Cooperative power allocation with partial channel information in multi‐cell multi‐user dual‐hop MIMO relay systems

This paper considers cooperative power allocation with the use of partial channel state information (CSI) in a multi‐user dual‐hop relay system with multiple antennas. The end‐to‐end capacity can be improved by dynamically allocating the transmit power of the base station and relay according to co‐channel interference caused by the adjacent relays. The proposed scheme allocates the transmit power in association with the eigenvalues and angle difference between the eigenvectors of transmit correlation matrices of the desired and interference channel. It is shown by means of upper‐bound analysis that the end‐to‐end capacity of the proposed scheme can be maximized in highly correlated channel environments when the principal eigenvectors of transmit correlation matrices of the desired and interference channel are orthogonal to each other. It is also shown that the proposed scheme is robust to the channel estimation error. Finally, the performance of the proposed scheme is verified by the computer simulation. Copyright © 2010 John Wiley & Sons, Ltd.     

Successive relaying for large MIMO amplify‐and‐forward relay networks

In this paper, we consider a large relay network with one source, K relays and M users, where the source and relays are equipped with W and N antennas, respectively. We propose an amplify‐and‐forward successive relaying protocol in which the relays are divided into 2 groups to successively help transmission to M users. Achievable sum rate of the proposed protocol is derived and found to scale as when N and M are fixed and K→∞ . On the other hand, when M and K are fixed and N→∞, the achievable sum rate scales as . Therefore, the scaling law of the achievable sum rate coincides with the capacity scaling law of the considered network. Then, both precoding at the source and grouping of the relay nodes are jointly optimized to further improve the proposed protocol. Numerical results show that the proposed successive relaying protocol can outperform the conventional 2‐slot relaying protocol and the proposed joint optimization scheme for source precoding and relay grouping bring considerable rate gain.     

Effect of imperfect channel state information and co‐channel interferences on two‐hop fixed gain amplify‐and‐forward relay networks with beamforming

This letter investigates the joint effects of imperfect channel state information and co‐channel interferences on a two‐hop fixed gain amplify‐and‐forward (AF) relay network with beamforming. Specifically, the analytical expressions of the outage probability and the average symbol error rate for the AF relaying are derived. Moreover, the asymptotic analysis at high signal‐to‐noise ratio is also presented to reveal the diversity order and array gain of the considered AF relay system. Finally, computer simulations are given to confirm the validity of the analytical results. Copyright © 2014 John Wiley & Sons, Ltd.     

An architecture for amplify‐and‐forward cooperative systems with real modulations

This paper proposes a simple architecture for half‐duplex cooperative systems which use amplify‐and‐forward (AF) as a relay strategy and one‐dimensional modulations for source messages. The proposed solution uses the two orthogonal channels of quadrature modulation in order to allow a node to behave simultaneously as a source and a relay. We demonstrate that the new scheme has a similar performance to the conventional orthogonal amplify‐and‐forward protocol without suffering from bandwidth loss, and avoids error propagation problems of previously reported AF superposition schemes. The proposed technique is suitable for applications with low spectral efficiencies and practical adaptive systems where real modulations are implemented based on a quadrature modulation core. Copyright © 2009 John Wiley & Sons, Ltd.     

Two‐pilot channel estimation scheme for amplify‐and‐forward relay networks

In amplify‐and‐forward relay networks, the equivalent channel to the destination node is not independent of equivalent noise and the equivalent noise does not follow a Gaussian distribution. Therefore, it is difficult to directly estimate the equivalent channel based on traditional optimal rules. In this paper, we propose a two‐pilot estimation (TPE) scheme that decomposes a non‐Gaussian noise channel estimation problem into two channel estimation problems in Gaussian noise. In TPE scheme, the relay‐destination channel is first estimated by one pilot and the other pilot is used to estimate the equivalent channel with the aid of the estimated relay‐destination channel. Simulation results show that the TPE scheme can achieve less estimation error and larger system throughput than other existing channel estimators in slow fading case. Copyright © 2012 John Wiley & Sons, Ltd.     

Decode‐and‐forward with partial relay selection

Exact expressions for outage probability and symbol error rate are presented for a decode‐and‐forward cooperative network with partial relay selection. An independent but not identically distributed Nakagami‐m fading environment is considered. Numerical and simulated results show the validity of the analytical results. Copyright © 2010 John Wiley & Sons, Ltd.     

Ergodic capacity analysis of cooperative amplify‐and‐forward relay networks over generalized fading channels

This paper presents two new methods for evaluating the ergodic channel capacities of cooperative non‐regenerative multirelay networks in a myriad of fading environments and under three distinct source‐adaptive transmission policies: (i) optimal rate adaptation with a fixed transmit power; (ii) optimal joint power‐and‐rate adaptation; and (iii) truncated channel inversion with fixed rate. In contrast to the previous related works, our proposed unified analytical frameworks that are based on the moment generating function and/or the cumulative distribution function of end‐to‐end signal‐to‐noise ratio allow us to gain insights into how power assignment during different transmission phases, relay node placement, fade distributions, and dissimilar fading statistics across the distinct communication links impact the ergodic capacity, without imposing any restrictions on the channel fading parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

Enhancing secrecy capacity of multi‐relay wiretap systems with partial channel state information

Distributed precoding has provento be capable of enhancing the secrecy capacity of the multi‐relay wiretap system. An iterative distributed precoding and channel state information (CSI) sharing scheme can be used to reduce the CSI overhead at each relay node. However, in practical applications, the CSI of each relay node cannot be perfectly known to themselves, especially that of the relay‐eavesdropper channels. Thus, partial CSI for the relay‐eavesdropper links is assumed, and the corresponding distributed precoding and CSI sharing schemes are investigated. Under the assumption that the average value of the relay‐eavesdropper channel is known at each relay node, an extended iterative distributed precoding and CSI sharing scheme is proposed. Simulation results demonstrate that with the increase of the power ratio of the constant part to the random part of the relay‐eavesdropper channels, the proposed scheme with partial CSI performs increasingly close to the one with perfect CSI in secrecy capacity. Copyright © 2014 John Wiley & Sons, Ltd.     

Cognitive cooperative networks with decode‐and‐forward relay selection under interference constraints of multiple primary users

In this paper, we study the performance of cognitive cooperative radio networks under the peak interference power constraints of multiple primary users (PUs). In particular, we consider a system model where the secondary user communication is assisted by multiple secondary relays (SRs) that operate in the decode‐and‐forward mode to relay the signal from a secondary transmitter to a secondary receiver. Moreover, we assume that the transmit powers of the secondary transmitter and the SRs are subject to the peak interference power constraints of multiple PUs that operate in their coverage range. Given this system setting, we first derive the cumulative distribution function of the instantaneous end‐to‐end signal‐to‐noise ratio. Then, we obtain a closed‐form expression for the outage probability and an exact expression for the symbol error probability of the considered network. These tractable formulas enable us to examine the impact of the presence of multiple PUs on the performance of the considered spectrum sharing system. Furthermore, our numerical results show that system performance is improved significantly when the number of SRs increases or the channel mean power from the secondary user to the PUs is low. Also, any increase in the number of PUs in the coverage range of the secondary transmitter or the SRs leads to degradation in system performance. Finally, Monte Carlo simulations are provided to verify the correctness of our analytical results. Copyright © 2013 John Wiley & Sons, Ltd.     

Iterative receiver for amplify‐and‐forward relay networks with unknown noise correlation

For amplify‐and‐forward relay networks, we propose an iterative scheme to estimate channel and detect information symbols for the multi‐antenna destination in spatially correlated noise. The equivalent channel coefficients and noise covariance are estimated by expectation–maximization algorithm. In addition, we discuss the initialization of iteration and analyze the modified Cramér–Rao bound to show the performance of the proposed iterative estimation. Moreover, on the basis of the structure of the proposed iterative estimator, a joint channel estimation and detection receiver is also provided. Finally, simulation results show that the proposed channel estimator and receiver can achieve the optimal performances in amplify‐and‐forward relay networks with unknown noise correlation. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance analysis of selective cooperation in amplify‐and‐forward relay networks over identical Nakagami‐m channels

In cooperative communications, multiple relays between a source and a destination can increase the diversity gain. Because all the nodes must use orthogonal channels, multiple‐relay cooperation becomes spectrally inefficient. Therefore, a bestrelay selection scheme was recently proposed. In this paper, we analyzed the performance of this scheme for a system with the relays operating in amplify‐and‐forward mode over identical Nakagami‐m channels using an exact source–relay–destination signal‐to‐noise ratio (SNR).We derived accurate closed‐form expressions for various system parameters including the probability density function of end‐to‐end SNR, the average output SNR, the bit error probability, and the channel capacity. The analytical results were verified through Monte Carlo simulations. Copyright © 2011 John Wiley & Sons, Ltd.     

Performance analysis of spectrally efficient amplify‐and‐forward opportunistic relaying scheme for adaptive cooperative wireless systems

In this paper, we propose an adaptive amplify‐and‐forward (AF) relaying scheme that selects the best relay among the available relay nodes opportunistically to cooperate with a source node for improvement of the spectral efficiency. This improvement can be achieved by introducing a policy that gives the useful cooperative regions and defines a switching threshold signal‐to‐noise ratio that guarantees the bit error rate (BER) of cooperative transmission is below the target. We model all links as independent non‐identically distributed Rayleigh fading channels. We then derive closed‐form expressions for the average spectral efficiency, average BER, and outage probability when an upper bound for the signal‐to‐noise ratio of the end‐to‐end relay path is applied and adaptive discrete rate is considered. Numerical and simulation results show that the proposed scheme, compared with the outage‐based AF incremental relaying, AF fixed relaying, and the conventional direct transmission, can achieve the maximum average spectral efficiency while maintaining the average BER and outage probability. Copyright © 2013 John Wiley & Sons, Ltd.     

User selection scheme with limited feedback processing and outdated CSI in multiuser relay networks

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.     

Variable‐rate adaptive modulation with optimum switching thresholds for cooperative systems with relay selection

In this paper, the performance of variable‐rate adaptive modulation schemes in the amplify‐and‐forward cooperative systems with relay selection is analyzed over Rayleigh fading channels. We consider constant power and discrete‐rate adaptive multi‐level modulation techniques. The switching levels required for discrete‐rate adaptive modulation have been determined for two schemes, namely fixed switching levels and optimum switching levels, both respecting a target bit error rate requirements, where in the later scheme, the switching levels are optimally determined in a way that the average spectral efficiency of the system is maximized. Two M‐ary modulation schemes, namely quadrature amplitude modulation and phase shift keying, are considered. Closed‐form expressions are derived for three performance metrics, namely average spectral efficiency, outage probability, and average bit error rate, for two cases: independent and identically distributed fading relay links and independent and non‐identically distributed links. It is shown that, compared with using fixed switching levels, employing optimum switching levels provides a slight improvement in the spectral efficiency and moderate improvements in the signal‐to‐noise ratio gain and in the outage probability of the system. It is also shown that compared with the independent and identically distributed links, independent and non‐identically distributed relay links yield a slight increase in the signal‐to‐noise ratio gain and a slight decrease in the diversity order of the system. Copyright © 2011 John Wiley & Sons, Ltd.     

QoS‐driven jointly optimal subcarrier pairing and power allocation for OFDM amplify‐and‐forward relay systems

In this paper, we investigate the quality‐of‐service (QoS) driven subcarrier pairing and power allocation for two‐hop amplify‐and‐forward OFDM relay systems. By integrating the concept of effective capacity, our goal is to maximize the system throughput subject to a given delay QoS constraint. We propose a jointly optimal subcarrier pairing and power allocation scheme, which can be implemented with two separate steps. First, pair the subcarriers over the source‐relay channel and relay‐destination channel by the descending order of the subcarriers’ channel gains. Second, by making use of the derived equivalent end‐to‐end channel gains of the subcarrier pairs, optimally allocate power over the subcarrier pairs, and then optimally partition the power of the subcarrier pairs between the source and the relay. The simulation results show that our proposed scheme can efficiently provide different levels of delay QoS guarantees, even if under stringent delay QoS constraints. The simulation results also verify that our proposed scheme shows significant superiorities over the other existing schemes. Copyright © 2013 John Wiley & Sons, Ltd.     

Cooperative beamforming and relay selection in cognitive radio systems

In this paper, a new scheme is suggested for cooperative beamforming (BF) and relay selection in CR networks, where a pair of secondary users communicates with each other assisted by some multiple antenna relay nodes. The goal of the algorithm is to maximize signal to interference plus noise ratio of secondary user receiver subject to limited interference caused for primary user receiver and power constraints of relay nodes. The relay selection and BF optimization problem is solved separately by employing convex semidefinite programming through rank‐one relaxation. It is shown that our proposed algorithm outperforms conventional relay selection and BF schemes. Copyright © 2014 John Wiley & Sons, Ltd.     

Nearly optimal linear transceiver design for amplify‐and‐forward MIMO multiple‐relay systems under MMSE criterion

In this paper, we consider amplify‐and‐forward multiple‐input multiple‐output multiple‐relay systems, where all the nodes have multiple antennas. For enhancing link reliability, we address the problem of designing optimal linear transceiver to minimize the mean squared error (MSE) of symbol estimations subject to the total relay transmit power constraint. This problem is highly complex and has not been solved in the literature. We first simplify this optimization problem to one that takes a singular value vector and a unitary matrix as optimization variables. Then based on the analyses for the simplified problem, we develop an iterative algorithm consisting of one boundary optimization and one unitary matrix constrained optimization. We show analytically that the proposed iterative algorithm always converges, and the MSE is monotonically decreasing from one iteration to the next. Finally, numerical results demonstrate the nearly optimal performance of the proposed scheme. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance of joint transmit and receive antenna selection in dual hop amplify‐and‐forward relay network over Nakagami‐m fading channels

In this paper, performance of joint transmit and receive antenna selection in each hop of dual hop amplify‐and‐forward relay network is analyzed over flat and asymmetric Nakagami‐m fading channels. In the network, source, relay, and destination are equipped with multiple antennas. By considering relay location, we derive exact closed‐form cumulative distribution function, moment generating function, moments of end‐to‐end signal‐to‐noise ratio and closed form symbol error probability expressions for fixed and channel state information‐based relay gains. We also derive the asymptotical outage probability and symbol error probability expressions to obtain diversity order and array gain of the network. Analytical results are validated by the Monte Carlo simulations. Copyright © 2012 John Wiley & Sons, Ltd.     

Nelder‐Mead–based power optimization for secrecy enhancement in amplify‐and‐forward cooperative relay networks

Cooperative communication based on relaying nodes has been considered as a promising technique to increase the physical layer security (PLS) performance in wireless communications. In this paper, an optimal power allocation (OPA) scheme based on Nelder‐Mead (NM) algorithm is proposed for improving the secrecy rate of amplify‐and‐forward (AF) cooperative relay networks employing cooperative jamming (CJ) scheme. The proposed hybrid jamming scheme allows the source and selected relay to transmit the jamming signal along with the information to confound the eavesdropper. The path selection probability of ant colony optimization (ACO) algorithm is used for selecting the relay for transmission. The performance based on secrecy rate is evaluated for “n” trusted relays distributed dispersedly between the source and destination. Gradient‐based optimization and three‐dimensional exhaustive search methods are used as benchmark schemes for comparison of the proposed power optimization algorithm. The secrecy performance is also compared with conventional AF scheme and CJ scheme without power optimization (EPA). The impact of single and multiple relays on secrecy performance is also evaluated. Numerical results reveal that, compared with the gradient method and exhaustive search algorithm, the proposed power allocation strategy achieves optimal performance. Also, the derived OPA results show a significantly higher secrecy rate than the EPA strategy for both CJ and AF schemes.     

Nth best relay selection with network coding in two‐way relay systems

In this paper, we propose an Nth best relay selection (Nth‐RS) scheme for analog network coding in two‐way relay systems. In traditional two‐way single‐relay selection schemes, only the best one is selected to forward network‐coded signals. However, in practical applications, the best relay may be unavailable because of the scheduling or overload constraints. In this case, we investigate a more general scheme, where the Nth best but available relay is selected. To evaluate the transmission of reliability, the expression of outage probability in exponential–integral form and its asymptotic expression in closed form are presented. Moreover, the upper bound and lower bound of outage probability are also derived. The analysis reveals that the diversity order of Nth‐RS equals to (M ? N + 1), where M is the number of relay nodes, and the results are verified by simulations. In order to improve system performance, transmit power between sources and relay is optimally allocated to minimize the upper bound of outage probability under total power constraint. Simulation results show that Nth‐RS scheme with proposed power allocation can achieve substantial improvement over equal power allocation scheme. Copyright © 2012 John Wiley & Sons, Ltd.