Based on its key performance indicators (KPIs), the sixth generation (6G) networks are expected to be quite challenging in terms of security due to the ever-growing demand for virtualization. Therefore, to increase the communication secrecy in a distributed 6G network, we propose an intelligent zero trust framework that protects the radio access network (RAN) from threats. To comply with such a distributed network, the proposed zero trust model accommodates the secrecy modules in different nodes, such as the base station, the core network, and the cloud, to monitor the network while executing a hierarchical and distributed threat detection. Thereby, the distributed modules collaborate to efficiently decide and act against the suspected RAN threats. Our simulation results show that our zero trust framework based on a distributed security modules is robust against the attacks, while exhibiting low detection time and low false positive.

Next generation networks will support diverse use cases that require higher flexibility in the resource allocation. To this end, we propose a new waveform referred to as Lagrange Vandermonde division multiplexing (LVDM) that generalizes the zero padding orthogonal frequency division multiplexing (ZP-OFDM) while guaranteeing the perfect recovery of the transmitted signal. The LVDM transceiver design relies on the signature roots that have been judiciously selected to 1) provide a flexible resource allocation and 2) keep the complexity low where one-tap equalization is adopted. Carrying out the simulations in both frequency selective and 3GPP channels, our proposed waveform outperforms the ZP-OFDM where performance results are shown in terms of bit error ratio (BER). The LVDM achieves a signal-to-noise ratio gain of 5 dB over the ZP-OFDM when applying the optimized power allocation over subcarriers.

Lagrange Vandermonde division multiplexing (LVDM), that generalizes yet outperforms the zero padding orthogonal frequency division multiplexing (ZP-OFDM) while guaranteeing the perfect recovery of the transmitted signal, has been proposed recently as a new waveform. Therein, the transceiver design relies on the signature roots that have been optimized to keep the implementation complexity low where a simple one-tap equalization has been performed in frequency selective channels. While the beyond 5G solutions ought to be designed to overcome the intercarrier interference in high mobility, a new advanced receiver for LVDM that deals with doubly selective channels is needed. We propose two new schemes using time-frequency domain equalization where performance and implementation complexity have been discussed for each. Simulation results show that both schemes drastically boost the performance while keeping the complexity low compared to the state of the art solutions.

Reliability and latency challenges in future mixed sub-6 GHz/millimeter wave (mmWave) fifth generation (5G) cell-free massive multiple-input multiple-output (MIMO) networks is to guarantee a fast radio resource management in both uplink (UL) and downlink (DL), while tackling the corresponding propagation imbalance that may arise in blockage situations. In this context, we introduce a semi-blind UL/DL decoupling concept where, after its initial activation, the central processing unit (CPU) gathers measurements of the Rician K-factor-reflecting the line-of-sight (LOS) condition of the user equipment (UE)-as well as the DL reference signal receive power (RSRP) for both 2.6 GHz and 28 GHz frequency bands, and then train a non-linear support vector machine (SVM) algorithm. The CPU finally stops the measurements of mmWave definitely, and apply the trained SVM algorithm on the 2.6 GHz data to blindly predict the target frequencies and access points (APs) that can be independently used for the UL and DL. The accuracy score of the proposed classifier reaches 95% for few training samples.

In a cost-effective implementation, we address the problem of cross-interference in cognitive multiple-input multiple-output (MIMO) spectrum sharing where mutual cochannel interference (CCI) between the primary and secondary systems is of major concern. First, we propose an optimal transmit-antenna selection (TAS) strategy that rather maximizes the received signal-to-interference-plus-noise ratio (SINR) after maximum-ratio combining (MRC). We statistically analyze the combined SINR structure and derive new expressions of its cumulative and probability distribution functions. Interestingly, we accurately show the substantial gains offered by the proposed TAS strategy in terms of the secondary system outage performance compared to the existent state-of-the-art TAS strategies. Finally, our analytical results are validated by simulations and some important insights are also underlined.

In this paper, we address the issue of automatic tracking areas (TAs) planning in fifth generation (5G) ultra-dense networks (UDNs). By invoking handover (HO) attempts and measurement reports (MRs) statistics of a 4G live network, we first introduce a new kernel function mapping HO attempts, MRs and inter-site distances (ISDs) into the so-called similarity weight. The corresponding matrix is then fed to a self-tuning spectral clustering (STSC) algorithm to automatically define the TAs number and borders. After evaluating its performance in terms of the Q-metric as well as the silhouette score for various kernel parameters, we show that the clustering scheme yields a significant reduction of tracking area updates and average paging requests per TA; optimizing thereby network resources.

Due to increasing quality-of-service (QoS) demand in already congested radio spectrum, there is a need for designing energy-efficient free space optical (FSO) communication networks. Considering a realistic fading model incorporating the fluctuations in angle-of-arrival, we minimize the outage probability for error free transmission of high data volumes through optimizing the power allocation (PA) and relay placement (RP) in a dual-hop decode-and-forward (DF) relay-assisted cooperative FSO communication with coherent detection and direct link unavailability. As this problem is nonconvex, first the optimal PA between source and relay is obtained using a global optimization algorithm. Also, a closed form for the solution is obtained using a tight analytical approximation with the assumption that atmospheric turbulence over both the links is nearly same. Next, we optimize the RP followed by the outage probability is jointly minimized using alternating optimization algorithm. Numerical results validate the outage analysis and provide key insights on optimal PA and RP yielding an outage enhancement of around 37% over the benchmark scheme.

Multiple‐input multiple‐output is being considered as a highly interesting topic for the new generation of wireless communication systems. The major challenge is how to enhance the link reliability while neither affecting the spectral efficiency nor the quality of service. To reduce the complexity of the maximum likelihood (ML) receiver, efforts have been focused on zero forcing (ZF) decoder using lattice reduction‐aided (LRA) techniques. In this study, the authors propose a method that improves performance using the LRA‐based precoding technique without increasing the decoder complexity. Their approach is to exploit the channel matrix to generate matrices for both the precoding and LRA detection technique. This makes it possible to achieve maximum orthogonality for the generated channel matrices while reducing considerably the noise effect. They focus on the use of the LRA‐ZF for the ease of its implementation and low complexity. The proposed method outperforms LRA‐minimum mean square error‐based receiver and gets closer to ML receiver performance. The performance and complexity of the proposed scheme are discussed.

In this paper, we investigate a trade-off between energy consumption and physical layer security in a dual-hop untrusted multiple amplify-and-forward relaying network. Considering the presence of the direct link, we propose an adaptive switching scheme between the direct and the relaying modes. Since relay nodes are energy-constrained, the direct link is prioritized. With outdated channel state information (CSI), to save energy while forwarding the source information, we adopt a greedy relay selection scheme and derive a tight upper bound on the secrecy outage probability. We quantify the impact of outdated CSI on the secrecy performance and the secrecy gains of our proposed mode switching scheme over its fixed transmission counterparts.

This paper considers a massive MIMO network that includes one multiple-antenna base station, one multiple-antenna eavesdropper, and K single-antenna users. The eavesdropper operates in passive mode and tries to overhear the confidential information from one of the users in the down-link transmission. In order to secure the confidential information, two artificial noise (AN)-aiding schemes are proposed. In the first scheme, AN is injected into the downlink training signals to prevent the eavesdropper from obtaining the correct channel state information of the eavesdropping link. In the second scheme, AN is deployed in both downlink training phase and payload data transmission phase to further degrade the eavesdropping channel. Analytical expressions and tight approximations of the achievable secrecy rate of the considered systems are derived with taking imperfect channel estimation and two types of precoding, i.e., maximum-ratio-transmission and zero-forcing, into consideration. Optimization algorithms for power allocation are proposed to enhance the secrecy performance of the proposed AN-aiding schemes. The results reveal that deploying AN in the downlink training phase of massive MIMO networks does not affect the downlink channel estimation process at users while enabling the system to suppress the downlink channel estimation process at eavesdropper. As a consequence, the proposed AN-aided schemes improve the system performance significantly. Furthermore, implementing AN in both phases allows the considered system having a flexible solution to maximize its secrecy performance at the price of higher complexity.

This paper studies cooperative transmission for securing a two‐hop network where the eavesdropper can wiretap the relay channels. With outdated channel state information (CSI), we propose an opportunistic relaying with destination‐based jamming scheme, where one relay is chosen to forward the confidential signal and the destination sends jamming signal to confuse the eavesdropper. To reveal the capability of the proposed scheme in improving the system secrecy, we investigate the two common performance metrics under different assumptions on the eavesdropper CSI. First, lower bound on the ergodic secrecy capacity (ESC) is derived assuming perfect knowledge of the eavesdropper channel gains. We, then, extend our results to the scenario where only the legitimate channel gain is known. Here, lower bound on the secrecy outage probability (SOP) is obtained. To further improve the utility of the destination, under total power constraint, a power allocation between data signal and jamming noise is considered. Both the analysis and simulation results indicate that the system performance is significantly improved in terms of both ESC and SOP when the jamming power at the destination is adjusted according to the selected relay position. We verify that, with outdated CSI, the secrecy performance of opportunistic relaying is severely degraded.

This paper studies the secrecy performance of an untrusted amplify-and-forward relaying network where both the relaying and the direct links are used to convey the source's information. Since relays act as eavesdroppers, a source-based jamming technique is proposed to keep the source's message secret from these helper nodes. We analyze the secrecy capacity of partial relay selection based on outdated channel state information. The study is conducted analytically by deriving the lower-bound expression for the ergodic secrecy capacity (ESC). We demonstrate through the simulation results the tightness of the derived bound. The use of the direct link is shown to improve the secrecy performance of the proposed scheme. In this case, we found that ESC performance with maximum ratio combining at the destination is better than that with selection combining. The system performance worsens as the correlation coefficient decreases. Furthermore, the maximum of ESC is achieved when relays are in the middle between the source and the destination with equal power allocation between the information and the jamming signals at the source.

We study a relay and user selection scheme to improve the secrecy performance of cooperative multi-user multiple amplify-and-forward (AF) relay network. Nevertheless, relay transmission is highly susceptible to eavesdropping since confidential information is broadcasted twice, i.e., by the source and the relay. In particular, we propose a cooperative jamming aided secure transmission through the relay path. Furthermore, with the presence of the direct path, a transmission mode selection scheme is elaborated in such a way to reduce the amount of information leaked to non-selected users in the system, who may overhear the source message as eavesdroppers. Specifically, the selected relay is only activated when it has better secrecy performance than the source. We study the impact of outdated channel state information (CSI) on the system by deriving a closed-form lower bound for the secrecy outage probability (SOP). By investigating the effect of the jamming power level and the correlation coefficients imbalance, a comparison reveals that the proposed scheme outperforms the fixed transmission modes and its signal-to-noise ratio based counterpart. It is shown that the secrecy outage probability can be decreased as the number of destinations grows unless the number of relays is small. Importantly, optimal relay location and jamming power allocation significantly enhance the secrecy performance.

In a spectrum-sharing underlay environment, we carry out a comprehensive outage probability analysis for a secondary (unlicensed) system operating with a limited feedback from the primary (licensed) system. We focus on single-user single- input multiple-output (SIMO) secondary communications where the direct link is being assisted by a set of single-antenna decode-and- forward (DF) relay nodes acting in a halfduplex selective-and-incremental relaying mode. For practicality reasons, we consider that the secondary system transmitting nodes are only capable of acquiring a limited interference channel state information (I-CSI) of the underlying cognitive radio interference channels. Accordingly, the cumulative distribution functions (CDF) of the received signal-to-noise ratio (SNR) at the secondary receiving nodes are devised in recursive and tractable closed-form expressions. These statistics are then used to derive the exact end-to-end secondary system outage probability. Also, we investigate the asymptotic outage analysis of the secondary network, and derive the achievable diversity-and-multiplexing tradeoff (DMT). Finally, the analytical and simulation results are compared and interestingly shown to perfectly match while revealing that limited I-CSI acquisition by the secondary system is more favored than its spectrum-consuming counterpart.

We investigate the spectrum sharing possibility as an incentive to enhancing the physical layer security. The concept behind this is that a legitimate source-destination pair, communicating in the presence of a passive eavesdropper, stimulates the help of another source-destination node looking for transmission opportunities, referred to as secondary network (SN). We thus propose a cooperative scheme, whereby the secondary transmitter will act as a friendly jammer to confound the eavesdropper and be granted to share the spectrum of the legitimate pair, referred to as primary network (PN), as a reward. To evaluate the performance of our cooperative scheme, we derive the closed-form expression of the secrecy outage probability (SOP). However, to assess its usefulness, we introduce the mutual outage probability (MOP) and we carry out analysis to derive its expression. We show that a satisfactory SOP is a necessary condition for triggering the cooperation, whereas a satisfactory MOP is a sufficient condition to guarantee that the PN-SN collaboration leads to a win-win situation. Results have been presented considering outdated channel state information, thus giving more insights for practical setups.

In this paper, we investigate the spectrum sharing incentive to enhance the physical layer security. The concept behind is that a legitimate source- destination pair, communicating in the presence of a passive eavesdropper, stimulates the help of another source-destination nodes looking for transmission opportunities, referred to as secondary network (SN). We thus propose a cooperative scheme, whereby the secondary transmitter will act as a friendly jammer to confound the eavesdropper and be granted to share the spectrum of the legitimate pair, referred to as primary network (PN), as a reward. To evaluate the performance of our cooperative scheme, we derive the closed form expression of the secrecy outage probability (SOP). However, to assess its usefulness, we introduce the mutual outage probability (MOP) and we carry out analysis to derive its expression. We show that a satisfactory SOP is a necessary condition for triggering the cooperation, whereas a satisfactory MOP is a sufficient condition to guaranty that the PN-SN collaboration leads to a win-win situation.

In a network with multiple relays, relay selection has been shown as an effective scheme to achieve diversity as well as to improve the overall throughput. This paper studies the impact of using outdated channel state information for relay selection on the performance of a network where two sources communicate with each other via fixed-gain amplify-and-forward relays. For a Rayleigh faded channel, closed-form expressions for the outage probability, moment generating function and symbol error rate are derived. Simulations results are also presented to corroborate the derived analytical results. It is shown that adding relays does not improve the performance if the channel is substantially outdated. Furthermore, relay location is also taken into consideration and it is shown that the performance can be improved by placing the relay closer to the source whose channel is more outdated.

In a spectrum sharing relay-aided secondary network, the design objective shifts from minimizing the violation of the primary system constraints when using the outdated channel state information (CSI), to making the best use of the available (imperfect) channel gain while maintaining the primary system Quality of Service (QoS). To address this trend, we propose a new power allocation based on a linear regression model and we carry out the outage analysis as well as the asymptotical analysis where we deduce the diversity order of the secondary network. The performance results show that the proposed power allocation strategy outperforms the existing schemes in the literature and provides an accurate tight approximation of the ideal case under perfect CSI while respecting the primary system QoS and using outdated CSI.

Abstract Consider a multi‐user underlay cognitive network where multiple cognitive users concurrently share the spectrum with a primary network with multiple users. The channel between the secondary network is assumed to have independent but not identical Nakagami‐m fading. The interference channel between the secondary users (SUs) and the primary users is assumed to have Rayleigh fading. A power allocation based on the instantaneous channel state information is derived when a peak interference power constraint is imposed on the secondary network in addition to the limited peak transmit power of each SU. The uplink scenario is considered where a single SU is selected for transmission. This opportunistic selection depends on the transmission channel power gain and the interference channel power gain as well as the power allocation policy adopted at the users. Exact closed form expressions for the moment‐generating function, outage performance, symbol error rate performance, and the ergodic capacity are derived. Numerical results corroborate the derived analytical results. The performance is also studied in the asymptotic regimes, and the generalized diversity gain of this scheduling scheme is derived. It is shown that when the interference channel is deeply faded and the peak transmit power constraint is relaxed, the scheduling scheme achieves full diversity and that increasing the number of primary users does not impact the diversity order. Copyright © 2014 John Wiley & Sons, Ltd.

Opportunistic relaying in cooperative communication depends on careful relay selection. However, the traditional centralized method used for opportunistic amplify-and-forward protocols requires precise measurements of channel state information at the destination. In this paper, we adopt the max-min criterion as a relay selection framework for opportunistic amplify-and-forward cooperative communications, which was exhaustively used for the decode-and-forward protocol, and offer an accurate performance analysis based on exact statistics of the local signal-to-noise ratios of the best relay. Furthermore, we evaluate the asymptotical performance and deduce the diversity order of our proposed scheme. Finally, we validate our analysis by showing that performance simulation results coincide with our analytical results over Rayleigh fading channels, and we compare the max-min relay selection with their centralized channel state information-based and partial relay selection counterparts.

In this paper, we carry out an exact outage analysis for a secondary (unlicensed) system operating under a strict primary (licensed) system outage constraint. We focus on single-user single-input multiple-output (SIMO) secondary communications where the direct link is being assisted by a cluster of single-antenna decode-and-forward (DF) relay nodes acting in a half-duplex selective-and-incremental relaying mode. Firstly, we derive a transmit power model for the secondary system where the source and relays adapt their transmit power based on: 1) a perfect acquisition of the underlying interference channel state information (I-CSI), and 2) an interference constraint that is either fixed or proportional to the primary system outage probability. Secondly, the cumulative distribution functions (CDF)s of the received signal-to-noise ratio (SNR) at the secondary receiving nodes are devised in a recursive and tractable closed-form expressions. These statistics are used to derive the exact end-to-end secondary system outage probability. The analytical and simulation results are then compared and interestingly shown to perfectly match, while revealing that with a moderate number of primary and secondary receive antennas, the secondary system spectral efficiency is amply enhanced as opposed to being severely degraded in the single receive antenna case.

Consider a multi-user underlay cognitive network where multiple cognitive users, having limited peak transmit power, concurrently share the spectrum with a primary network with multiple users. The channel between the secondary network is assumed to have independent but not identical Nakagami-m fading. The interference channel between the secondary users and the primary users is assumed to have Rayleigh fading. The uplink scenario is considered where a single secondary user is selected for transmission. This opportunistic selection depends on the transmission channel power gain and the interference channel power gain as well as the power allocation policy adopted at the users. Exact closed form expressions for the momentgenerating function, outage performance and the symbol-error-rate performance are derived. The outage performance is also studied in the asymptotic regimes and the generalized diversity gain of this scheduling scheme is derived. Numerical results corroborate the derived analytical results.

In this paper, we investigate the impact of using outdated channel state information for incremental opportunistic relay selection in a spectrum sharing cognitive network. In our work, we present the outage performance of two schemes where the power allocation is based on the instantaneous outdated channel state information and the statistical knowledge of the interference links, referred to as CSI-based scheme and LF-based scheme, respectively. By investigating the effect of the rank of the selected relay and the correlation coefficients imbalance on outage performance, a comparison reveals that LF-based scheme outperforms its CSI-based counterpart. We validate our analysis with simulation results in a Rayleigh fading environment.

We consider in this paper, a spectrum sharing cognitive radio system with a ratio selection scheme; where one out of N independent-and-identically-distributed transmit antennas is selected such that the ratio of the secondary transmitter (ST) to the secondary receiver (SR) channel gain to the interference from the ST to the primary receiver (PR) channel gain is maximized. Although previous works considered perfect, outdated, or partial channel state information at the transmitter, we stress that using such assumptions may lead to a feedback overhead for updating the SR with the ST-PR interference channel estimation. Considering only statistical knowledge of the ST-PR channel gain, we investigate a ratio selection scheme using a mean value (MV)-based power allocation strategy referred to as MV-based scheme. We first provide the exact statistics in terms of probability density function and cumulative distribution function of the secondary channel gain as well as of the interference channel gain. Furthermore, we derive exact cumulative density function of the received signal-to-noise ratio at the SR where the ST uses a power allocation based on instantaneous perfect channel state information (CSI) referred to as CSI-based scheme. These statistics are then used to derive exact closed form expressions of the outage probability, symbol error rate, and ergodic capacity of the secondary system when the interference channel from the primary transmitter (PT) to the SR is ignored. Furthermore, an asymptotical analysis is also carried out for the MV-based scheme as well as for the CSI-based scheme to derive the generalized diversity gain for each. Subsequently, we address the performance analysis based on exact statistics of the combined signal-to-interference-plus-noise ratio at the SR of the more challenging case; when the PT-SR interference channel is considered. Numerical results in a Rayleigh fading environment manifest that the MV-based scheme outperforms the CSI-based scheme provided that a low interference power constraint is deployed, implying that the MV-based scheme is more suitable for practical systems.

Having multiple secondary users (SUs) can be exploited to achieve multiuser diversity and improve the throughput of the underlay secondary network. In the cognitive setting, satisfying the interference constraint is essential, and thus, a scheduling scheme is considered where some SUs are preselected based on the low interference power. From this subset, the SU that yields the highest throughput is selected for transmission. This scheduling scheme helps to lower the interference power while giving good throughput. For an independent but not identically distributed Nakagami-m fading channel, we obtain exact closed-form expressions of the capacity of this scheduling scheme. Furthermore, the scheduling time of SUs is characterized and closed-form expressions for the mean time after which a SU is selected for transmission are obtained. Numerical simulations are performed to corroborate the derived analytical results. Our results show that at low interference threshold, increasing transmit power of the SUs is not beneficial and results in reduced capacity. Furthermore, the channel idle time (i.e. time that no user is utilizing the channel) reduces with increasing the number of SUs.

In this paper, we consider a spectrum sharing cognitive radio system with ratio selection using a mean value-based power allocation strategy. We first provide the exact statistics in terms of probability density function and cumulative density function of the secondary channel gain as well as of the interference channel gain. These statistics are then used to derive exact closed form expression of the secondary outage probability. Furthermore, asymptotical analysis is derived and generalized diversity gain is deduced. We validate our analysis with simulation results in a Rayleigh fading environment.

ABSTRACT Mobile users with single antennas can still take advantage of spatial diversity through cooperative space–time‐encoded transmission. In this paper, we considered a scheme in which a relay chooses to cooperate only if its source–relay channel is of an acceptable quality, and we evaluate the usefulness of relaying when the source acts blindly and ignores the decision of the relays whether they may cooperate or not. In our study, we consider the regenerative relays in which the decisions to cooperate are based on a targeted end‐to‐end data rateR. We derived the end‐to‐end outage probability for a transmission rateRand a code rateρand look at a power allocation strategy between the source and the relays in order to minimize the end‐to‐end outage probability at the destination for high signal‐to‐noise ratio, by using the golden section search method. Performance results show that the computer simulations‐based results coincide with our analytical results. Copyright © 2011 John Wiley & Sons, Ltd.

Cooperative relay technology was recently introduced into cognitive radio networks in order to enhance network capacity, scalability, and reliability of end-to-end communication. In this paper, we investigate an underlay cognitive network where the quality of service of the secondary link is maintained by triggering an opportunistic regenerative relaying once it falls under an unacceptable level. We first provide the exact cumulative density function (CDF) of received signal-to-noise (SNR) over each hop with co-located relays. Then, the CDFs are used to determine very accurate closed-form expression for the outage probability for a transmission rate R. We validate our analysis by showing that simulation results coincide with our analytical results in Rayleigh fading channels.

In a spectrum sharing underlay context, we investigate the exact derivation of the outage probability for relay-aided cognitive radio communications. To give more degrees of freedom to the secondary system in acquiring a targeted quality-of-service (QoS) under the primary system interference constraint, the secondary link is assisted by a set of relays acting in a two-hop decode-and-forward selective relaying mode. By means of the cumulative distribution functions of the received signal-to-noise ratio (SNR) at the secondary receiver, we derive the end-to-end outage probability of the secondary system in its closed form. The analytical and simulation results are then compared and interestingly shown to perfectly match over the entire interference threshold region.

In this paper, we investigate an opportunistic relaying scheme where the selected relay assists the source-destination (direct) communication. In our study, we consider a regenerative opportunistic relaying scheme in which the direct path may be considered unusable, and the destination may use a selection combining technique. We first derive the exact statistics of each hop, in terms of probability density function (PDF). Then, the PDFs are used to determine accurate closed form expressions for end-to-end outage probability for a transmission rate R. Furthermore, we evaluate the asymptotical performance analysis and the diversity order is deduced. Finally, we validate our analysis by showing that performance simulation results coincide with our analytical results over different network architectures.

In this paper, we investigate an opportunistic relaying scheme where the selected relay assists the source-destination (direct) communication. In our study, we consider a regenerative opportunistic relaying scheme in which the direct path can be considered unusable, and takes into account the effect of the possible erroneously detected and transmitted data at the best relay. We first derive the exact statistics of each hop, in terms of probability density function (PDF). Then, the PDFs are used to determine accurate closed form expressions for end-to-end bit-error rate (BER) of binary phase-shift keying (BPSK) modulation where the detector may use maximum ration combining (MRC) or selection combining (SC). Finally, we validate our analysis by showing that performance simulation results coincide with our analytical results over linear network (LN) architecture and considering Rayleigh fading channels.

Opportunistic relaying in cooperative communication depends on careful relay selection. However the traditional centralized method used for opportunistic Amplify-and-Forward (OAF) protocols requires precise measurements of channel state information (CSI) at the destination (CSI-OAF). In this paper, we propose a distributed relay selection framework for opportunistic AF (D-OAF) cooperative communications and offer an accurate performance analysis based on exact statistics of the local signal to-noise ratios of the best relay. Furthermore, we validate our analysis by showing that performance simulation results coincide with our analytical results over different network architectures, and we compare our new results with their CSI-based counter parts.

In this paper, we investigate an incremental opportunistic relaying scheme where the selected relay chooses to cooperate only if the source-destination channel is of an unacceptable quality. In our study, we consider regenerative relaying in which the decision to cooperate is based on a signal-to-noise ratio (SNR) threshold and takes into account the effect of the possible erroneously detected and transmitted data at the best relay. We derive a closed-form expression for the end-to-end bit-error rate (BER) of binary phase-shift keying (BPSK) modulation based on the exact probability density function (PDF) of each hop. Furthermore, we evaluate the asymptotic error performance and the diversity order is deduced. We show that performance simulation results coincide with our analytical results.

In this paper, we investigate a dual-hop decode-and-forward opportunistic relaying scheme where the selected relay chooses to cooperate only if the source-destination channel is of an unacceptable quality. We first derive the exact statistics of received signal-to-noise (SNR) over each hop with co-located relays, in terms of probability density function (PDF). Then, the PDFs are used to determine very accurate closed-form expression for the outage probability for a transmission rate R. Furthermore, we perform asymptotic analysis and we deduce the diversity order of the scheme. We validate our analysis by showing that performance simulation results coincide with our analytical results over different network architectures.

We consider a multiple access MAC fading channel with two users communicating with a common destination, where each user mutually acts as a relay for the other one as well as wishes to transmit his own information as opposed to having dedicated relays. We wish to evaluate the usefulness of relaying from the point of view of the system's throughput (sum rate) rather than from the sole point of view of the user benefiting from the cooperation as is typically done. We do this by allowing a trade-off between relaying and fresh data transmission through a resource allocation framework. We develop capacity expressions for our scheme and derive the rate-optimum power allocation in closed form for distributed frameworks. In the distributed scenario, partially statistical and partially instantaneous channel information is exploited. Our results indicate that the sum rate is maximized when both mobiles act selfishly.

Mobile users with single antennas can still take advantage of spatial diversity through cooperative space-time encoded transmission. In this paper, we consider a scheme in which a relay chooses to cooperate only if its source-relay channel is of an acceptable quality and we evaluate the usefulness of relaying when the source acts blindly and ignores the decision of the relays whether they may cooperate or not. In our study, we consider the regenerative relays in which the decisions to cooperate are based on a signal-to-noise ratio (SNR) threshold and consider the impact of the possible erroneously detected and transmitted data at the relays. We derive the end-to-end bit-error rate (BER) expression and its approximation for binary phase-shift keying modulation and look at two power allocation strategies between the source and the relays in order to minimize the end-to-end BER at the destination for high SNR. Some selected performance results show that computer simulations based results coincide well with our analytical results.

Mobile users with single antennas can still take advantage of spatial diversity through cooperative space-time encoded transmission. In this paper, we consider a scheme in which the relay chooses to cooperate only if the source-relay channel is of an acceptable quality and we evaluate the usefulness of relaying when the source acts blindly and ignores the decision of the relay whether it may cooperate or not. In our study, we consider a regenerative relay in which the decision to cooperate is based on a signal-to-noise ratio (SNR) threshold and consider the impact of the possible erroneously detected and transmitted data at the relay. We derive the end-to-end bit-error rate (BER) for binary phase-shift keying modulation and look at two power allocation strategies between the source and the relay in order to minimize the end-to-end BER at the destination for high SNR. Some selected performance results show that computer simulations based results coincide with our analytical results.

Mobile users with single antennas can still take advantage of spatial diversity through repetition based cooperative transmission. In this paper, we consider a scheme in which the relay chooses to cooperate only if the source-destination channel is of an unacceptable quality. In our study, we consider a regenerative relay in which the decision to cooperate is based on a signal-to-noise ratio (SNR) threshold and consider the effect of the possible erroneously detected and transmitted data at the relay. We derive an expression for the end-to-end bit-error rate (BER) of binary phase-shift keying (BPSK) modulation and look at the optimal strategy to minimize this end-to-end BER at the destination for high SNR. Some selected performance results show that computer simulations based results coincide with our analytical results.

We consider a multiuser MAC fading channel with two users communicating with a common destination, where each user mutually acts as a relay for the other one as well as transmits his own information. We propose a power control-enhanced cooperative transmission scheme allowing each user to allocate a certain amount of power for his own transmitted data while the rest is devoted to relaying. The underlying protocol is based on a modification of the so-called non-orthogonal amplify and forward (NAF) protocol in Azarian, K. et al, (2005). We develop capacity expressions for our scheme and derive the rate-optimum power allocation, in closed form. Our results indicate that even in a mutual cooperation setting like ours, on any given realization of the channel, one of the users will always allocate zero power to relaying the data of the other one, and thus act selfishly.

Mobile users with single antennas can use spatial transmission diversity through cooperative space-time encoded transmission. This paper presents an end-to-end performance analysis of two-hop asynchronous cooperative diversity with regenerative relays over Rayleigh block-flat-fading channels. We present a preceding frame-based scheme with packet-wise encoding which enables best synchronization and channel estimation. We derive the bit-error rate and the end-to-end bit-error rate expressions for binary phase-shift keying. We present the performance of the frame-error rate and the end-to-end frame-error rate. Finally, comparisons between three system configurations, differing by the amount of cooperation, are presented. The influence of the amount of cooperation is small (about 2dB), Furthermore, simulations show that the analytical results are correct at all SNRs.