低轨卫星移动通信系统中译码前传协作策略的性能研究

卫星移动通信信道是典型的衰落信道,其信道特性在不同的环境中通常被表述为三种衰落形式,即Rayleigh 衰落、Rician衰落和遮蔽Rician衰落.为了保证可靠的通信,系统需要采用具有较强抗衰落能力的技术来进行信号的传输.近年来,协作分集技术因其能显著地提高无线通信系统的性能而吸引了大量的研究关注.莱曼等人的研究中提出了多种协同策略,包括放大前传,译码前传和编码协同等.本文中提出将协作分集技术应用在低轨卫星移动通信系统用户下行链路(卫星和终端用户之间的通信链路)中以减小信道衰落的影响.建立系统模型,分析得出该系统采用译码前传协作策略的通用误码率公式,由此推导出用户下行链路在Rayleigh衰落信道、Rician衰落信道、遮蔽Rician衰落信道条件下的系统误码率公式.仿真结果表明,理论分析和仿真结果完全吻合,本系统相对于传统无协作系统有较大的性能优势.     

信道估计错误对LDPC-OFDM系统译码性能的影响

LDPC码应用于OFDM系统能够得到非常好的性能,然而当信道估计信息发生错误将直接影响译码性能.为此,研究了信道估计误差对LDPC-OFDM系统性能的影响.理论和仿真得出了以下结论:信道估计误差可带来译码过程外部信息信噪比的提高或降低,由此加速成功译码或者导致译码失败.     

多用户SIMO-OFDM系统中的协作分集性能分析

协作分集使得在多用户环境下,具有单天线的多个终端可以共享彼此的天线,以形成一个虚拟的天线阵列结构,实现空域分集。首先介绍了多用户SIMO—OFDM系统中的两种协作方式,放大前传和译码前传。然后针对源结点和目的结点之间是否存在路径损耗,提出了一种基于信道状态信息的选择中继协作方式,并对系统性能进行了理论和仿真分析。仿真结果表明,这种基于信道状态信息的选择中继协作方式可以获得较好的系统性能。     

Turbo编码OFDM系统中一种判决反馈的WLMS信道跟踪算法

在Turbo译码时对信息位和校验位都进行MAP译码,避免了传统基于译码再编码判决反馈信道估计和跟踪算法中反馈信号重构时的错误传播问题,降低了反馈信号的错误概率,提高了信道估计和跟踪的性能,解决了在信道变化较快条件下文献[8]提出的维纳LMS(WLMS)算法信噪比门限过高的问题。仿真结果表明,算法性能接近理想判决反馈时的性能限,与直接判决反馈的维纳LMS算法相比,在系统误比特率为10~(-4)条件下系统性能约提高2dB。     

SNR估计误差对BICM-ID系统性能影响的研究

在BICM-ID(Bit-Interleaved Coded Modulation with Iterative Decoding,BICM-ID)系统或其他级联码系统中实现迭代检测接收机需要对信道的信噪比(SNR)进行估计。该文研究信噪比估计误差对BICM-ID系统性能的影响,分析各种译码算法对信噪比估计误差的敏感程度。理论分析和仿真结果表明max-log-APP算法对信噪比估计具有最好的鲁棒性,log-APP及其他简化算法对SNR估计误差的鲁棒性在SNR过估计时比SNR欠估计时好,原因在于各个算法所采用的修正函数不同。满足算法性能没有大的损失的SNR估计误差精度范围随着信道条件的改善而增大,通常在-2dB到+6dB之间。因此在非理想的信道状态信息条件下,max-log-APP算法是BICM-ID系统的最佳译码算法。     

基于机会中继和最大比合并下的协作通信系统

分析了机会中继和最大比合并下的协作通信系统性能.首次给出了Nakagami衰落信道下的符号错误率和中断概率的精确闭式结果.通过对中断概率的近似结果分析发现对于任意一条源节点-中继-目的节点链路,分集增益均由源节点-中继链路和中继-目的节点链路中的深衰落链路决定.     

多天线认知系统中最优协作用户数的选择研究

针对非理想控制信道传输错误对认知系统中多天线多用户协作频谱感知性能影响的问题,提出一种使得认知系统误检概率最小的最优协作用户数选择方案,并推导给出其闭式表达式。该方案首先根据单根天线错误检测概率最小的原则推导出认知用户单根天线最优的判决门限。然后利用“K秩”准则对多天线进行合并,根据认知用户错误检测概率最小的原则推导出最优的“K”值。最后根据认知系统误检概率最小的原则推导出最优的协作感知用户数。通过仿真验证了该方案理论的正确性,并分析给出了控制信道错误概率对认知系统检测性能的影响。相比于传统的协作检测算法,本方案具有更好的检测性能。      

非理想信道下编码-预编码OFDM迭代译码性能界

针对信道估计精度会影响迭代译码性能的问题,分析了非理想信道估计下信道编码与预编码结合的OFDM系统迭代接收机的性能。利用信道估计误差强度来表征信道估计质量,推导了信道参数不理想时系统误码率的一般上界,并给出了一种经验下界。理论分析及仿真表明,信道估计误差不损失系统的分集增益,只造成编码增益下降。在各种系统参数选取下,误码率仿真曲线均落入理论分析上下界之间,且两者间距仅为0.6 dB左右,表明所提出的上下界可有效预测实际系统的渐近误码率性能。     

非对称信道下协作通信系统中断概率的研究

首先建立了非对称信道下协作系统的数学模型,考虑了源节点到目的节点的直通链路,利用特征函数法推导出协作系统的平均中断概率公式,通过仿真验证了该中断概率公式的精确性,理论和仿真都表明协作系统的中断概率和中继节点的个数、位置分布以及协作系统的功率分配因子有着密切关系,并且考虑源到目的节点的直通链路相对于不考虑直通链路的情况具有更高的性能增益;然后又在此公式基础卜就协作系统中继节点的位置分布对最优功率分配因子的影响进行了仿真分析,并指出协作系统的最优功率分配因子受制于中继节点的位置分布情况.     

TD-LTE-Advanced系统SRS信道估计误差性能研究

在实际TD-LTE-Advanced系统中,针对非理想SRS信道,结合SRS信道估计误差的理论,建立SRS信道估计误差的模型,深入研究SRS误差对MIMO的性能影响。仿真分析显示,存在SRS误差的实际信道对协作多点传输（CoMP）有较大影响,但是相对其他基于波束赋形的MIMO模式仍能保证较高的增益。文章的研究工作分析非理想信道估计对系统性能的影响,并评估非理想SRS信道下多种MIMO模式的系统性能。     

Performance analysis and power allocation for decode‐and‐forward cooperative communications over Rician fading channel

This paper derives the asymptotic symbol error rate (SER) and outage probability of decode‐and‐forward (DF) cooperative communications over Rician fading channels. How to optimally allocate the total power is also addressed when the performance metric in terms of SER or outage probability is taken into consideration. Analysis reveals the insights that Rician factor has a great impact on the system performance as compared with the channel variance, and the relay–destination channel quality is of importance. In addition, the source–relay channel condition is irrelevant to the optimal power allocation design. Simulation and numerical evaluation substantiate the tightness of the asymptotic expressions in the high‐SNR regions and demonstrate the accuracy of our theoretical analysis. Copyright © 2011 John Wiley & Sons, Ltd.     

时变信道下ADF协作车载通信误码率分析

智能车载协作系统中车辆快速移动使得无线通信信道具有时变特性,为有效评估系统的误码性能,给出了符合车载时变信道的一阶自回归(AR1)模型,提出了一种基于AR1模型的自适应解码转发(ADF)协作误码率分析方法.该方法通过AR1模型的多普勒频偏相关系数来刻画时变信道特性,根据中继译码结果自适应选择是否协作转发,提升了智能交通系统的可靠性.此外,利用矩生成函数(MGF)推导出ADF协作下多进制正交幅度调制(M-QAM)信号误码率封闭表达式,并分析了车载移动速度和信道状态信息(CSI)估计精度对误码性能的影响.数值仿真结果表明,车载系统能通过增加CSI估计精度,有效地减少车载快速移动引起的误码平顶值.该方法相对于放大转发(AF)协作通信方式,平均误码性能提高约8.7 dB.     

Hybrid decode-amplify-forward (HDAF) scheme in distributed Alamouti-coded cooperative network

In this article, a signal-to-noise ratio (SNR)-based hybrid decode-amplify-forward scheme in a distributed Alamouti-coded cooperative network is proposed. Considering a flat Rayleigh fading channel environment, the MATLAB simulation and analysis are carried out. In the cooperative scheme, two relays are employed, where each relay is transmitting each row Alamouti code. The selection of SNR threshold depends on the target rate information. The closed form expressions of symbol error rate (SER), the outage probability and average channel capacity with tight upper bounds are derived and compared with the simulation done in MATLAB environment. Furthermore, the impact of relay location on the SER performance is analysed. It is observed that the proposed hybrid relaying technique outperforms the individual amplify and forward and decode and forward ones in the distributed Alamouti-coded cooperative network.     

Performance of Decode-and-Forward Cooperative Communications with Multiple Dual-Hop Relays over Nakagami-m Fading Channels

In this letter, we examine the symbol-error-rate (SER) performance of decode-and-forward (DF) cooperative communications. Our focus is on the scenario in which multiple dual-hop relays are employed and the channel environment is described by Nakagami-m fading. Cooperative diversity is observed from the derived error-rate expressions, and some insight into how channel conditions in the relay nodes affect the SER performance is obtained. In addition, with the knowledge of the partial channel state information (CSI) at the transmitting sides, we derive an optimal power allocation scheme to further enhance the SER performance. Simulation results are finally presented to demonstrate the correctness of our analysis and perception for the considered DF protocol.     

Ser performance analysis of decode-and-forward cooperative scheme in satellite mobile channel

In this paper, Symbol-Error-Rate (SER) performance analysis is provided for a Decode-and-Forward (DF) cooperative scheme in satellite mobile channel environment. We present a satellite mobile cooperative communication system model and derive two generalized error probability expressions with Cyclical Redundancy Check (CRC) or not. We also derive and simulate SER of the proposed system over different satellite mobile channels. The results show that the analytical results are in great accordance with the ones obtained by simulation. Also, it was shown that, whether or not adopt CRC depends on the channel link quality between the source node and the relay node.     

SER PERFORMANCE ANALYSIS OF DECODE-AND-FORWARD COOPERATIVE SCHEME IN SATELLITE MOBILE CHANNEL

In this paper,Symbol-Error-Rate (SER) performance analysis is provided for a Decodeand-Forward (DF) cooperative scheme in satellite mobile channel environment.We present a satellite mobile cooperative communication system model and derive two generalized error probability expressions with Cyclical Redundancy Check (CRC) or not.We also derive and simulate SER of the proposed system over different satellite mobile channels.The results show that the analytical results are in great accordance with the ones obtained by simulation.Also,it was shown that,whether or not adopt CRC depends on the channel link quality between the source node and the relay node.     

Symbol error rate analysis and power allocation for hybrid cooperation in Rayleigh fading environment

In this paper, we investigate the symbol error rate (SER) performance and power allocation for hybrid cooperation (HC) system in a Rayleigh fading environment. Specifically, we derive closed‐form and approximate SER expressions for the HC system by using the moment generating function (MGF)‐based approach. However, the resultant SER expressions contain an MGF of the harmonic mean of two independent random variables, which is not tractable in SER analysis. Then, we present simple and tight bounds on the SER to show the asymptotic behavior of the HC system. It is shown that the HC system achieves full diversity order. On the basis of the tight SER bound, the power allocation is investigated. It is found that the optimal power allocation (OPA) of the HC system does not depend on the fading parameter of the source‐to‐destination channel and it only depends on the channel links related to the relay. With the increase of the ratio of mean channel power gains (i.e., the ratio of the relay‐to‐destination channel gain to the source‐to‐relay channel gain), more performance gain can be acquired by using the OPA scheme. Finally, the analytical results are validated through computer simulations. Copyright © 2012 John Wiley & Sons, Ltd.     

Cooperative Communication Protocols in Wireless Networks: Performance Analysis and Optimum Power Allocation

In this paper, symbol-error-rate (SER) performance analysis and optimum power allocation are provided for uncoded cooperative communications in wireless networks with either decode-and-forward (DF) or amplify-and-forward (AF) cooperation protocol, in which source and relay send information to destination through orthogonal channels. In case of the DF cooperation systems, closed-form SER formulation is provided for uncoded cooperation systems with PSK and QAM signals. Moreover, an SER upper bound as well as an approximation are established to show the asymptotic performance of the DF cooperation systems, where the SER approximation is asymptotically tight at high signal-to-noise ratio (SNR). Based on the asymptotically tight SER approximation, an optimum power allocation is determined for the DF cooperation systems. In case of the AF cooperation systems, we obtain at first a simple closed-form moment generating function (MGF) expression for the harmonic mean to avoid the hypergeometric functions as commonly used in the literature. By taking advantage of the simple MGF expression, we obtain a closed-form SER performance analysis for the AF cooperation systems with PSK and QAM signals. Moreover, an SER approximation is also established which is asymptotically tight at high SNR. Based on the asymptotically tight SER approximation, an optimum power allocation is determined for the AF cooperation systems. In both the DF and AF cooperation systems, it turns out that an equal power strategy is good, but in general not optimum in cooperative communications. The optimum power allocation depends on the channel link quality. An interesting result is that in case that all channel links are available, the optimum power allocation does not depend on the direct link between source and destination, it depends only on the channel links related to the relay. Finally, we compare the performance of the cooperation systems with either DF or AF protocol. It is shown that the performance of a systems with the DF cooperation protocol is better than that with the AF protocol. However, the performance gain varies with different modulation types and channel conditions, and the gain is limited. For example, in case of BPSK modulation, the performance gain cannot be larger than 2.4 dB; and for QPSK modulation, it cannot be larger than 1.2 dB. Extensive simulation results are provided to validate the theoretical analysis.     

Adaptive Path Selection Scheme with Combining for Multiple Relaying Cooperative Communications Networks

In future communications, cooperative communications with relay networks will be one of the most effective schemes to enlarge the coverage area and to boost the data rate. In the recent research results, the path selection, power allocation, and relay protocols on relay networks are the most important factors to improve the system performance. However, the channel quality of the direct transmission path and the relaying path has an influential effect on the performance of relay networks. Therefore, in this paper, we propose a best relaying path selection (BRPS) scheme to obtain the path diversity to improve the system capacity and data rate for cooperative networks (CNs). Simulation results show that the more the relay nodes are selected, the lower the bit error rate (BER) is. The proposed BRPS scheme obtains a high concession between both BER and system capacity for CNs.     

Performance Analysis and Optimal Power Allocation for Space–Time and Amplify-and-Forward Cooperative System

In this paper, we consider a space?Ctime and amplify-and-forward (ST-AF) cooperative system which consists of two-antenna source, single-antenna relay and destination. Source transmits Alamouti space?Ctime coding symbols to destination with cooperation of relay which adopts AF strategy. Closed-form symbol error rate (SER) is derived for the ST-AF system with PSK signals. Moreover, a SER approximation is developed to show the asymptotic performance of the ST-AF cooperative system in medium and high SNR regimes. For comparison, the SER approximation of another cooperative space?Ctime coding (C-STC) scheme is also derived. Theoretical analysis shows that the ST-AF can obtain more diversity gain and achieve higher diversity order than C-STC. Statistical optimal power allocation (OPA) algorithm for the ST-AF cooperative system is presented based on the SER performance. It turns out the OPA only depends on the channel links related to the relay, not depend on the direct link between source and destination. Finally, numerical simulations validate the theoretical analysis.