Multi-antenna Relay Systems Research Articles

Joint User Scheduling, Relay Selection, and Power Allocation for Multi-Antenna Opportunistic Beamforming Systems.

Opportunistic beamforming (OBF) is a potential technique in the fifth generation (5G) and beyond 5G (B5G) that can boost the performance of communication systems and encourage high user quality of service (QoS) through multi-user selection gain. However, the achievable rate tends to be saturated with the increased number of users, when the number of users is large. To further improve the achievable rate, we proposed a multi-antenna opportunistic beamforming-based relay (MOBR) system, which can achieve both multi-user and multi-relay selection gains. Then, an optimization problem is formulated to maximize the achievable rate. Nevertheless, the optimization problem is a non-deterministic polynomial (NP)-hard problem, and it is difficult to obtain an optimal solution. In order to solve the proposed optimization problem, we divide it into two suboptimal issues and apply a joint iterative algorithm to consider both the suboptimal issues. Our simulation results indicate that the proposed system achieved a higher achievable rate than the conventional OBF systems and outperformed other beamforming schemes with low feedback information.

Joint Transceiver Optimization for Multiuser Multi-Antenna Relay Systems With Energy Harvesting

In this paper, a multiuser multi-antenna relay system with wireless power transfer is investigated, where the relay node harvests energy from the radio frequency signal sent from the source node and utilizes the harvested energy to forward the information signal to users. Under the transmit power constraints at all nodes, the optimization problem is formulated which maximizes the achievable sum rate of the relay system by optimizing the time switching (TS) factor and the source and the relay precoding matrixes. We divide the complex non-convex objective problem into several subproblems which are solved by the proposed alternating optimization (AO) scheme. We also propose a novel power allocation scheme across subchannels to maximize the sum rate. Since the objective function to optimize the relay power allocation is not convex, difference of convex programming is adopted to transform the original non-convex problems into convex optimization problems. Simulation results are provided to verify the analysis and demonstrate the efficiency of the proposed AO scheme.

Secure beamforming for two-way multiantenna relay systems

In two-way multiantenna relay systems, confidential information can be easily intercepted by eavesdroppers. To overcome this problem, two secure beamforming schemes are proposed: secrecy sum rate maximization beamforming (SSRMB) and null space beamforming (NSB). For the SSRMB scheme, the nonconvex secrecy sum rate maximization problem is solved using a successive convex approximation strategy, which is convergent. The obtained beamforming solution is locally optimal to the original problem. For the special scenario where the number of antennas at the relay is more than the number of eavesdroppers, the NSB scheme with a low complexity is proposed. The beamforming solution is obtained by solving a generalized Rayleigh quotient problem. Simulation results demonstrate that the proposed schemes can effectively improve the security performance, and the SSRMB scheme has a fast convergence rate.

QoS Constrained Optimization for Multi-Antenna AF Relaying With Multiple Eavesdroppers

In this letter, we study physical-layer secure transmission for a multi-antenna relay system in the presence of multiple eavesdroppers, with emphasis on quality-of-service (QoS) based optimization. In particular, our ultimate goal is to minimize relay transmit power and meanwhile guarantee the fulfillment of QoS constraints with regard to both legitimate receiver and eavesdroppers. Instead of directly dealing with this intricate problem, we first prove that the optimal relay precoder possesses a generalized channel matching structure, which allows us to transform the original problem into a tractable semi-definite programming (SDP). We then rigorously verify the optimality of this approach, and further discuss its extension to the scenario with only channel statistics. Simulations are finally performed to demonstrate the superiority of the proposed optimal design.

Statistical Analysis of Multiantenna Relay Systems and Power Allocation Algorithms in a Relay With Partial Channel State Information

The performance of a dual-hop MIMO relay network is studied in this paper. The relay is assumed to have access to the statistical channel state information of its preceding and following channels and it is assumed that fading at the antennas of the relay is correlated. The cumulative density function (cdf) of the received SNR at the destination is first studied and closed-form expressions are derived for the asymptotic cases of the fully-correlated and non-correlated scenarios; moreover, the statistical characteristics of the SNR are further studied and an approximate cdf of the SNR is derived for arbitrary correlation. The cdf is a multipartite function which does not easily lend itself to further mathematical calculations, e.g., rate optimization. However, we use it to propose a simple power allocation algorithm which we call "proportional power allocation". The algorithm is explained in detail for the case of two antennas and three antennas at the relay and the extension of the algorithm to a relay with an arbitrary number of the antennas is discussed. Although the proposed method is not claimed to be optimal, the result is indistinguishable from the benchmark obtained using exhaustive search. The simplicity of the algorithm combined with its precision is indeed attractive from the practical point of view.

Power and Spectral Efficiency of Multi-Pair Massive Antenna Relaying Systems With Zero-Forcing Relay Beamforming

We consider a multi-pair multi-antenna relay system, delivering data from multi-sources to their destinations simultaneously with the help of a massive antenna relay. We analyze the spectral efficiency and power scaling laws under perfect and imperfect channel state information (CSI) for zero-forcing relay beamforming. The analysis shows that the transmit power can be scaled down by $1/M^{\alpha}$ at the users and by $1/M^{\beta}$ at the relay as the number $M$ of relay antennas increases, where $(\alpha,\beta)=(t,1)$ or $(1,t)$ for $0\leq t\leq 1$ in the case of perfect CSI and $(\alpha,\beta)=(t,1-t)$ or $(1/2,t)$ for $0\leq t\leq 1/2$ in the case of imperfect CSI.

Robust relay design for two-way multi-antenna relay systems with imperfect CSI

The paper investigates the problem of designing the multiple-antenna relay in a two-way relay network by taking into account the imperfect channel state information (CSI). The objective is to design the multiple-antenna relay based upon the CSI estimates, where the estimation errors are included to attain the robust design under the worst-case philosophy. In particular, the worst-case transmit power at the multiple-antenna relay is minimized while guaranteeing the worst-case quality of service requirements that the received signal-to-noise ratio (SNR) at both sources are above a prescribed threshold value. Since the worst-case received SNR expression is too complex for subsequent derivation and processing, its lower bound is explored instead by minimizing the numerator and maximizing the denominator of the worst-case SNR. The aforementioned problem is mathematically formulated and shown to be nonconvex. This motivates the pursuit of semidefinite relaxation coupled with a randomization technique to obtain computationally efficient high-quality approximate solutions. This paper has shown that the original optimization problem can be reformulated and then relaxed to a convex problem that can be solved by utilizing suitable randomization loop. Numerical results compare the proposed multiple-antenna relay with the existing non-robust method, and therefore validate its robustness against the channel uncertainty. Finally, the feasibility of the proposed design and the associated influencing factors are discussed by means of extensive Monte Carlo simulations.

A Unified Analysis of Spectral Efficiency for Two-Hop Relay Systems With Different Resource Configurations

In this paper, we analyze in a unified way the achievable rate and spectral efficiency of multicarrier and multiantenna two-hop relay systems with an interference-free linear transceiver. We consider two kinds of decode-and-forward (DF) relay systems with three nodes. In the first kind of relay systems where only the relay node employs the full channel state information (CSI) of both the source–relay (S–R) link (first hop) and the relay–destination (R–D) link (second hop), the relay eliminates the interference of these links. In the second kind of systems, where the source node employs the CSI of the first hop, the destination node employs the CSI of the second hop, whereas the relay node employs the full CSI of both the hops; these nodes can jointly eliminate the interference. For the multicarrier relay or multiantenna relay systems supporting different diversity and multiplexing gains and under the aforementioned assumptions for the CSI, we find that the achievable rate of each hop or its lower bound can be unified in the generalized mean of the eigenvalues of an equivalent channel correlation matrix. Furthermore, by using the properties of generalized mean and by resorting to the random matrix theory, we derive the asymptotic spectral efficiency of the multicarrier or multiantenna systems with different CSI assumptions. Finally, we provide simulation results to validate our analytical results. Our studies show that the multicarrier systems using code-division multiplexing (CDM) or the multiantenna systems can benefit more from exploiting the CSI at all the three nodes than the multicarrier systems using frequency-division multiplexing (FDM), particularly, in the case of a high load factor.

Reduced Complexity Beamforming with Optimal Power Allocation in Two-Way Multi-Antenna Relay Systems

This paper introduces a new beamforming method for a two-way relay system, where two single-antenna terminals exchange messages via a multi-antenna amplify-and-forward (AF) relay operating in the half-duplex mode. The optimization problem under consideration is to design a relay beamforming matrix that maximizes the achievable weighted sum rate under the relay power constraint, given the channel state information. This problem is non-convex, and iterative optimization algorithms and suboptimal beamformers with reduced complexity have been developed in previous works. In this paper, we develop an alternative suboptimal scheme by converting the original non-convex optimization into convex optimization. This conversion is made possible through some changes of variables and an additional condition. The simulation results demonstrate that the proposed method performs almost as well as the optimal scheme and can yield larger achievable rates than conventional suboptimal beamformers.

Performance Analysis of Two-Step Bi-Directional Relaying with Multiple Antennas

In this paper we study decode-and-forward multi-antenna relay systems that achieve bi-directional communication in two time slots. We investigate different downlink broadcast schemes which employ binary or analog network coding at the relay. We also analyze and compare their performances in terms of diversity order and symbol error probability. It is shown that if exact downlink channel state information is available at the relay, using analog network coding in the form of multi-antenna maximal-ratio transmit beamforming to precode the information vectors at the relay gives the best performance. Then, we propose a Max-Min antenna selection with binary network coding scheme that can approach this performance with only partial channel state information.

Performance analysis of multi-antenna relay communication systems with MRC

The performance of multi-antenna relay communication systems is investigated in this letter. The source broadcasts the signal to all the relays and the destination. Decode-and-forward scheme is adopted at the relays, and the destination employs maximum ratio combining (MRC) technique to maximize the received signal-to-noise ratio. Closed-form expressions of outage probability are derived in the MRC case and minimum routes MRC case. An adaptive grouping algorithm of relay antennas is also presented under the constraint of total antenna number. Simulation results show that the analytical curves agree with the simulated ones very well, and the performance of the proposed grouping algorithm is very close to the upper bound mentioned in other papers. Copyright © 2012 John Wiley & Sons, Ltd.

Optimal Linear MMSE Beamforming for Two Way Multi-Antenna Relay Systems

This letter addresses the beamforming issues for two way relay systems assuming that two source nodes S1 and S2 are equipped with single antenna each and relay node R with multiple antennas. We derive the optimal linear beamforming matrix at R in closed form based on convex optimization techniques, aiming to minimize the weighted mean squared error (MSE). To reduce complexity for implementation, two alternative relay schemes are also presented. In addition, all the proposed schemes' performance are analyzed and compared in terms of mean squared error (MSE) and computation complexity.

Sum rate analysis of multiantenna multiuser relay channel

In this study, the authors introduce two types of relay operations such as a half-duplex relay (HDR) and a full-duplex relay (FDR) and analyse the corresponding sum rate of a multiantenna multiuser relay system. Especially for FDR, a precoding scheme based on block diagonalisation (BD) is proposed to suppress the self-interference as well as to support multiple users at the same time slot. Simulation results show that the proposed FDR scheme enhances significantly the sum rate performance compared to HDR.