新的半双工中继协议及其资源分配

传统的基于放大转发或解码转发的时分半双工中继协议避免了对于中继节点的全双工要求,却带来了频谱效率的降低.基于广播信道及多址接入信道的概念,提出了一种新的半双工中继协议,并对该协议下的资源分配问题进行了研究.结果表明,与基于放大转发或解码转发的半双工中继协议相比,新的协议能够提高频谱效率.     

一种新的协同网络增强型中继策略

分析了三个终端构成的协同网络中继协议,根据终节点反馈的两比特信息。提出了一种增强选择性放大转发（ISAF）协议,同时推导出了其中断概率。仿真结果证明,新提出的（ISAF）协议在中断概率上都要比增强型放大转发（IAF）协议要低,同时也拥有较好的频谱效率。     

同频干扰环境下双向AF中继性能分析及参数优化

双向中继（two-way relaying, TWR）作为提高中继网络频谱效率的一种有效手段,近年来受到广泛关注。本文研究同频干扰环境下三时隙TWR策略的中断概率性能,中继采用放大转发（amplify-and-forward, AF）策略。首先,理论推导了策略中断概率下界的闭合表达式和高信噪比下的渐进表达式。仿真表明,理论结果能够匹配策略的实际中断概率性能。根据理论结果,以最小化中断概率为目标对中继节点功率分配和中继位置参数进行优化。提出三种优化问题,即（1）给定中继位置,中继功率分配优化问题,（2）给定中继功率分配方案,中继位置优化问题,（3）联合优化中继功率分配和中继位置。仿真结果表明,联合优化能够实现最优的中断概率性能。      

一种新的VMIMO协作中继选择方案

提出了一种无需信道估计的中继选择方案来实现虚拟多路输入输出（VMIMO）小区间的协作复用通信。在该方案中,VMIMO协作复用采用放大转发（AF）方式,中继VMIMO小区首先把接收到的从源VMIMO小区发射的信号和噪声同时进行线性放大,再向目的小区转发。仿真结果表明,与需要进行信道估计的基于瞬时信噪比的中继选择方案相比,所提出的方案其系统遍历容量和中断容量与前者几乎相同,但是后者却可以减小中继小区的运算复杂度。     

协作通信中的解码转发中继策略优化

在单中继协作通信系统模型下,研究了直接传输、固定解码转发（ DF）中继、选择DF中继和增量中继协议。为了提高协作通信系统的频谱效率和中断性能,通过比较源-目的端和中继-目的端的瞬时信道增益来判定是否需要采用协同通信模式,提出一种改进的解码转发方式（ IDF）。通过将选择DF中继和增量中继的优点结合,提出了一种增量选择中继（ ISR）策略,并给出了2种改进协议任意信噪比的中断概率表达式。仿真结果表明,与传统的单中继解码转发协议相比, IDF和ISR均具有更低的中断概率, ISR具有最低的中断概率。     

无线协作网络中的最优中继选择方案及中断概率分析

从物理层安全的角度出发,在含有协作中继的通信网络中,通过选择最优中继来传递信息可有效提升系统性能。讨论了在含有多个单天线窃听用户的多中继通信系统中对最优中继的选择方案,并对系统采用放大转发（amplify-and-forward,AF）协议和解码转发（decode-and-forward,DF）协议这两种不同的情况分别进行了讨论和比较,同时还对比了不含中继的直接传输情况。理论分析和仿真结果表明,最优中继选择方案可有效保障系统的安全性能。     

放大转发方式下中继选择策略的性能分析

研究了几种主要的协同中继选择策略;通过仿真对比了在放大转发方式（Amplifier-and-forward AF）下全部协同、部分协同、机会中继的中断概率性能;最后归纳总结了中继节点数目、信噪比及目标信息速率对上述中继选择策略中断概率性能的影响情况。     

混合中继转发机制下的资源分配

在基于放大转发( AF)和译码转发( DF)的混合中继转发机制模型下,为了使系统获得最大和速率,提出了相应的资源分配方案,在子载波对混合中继协议的判断选择和最优功率分配算法的基础上讨论了等效信道增益模型和非等效信道增益模型。在非等效信道增益模型中,为了降低计算复杂度提出了一种次优算法。在该机制模型下,系统自适应地选择AF或者DF转发,既克服了两种单一转发模式存在的弊端,又能获得更大的和速率,从而提高了资源利用率。仿真结果表明,当系统功率等因素变化时,该分配方案下的混合中继转发模型与传统的AF和DF模型相比系统和速率分别提高了60%和8%以上,充分说明了该系统的优越性。     

多载波码分多址两跳放大转发中继系统的能量效率与频谱效率关系

本文研究了多载波码分多址两跳放大转发中继系统的能量效率与频谱效率关系,利用随机矩阵理论的渐近分析推导了系统分别采用协作式和分布式放大转发中继时,系统能量效率与频谱效率关系的显式表达式。分析了不同系统参数对能量效率的影响。此外,还分析了系统要使频谱效率最优和能量效率最优时使用系统资源的区别。最后,通过仿真验证了理论分析的有效性。结果表明,能量效率与负载因子和中继个数的关系不是单调的,存在一个使能量效率最优的负载因子和中继个数,此外,使能量效率最优的负载因子和中继个数总小于或等于使频谱效率最优的负载因子和中继个数。      

分布式环境中协作分集的移动中继动态选择和切换策略研究

在协作无线通信系统中,中继节点的移动会大大降低系统的性能,目前该方面的结果很少。本文探讨分布式环境中协作分集的移动中继选择算法,在放大转发（AF）协作通信模式下,给出了基于信道统计状态信息的功率分配和中继的动态选择策略。针对单中继情况提出了中继切换方案,针对多中继情况提出了动态剔除、补充中继的方案,并导出了分集增益及系统容量的计算公式。通过模拟仿真分析,该方案能有效的降低中断概率,提高系统分集增益,扩大系统容量,实现良好的整体性能。      

Mixed AF and DF cooperative relay systems and their performance bounds analyses

This paper analyzes the performance bounds of a wireless relay system consisting of several relay stations working on both amplifier‐and‐forward (AF) and decode‐and‐forward (DF) protocols. We want to study the outage probability behavior of the proposed mixed AF and DF relay systems under independent Nakagami‐m fading channels. In particular, we will derive the lower and upper bounds of outage probability of the mixed AF and DF relay systems based on maximal ratio combining diversity reception. The results give optimal configuration of AF and DF relays under a specific channel condition, thus helping us to design an optimized mixed AF and DF relay system in a generic fading environment. The trade‐off between complexity and performance is discussed in this paper. In addition, we will use computer simulations to verify the effectiveness of the proposed mixed AF and DF relay configurations. Finally, the power allocation issues for such a mixed AF and DF relay system will also be discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Particle swarm optimization–based power allocation for Alamouti amplify and forward relaying protocol

The performance of wireless communication systems is improved over flat fading channel by using Alamouti coding scheme, which provides the quality of diversity gain. In this paper, performance analysis of symbol error rate (SER) and particle swarm optimization (PSO)–based power allocation (PA) for Alamouti amplify and forward (AF) relaying protocol using maximum ratio combining (MRC) technique is presented. Analytical expression of SER upper bound and SER approximation is derived for Alamouti AF relaying protocol with quadrature phase shift keying (QPSK) modulation over Rayleigh fading channel and Rician fading channel. In addition, PSO‐based optimum PA factor is calculated on the basis of the minimum SER of proposed method. PSO‐based optimum PA gives 0.5 dB of improved signal‐to‐noise ratio (SNR) compared with the equal power allocation (EPA). The theoretical approximate SER result is compared with the simulated SER. The proposed protocol provides full diversity gain and reduces SER compared with the existing AF and decode and forward (DF) relaying protocols over Rayleigh fading channel and Rician fading channel.     

BLUE‐Based Channel Estimation Technique for Amplify and Forward Wireless Relay Networks

The best linear unbiased estimator (BLUE) is most suitable for practical application and can be determined with knowledge of only the first and second moments of the probability density function. Although the BLUE is an existing algorithm, it is still largely unexplored and has not yet been applied to channel estimation in amplify and forward (AF)‐based wireless relay networks (WRNs). In this paper, a BLUE‐based algorithm is proposed to estimate the overall channel impulse response between the source and destination of AF strategy‐based WRNs. Theoretical mean square error (MSE) performance for the BLUE is derived to show the accuracy of the proposed channel estimation algorithm. In addition, the Cramér‐Rao lower bound (CRLB) is derived to validate the MSE performance. The proposed BLUE channel estimation algorithm approaches the CRLB as the length of the training sequence and number of relays increases. Further, the BLUE performs better than the linear minimum MSE estimator due to the minimum variance characteristic exhibited by the BLUE, which happens to be a function of signal‐to‐noise ratio.     

Cooperative quadrature physical layer network coding in wireless relay networks

This paper proposes a cooperative quadrature physical layer network coding (CQPNC) scheme for a dual‐hop cooperative relay network, which consists of two source nodes, one relay node and one destination node. All nodes in the network have one antenna, and the two source nodes transmit their signals modulated with quadrature carriers. In this paper, a cooperative quadrature physical layer network coded decode‐and‐forward (DF) relay protocol (CQPNC‐DF) is proposed to transmit the composite information from the two source nodes via the relay node to the destination node simultaneously to reduce the number of time slots required for a transmission. The proposed CQPNC‐DF relay protocol is compared with time‐division multiple‐access amplify‐and‐forward (TDMA‐AF), TDMA‐DF, cooperative network coded DF (CNC‐DF) and cooperative analog network coded AF (CANC‐AF) relay protocols to demonstrate its effectiveness in terms of bit error rate (BER) and system throughput under different propagation conditions. The simulation results reveal that the proposed CQPNC‐DF relay protocol can significantly improve the network performance. Compared with two TDMA schemes and CNC‐DF, the proposal can provide up to 100% and 50% throughput gains, respectively. Moreover, no matter what the scene, the proposed scheme always has the lowest BER in the low SNR region. Copyright © 2013 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.     

Exact analysis of the multihop multibranch hybrid AF/DF relaying networks

In this paper, the exact analysis of a multihop multibranch (MHMB) relaying network is investigated wherein each relay can operate in amplify‐and‐forward (AF) or decode‐and‐forward (DF) modes depending upon the decoding result of its received signal. If a relay decodes the received signal correctly, it works in DF mode; otherwise, the relay operates in AF mode. Therefore, we name such relaying network as hybrid amplify‐and‐forward and decode‐and‐forward (hybrid AF/DF) relaying network. We first investigate the signal transmission from source to destination node via n number of relays in a hybrid AF/DF MHMB mode. Then, we obtained the statistical features and analyze the end‐to‐end signal‐to‐noise ratio (SNR). Finally, a comprehensive performance analysis is conducted by using maximal ratio combining (MRC) scheme at the destination node. For comparison, we also obtained the results using selection combining (SC) scheme at the destination node. To the best of our knowledge, very few works in the literature have considered a general system model of MHMB relaying network wherein each relay can operate in AF or DF modes, that is, a hybrid AF/DF relaying network. Accordingly, the analysis of our system model is not only novel and exact, but also is comprehensive and can be employed in the future works.     

Cooperative multiple relay system for frequency selective environment: outage analysis and power allocation

In this paper, we propose a novel cooperative relaying scheme with multiple relays for frequency selective wireless environment. In our proposed scheme, the frequency selective wireless channel is divided into flat fading sub‐channels. Cooperative relaying is then employed over each sub‐channel to improve the system diversity order. We then investigate the asymptotic behavior of the outage probability and show that the proposed scheme achieves full diversity order in both amplify and forward (AF) and adaptive decode and forward (ADF) relaying scenarios. Furthermore, we propose a power allocation strategy to minimize the system outage probability. Simulation results confirm our analysis and show that the proposed power allocation method outperforms uniform power allocation. Copyright © 2010 John Wiley & Sons, Ltd.     

A Novel Adaptive and Cooperative Scheme to Save Energy for Wireless Sensor Network

In this paper, we evaluate the performance of a novel cooperative adaptive relaying scheme called ISDF (Incremental Selective Decode and Forward). We compare the energy efficiency of this underlying scheme with direct transmission (DT), Fixed and adaptive relaying schemes in Wireless Sensor Network (WSN). The system is constrained by a minimum value of Signal-to- Noise Ratio (SNR) and the end-to-end throughput. We consider a three-node cooperative system. The relay is equidistant to the source and destination, and then we study different positions of the latter. The destination receives two copies of the message: one from the source and the other from the relay. Then it combines them by using maximal ratio combining (MRC). The proposed scheme differs from other schemes by the combination of the selective decode and forward scheme with the incremental scheme which requires the presence of a feed-back from the destination. The analysis reveals that the proposed ISDF provides good energy efficiency compared to DT and other cooperative schemes. It also shows that DF cooperative scheme (decode and forward , either with feedback or no, is outperformed by AF (amplify and forward) scheme especially when the relay is placed near the destination.

     

On the achievable diversity-multiplexing tradeoff in half-duplex cooperative channels

We propose novel cooperative transmission protocols for delay-limited coherent fading channels consisting of N (half-duplex and single-antenna) partners and one cell site. In our work, we differentiate between the relay, cooperative broadcast (down-link), and cooperative multiple-access (CMA) (up-link) channels. The proposed protocols are evaluated using Zheng-Tse diversity-multiplexing tradeoff. For the relay channel, we investigate two classes of cooperation schemes; namely, amplify and forward (AF) protocols and decode and forward (DF) protocols. For the first class, we establish an upper bound on the achievable diversity-multiplexing tradeoff with a single relay. We then construct a new AF protocol that achieves this upper bound. The proposed algorithm is then extended to the general case with (N-1) relays where it is shown to outperform the space-time coded protocol of Laneman and Wornell without requiring decoding/encoding at the relays. For the class of DF protocols, we develop a dynamic decode and forward (DDF) protocol that achieves the optimal tradeoff for multiplexing gains 0/spl les/r/spl les/1/N. Furthermore, with a single relay, the DDF protocol is shown to dominate the class of AF protocols for all multiplexing gains. The superiority of the DDF protocol is shown to be more significant in the cooperative broadcast channel. The situation is reversed in the CMA channel where we propose a new AF protocol that achieves the optimal tradeoff for all multiplexing gains. A distinguishing feature of the proposed protocols in the three scenarios is that they do not rely on orthogonal subspaces, allowing for a more efficient use of resources. In fact, using our results one can argue that the suboptimality of previously proposed protocols stems from their use of orthogonal subspaces rather than the half-duplex constraint.     

An amplify‐and‐forward and decode‐and‐forward mixed relay communication system and its performance analysis

A kind of amplify‐and‐forward (AF) and decode‐and‐forward (DF) mixed relay communication system is proposed in this letter. The source broadcasts the signal to all the relays. Relays that can decode the signal adopt DF scheme to retransmit the signal, while the rest adopt AF scheme for retransmission. The destination employs maximum ratio combining technique to maximize the received signal‐to‐noise ratio. Another situation concerned in this letter is that when the relay cannot decode the source signal, it may retransmit the interference signal with AF scheme. Closed‐form expressions of outage probability are derived. Simulation results show that the analytical curves agree with the simulated ones very well, and the AF‐DF mixed relay system can improve the availability of the relays. Copyright © 2013 John Wiley & Sons, Ltd.