无线传感网物理层协作分集的性能分析

该文针对无线传感网中的远距传输问题,研究了一种无需网络同步和正交信道的协作分集方法的性能,给出了在两种典型信道中当解码转发存在误差传播时远程目的节点的误码率及分集指数;分析了当转发节点间为白高斯信道时增加协作节点数能够提高目的节点性能的条件。研究结果表明,当转发节点间为白高斯信道时,只要转发节点处于正常工作点,增加节点数就能提高目的节点的性能;当转发节点间为瑞利平衰落信道时,只有当转发节点处于一定位置时,协作分集相对于直接发送或传统空间分集才有性能增益;当转发节点间为瑞利平衰落信道时增加跳数性能更好,为白高斯信道时增加分支数更有效;当转发节点间为瑞利平衰落信道时,误差传播将使目的节点的分集指数为1,但在较低信噪比条件下对分集性能影响很小。     

多传感器节点分布式协作调制识别算法

调制方式的自动识别是保证合法通信的关键措施之一,在民用和军用领域都有重要的作用。本文提出了一种在无线传感器网络中多个传感器节点分布式协作识别数字调制信号的新方法。为了克服在衰落信道中低信噪比时单接收节点调制识别率低的缺点,实现对MASK,MFSK,BPSK,QPSK以及OFDM这几种典型调制方式的正确识别,首先利用网络中相互协作的多个传感器,从提高网络识别性能出发,在每个传感器节点能耗最小的前提下,根据接收信噪比的大小设计有效的协作方案,得到反映调制类型显著差异的特征参数的新组合,然后利用径向基神经网络对数字调制信号进行识别,并就不同的网络条件,给出了不同的协作方案。仿真结果表明,与单节点调制识别相比,本文设计的调制识别方法在衰落信道中具有更高的识别率,并且节点系统更灵活可靠。      

一种超宽带模拟合并转发协作通信系统

针对频率选择性衰弱信道下协作通信系统信道估计复杂的问题,提出了一种模拟合并放大转发超宽带(UWB)协作通信方案.该方案在源节点处对发送符号进行实数分布式空时编码并附加传输参考,在中继节点处利用传输参考对多径分量进行模拟合并,从而构建了一种改进型非正交放大转发协作通信系统.仿真结果证明,相对于无协作通信系统,模拟合并转发...     

基于定向天线的多中继DF协作通信系统性能分析

为提高多中继协作通信系统可靠性,将定向天线和多中继协作通信系统相结合,提出基于定向天线的解码转发最佳中继选择协作策略.首先理论推导出定向发送一定向接收模式下通信系统中断概率闭合表达式,并推导出定向发送一全向接收、全向发送一全向接收模式下的系统中断概率计算式.然后对3种模式下,协作通信系统中断概率和信噪比、中继节点数、天线增益、信道衰落系数以及信道容量之间的关系进行了数值仿真分析.理论分析和仿真结果表明,采用定向天线的协作通信系统可以有效降低协作通信系统中断概率,提高频谱利用率,并且存在最佳中继节点和最优功率分配系数使得中断概率进一步下降.     

CE-OFDM调制中继选择AF系统的误码性能

恒包络（CE）正交频分复用（OFDM）能够开发OFDM的优点,并能消除OFDM峰均功率比高、对功率放大器非线性失真敏感的缺点。反馈时延等因素会恶化采用OFDM调制的中继选择放大转发（AF）协作系统的误码性能。最小均方误差（MMSE）维纳信道预测器能够减轻反馈时延对无线系统误码性能的恶化。为此,提出了一种能够克服OFDM缺点和反馈时延影响的、采用CE-OFDM调制和MMSE信道预测的中继选择AF协作系统方案,并推导其在瑞利块衰落信道上矩形正交幅度调制下的平均误比特率（ABER）下界表达式。数值计算和仿真结果表明：当调制系数较小时,上述系统的ABER下界具有较高的准确性,可用于CE-OFDM调制和MMSE信道预测的中继选择AF协作系统的设计。     

无线传感器网络中协作通信的能耗优化方法研究

针对能量受限的无线传感器网络,该文综合考虑了协作节点数量和调制方式对系统能量有效性的影响,提出一种能量最优的综合优化方法。文中首先给出了在Rayleigh衰落信道环境下,协作通信系统采用二相相移键控(BPSK)和M进制正交幅度调制(MQAM)时误码率的闭式表达,同时对协作通信的系统能耗进行了分析。在此基础上,根据能耗最小化原则对协作节点数量和调制方式进行了联合优化。仿真结果表明,与调制方式固定或协作节点数固定的系统相比,该方案能进一步降低协作通信的系统能耗。     

基于不完美信道估计的闭环MIMO-MRC跨层设计

为提高无线通信的频谱效率,提出一种MIMO-MRC(multiple-input multiple-output-maximal ratio combining)跨层方案。是物理层的自适应调制(adaptive modulation,AM)和数据链路层的自动重传(automatic repeat request,ARQ)协作,发射端利用估计信道信息反馈,自适应调节调制模式,选择最优发射权矢量和自动重传发射数据。分析了估计误差对MIMO-MRC跨层系统的影响,给出了MIMO-MRC系统在信道估计存在误差时的频谱效率和中断概率的闭合表达式。通过仿真实验证明,对比SISO(single-input single-output)跨层系统和A lamouti's跨层系统,MIMO-MRC跨层系统的性能有明显提高,可获得约3 d B的分集增益。     

认知中继网络的多中继选择方法

将中继协作技术用于认知无线电网络可有效对抗信道衰落,提高频谱利用率,然而如何选择中继协助源节点传输信息是一个亟待解决的问题。该文根据多中继协作时目的节点信噪比的一阶偏导数特征,定义了中继协作效率,设计了基于协作效率的迭代中继选择方案。考虑到认知中继网络的非集中式结构,该文将虚拟计时器引入中继节点,提出了基于迭代中继选择方案的分布式算法,通过各中继基于协作效率的虚拟倒计时及信息交互实现了分布式迭代中继选择。复杂度分析和数值仿真结果表明,该算法复杂度较低,且性能接近最优解。     

散射通信的协作分集技术研究

对协作分集进行简单介绍,提出了将协作分集应用到散射通信中的方案,分析了协作分集在散射通信中的应用可行性。并对其中的放大转发和译码转发在只有一个中继节点的情况下进行仿真,仿真结果证明在大信噪比时译码转发的性能优于放大转发。且协作分集应用到散射通信中在相同的信噪比下,误码率更低,提高了系统性能。利用协作分集技术对散射通信进行组网,较直接组网设备复杂度低,易于实现。     

一种提高协作通信性能的新方法

传统的无线通信中,由于外部环境的干扰,信道的不稳定,网络的中断率比较高。在信道条件较差时,通信性能较差。本文利用无线通信节点间的合作,通过不同节点对信息的中继放大、解码（或者网络编码）,对直接传输的无线信道性能进行改进,大大提高了网络性能,降低网络的中断率;通过比较不同频谱效率下的不同转发方式的中断率性能,选择合适的传输方法。仿真实验得到：频谱效率小于一定值的情况下,利用放大和网络编码转发方式的性能大大高于直接传输方式。     

异步协同系统频选信道下基于线性预处理的DSTC结构设计

在协同中继系统中,应用分布式空时码(Distributed Space Time Coding, DSTC),可以在有效提高系统效率的同时获得全协同分集。但是,各中继节点的异步传输和节点间的多径衰落会破坏空时码字的结构,使之不能获得全分集。本文针对两中继的异步协同系统,提出了一种频率选择性信道下的基于线性预处理的DSTC传输结构。在此传输结构中,源节点对发送数据块进行预处理后发送给中继节点,中继节点对接收信号进行简单的共轭重排等处理,使得在目的节点形成DSTC的结构。其中,为抵抗异步传输和多径衰落引入的符号间干扰(Inter-symbol Interference, ISI),在源节点处和中继节点处均加入循环前缀(Cyclic Prefix, CP)。于是目的节点对接收到的信号进行DFT处理后,可以运用ML算法对数据信息进行检测。理论分析和仿真表明,当存在定时误差和节点间为频率选择性信道时,目的节点运用ML检测算法该传输结构可获得全空间分集和全多径分集。然后,本文考虑了信道各径延迟为整数倍符号周期的情况,并且证明了该传输结构的分集增益只与节点间信道的有效信道长度有关。      

Decode-and-Forward Cooperative Diversity with Power Allocation in Wireless Networks

We study power allocation for the decode-and-forward cooperative diversity protocol in a wireless network under the assumption that only mean channel gains are available at the transmitters. In a Rayleigh fading channel with uniformly distributed node locations, we aim to find the power allocation that minimizes the outage probability under a short-term power constraint, wherein the total power for all nodes is less than a prescribed value during each two-stage transmission. Due to the computational and implementation complexity of the optimal solution, we derived a simple near-optimal solution. In this near-optimal scheme, a fixed fraction of the total power is allocated to the source node in stage I. In stage II, the remaining power is split equally among a set of selected nodes if the selected set is not empty, and otherwise is allocated to the source node. A node is selected if it can decode the message from the source and its mean channel gain to the destination is above a threshold. In this scheme, each node only needs to know its own mean channel gain to the destination and the number of selected nodes. Simulation results show that the proposed scheme achieves an outage probability close to that for the optimal scheme obtained by numerical search, and achieves significant performance gain over other schemes in the literature     

A novel relay selection algorithm using ant colony optimization with artificial noise for secrecy enhancement in cooperative networks

Cooperative communication is one of the fastest growing research areas of today. It can efficiently mitigate the effect of shadowing and fading with the help of relays and proper relay selection technique. In this paper, a novel relay selection scheme combined with artificial noise (AN) is devised to enhance the secrecy of cooperative networks with amplify‐and‐forward scheme, over Rayleigh fading channels in the presence of a passive eavesdropper. The probability of path selection of ant colony optimization algorithm is used for selecting the best relay with high end‐to‐end signal‐to‐noise ratio. The probability of choosing a path depends on the significance of channel gain (G) and fading coefficients (h). The proposed algorithm finds the best relay in the following wireless scenarios: when (i) both channel gain and fading coefficients are significant; (ii) only fading coefficients are significant; and (iii) only channel gain is significant. Because the direct links between source and destination and source and eavesdropper are considered, AN along with the information is sent by both the source and the selected relay. The performance is evaluated based on secrecy rate (R_s); for the relays randomly placed between the source and destination and for different eavesdropper's location. The results show that the proposed relay selection scheme achieves better secrecy for different wireless scenarios compared with traditional schemes. With the help of AN, the secrecy rate can be made positive even when the eavesdropper lies near to source.     

物理层网络编码分组的机会中继

为提升物理层网络编码方案的抗衰落性能,该文提出了一种基于物理层网络编码的机会中继方案(Opportunistic Relaying based-on Physical-layer Network Coding,PNC-OR),该方案利用物理层网络编码的基本思想、有效提升网络吞吐的同时,通过中继节点的分布式选择,也能够使系统获得多用户分集增益,提高了系统的抗衰落性能。针对双向无线中继信道中端到端信息交换的情形,推导了准静态衰落环境下PNC-OR中多个目的节点接收信息的和容量。数值结果显示：和机会中继、传统网络编码两种方案相比,PNC-OR具有更高的频谱效率,并且随着中继节点的增多,频谱效率也越高。     

SER Performance of Amplify-and-Forward Cooperative Diversity Over Asymmetric Fading Channels

In this paper, the performance of amplify-and-forward (AF) cooperative diversity is analyzed over asymmetric fading channels. The source–relay and the relay–destination links experience Rayleigh fading while the source–destination link is subject to generalized Gamma fading. First, the probability density function (PDF) and the moment generating function (MGF) of the source–relay–destination link and the MGF of the source–destination link are derived. Then, the symbol error rate (SER) is determined based on the MGF of the total end-to-end signal-to-noise ratio (SNR). Moreover, the SER performance of N-relay assisted AF cooperative diversity is illustrated for M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM). Based on the derived MGF expressions, the numerical results are obtained by varying the modulation types and channel parameters for different scenarios.     

非正交选择译码转发协同

为了提高两跳中继网络的传输速率和协同分集增益,该文提出一种非正交的选择译码转发策略传输数据。单节点协同时,协同节点仅在正确译码时采用和发送节点非正交的时序转发;在多节点协同时,采用一种节点选择算法选择译码正确且信道条件最佳的节点用于非正交转发。这种协同策略可获得和非正交放大转发相同的分集复用折衷性能,但其实现更简单,且在低信噪比时中断性能更好。     

采用LDPC码的分布式联合信源信道网络编码

提出了采用低密度奇偶校验码的分布式联合信源信道网络编码方案,应用于两源一中继一目的节点的无线传感器网络中.在方案中,信源节点通过传输系统信道码的校验位与部分信息位,同时实现了信源压缩与信道纠错.中继节点有效利用数据的相关性进行译码,并进行部分数据比特删余,减少因中继端网络编码引起的错误传播,仿真验证了方案的有效性.应用了不等差错保护思想,更贴近实际应用场景,利于目的节点进行更好的低误差解码.     

Space-time cooperative diversity scheme using full feedback

This article proposes a new space-time cooperative diversity scheme called full feedback-based cooperative diversity scheme (FFBCD). In contrast to the conventional adaptive space-time cooperative diversity schemes that utilize the feedback from only the destination node, the new scheme utilizes the feedback from both the destination node and the cooperation node. With the feedback from the destination node, the occasional successful reception of the destination node in the information distribution stage can be detected, thus avoiding unnecessary retransmissions in the information delivery stage. The feedback from the cooperation node indicates the receiving state of the cooperation node in the information distribution stage, and the source node and the cooperation node will not perform cooperative retransmission during the information delivery stage unless the cooperation node is received successfully in the information distribution stage. In this way the new scheme can reduce the number of transmission attempt and improve the channel utilization. The expressions of the average number of transmission attempt are given. Numerical approximations and simulation results both show that the new scheme performs better than the non-cooperative scheme and the conventional adaptive space-time cooperative diversity scheme.     

BER-Constrained Incremental Relaying with Relay Selection in Cooperative Wireless Networks

One of the main drawbacks of cooperative communication systems with half-duplex relaying is inefficient use of spectrum. Incremental relaying is an effective technique that overcomes this limitation, particularly in the high-SNR regime. In this paper, for a multi-relay cooperative network with arbitrary number of relay nodes, we propose a spectrally-efficient incremental relaying scheme that eliminates redundant relay transmissions such that an average end-to-end bit error rate (BER) constraint is satisfied. For detect-and-forward relaying, under the assumption of error propagation from the relays to the destination, we analyze the performance of the system in terms of the end-to-end BER, the average spectral efficiency and the outage probability for Rayleigh fading environment. The performance of the system depends on a single threshold employed by the relays and the destination. We optimize this threshold such that the average spectral efficiency of the system is maximized under an average end-to-end BER constraint. This optimization criterion results in a constant-BER, variable-spectral efficiency relaying scheme. The proposed scheme makes use of spectrum as efficiently as possible, while providing the required quality of service at the destination.     

A reliable relay selection scheme for SSK modulation in cooperative communication systems

Space shift keying (SSK) modulation is a scheme in which the indices of active transmit antennas play an essential role in carrying information bits. In SSK modulation, the communication channel acts as modulating unit. Therefore, using the conventional relay selection schemes based on channel coefficients are impossible in cooperative systems using SSK modulation. In this paper, we propose a novel relay selection scheme that is applicable in cooperative systems using SSK modulation. The proposed scheme is based on calculating and measuring the Euclidean distances between received signal and all active or inactive channel coefficients. The proposed relay selection scheme is applied for a cooperative communication system with multiple relay nodes and a direct link between source and destination. The analytical expressions are derived to calculate the average bit error rate over Rayleigh fading channels. The analytical and simulation results show that this relay selection scheme can provide a full cooperative diversity order, for SSK modulation. In addition, we show that this scheme can be used at destination as a signal selection method, to select the best received signal from different paths. The analytical results are validated using Monte Carlo simulation studies.