the probability density function (PDF) of the sum of independent K RVs are obtained, while for arbitrary values of the shaping parameters, a corresponding PDF expression is derived in terms of fast converging infinite series. Furthermore, an infinite series representation for the PDF of the sum of two arbitrarily correlated K RVs is derived. The proposed analysis is employed to the performance analysis of equal-gain combining (EGC) receivers operating over composite fading/shadowing channels modeled by the K distribution. More specifically, the outage and the average bit error probabilities, as well as the average channel capacity of EGC receivers operating over such composite environment are studied. Considering different channel fading/shadowing conditions and correlation effects, various numerical performance evaluation results are presented. These results are complemented by equivalent computer simulated ones that validate the accuracy of the proposed analysis.

The spectral efficiency of L-branch selection diversity (SD) receiver with different adaptive transmission techniques operating over generalised-Gamma (GG) fading channels is studied. Novel expressions for important adaptive transmission techniques are obtained, namely optimal power and rate adaptation, optimal rate adaptation, channel inversion with fixed rate (CIFR) and truncated CIFR. Furthermore, simplified expressions for the capacities are obtained for SD reception in Nakagami-m as well as Weibull fading channels. The derived closed-from expressions extend previously published results to the GG fading model. Numerical results are given to demonstrate the usefulness of the theoretical approach.

The correlated bivariate generalized-K (KG) distribution, with not necessarily identical shaping and scaling parameters, is introduced and studied. This composite distribution is convenient for modeling multipath/shadowing correlated fading environments when the correlations between the signal envelopes and their powers are different. Generic infinite series expressions are derived for the probability density function (PDF), the cumulative distribution function (CDF) and the joint moments. Assuming identical shaping parameters, simpler expressions for the PDF, CDF and the characteristic function (CF) are provided, while the joint moments are derived in closed form. Furthermore, the PDFs of the product and ratio of two correlated KGrandom variables are obtained. Capitalizing on these theoretical expressions for the statistical characteristics of the correlated KGdistribution, the performance analysis of various diversity reception techniques, such as maximal ratio combining (MRC), equal gain combining (EGC) and selection diversity (SD), over bivariate KGfading channels is presented. For the SD, the outage probability is studied, while for the MRC and EGC the average bit error probability is obtained. The proposed analysis is accompanied by numerical results, clearly demonstrating the usefulness of the theoretical approach as well as the appropriateness of the KGdistribution to model multipath/shadowing fading channels.

In this paper the Shannon capacity of generalized-gamma (GG) fading channels is studied, under different adaptive transmission techniques. The GG is a generic distribution, which is considered to be appropriate for modeling fading/shadowing phenomena. Hence, assuming single (i.e., no diversity), as well as, dual-branch selection diversity receivers, novel expressions for the capacity under three adaptive transmission policies, namely optimal power and rate adaptation (OPRA), channel inversion with fixed rate (CIFR) and truncated CIFR (TIFR), are obtained. The derived expressions are in closed form and generalize previously presented results. The proposed analysis is accompanied by numerical results, clearly demonstrating the usefulness of the theoretical approach.

The performance of dual-branch equal-gain combining (EGC) and maximal-ratio combining receivers operating over a composite correlated fading environment, modelled by the generalised Gamma (GG) distribution, is analysed. The moments of the output signal-to-noise ratio are derived in closed form for both types of receivers, and by employing the Padé approximants method, the average bit error probability is studied for a great variety of modulation schemes. Furthermore, based on the statistic of the product of two correlated GG random variables, a tight union upper bound for the outage probability of the EGC is obtained, whereas for the special case of Weibull fading a simpler bound is derived in closed form. The proposed mathematical analysis is complemented by various, numericaly evaluated performance results, whereas simulations verify the correctness of the proposed analysis.

The correlated bivariate K-distribution with arbitrary and not necessarily identical parameters is introduced and analyzed. Novel infinite series expressions for the joint probability density function and moments are derived for the general case where the associated bivariate distributions, i.e., Rayleigh and gamma, are both arbitrary correlated. These expressions generalize previously known analytical results obtained for identical parameter cases. Furthermore, considering independent gamma distributions, the cumulative distribution and characteristic functions are analytically obtained. Although the derived expressions can be used in a wide range of applications, this letter focuses on the performance analysis of dual branch diversity receivers. Specifically, the outage performance of dual selection diversity receivers operating over correlated K fading/shadowing channels is analytically evaluated. Moreover, for low normalized outage threshold values, closed-form expressions are obtained.

A detailed performance analysis for the most important diversity receivers operating over a composite fading channel modeled by the generalized-K (Kg) distribution is presented. The Kg distribution has been recently considered as a generic and versatile distribution for the accurate modeling of a great variety of short term fading in conjunction with long term fading (shadowing) channel conditions. For this relatively new composite fading model, expressions for important statistical metrics of maximal ratio combining (MRC), equal gain combining (EGC), selection combining (SC) and switch and stay combining (SSC) diversity receivers are derived. Using these expressions and by considering independent but not necessarily identical distributed fading channel conditions, performance criteria, such as average output signal-to-noise ratio, amount of fading and outage probability are obtained in closed form. Moreover, following the moments generating function (MGF) based approach for MRC and SSC receivers, and the Pade approximants method for SC and EGC receivers, the average bit error probability is studied. The proposed mathematical analysis is complemented by various performance evaluation results which demonstrate the accuracy of the theoretical approach.