全双工MIMO中继系统高性能的波束成型算法

为了更好地抑制全双工多入多出技术(MIMO)中继系统的自干扰,提高信息传输速率,提出了一种新型的波束成型组合算法。该组合在源节点和目的节点采用奇异值分解的波束成型向量,而在中继站采用基于最大化信干噪比接收波束成型来抑制中继端的自干扰,以及最大化信泄噪比发射波束成型矩阵来减少中继发送端泄漏到中继接收机的信号组合波束成型算法。为了降低复杂度,随后引入了交替迭代结构来进一步优化中继接收和发送波束成型矩阵,并比较了不同组合波束成型方案的和速率性能。仿真结果表明,与现存的波束成型组合相比,当干噪比较低时,提出的组合算法能够提供更优的和速率性能。     

直达链路下的能量采集中继选择系统的吞吐量分析

针对三节点传输系统，研究了基于功率分配能量采集的全双工放大转发协作中继方案。考虑源和目的地之间存在直达链路，全双工中继节点采用天线选择技术以最小化中继自干扰，目的节点采用最大比合并技术以最大化目的地接收信噪比。在延迟受限传输模式下，运用高斯-切比雪夫积分推出了系统吞吐量的近似闭合表达式。在延迟容忍传输模式下，运用凸优化推出了系统吞吐量下界的闭合表达式。在多中继情况下考虑了最佳中继选择策略和机会式中继选择策略。数值分析和蒙特卡洛仿真表明，增加中继个数或者减小速率阈值均能提高系统吞吐量。      

非对称MIMO双向中继干扰信道的干扰对齐算法

针对非对称的多用户MIMO双向中继干扰信道,本文提出了一种简化的基于最大信干噪比的干扰对齐算法,给出了干扰对齐的可行性条件和干扰抑制方案,并分析了系统容量和自由度。本文算法的中继节点不必做复杂的信号处理,大大降低了整个系统的信号处理复杂度;同时,本文算法是基于最大信干噪比方案,与现有文献的中继迫零方案比较,大大提高了系统在中低信噪比时的容量;进一步,通过仿真验证表明,基于本文算法的K用户对双向中继干扰网络可以达到。      

面向OFDM的同时同频全双工双向高谱效中继方案

针对同时同频全双工双向中继网络,该文提出一种对中继剩余自干扰信号具有鲁棒性的双向中继传输方案。该文首先对中继剩余自干扰信号进行分析,将无限迭代的剩余自干扰信号建模成等效多径信号,并利用OFDM的循环前缀对抗等效多径现象,以降低中继剩余自干扰信号对系统传输性能的影响。在等效多径方案的基础上,以系统信干噪比最大化为目标,推导出全双工双向中继传输的最佳放大因子求解方法。最后,通过仿真验证所提出的双向中继传输方案的有效性。

     

直达链路下的全双工能量采集中继系统的吞吐量分析

针对三节点传输系统,研究了基于时间切换能量采集的全双工放大转发协作中继方案。考虑源和目的地之间存在直达链路,全双工中继节点采用天线选择技术以最小化中继自干扰,目的节点采用最大比合并技术以最大化目的地接收信噪比。在延迟受限传输模式下,推出了自干扰信道增益为常数时系统吞吐量近似闭合表达式。运用最佳目的地选择策略和自适应中继选择策略讨论了系统吞吐量性能。如图4所示,在源发送功率为30 dB时,就系统吞吐量而言,本文算法比其他两种算法分别提高了0.22 bits/s/Hz和0.68 bits/s/Hz。数值分析和蒙特卡洛仿真表明,增加中继个数,增加目的地个数或者增大能量采集效率均能够提升系统吞吐量性能。      

全双工MIMO中继的自干扰抑制

在全双工多输入多输出(MIMO)中继系统中,中继发送端和接受端之间的自干扰降低了系统的信道容量,严重影响了中继系统的传输性能。从理论上来讲,全双工中继方案的核心问题就是自干扰抑制。通过在中继处设计收发矢量来抑制自干扰信号,但与已有的基于奇异值分解(SVD)的抑制方案不同,研究了一种旨在放大中继收发端有用信号与自干扰信号功率比(SIR)的抑制方法,将收发抑制矢量进行分开设计,通过公式推导,求出最优的收发抑制矢量。仿真结果表明该方案改善了中继系统的信道容量,自干扰抑制效果明显。      

基于分布式天线的全双工中继系统最大化和速率波束成形设计

基于分布式天线的全双工中继系统结合了全双工中继两跳同时同频传输的能力和分布式天线高效覆盖的特性,为提升小区边缘和严重阴影衰落区域的频谱效率提供了一种有效途径。在自干扰抵消非理想的多用户场景下,利用分布式多天线波束成形可实现对系统中自干扰和多用户干扰的联合抑制。为此,该文首先建立了在各分布式天线节点独立发射功率约束下最大化多用户端到端和速率的最优化系统模型,进而提出一种双层迭代算法,解决原问题的非凸性求解难题。仿真结果验证了算法的有效性,表明在多用户分布式天线全双工中继系统中,所提波束成形设计能够有效抑制自干扰和多用户干扰,显著提高系统频谱效率。     

多用户MIMO双向中继信道鲁棒预编码

在多用户MIMO双向中继系统中,若使用非理想信道模型,中继处仅能获得部分下行信道状态信息,这样将导致系统性能大幅度下降。由此提出基于最小信干噪比最大化的鲁棒预编码方案。仿真表明,该方案不仅可以使最差信干噪比最大化,也有效地改善了系统的比特误码率性能。     

互信息量最大化和低复杂度的全双工中继网络自干扰消除和干扰对齐算法

该文针对多用户全双工中继干扰信道,同时考虑全双工中继自干扰消除及用户端干扰消除问题。首先,提出了基于最大化全局互信息量的自干扰消除和干扰对齐算法,并具体给出了自干扰消除矩阵的具体求解算法和方案。进一步,给出了信号对齐和干扰抑制的可行性条件,并分析了系统的互信息量和所受干扰噪声功率,以及系统的自由度。理论分析和仿真结果显示,与现有的典型全双工中继方案相比,该文算法可以提高系统互信息量和自由度,并有较低误码率;另外,该文算法的中继只需进行简单的功率约束,不需做复杂的信号处理,降低了整个系统的信号处理复杂度。     

中继增强的无线蜂窝多小区系统的联合调度与功率控制算法

针对采用全局频率复用的中继增强的无线蜂窝多小区系统,该文考虑多种通信模式并存的混合场景,提出了一种干扰感知的联合资源分配策略。以最大化系统总吞吐量为目标,同时考虑小区间干扰对中继节点与移动站点的影响,以及基站与中继节点各自的发射功率约束。为了降低计算复杂度,针对用户与中继节点配对问题提出了一种基于小区间干扰的调度算法;针对功率控制问题分别提出了一种基于符号规划的最优功率分配算法和一种次优的最小能耗功率分配算法。仿真结果表明,该文所提算法逼近最优资源分配,在系统吞吐量与能量效率等性能方面具有显著优势。     

The Effect of Noise Correlation in Amplify-and-Forward Relay Networks

In wireless relay networks, noise at the relays can be correlated possibly due to common interference or noise propagation from preceding hops. A parallel relay network with noise correlation is considered in this network. For the relay strategy of amplify and forward (AF), the optimal rate maximizing relay gains when correlation knowledge is available at the relays are determined. Interestingly, it is shown that, on average, noise correlation is beneficial regardless of whether the relays know the noise covariance matrix. However, the knowledge of correlation can greatly improve the performance. Typically, the performance improvement from correlation knowledge increases with the relay power and the number of relays. With perfect correlation knowledge the system is capable of canceling interference if the number of interferers is less than the number of relays. For a two-hop multiple-access parallel network, closed-form expressions for the maximum sum rate and the optimal relay strategy are determined. Relay optimization for networks with three hops is also considered. Based on the result of two-hop network with noise correlation, an iterative algorithm is proposed for solving the relay optimization problem for three-hop networks.      

Optimal designs of collaborative relay‐assisted multiuser beamforming for cellular systems

With careful calculation of signal forwarding weights, relay nodes can be used to work collaboratively to enhance downlink transmission performance by forming a virtual multiple‐input multiple‐output beamforming system. Although collaborative relay beamforming schemes for single user have been widely investigated for cellular systems in previous literatures, there are few studies on the relay beamforming for multiusers. In this paper, we study the collaborative downlink signal transmission with multiple amplify‐and‐forward relay nodes for multiusers in cellular systems. We propose two new algorithms to determine the beamforming weights with the same objective of minimizing power consumption of the relay nodes. In the first algorithm, we aim to guarantee the received signal‐to‐noise ratio at multiusers for the relay beamforming with orthogonal channels. We prove that the solution obtained by a semidefinite relaxation technology is optimal. In the second algorithm, we propose an iterative algorithm that jointly selects the base station antennas and optimizes the relay beamforming weights to reach the target signal‐to‐interference‐and‐noise ratio at multiusers with nonorthogonal channels. Numerical results validate our theoretical analysis and demonstrate that the proposed optimal schemes can effectively reduce the relay power consumption compared with several other beamforming approaches. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance Analysis of Joint Relay and Transmit–Receive Antenna Selection for Low Density Parity Check-Asynchronous Distributed Space Time Block Coded Cooperative Diversity System

In this work we propose a joint relay and transmit–receive antenna selection method for amplify and forward asynchronous distributed space time block coded (ADSTBC) system. To the LDPC coded signal received from the source at the relays, we apply linear dispersion codes which are optimized over the delay profile of the system to produce the ADSTBC. Selection of the best transmit–receive antenna pair between source and selected relays, and that of between the selected relays and destination is done on the basis of instantaneous signal to noise ratio (SNR) of the respective paths. The relays and transmit-receive antenna pairs are selected by maximizing the end-to-end instantaneous SNR. Performance analysis of the system is done on the basis of pair wise error probability and outage probability (P_out).     

Efficient Beamforming by SLNR Maximization for Dual-Hop Relay Assisted MIMO Network

Relay beamforming is a key signal processing technique to mitigate interference in single and multiuser multiple input and multiple output (MIMO) wireless communication networks. In MIMO relay networks interference cancellation is an essential task of the study to get optimal network capacity. In this paper, the problem of interference resulted from leakage signal from desired signal for intended antenna, which causing interference to other antennas of the same user with multiple antennas is tackled. The criterion of signal to leakage plus noise ratio (SLNR) maximization is considered to design a vector by vector relay downlink precoding based on Fukunaga Koontz transform. This can control the interference among multiple antennas and maximize SLNR. However, matched filter beamforming is utilized at the relay receiving side that can increase output signal to noise ratio at the relay node. The channel state information of both the channel between source to relay and relay to destination is assumed at the relay node. The effectiveness of the proposed technique is studied and compared with conventional relay precoding techniques. Simulation is carried out in MATLAB environment using ideal channel conditions. This study demonstrates that, the proposed scheme for relay assisted MIMO networks can improve overall system performance in terms of ergodic capacity.     

Optimal Timing at the Relay in OFDM Based Two Way Relay Systems

In amplify and forward (AF) two way relay networks (TWRN), two sources are allowed to transmit simultaneously in the multiple access time slot. Signal misalignment at the relay occurs due to the inaccurate time slot synchronization and different propagation delay. In broadband wireless communications, the misalignment could cover several symbol intervals. In orthogonal frequency division multiplexing (OFDM) based TWRN, such signal misalignment raises question on how to choose the fast Fourier transform (FFT) window at the relay node in order to minimize the interference which will be forwarded to the sources. In this paper, the timing issue of the received superimposed signal at the relay for both sources in OFDM based AF–TWRN is investigated. The optimal timing that minimizes the total interference power at the relay is studied. The imperfect timing induced interference power at each timing point is derived. An efficient estimator is proposed for the relay to decide where to establish the FFT window boundary in order to introduce the minimum interference plus noise power based on the superimposed training block from the two source nodes. The estimator is a sliding window estimator measuring the total interference plus noise power at each timing position, and the timing position which minimizes the metric function is the optimal position to establish the FFT window. Finally, the performance of the proposed estimator is evaluated by computer simulation in the presence of different amount of signal misalignment.     

Performance study of hybrid decode–amplify–forward (HDAF) relaying scheme for physical layer security in wireless cooperative network

In this paper, the secrecy performance and power allocation of the signal‐to‐noise ratio‐based hybrid decode–amplify–forward (HDAF) relaying protocol in wireless cooperative network are investigated to get security at physical layer. The performance metrics considered are secrecy rate and intercept probability. The Ergodic secrecy rate is approximated theoretically. The effect of relay and eavesdropper locations on the secrecy performance of the system is analyzed. It is found that maximum secrecy rate is obtained for the relay close‐to‐destination case and minimum for the relay close‐to‐eavesdropper case. Jamming schemes are superior in secrecy rate performance than without jamming schemes. To enhance the secrecy rate further with the optimized relay and jammer powers, invasive weed optimization (IWO) algorithm‐based power allocation is proposed. Here, maximizing the secrecy rate is defined as the cost function for the proposed IWO algorithm‐based power allocation. Comparative study is done over the conventional equal and proposed power allocation schemes for validation. The proposed power allocation scheme proved to be superior. Copyright © 2016 John Wiley & Sons, Ltd.     

基于空间投影的全双工MIMO中继站自反馈干扰抑制

建立由基站、全双工MIMO中继站和用户站组成的系统模型,获得MIMO自反馈干扰矩阵.当自反馈干扰信号最大多径时延大于符号周期时,根据自反馈干扰矩阵张成空间,设计并添加信号空间投影滤波器到中继站中实现自反馈干扰抑制.针对自反馈干扰矩阵是否满秩,滤波器设计分别采用子空间和零空间投影算法,并讨论了不同投影算法下对中继站接收信号误码率影响和自干扰信号的残留量.理论分析和仿真结果表明,所提出的空间投影算法对自反馈干扰的抑制有较理想的效果.     

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.     

协作分集中中继选择算法的研究

协作分集是近几年无线通信技术的研究热点.在协作通信中合理地选择中继能给系统带来更大的增益.本文介绍了协作时机的选择和协作关系的形成,重点讨论了3种主要的中继选择方法:基于位置信息、基于平均接收信噪比和基于瞬时信道状态的中继选择算法,并对这些方法进行了分析和比较.