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Cooperative diversity systems with optimal and suboptimal antenna and relay selection have been paid significant attention for more than half a decade. However, optimal antenna and relay selection strategies require global channel state information (CSI), which is a cumbersome process. In this paper, we propose a diversity-optimal and three suboptimal transmit–receive antenna and relay selection strategies, named S1, S2, and S3, for decode-and-forward cooperative relaying systems, equipped with Nt and Nr antennas at the source (S) and the destination (D), respectively, and considering a multirelay scenario. Furthermore, we study the symbol error probability (SEP) for these strategies, assuming M-ary phase-shift keying signaling over Rayleigh fading channels. In addition, we perform diversity order analysis through closed-form asymptotic SEP expressions. Since the proposed suboptimal strategies, i.e., S2 and S3, involve the so-called switch-and-examine combining scheme, we compare the complexity of S1, S2, and S3 in terms of the average number of CSI required at D to select the best transmit–receive antenna pair and relay. From the derived SEP expressions, it can be concluded that all three suboptimal strategies achieve the same diversity order of N_{t}N_{r}+N, where N$ is the number of relays, except for the case when the switching threshold signal-to-noise ratio (SNR) is much lower than the average SNR in S2 and S3. Finally, the diversity-optimal strategy achieves full diversity order of N_{t}N_{r}+N\min(N_{t},N_{r}). [ABSTRACT FROM AUTHOR]

A novel analytical framework for evaluating the statistics of the product of independent random variables is proposed. Compared with other methods, which use either infinite series or special functions, the new method provides simple and efficient closed-form approximations, in terms of elementary functions, such as powers and exponentials. Numerical results, which are used to check the accuracy of the new approximations, show that it is quite accurate in most regions of interest. As an application, the proposed analytical results can be efficiently used in wireless communications theory to evaluate in closed form the outage probability of cascaded fading channels, as well as the rate offset of the hybrid automatic repeat request (H-ARQ) transmission. Numerical examples show that the derived expressions provide significant insights on the behaviors of important system parameters as the outage probability and the rate offset. [ABSTRACT FROM PUBLISHER]

We argue for incorporating Space Division Multiple Access (SDMA) into cooperative relaying, in order to bypass orthogonal relaying transmissions and thus achieve diversity gain without any cost on the available degrees of freedom. In particular, we propose a cooperative relaying scheme that utilizes two single-antenna relays and multiple antennas at the destination terminal, in order to spatially separate the concurrently arriving signals. The whole concept is based upon two key elements: a) To combat the half-duplex constraint by having two relays transmitting alternatively, i.e., the one receiving while the other transmitting and vice versa, and b) to spatially separate the signals arriving concurrently at the destination using the well-known optimum combining technique. Closed-form expressions for the average capacity and outage probability are provided. Numerical results manifest that the proposed model outperforms orthogonal relaying as well as distributed space-time coding in terms of average capacity and outage probability, owing to the former's advantage of higher spectral efficiency. [ABSTRACT FROM PUBLISHER]

This work is devoted to the study of dispersed spectrum cognitive radio (CR) systems over independent and nonidentically distributed (i.n.i.d.) generalized fading channels. More specifically, this is performed in terms of the high-order statistics (HOS) of the channel capacity over \eta-\mu fading channels. A generic analytic expression is derived for the corresponding $n$th statistical moment, which is subsequently employed for deducing exact closed-form expressions for the first four moments. Using these expressions, important statistical metrics, such as the amount of dispersion, amount of fading, skewness, and kurtosis, are derived in closed form and can be efficiently used in providing insights on the performance of dispersed CR systems. The obtained numerical results reveal interesting outcomes that could be useful for the channel selection, either for sharing or aggregation in heterogeneous networks, which is the core structure of future wireless communication systems. [ABSTRACT FROM AUTHOR]

A multiuser dual-hop relaying system over mixed radio frequency/free-space optical (RF/FSO) links is investigated. Specifically, the system consists of m single-antenna sources, a relay node equipped with n≥ m receive antennas and a single photo-aperture transmitter, and one destination equipped with a single photo-detector. RF links are used for the simultaneous data transmission from multiple sources to the relay. The relay operates under the decode-and-forward protocol and utilizes the popular V-BLAST technique by successively decoding each user's transmitted stream. Two common norm-based orderings are adopted, i.e., the streams are decoded in an ascending or a descending order. After V-BLAST, the relay retransmits the decoded information to the destination via a point-to-point FSO link in m consecutive timeslots. Analytical expressions for the end-to-end outage probability and average symbol error probability of each user are derived, while closed-form asymptotic expressions are also presented. Capitalizing on the derived results, some engineering insights are manifested, such as the coding and diversity gain of each user, the impact of the pointing error displacement on the FSO link and the V-BLAST ordering effectiveness at the relay. [ABSTRACT FROM AUTHOR]

We study the outage performance of orthogonal frequency-division multiplexing dual-hop opportunistic amplify-and-forward relaying in the presence of I/Q imbalance (IQI) in all nodes. We derive a closed-form expression for the end-to-end outage probability for the general case where each node suffers from a different IQI level and assuming that the distance between source and relay is not the same for all relays. Furthermore, we consider the more general case where there is a direct link between the source and the destination and we derive a closed-form expression for the end-to-end outage probability for both maximum-ratio and selection combining at the destination. To gain more insights, we analyze a special case where all relays lie on the perpendicular line midway between source and destination and all nodes experience the same IQI level. Our simulations show excellent match to the analytical results and both demonstrate that uncompensated IQI can be detrimental but it can also be effectively mitigated using a few opportunistic relays. In summary, the main contributions of this paper are analyzing the effect of IQI on the outage probability of an opportunistic relaying system and determining the number of relays needed to effectively mitigate uncompensated IQI. [ABSTRACT FROM AUTHOR]

We investigate the ergodic capacity of amplify-and-forward (AF) dual-hop relaying systems in composite Nakagami-m/inverse-Gaussian fading channels. This type of fading, which is known in the literature as \cal G fading, has recently attracted increasing research interest due to its ability to better approximate the Nakagami- m/lognormal model, compared with the Nakagami- m/gamma model. We study both fixed- and variable-gain relaying systems and present analytical upper and lower bounds for the ergodic capacity of dual-hop relaying systems with not necessarily identical hops; these bounds provide an efficient means to evaluate the ergodic capacity of AF dual-hop relaying systems over \cal G fading channels. We also establish sufficient conditions for the existence of the bounds, depending on the values of the fading parameters. In both cases, our simulation results demonstrate that the proposed upper and lower bounds remain relatively tight for different fading conditions. [ABSTRACT FROM PUBLISHER]

The outage probability is an important performance measure for cooperative diversity schemes. However, in mobile environments, the outage probability does not completely describe the behavior of cooperative diversity schemes since the mobility of the involved nodes introduces variations in the channel gains. As a result, the capacity outage events are correlated in time and second-order statistical parameters of the achievable information-theoretic capacity such as the average capacity outage rate (AOR) and the average capacity outage duration (AOD) are required to obtain a more complete description of the properties of cooperative diversity protocols. In this paper, assuming slow Rayleigh fading, we derive exact expressions for the AOR and the AOD of three well-known cooperative diversity protocols: variable-gain amplify-and-forward, decode-and-forward, and selection decode-and-forward relaying. Furthermore, we develop asymptotically tight high signal-to-noise ratio (SNR) approximations, which offer important insights into the influence of various system and channel parameters on the AOR and the AOD. In particular, we show that on a double-logarithmic scale, similar to the outage probability, the AOR asymptotically decays with the SNR with a slope that depends on the diversity gain of the cooperative protocol, whereas the AOD asymptotically decays with a slope of -1/2 independent of the diversity gain. [ABSTRACT FROM AUTHOR]

This paper elaborates on the ergodic capacity of fixed-gain amplify-and-forward (AF) dual-hop systems, which have recently attracted considerable research and industry interest. In particular, two novel capacity bounds that allow for fast and efficient computation and apply for nonidentically distributed hops are derived. More importantly, they are generic since they apply to a wide range of popular fading channel models. Specifically, the proposed upper bound applies to Nakagami-m, Weibull, and generalized-K fading channels, whereas the proposed lower bound is more general and applies to Rician fading channels. Moreover, it is explicitly demonstrated that the proposed lower and upper bounds become asymptotically exact in the high signal-to-noise ratio (SNR) regime. Based on our analytical expressions and numerical results, we gain valuable insights into the impact of model parameters on the capacity of fixed-gain AF dual-hop relaying systems. [ABSTRACT FROM PUBLISHER]

In this paper, the decremental effect of beamforming with feedback delay on the performance of a two-hop amplify-and-forward (AF) relay network over Rayleigh-fading channels is investigated. An antenna configuration in which the source and the destination are equipped with multiple antennas, whereas the relay is equipped with a single antenna, is assumed. We derive new expressions for the outage probability and the average bit error rate (BER), which are useful for a large number of modulation schemes. To gain further insights, simple outage probability and average BER approximations at high signal-to-noise ratio (SNR) are also presented. It is shown that, whenever a feedback delay exists, the network is not capable of offering diversity gains. Furthermore, source and relay power allocation results show significantly different behavior with feedback delay. Numerical results supported by simulations are provided to show that feedback delay can severely degrade the performance of the considered AF relay system. [ABSTRACT FROM PUBLISHER]

This correspondence explores the ergodic capacity of multiple-input multiple-output (MIMO) systems operating in generalized-K fading conditions. Using some recent results on majorization theory, we derive an analytical capacity bound which is applicable for arbitrary values of the signal-to-noise ratio (SNR) and number of antenna elements. In addition, we deduce simple bound approximations in the high-SNR regime and demonstrate that the effects of small and large-scale fading are decoupled. A similar statistical analysis is carried out for MIMO channels under K-fading, which represents a special case of generalized-K fading that can be tackled via the Wishart matrix theory. The implications of the model parameters on the bound performance are also investigated via Monte Carlo simulations. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

This paper proposes the integration of non-orthogonal multiple access (NOMA) and broadcasting with buffer-aided (BA) full-duplex (FD) relaying to improve the spectral efficiency of a single-source two-user network. Furthermore, opportunistic relay selection is employed, activating one or two FD relays to support the end-to-end communication. The network performance is improved through buffers, allowing to switch to half-duplex (HD) transmission/reception when FD operation is not feasible. A hybrid BA FD/HD NOMA relay selection algorithm is proposed, herein called $\mathop {\mathrm{BAHyNOMA}}$ , orchestrating the three aforementioned techniques, towards a simple yet efficient and robust relay selection. $\mathop {\mathrm{BAHyNOMA}}$ is evaluated in terms of outage probability, average sum-rate and average delay, and it is observed that the average sum-rate can be doubled, while maintaining low latency. [ABSTRACT FROM AUTHOR]

We investigate the performance of mixed free-space optical (FSO)/millimeter-wave (mmWave) relay networks with interference at the destination. The FSO/mmWave channels are assumed to follow Málaga- $\mathcal {M}$ /generalized- $\cal K$ fading models with pointing errors in the FSO link. The H-transform theory, wherein integral transforms involve Fox’s H-functions as kernels, is embodied to unifying the performance analysis framework that encompasses closed-form expressions for the outage probability, the average bit error rate (BER), and the average capacity. By virtue of some H-transform asymptotic expansions, the high signal-to-interference-plus-noise ratio (SINR) analysis reduces to easy-to-compute expressions for the outage probability and BER, which reveals inside information for the system design. We finally investigate the optimal power allocation strategy that minimizes the outage probability. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the performance of energy detection-based spectrum sensing over $\mathcal {F}$ composite fading channels. To this end, an analytical expression for the average detection probability is first derived. This expression is then extended to account for collaborative spectrum sensing, square-law selection diversity reception, and noise power uncertainty. The corresponding receiver operating characteristics (ROC) are analyzed for different conditions of the average signal-to-noise ratio (SNR), noise power uncertainty, time-bandwidth product, multipath fading, shadowing, number of diversity branches, and number of collaborating users. It is shown that the energy detection performance is sensitive to the severity of the multipath fading and the amount of shadowing, whereby even small variations in either of these physical phenomena can significantly impact the detection probability. As a figure of merit to evaluate the detection performance, the area under the ROC curve (AUC) is derived and evaluated for different multipath fading and shadowing conditions. Closed-form expressions for the differential entropy and cross entropy are also formulated and assessed for different average SNR, multipath fading, and shadowing conditions. Then, the relationship between the differential entropy of $\mathcal {F}$ composite fading channels and the corresponding ROC/AUC is examined where it is shown that the average number of bits required for encoding a signal becomes small (i.e., low differential entropy) when the detection probability is high or when the AUC is large. The difference between composite fading and traditional small-scale fading is emphasized by comparing the cross entropy for Rayleigh and Nakagami- $m$ fading. A validation of the analytical results is provided through a careful comparison with the results of some simulations. [ABSTRACT FROM AUTHOR]

This paper considers the bit loading problem for communication systems that utilize orthogonal frequency division multiplexing (OFDM) in conjunction with precoding (POFDM) or time-domain interleaving (IOFDM). In particular, we propose a new bit loading algorithm for P/I-OFDM that has substantially higher effective throughput and less computational complexity, when compared to bit loading in conventional OFDM systems. The obtained results show that the effective throughput of P/I-OFDM can be more than fourfold the conventional OFDM while the complexity is less than $1.5\%$. Moreover, the results show that the peak-to-average power ratio (PAPR) properties of the considered systems are preserved under the adaptation process. [ABSTRACT FROM AUTHOR]

We present a novel framework for analyzing and optimizing the outage performance of a multiple amplify-and-forward relay network, where $N$ relays, each with finite cache capacity, are deployed to assist the transmission from a base station equipped with $L$ antennas to the destination. In this network, if the requested file is cached at the relays and the associated relay channels are in good condition, then the data are directly transmitted from the relays to the destination; otherwise, traditional two-hop transmission is performed. Based on this concept, we propose a relay-selection criterion to choose the best relay, which maximizes the received signal-to-noise ratio at the destination. For this criterion, we derive exact and asymptotic analytical expressions for the system outage probability in Nakagami- $m$ fading. To further improve the network outage performance, we optimize the cache placement, which minimizes the outage probability. Numerical and simulation experiments are performed to validate the analysis and demonstrate the conditions where the proposed cache placement strategy outperforms the existing ones. [ABSTRACT FROM AUTHOR]

To study the feasibility of network non-orthogonal multiple access (N-NOMA) techniques, this paper proposes a N-NOMA scheme for a downlink coordinated multipoint (CoMP) communication scenario, with randomly deployed users. In the considered N-NOMA scheme, superposition coding (SC) is employed to serve cell-edge users as well as users close to base stations (BSs) simultaneously, and distributed analog beamforming by the BSs to meet the cell-edge user’s quality of service requirements. The combination of SC and distributed analog beamforming significantly complicates the expressions for the signal-to-interference-plus-noise ratio at the receiver, which makes the performance analysis particularly challenging. However, by using rational approximations, insightful analytical results are obtained in order to characterize the outage performance of the considered N-NOMA scheme. Computer simulation results are provided to show the superior performance of the proposed scheme as well as to demonstrate the accuracy of the analytical results. [ABSTRACT FROM AUTHOR]

Buffer-aided relays can improve the diversity of multi-hop networks, however, they increase packet delays. Thus, various delay-aware protocols have been developed, but without considering the transmission diversity. Moreover, most works adopt ideal assumptions, such as symmetric links, perfect channel state information (CSI), and error-free feedback channels. So, we propose a low-complexity (LoCo) link selection algorithm, herein called $\mathop {\mathrm {LoCo\!\!-\!\!Link}}$. The proposed algorithm may experience delays during CSI updates, and hence, by using outdated CSI its performance may deteriorate. To alleviate this issue, we next propose a distributed version of $\mathop {\mathrm {LoCo\!\!-\!\!Link}}$ ($\mathop {\mathrm {d\!\!-\!\!LoCo\!\!-\!\!Link}}$) dealing with outdated CSI. In both algorithms, the source performs broadcasting toward multiple relays; when the packets are transmitted by a relay to the destination, they are discarded from all other relays. This coordination relies on feedback channels. For non error-free feedback channels, we propose a scheme in which the relays listen to the transmission of the best relay and drop duplicate packets. Results show that $\mathop {\mathrm {LoCo\!\!-\!\!Link}}$ surpasses other algorithms, by decreasing the delay in asymmetric networks. Moreover, $\mathop {\mathrm {d\!\!-\!\!LoCo\!\!-\!\!Link}}$ avoids diversity losses due to outdated CSI, while the effect of non error-free feedback channels is mitigated by taking advantage of the inter-relay channels. [ABSTRACT FROM AUTHOR]

In networks with multiple wireless nodes and especially in fast changing environments, the continuous acquisition of channel state information (CSI) may become infeasible. In this correspondence paper, we study the relay selection problem for a cooperative network, where full-duplex relays are equipped with buffers and only statistical CSI is available at the transmitters (CSIT). For this setting, a novel relay selection algorithm is proposed, named $\mathop {\mathrm{HyQoS}}$ , relying only on statistical CSIT and performing power adaptation. $\mathop {\mathrm{HyQoS}}$ solves an optimization problem for selecting the optimal set of relays. We consider two optimization approaches, first, minimization of the total power consumed in the network, and, second, maximization of the throughput per energy unit, while guaranteeing a specific quality of service, in terms of successful transmission probability. The power allocation problem is formulated as a nonlinear optimization one, taking into account inter-relay interference and residual loop interference. Numerical results show that $\mathop {\mathrm{HyQoS}}$ can reduce the power expenditure and increase the throughput, by using only statistical CSIT. [ABSTRACT FROM AUTHOR]

In this paper, we study the impact of correlated fading on the secrecy performance of multiple decode-and-forward (DF) relaying with outdated relay selection. It is assumed that the information transmission, assisted by $N$ DF relays from the source to the destination, can be overheard by an eavesdropper. Particularly, we consider the realistic scenario where the eavesdropper's and the main channels are correlated. In order to enhance the network security, the best relay is selected among $N$ available DF relays to assist the secure transmission. Due to the time-varying channel environments, we note that the selected relay may be outdated. In order to study the impact of both channel correlation and outdated relay selection on the secrecy performance, we first derive an analytical expression for the secrecy outage probability (SOP). Also, we derive the asymptotic expression for the SOP in the high main-to-eavesdropper ratio regime. Numerical results are provided to demonstrate the correctness of our analytical expressions. [ABSTRACT FROM AUTHOR]

This paper quantifies the impact of correlated fading on secure communication of multiple amplify-and-forward (AF) relaying networks. In such a network, the base station (BS) is equipped with multiple antennas and communicates with the destination through multiple AF relays, while the message from the relays can be overheard by an eavesdropper. We focus on the practical communication scenario, where the main and eavesdropper’s channels are correlated. In order to enhance the transmission security, transmit antenna selection is performed at the BS, and the best relay is chosen according to the full- or partial-relay selection criterion, which relies on the dual-hop relay channels or the second-hop relay channels, respectively. For these criteria, we study the impact of correlated fading on the network secrecy performance, by deriving an analytical approximation for the secrecy outage probability and an asymptotic expression for the high main-to-eavesdropper ratio. From these results, it is concluded that the channel correlation is always beneficial to the secrecy performance of full relay selection. However, it deteriorates the secrecy performance if partial-relay selection is used, when the number of antennas at the BS is less than the number of relays. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a unified approach to model wireless channels by the mixture of Gaussian (MoG) distribution. The proposed approach provides an accurate approximation for the envelope and the signal-to-noise ratio (SNR) distributions of wireless fading channels. Simulation results have shown that the proposed model can accurately characterize multipath and composite fading channels. We utilize the well-known expectation–maximization (EM) algorithm to estimate the parameters of the MoG distribution and further utilize the Bayesian information criterion (BIC) to determine the number of mixture components automatically. We employ the Kullback–Leibler (KL) divergence and the mean-square-error (MSE) criteria to demonstrate that the proposed distribution provides both high accuracy and low computational complexity. Additionally, we provide closed-form expressions or approximations for several performance metrics used in wireless communication systems, including the moment generating function (MGF), the raw moments, the amount of fading (AF), the outage probability, the average channel capacity, and the probability of energy detection for cognitive radio (CR). Numerical analysis and Monte Carlo simulation results are presented to corroborate the analytical results. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate the performance of cooperative spectrum sensing (CSS) with multiple-antenna nodes over generalized and composite fading channels. To this end, we approximate the probability density function (pdf) of the signal-to-noise ratio (SNR) of various fading channels using the mixture Gamma (MG) distribution. Based on this, we derive an exact closed-form expression and a generic infinite series representation for the corresponding probability of energy detection, along with a finite upper bound for the involved truncation error. Both expressions have a relatively simple algebraic form that gives them convenience in handling both analytically and numerically. Furthermore, the composite effect of multipath fading and shadowing scenarios in CSS is mitigated by applying an optimal fusion rule that minimizes the total error rate (TER), where the optimal number of nodes is derived under the Bayesian criterion, assuming erroneous feedback channels. We also extend the derived average detection probability to include diversity reception techniques, namely, maximal-ratio combining, square-law combining, and square-law selection (SLS). For the SLS, we demonstrate the existence of an error rate floor as the number of antennas of the cognitive radio nodes increases in erroneous decision feedback channels. Accordingly, we derive the optimal rule for the number of antennas that minimizes the TER in the SLS framework. Monte Carlo simulations are presented to corroborate the analytical results and to provide illustrative performance comparisons and insights between different composite fading channels. [ABSTRACT FROM PUBLISHER]

In this paper, we consider underlay cognitive radio (CR) networks with one primary receiver, one cognitive transmitter–receiver pair, and one energy harvesting relay. The transmission power of the secondary source is opportunistically determined by its interference to the primary receiver, and the relay transmission is powered by the energy harvested from the radio-frequency observations at the relay. For the considered CR networks with simultaneous wireless information and power transfer (SWIPT), we derive analytical expressions for the outage probability, as well as their high signal-to-noise ratio (SNR) approximations in closed form. The developed analytical results demonstrate that the use of SWIPT will not cause any loss in diversity gain, but the outage probability achieved by the SWIPT-CR scheme asymptotically decays as \log \mboxSNR/\mboxSNR, whereas a decaying rate of \mbox1/\mboxSNR is achieved by a conventional CR network. Computer simulation results are also provided to demonstrate the accuracy of the presented analysis. [ABSTRACT FROM AUTHOR]

In this paper, a downlink single-cell non-orthogonal multiple access (NOMA) network with uniformly deployed users is considered and an analytical framework to evaluate its performance is developed. Particularly, the performance of NOMA is studied by assuming two types of partial channel state information (CSI). For the first one, which is based on imperfect CSI, we present a simple closed-form approximation for the outage probability and the average sum rate, as well as their high signal-to-noise ratio (SNR) expressions. For the second type of CSI, which is based on second order statistics (SOS), we derive a closed-form expression for the outage probability and an approximate expression for the average sum rate for the special case two users. For the addressed scenario with the two types of partial CSI, the results demonstrate that NOMA can achieve superior performance compared to the traditional orthogonal multiple access (OMA). Moreover, SOS-based NOMA always achieves better performance than that with imperfect CSI, while it can achieve similar performance to the NOMA with perfect CSI at the low SNR region. The provided numerical results confirm that the derived expressions for the outage probability and the average sum rate match well with the Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Cognitive radio has emerged as an essential recipe for future high-capacity, high-coverage multitier hierarchical networks. Securing data transmission in these networks is of the utmost importance. In this paper, we consider the cognitive wiretap channel and propose multiple antennas to secure the transmission at the physical layer, where the eavesdropper overhears the transmission from the secondary transmitter to the secondary receiver. The secondary receiver and the eavesdropper are equipped with multiple antennas, and passive eavesdropping is considered where the channel state information (CSI) of the eavesdropper's channel is not available at the secondary transmitter. We present new closed-form expressions for the exact and asymptotic secrecy outage probability. Our results reveal the impact of the primary network on the secondary network in the presence of a multiantenna wiretap channel. [ABSTRACT FROM PUBLISHER]

In this paper, an energy harvesting dual-hop relaying system without/with the presence of co-channel interference (CCI) is investigated. Specifically, the energy constrained multi-antenna relay node is powered by either the information signal of the source or via the signal receiving from both the source and interferer. In particular, we first study the outage probability and ergodic capacity of an interference free system, and then extend the analysis to an interfering environment. To exploit the benefit of multiple antennas, three different linear processing schemes are investigated, namely, 1) Maximum ratio combining/maximum ratio transmission (MRC/MRT), 2) Zero-forcing/MRT (ZF/MRT) and 3) Minimum mean-square error/MRT (MMSE/MRT). For all schemes, both the systems outage probability and ergodic capacity are studied, and the achievable diversity order is also presented. In addition, the optimal power splitting ratio minimizing the outage probability is characterized. Our results show that the implementation of multiple antennas increases the energy harvesting capability, hence, significantly improves the systems performance. Moreover, it is demonstrated that the CCI could be potentially exploited to substantially boost the performance, while the choice of a linear processing scheme plays a critical role in determining how much gain could be extracted from the CCI. [ABSTRACT FROM PUBLISHER]

This paper proposes relay selection to increase the physical layer security in multiuser cooperative relay networks with multiple amplify-and-forward relays, in the presence of multiple eavesdroppers. To strengthen the network security against eavesdropping attack, we present three criteria to select the best relay and user pair. Specifically, criteria I and II study the received signal-to-noise ratio (SNR) at the receivers, and perform the selection by maximizing the SNR ratio of the user to the eavesdroppers. To this end, criterion I relies on both the main and eavesdropper links, while criterion II relies on the main links only. Criterion III is the standard max-min selection criterion, which maximizes the minimum of the dual-hop channel gains of main links. For the three selection criteria, we examine the system secrecy performance by deriving the analytical expressions for the secrecy outage probability. We also derive the asymptotic analysis for the secrecy outage probability with high main-to-eavesdropper ratio. From the asymptotic analysis, an interesting observation is reached: for each criterion, the system diversity order is equivalent to the number of relays regardless of the number of users and eavesdroppers. [ABSTRACT FROM AUTHOR]

In this paper, the outage behavior of dual-hop multiuser multirelay cognitive radio networks under spectrum-sharing constraints is investigated. In the proposed cognitive radio network, the secondary network is composed of one secondary-user (SU) source that communicates with one out of L destinations through a direct link and also via the help of one out of N relays by using an efficient relay–destination selection scheme. Additionally, a selection combining (SC) scheme to select the best link (direct or dual-hop link) from the SU source is employed at the selected SU destination. Adopting an underlay approach, the SU communication is performed accounting for an interference constraint, where the overall transmit power is governed by the interference at the primary-user (PU) receiver, as well as by the maximum transmission power available at the respective nodes. Closed-form expressions for the outage probability are derived, from which an asymptotic analysis reveals that the diversity order of the considered system is not affected by the interference and is equal to N + L for both decode-and-forward (DF) and amplify-and-forward (AF) relaying protocols. The analytical results are corroborated by Monte Carlo simulations, and insightful discussions are provided. [ABSTRACT FROM PUBLISHER]

We study the effect of outdated channel state information on the outage and error rate performance of amplify-and-forward (AF) relay selection, where only one out of the set of available relays is activated. We consider two variations of AF relay selection, namely best relay selection and partial relay selection, when the selection is based upon outdated channel estimates. For both these variations, closed-form expressions for the outage probability are obtained, along with approximate expressions for the symbol error rate in the medium to high signal-to-noise-ratio (SNR) regime. The diversity gain and coding gain of the above schemes are also explicitly derived. Numerical results manifest that the outage performance of AF relay selection is highly dependent on the level of correlation between the actual channel conditions and their corresponding (outdated) estimates. This result has a significant impact on the deployment of relay selection in practical applications, implying that a high feedback rate may be required in practice in order to attain the full benefits of relay selection. It is further shown that it may be preferable, in terms of outage and symbol error probability, not to include links in the relay selection process that experience a sufficiently high maximum Doppler shift, since in those cases partial relay selection outperforms best relay selection. [ABSTRACT FROM AUTHOR]

This paper provides a systematic characterization of Zero-Forcing (ZF) detectors over multiple-input multiple-output (MIMO) channels that experience both small and large-scale fading. In particular, we consider the generic K distribution (Rayleigh/gamma distribution) to model the composite fading fluctuations and also assume the general case of semi-correlated small-scale fading. In the following, novel exact analytical expressions for the achievable sum rate are derived, followed by asymptotic expressions in the high and low Signal-to-Noise ratio (SNR) regimes. In these limiting cases, two common and insightful affine expansions are studied followed by new, closed-form upper and lower bounds on the sum rate that remain tight for all SNRs. In the second part of the paper, we present exact tractable expressions along with first-order expansions for the symbol error rate (SER) and outage probability; we also quantify the performance of ZF detectors in terms of diversity order and array (or coding) gain. The implications of the model parameters on the ZF detector performance are investigated via Monte-Carlo simulations which also validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper provides a systematic mutual information (MI) and multichannel beamforming (MBF) characterization of optimized multiple-input multiple-output (MIMO) communication systems operating in Ricean fading. These optimized configurations are of high practical importance since, contrary to the common belief, benefit from the presence of direct Line-of-Sight (LoS) components and deliver maximum multiplexing gains, by deploying specifically designed antenna arrays at both ends. In the following, using elements from random matrix theory, novel analytical expressions are derived for the exact and asymptotic MI statistics while the prevalent Gaussian approximation is examined. Moreover, new explicit expressions for the marginal eigenvalues are deduced which are thereafter used to analyze the BF performance of the associated eigenmodes in terms of Signal-to-Noise ratio (SNR) outage probability. We note that all derived formulas are given in tractable determinant form and therefore allow for fast and efficient computation and also yield an excellent match with Monte-Carlo simulations, under different fading scenarios and model parameters. [ABSTRACT FROM PUBLISHER]

In selective cooperative relaying only a single relay out of the set of available relays is activated, hence the available power and bandwidth resources are efficiently utilized. However, implementing selective cooperative relaying in time-varying channels may cause frequent relay switchings that deteriorate the overall performance. In this paper, we study the rate at which a relay switching occurs in selective cooperative relaying applications in time-varying fading channels. In particular, we derive closed-form expressions for the relay switching rate (measured in Hz) for opportunistic relaying (OR) and distributed switch and stay combining (DSSC). Additionally, expressions for the average relay activation time for both of the considered schemes are also provided, reflecting the average time that a selected relay remains active until a switching occurs. Numerical results manifest that DSSC yields considerably lower relay switching rates than OR, along with larger average relay activation times, rendering it a better candidate for implementation of relay selection in fast fading environments. [ABSTRACT FROM PUBLISHER]

N/A [ABSTRACT FROM PUBLISHER]