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The cooperative aerial and device-to-device (D2D) networks employing non-orthogonal multiple access (NOMA) are expected to play an essential role in next-generation wireless networks. Moreover, machine learning (ML) techniques, such as artificial neural networks (ANN), can significantly enhance network performance and efficiency in fifth-generation (5G) wireless networks and beyond. This paper studies an ANN-based unmanned aerial vehicle (UAV) placement scheme to enhance an integrated UAV-D2D NOMA cooperative network.The proposed placement scheme selection (PSS) method for integrating the UAV into the cooperative network combines supervised and unsupervised ML techniques. Specifically, a supervised classification approach is employed utilizing a two-hidden layered ANN with 63 neurons evenly distributed among the layers. The output class of the ANN is utilized to determine the appropriate unsupervised learning method—either k-means or k-medoids—to be employed. This specific ANN layout has been observed to exhibit an accuracy of 94.12%, the highest accuracy among the ANN models evaluated, making it highly recommended for accurate PSS predictions in urban locations. Furthermore, the proposed cooperative scheme allows pairs of users to be simultaneously served through NOMA from the UAV, which acts as an aerial base station. At the same time, the D2D cooperative transmission for each NOMA pair is activated to improve the overall communication quality. Comparisons with conventional orthogonal multiple access (OMA) and alternative unsupervised machine-learning based-UAV-D2D NOMA cooperative networks show that significant sum rate and spectral efficiency gains can be harvested through the proposed method under varying D2D bandwidth allocations.

Motion Shield is an automatic crash notification system that uses a mobile phone to generate automatic alerts related to the safety of a user when the user is boarding a means of transportation. The objective of Motion Shield is to improve road safety by considering a moving vehicle’s risk, estimating the probability of an emergency, and assessing the likelihood of an accident. The system, using multiple sources of external information, the mobile phone sensors’ readings, geolocated information, weather data, and historical evidence of traffic accidents, processes a plethora of parameters in order to predict the onset of an accident and act preventively. All the collected data are forwarded into a decision support system which dynamically calculates the mobility risk and driving behavior aspects in order to proactively send personalized notifications and alerts to the user and a public safety answering point (PSAP) (112).

In this paper, an airborne multi-user (MU) multiple-input multiple-output (MIMO) communication system is investigated, consisting of multiple users sources, multiple users destinations, and an aerial platform acting as a decode-and-forward (DF) relay. In this context, a novel three-dimensional (3-D) geometry-based optimization method for the relay location is proposed and expressions for the outage probability are presented. The results highlight the impact of the relay position, power allocation, fading severity, and number of antennas on the system performance.

In this paper, a new mode selection (MS) scheme for device-to-device (D2D) enabled cellular networks is proposed. The MS is carried out using a predefined signal-to-interference plus noise ratio threshold and it aims to guarantee a minimum level of quality of service.With the new scheme, interfering effects are taken into consideration, while realistic assumptions for the acquisition of the channel information are made. Assuming a single-user case, an analytical framework is developed, which is based on the Markov-chain theory, and is used to study the system's performance. The various performance results that have been obtained reveal that with the proposed MS, the probability of mode switching can be reduced (as compared to other systems) and thus, an increase on the overhead and signal processing is avoided. Moreover, the new approach is applied in a multi-user communication scenario, where a joint MS, resource allocation (RA), and scheduling optimization problem is formulated. The solution to this problem is based on a greedy heuristic algorithm that properly assigns the available resources among cellular users and D2D pairs. The joint MS, RA, and scheduling algorithm considers limited SINR awareness and it provides significant performance improvement in terms of sum-rate.

This paper studies the multivariate Ricean distribution, which is the most important distribution for modeling the magnitude of the received signal when line-of-sight propagation conditions exist. In this paper, assuming different mean values for the Ricean distributed random variables, generic expressions for the multivariate Ricean probability density function, cumulative distribution function, and the joint moments are derived in terms of integral representations. As an application example, these results have been used to analyze the performance of a multi-branch selection diversity receiver, operating in spatially correlated Ricean fading environment. Moreover, an asymptotic closed-form expression has been also provided, which approximates quite well to the exact one, and has been used to study various performance metrics. Finally, a complexity analysis has been performed and simulation results have been also presented.

One of the most crucial emergencies that require instant action to be taken during traveling across water is the so-called man overboard (MOB). Thus, constant monitoring equipment needs to be installed for the fast notice and detection of the victim to be rescued, if an incident happens. Despite the fact that different installations such as radar sensors, thermal cameras etc., can be handy, a combination of these could be beneficial yet it would increase the complexity. Nevertheless, the full potential may be not reached yet. The key component to what needs to be done in order to achieve the utmost accuracy is artificial intelligence (AI). That is, with the aid of AI, one can deploy an automated surveillance system capable of making its own humanlike decisions regarding such incidents like MOB. To achieve this, fully organized real-time cooperation among the concerned system components is essential. The latter holds since in such dynamically changing operational environments like these, information must be distributed fast, errorless and reliably to the decision center. This study aims to analyze and demonstrate the outcome of an integrated sensor-based system that utilizes AI, implemented for ship incidents. Different machine learning algorithms were used where each one of them made use of information that originated from a cluster of radar sensors located remotely. In particular, the deployed system’s objective is to detect human motion so it can be used to protect against potentially fateful events during ship voyages.

Antenna selection (AS) techniques are considered as ideal candidates for vehicle-to-vehicle communications, since they improve system's performance and simultaneously satisfy the hardware and signal processing constraints that exist in these systems. However, the achieved gain over single antenna links is affected by the fast varying wireless channel, since AS is frequently performed using outdated versions of the signal-to-noise ratio (SNR). In this paper, we propose an AS technique that exploits the stationarity of large-scale fading. In this context, by employing 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 than small-scale fading. The performance of the proposed technique is analyzed using the criteria of outage probability and average output SNR. It is shown that the proposed scheme outperforms the corresponding one that is based on outdated CSI, especially in scenarios with mild fading/shadowing channel conditions. Moreover, the influence of correlated shadowing on the system's performance has been also analytically investigated. The main results have been also verified by empirical data based on measurement campaigns in non-stationary communication conditions.

This communication investigates the validity of application of a multiple-scattering model to a dual-polarized multiple-input-multiple-output land mobile satellite (LMS) channel based on a measurement campaign. Two LMS link scenarios in an urban pedestrian environment are examined. In the first one, the transmitter was located at the top of a tall building and in the second one, on an airship. The third-order scattering model is utilized to statistically characterize the measured channels, and the results reveal that the model provides an excellent fit to the LMS composite fading amplitude in the line-of-sight (LOS) and non-LOS scenarios. For this generic radio channel, asymptotic expressions for the theoretical probability density function and the cumulative distribution function are derived. The comparison of the third-order scattering model with existing LMS ones showed that it provides better overall accuracy together with a physical interpretation of the propagation mechanisms involved.

This paper considers a mixed free-space optical/radio frequency (FSO/RF) system, which facilitates the communication between a ground central unit (CU) and multiple ground users (GUs) via a hovering unmanned aerial vehicle (UAV) acting as a decode-and-forward (DF) aerial relay. It is considered that the CU employs multiple transmit apertures and is interconnected with the aerial relay via an FSO link with transmit aperture selection (TAS), whereas the relay communicates with the GUs over multiple-input multiple-output (MIMO) RF links with orthogonal space–time block coding (OSTBC) transmission. It is assumed that the optical channels are mainly affected by atmospheric attenuation as well as geometric and misalignment loss (GML). Besides, the atmospheric turbulence is assumed weak for short-range links. Also, fully correlated shadowed conditions are considered in the RF links and the MIMO channels are modeled using the Nakagami/Inverse Gamma (IG) composite fading distribution. Opportunistic GU scheduling is applied in the downlink and novel closed-form mathematical formulas for the outage probability (OP) are derived. The results demonstrate the theoretical derivations and the performance benefits of the proposed approach.

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 closed-form 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.

Unmanned aerial vehicles (UAVs) will be an integral part of the next generation wireless communication networks. Their adoption in various communication-based applications is expected to improve coverage and spectral efficiency, as compared to traditional ground-based solutions. However, this new degree of freedom that will be included in the network will also add new challenges. In this context, the machine-learning (ML) framework is expected to provide solutions for the various problems that have already been identified when UAVs are used for communication purposes. In this article, we provide a detailed survey of all relevant research works, in which ML techniques have been used on UAV-based communications for improving various design and functional aspects such as channel modeling, resource management, positioning, and security.

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 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.

The probability density function and cumulative distribution function of the received signal-to-noise ratio (SNR) and the received signal-to-interference ratio (SIR), for interference-limited systems is derived, at the nth nearest neighbour node in a Poisson point process wireless random network. The analytical expressions are given in terms of the Meijer G-function and reveal the impact of node spatial density, transmit power, interference power, and path-loss exponent on the connectivity probability of a broadcast wireless transmission. The analytical results are validated with computer simulation.

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.

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, 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 ${\cal 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.

Distributed-Multiple Input Multiple Output (D-MIMO) communication systems consist an attractive solution for networks with increased capacity demands. In these systems, the required information that needs to be exchanged among the network elements increases the data overhead and hence decreases the effective sum-rate (or throughput). Recently, it was shown that the total required overhead for D-MIMO networks can be reduced through its partitioning into smaller orthogonal D-MIMO segments. In this paper, a new scheme is proposed for further improving the effective sum-rate of D-MIMO networks by means of exploiting the spatial channels correlation within the D-MIMO network. Such effects can be observed in dense networks and the scope of the proposed correlation exploitation techniques is to avoid sending redundant feedback information. Numerical results indicate that important savings can be achieved when this novel method is applied under different wireless environments.

Distributed-Multiple Input Multiple Output (DMIMO) networks is a promising enabler to address the challenges of high traffic demand in future wireless networks. A limiting factor that is directly related to the performance of these systems is the overhead signaling required for distributing data and control information among the network elements. In this paper, the concept of orthogonal partitioning is extended to D-MIMO networks employing joint multi-user beamforming, aiming to maximize the effective sum-rate, i.e., the actual transmitted information data. Furthermore, in order to comply with practical requirements, the overhead subframe size is considered to be constrained. In this context, a novel formulation of constrained orthogonal partitioning is introduced as an elegant Knapsack optimization problem, which allows the derivation of quick and accurate solutions. Several numerical results give insight into the capabilities of D-MIMO networks and the actual sum-rate scaling under overhead constraints., IEEE Wireless Communications Letters

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 capacity of correlated generalized gamma (GG) fading channels with dual-branch selection diversity (SD) reception and channel side information (CSI) available at both the transmitter and the receiver is studied and analyzed. In particular, the capacity is evaluated under four adaptive transmission policies defined according to the operational CSI: 1) optimal power and rate adaptation (OPRA); 2) constant power with optimal rate adaptation (ORA); 3) channel inversion with fixed rate (CIFR); and 4) truncated CIFR (TIFR). Under these four adaptation policies, fast converging infinite series expressions for the capacity are derived and evaluated. Furthermore, for the simplifying cases of independent GG fading channels and/or single-channel reception (i.e., no diversity), closed-form expressions have been obtained. Previously published expressions are shown to be special cases of the derived expressions. Performance evaluation results obtained by means of numerical analysis clearly demonstrate the usefulness and accuracy of the analysis.

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 KG random variables are obtained. Capitalizing on these theoretical expressions for the statistical characteristics of the correlated KG distribution, the performance analysis of various diversity reception techniques, such as maximal ratio combining (MRC), equal gain combining (EGC) and selection diversity (SD), over bivariate KG fading 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 KG distribution to model multipath/shadowing fading channels.

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.

The bivariate double-Rayleigh distribution with non-identically distributed parameters is introduced and analyzed. Novel series expressions for the joint probability density, cumulative distribution, and the moments are derived. Based on this new distribution, a multichannel communication scenario is studied, where correlation exists between the actual channel and its corresponding estimate, under time varying channel assumption. In this context, the outage performance of the system under consideration is analytically evaluated, while for higher values of the average signal-to-noise ratio, closed-form results are derived. Finally, an analysis for total outage probability in varying-correlation conditions is also provided.

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.

In this paper a MMIC QPSK modulator is described. The bit rate of the baseband signal is 155 Mbits/sec, while the frequency of the carrier isfLO=10 GHz. The modulation is performed directly at the RF band and therefore an IF stage is not necessary. The design contains all the necessary sub-circuits except for the demultiplexer and the low-pass filters, which are used so that the bandwidth of the binary waves can be reduced. The mixers that are contained in the circuit are not the typical Gilbert cells and as a result they occupy a much smaller area (half). The circuit needs no external LO coupler and no RF chocks. The F-20 process of GEC Marconi was used for designing the modulator. The occupied area is approximately12 mm2. Section 1 presents fundamentals on Quadriphase Shift Keying (QPSK), while in Section 2 the characteristics of the F-20 process are described. In the following Sections the sub-circuits, the complete circuit, and its simulated results are presented.

In this paper two active MMIC mixers for RF front-end applications are described. A down-converter that converts an RF signal(fRF=10.45 GHz)into an IF signal(fIF=0.95 GHz)using an LO signal(fLO=9.5 GHz)and an up-converter that performs the opposite process have been fabricated. The down-converter is designed using the topology of a dual-gate pHEMT, while the up-converter is implemented in the form of a double balanced mixer using the topology of the Gilbert cell and the occupied areas are approximately0.78 mm2and3.86 mm2, respectively. Both mixers present conversion gain, very low input and output return losses, very good isolation between all of their ports and the required LO power is quite low, while the up-converter contains on chip, except for the dc-bias and matching sub-circuits, the required LO and RF baluns. Both circuits have been fabricated using the H-40 process of GEC-Marconi. Section 1 presents fundamentals on mixer theory and mixer design while in Section 2 the characteristics of H-40 process are described. In Section 3 and in Section 4 the designing, the simulated and the measured results of the down-converter and the up-converter are presented, respectively.

Summary The development of new hybrid diversity methods has been demanded by the continuous growth of information rates. These schemes achieve improved balance between hardware complexity and performance, in relation to conventional ones. In this paper, a new hybrid switched diversity system is proposed. It is called generalized multi-branch switch and examine combining. The new scheme is a generalization of the classic one known as switch and examine combining with considerably improved performance. The performance of this scheme is analyzed over independent and identically distributed Nakagami-m fading channels using well-known performance and complexity measures. More specifically, important performance metrics, such as the outage probability, the average bit error rate, and the capacity under different adaptive transmission policies, have been analytically studied. It is shown that the proposed approach offers an important improvement on the performance, without considerably affecting the complexity.

A detailed performance analysis of generalized-selection combining GSC(2,L) receivers operating over generalized-gamma fading channels is presented. For this class of receivers, a novel closed-form expression for the moments output signal-to-noise ratio is derived. Furthermore, infinite series representations for the moments-generating and the cumulative distribution functions are obtained. The proposed mathematical analysis is accompanied by various performance evaluation results. These theoretical results are complemented by equivalent computer simulated results, which validate the accuracy of the proposed analysis.

The performance of digital communication systems over generalized-K (K/sub G/) fading channels is analyzed and evaluated. Novel closed form expressions for the SNR statistics, the average Shannon's channel capacity and the bit error rate (BER) are derived. These expressions are used to study important performance criteria such as the outage performance, the average capacity and the BER for a great variety of modulation formats in K/sub G/ fading channels. The proposed mathematical analysis is accompanied with various performance evaluation results, which demonstrate the usefulness of the proposed approach.

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 Weibull-gamma (WG) distribution, which is appropriate for modelling fading environments when multipath is superimposed on shadowing, is introduced and studied. For this composite distribution the probability density, cumulative distribution, characteristic functions and the moments are derived in closed form. Furthermore, the average bit error and outage probabilities of a receiver operating over WG fading channels are assessed and compared with the corresponding performances obtained using other composite distributions.

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 Pade 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.

In this paper, we study a cooperative vehicle-to-vehicle (V2V) interference-limited communication system with decode-and-forward relaying operating over double-Nakagami (DN) fading channels. This distribution has been proposed as a fading model for scenarios where both the transmitter and the receiver are in motion, as is the case in V2V communications. For the purposes of our analysis, we introduce the bivariate DN distribution and derive statistical metrics for this distribution, such as the probability density function, the cumulative distribution function, and the moments. The new expressions have been employed to model the outdated channel state information (CSI) that is due to the fast time-varying nature of the wireless medium and the delay naturally existing between relay selection and data transmission instances. The analysis considers two relay selection schemes, according to the relay-destination channel quality, namely the well-known best relay selection (BRS) and a new one termed as threshold-based relay selection, which offers lower complexity than BRS. Using our analytical framework, exact and asymptotic performance results are obtained for the outage and the average symbol error probabilities, while the diversity order has been also evaluated. The numerical and simulation results corroborate our theoretical ones and quantify the impact of the outdated CSI, level of interference, and varying distances between the relay nodes and the final destination on the system's performance.