基于协作分集的HAPS应急通信系统性能研究

基于协作分集技术,提出一种发射功率受限的多用户双天线协作应急通信系统,理论分析了该系统和多用户单天线协作系统在高信噪比条件下能够达到的最大分集增益,仿真研究了不同HAPS雨衰场景下该系统和传统系统的误码性能.研究结果表明:该系统能够克服传统系统在高信噪比出现的“地板效应”,在10-3误码条件下取得2～3dB的性能增益,与理论研究结果吻合.     

瑞利衰落信道下扩频信号隐分集性能分析

介绍了瑞利衰落信道下采用扩频信号的失真接收机获取隐分集增益机理.在多重分集通道和每个分集通道存在离散多径的瑞利信道模型基础上,分别详细分析推导了单重分集通道和多重分集通道情况下的隐分集性能表达式,给出了不同情况下的误码性能理论曲线,并在硬件系统测试平台上,对扩频信号的隐分集性能进行了实测.测试结果表明该分析方法可行,可用于指导工程实践.     

基于SC分集的混合FSO/RF航空通信系统性能分析

研究了基于选择合并(SC)分集的混合激光/射频(FSO/RF)航空通信系统性能,该系统设计简单且在发射端无需信道状态信息(CSI)。激光链路服从平均孔径效应下的Exponentiated Weibull大气湍流分布模型,射频采用Nakagami-m衰落信道模型,推导求出混合FSO/RF航空通信系统接收端信噪比的累积分布函数,进一步利用Meijer′s G函数推导了混合FSO/RF通信系统中断概率以及平均误码率的闭合表达式,通过仿真对比分析了不同湍流强度、调制方式及分集通信方案对系统中断、误码性能的影响。仿真结果表明,混合FSO/RF航空通信系统有效地利用了FSO及RF链路在不同大气信道条件下的互补性质,具有更优的中断和误码性能;采用BPSK调制能有效降低大气湍流对混合FSO/RF航空通信系统的影响;信噪比高时,并行传输、分集接收的通信方案对系统误码性能的改善要优于低阶调制对误码性能的改善。     

基于中继放大转发的多天线基站下行传输性能分析

研究基站具有多根天线、中继与用户具有单天线的中继放大转发多用户系统,利用正交空时分组编码和机会主义调度策略同时获得空间分集和多用户分集.针对自适应增益和固定增益转发策略,首先导出独立不同分布瑞利衰落下系统中断概率和误码率的闭合表达式;随后根据高信噪比下系统的渐近性能确定系统的分集阶数,分析空间分集和多用户分集对系统性能的联合影响,并与单跳多天线基站下行传输系统进行性能比较;最后利用计算机仿真验证理论分析结果的正确性.     

瑞利衰落多重分集后LDPC的性能分析

针对瑞利信道条件下多个收发通道分集合并后LDPC的纠错性能问题,提出了一种通用的计算方法。该方法根据合并方式如最大比值合并、等增益合并、选择合并与各分集通道的平均输入信噪比计算合并后信噪比的概率密度函数,用多项式拟合的方法得到LDPC在AWGN信道中的纠错性能公式,对该式按合并信噪比的概率密度积分得到瑞利信道下LDPC的纠错能力表达式。以一种有限几何LDPC码为例计算其编码增益,分析了在最大比值合并条件下分集重数与LDPC编码增益的关系。     

莱斯衰落信道中的正交空时分组码的性能分析

在得到莱斯衰落下正交空时分组码接收信噪比的基础上,分析了在莱斯衰落信道条件下引入正交空时分组码的多天线系统的符号差错性能。理论分析及仿真结果表明在接收天线数量一定的情况下,增加发送天线的数量可以带来更大的分集增益,莱斯系数均值对系统的影响是一致的。     

基于DSTBC-MC-CDMA的无线网络协同发射分集系统

协同分集(cooperative diversity)技术通过使网络中各单天线用户共享彼此天线,形成虚拟的多天线阵列来实现发射或接收分集,可以有效地提高系统性能。该文提出无线网络中频率选择性衰落信道环境下的一种基于分布式空时分组码(Distributed Space Time Block Code, DSTBC)和MC-CDMA的协同发射分集方案,并给出了系统实现。建立了误码模型,探讨了协同用户间的信道状态信息(CSI)对系统误码性能的影响,分析了误码性能的上限,并给出了仿真结果。结果表明,DSTBC-MC-CDMA系统相对于未协同的MC-CDMA系统,获得了明显的性能增益。     

弱湍流中紫外光非直视分集接收技术的研究

根据弱湍流信道中对数正态分布模型,建立了紫外光非直视分集接收系统。采用开关键控（OOK）调制,在不同闪烁指数和接收天线数下,分别对比分析了最大比合并（MRC）、等增益合并（EGC）和选择性合并（SC）的误码性能。仿真结果表明,相比于无分集情况,采用三种合并方式的误码率性能有明显提升。在接收天线数相同的情况下,三种合并方式中,MRC的性能最优,其次是EGC,SC的性能最差。对比分析了不同接收天线数时的误码率性能,随着接收天线数的增加,三种合并方式的误码性能得到了较大改善。在弱湍流信道中,采用分集接收技术能够减轻衰落的影响,提高分集增益。     

一种协同分集方案的误码性能分析

协同分集(cooperative diversity)技术通过为网络中某些单天线用户寻找若干个用户作为"伙伴",并共享彼此天线,形成虚拟的多天线阵列,来实现多天线分集,结合分布式空时分组编码(Distributed Space Time Block Code,DSTBC),可以有效地提高系统性能.多载波码分多址(Multi-Carrier Code Division Multiple Access,MC-CDMA)技术将数据调制到各个子载波上发送,可以有效地抵抗信道频率选择性衰落的影响.本文提出了无线网络中频率选择性衰落信道环境下的一种基于分布式空时分组码和MC-CDMA的协同发射分集方案,建立了协同用户间的误码表示模型,基于该模型推导了协同分集方案误码性能的理论表达式,并分析了协同用户间的平均解码差错概率对系统方案误码性能的影响,同时给出了仿真结果.结果表明,DSTBC-MC-CDMA相对于未协同的MC-CDMA系统获得了明显的性能增益,同时仿真也较好地验证了理论结果.     

基于信道编码及空时编码级联的两用户协同分集

传统协同分集通过使网络中各单天线用户共享彼此天线,形成虚拟多天线阵列来实现空间分集,使得体积和功耗受限的网络终端也能获得分集增益,然而这并没有将信道编码和空时编码结合起来以使系统得到编码增益。为了能够获得编码增益来进一步改善系统性能,本文提出了一种基于信道编码和分布式空时分组码级联方式下的两用户协同分集方案,并且在准静态的瑞利衰落信道下对系统误码性能进行了理论推导和系统仿真,给出了误比特率的上限解析表达式。在协同用户间信道存在噪声的情况下,我们分别对CRC-DSTBC和CC-DSTBC级联下的发射方案进行了性能分析和系统仿真。仿真结果表明:即使协同用户间的信道存在噪声,本文所提出的协同分集方案与传统协同分集相比,不但获得了分集增益,同时也得到了编码增益,系统误比特率大大降低,从而显著提高了系统性能,并且这也和理论分析相吻合。     

The effect of Gaussian error in selection diversity combiners

Selection diversity combining (SDC) is one of the simplest and most commonly implemented diversity mechanism for mitigating the detrimental effects of deep fades experienced on wireless channels. While SDC improves the mean combined signal‐to‐noise ratio (SNR) over that of a single branch with increasing diversity order, its main advantage is the reduction of the probability of deep fades. The effect of Gaussian errors in the branch gain estimates on the SDC receiver performance is investigated by deriving new closed‐form expressions for the probability density function, cumulative distribution function and the moment generating function of the combiner output SNR statistic. Mathematical expressions for quantifying the degradation in the mean combined SNR, outage probability and the average symbol error rate of a broad class of binary and multilevel modulation schemes owing to imperfect branch SNR estimates in Rayleigh fading are also derived. It is shown that combiner errors affect the mean combined SNR negligibly in comparison to their effect on the deep fades. Copyright © 2001 John Wiley & Sons, Ltd.     

Spatial, polarization, and pattern diversity for wireless handheldterminals

This paper examines the antenna diversity configurations that improve the performance in handheld radios. Experiments using spatial, polarization, and pattern diversity were conducted for both line-of-sight (LOS) and obstructed outdoor and indoor multipath channels that experienced Ricean fading. Antenna separation, polarization, and pattern were varied independently to the extent possible. Envelope correlation, power imbalance, and diversity gain were calculated from the measurements. Diversity performance is measured by diversity gain, which is the difference in signal-to-noise ratio (SNR) between the output of a diversity combiner and the signal on a single branch, measured at a given probability level. Diversity gain increases with decreasing envelope correlation between the antenna diversity branches. However, diversity gain decreases as the power imbalance between diversity branches increases because a branch that has a weak signal has only a small contribution to the combined signal. Diversity gain values of 7-9 dB at the 99% reliability level were achieved in non-line-of-sight (NLOS) channels for all diversity configurations even with very small antenna spacings. The use of polarization diversity reduced polarization mismatches, improving SNR by up to 12 dB even in LOS channels     

基于分集接收技术的相干大气激光通信系统性能分析

分析了独立的双伽马衰落信道下采用分集接收技术的相干光通信系统的平均输出信噪比(SNR)、通信中断概率和平均误比特率。首先,利用多项式定理推导平均输出SNR的精确表达式;其次利用正随机变量的平均值不等式推导出SNR下界的概率密度函数(PDF),并进而得出通信中断概率和外差同步二进制相移键控(BPSK)平均误比特率的联合上界。回避了直接求取SNR的PDF,降低了性能分析的复杂度。结果表明,分集接收有效地克服了大气湍流对系统性能的影响,且分集路数越多,性能改善越明显,但强弱湍流区的性能差异也越大;所得到的性能上界与已有的Monte-Carlo模拟结果十分接近。     

Performance of antenna diversity multiuser receivers in CDMA channels with imperfect fading estimation

The performance of antenna diversity coherent and differentially coherent linear multiuser receivers is analyzed in frequency-nonselective Rayleigh fading CDMA channels with memory. The estimates of the complex fading processes are utilized for maximal-ratio combining and carrier recovery of the coherent multiuser receiver. To analyze the impact of channel estimation errors on the receiver performance, error probability is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimation as well as the fading-induced error probability floor. The impact of fading dynamics on the differentially coherent decorrelating receiver with equal-gain combining is quantified. While performance of multiuser receivers at lower SNR is determined by both the fading dynamics and the number of active CDMA users, performance at higher SNR is given by an error probability floor which is due to fading only and has the same value as in a single-user case. The comparison of the two receiver structures indicates that the coherent decorrelating receiver with diversity reception may be preferable to the differentially coherent one in nonselective fading CDMA channels with memory.     

基于随机相位误差Tikhonov分布的MC-DS-CDMA系统性能分析

本文针对工作于慢变频率选择性Rayleigh衰落信道中的部分相干的MC-DS-CDMA系统，基于随机相位误差的Tikhonov密度分布提出了误码率上边界的一种分析方法。仿真结果表明，随着环路信噪比的增加，相位误差损耗所产生的系统误码率性能损失变得越来越小，当环路信噪比高于系统信噪比(Eb/N0)20dB时，系统误码率性能损失小于0．2dB。在MC-DS-CDMA系统中，由于能用频率分集取代路径分集，因而接收机结构更易实现。     

Performance Analysis of Multiuser DS-CDMA in MIMO Systems Over Rayleigh Fading Channels

The performance of space-time transmit diversity is examined in a multiuser direct-sequence code-division multiple-access (DS-CDMA) system over fast- and slow-fading channels. The underlying space-time system employs transmit antennas and receive antennas at the mobile user and receiver base station, respectively. We consider the performance of the space-time multiuser system when using the linear decorrelator detector to combat the effect of multiuser interference. In our analysis, we derive a closed-form expression for the probability of bit error for both fast- and slow-fading channels. These theoretical results are shown to be very accurate when compared to system simulations. Both simulations and theoretical results prove that, regardless of the system load, the full diversity order of for fast-fading channels and for slow-fading channels is always maintained, and only a signal-to-noise ratio (SNR) loss is incurred. This SNR loss is proved to be a function of only the number of users (i.e., level of interference) and independent of the number of transmit and/or receive antennas. Using our theoretical results, we show that the loss in SNR from the single-user bound can be well approximated by , where represents the level of multiuser interference.     

频率选择性信道下高速数据传输中的单载波频域均衡技术

本文利用空时联合检测的方法，提出了在频率选择性信道下多天线发送的单载波频域均衡方案，它可以提供和一个发送天线多个接收天线单载波系统相同的分集增益，和单载波时域均衡相比具有低的复杂度。分析了信道估计误差对系统的影响，类似于基于迫零算法的线性均衡器的情形，在低信噪比下，信道估计误差对系统影响很大，给出了一种简单的克服方法。在不增加发射功率的情况下，使用信道编码可以进一步改善系统的性能。本方案的系统性能在6径典型城市信道模型下进行了评估，仿真的结果证明了本方案的有效性。     

Performance of generalized selection combining over Weibull fading channels

The literature is relatively sparse in performance analysis of diversity combining schemes over Weibull fading channels, despite the fact that the Weibull distribution is often found to be suitably fit for empirical fading channel measurements. In this paper, we capitalize on some interesting results due to Lieblein on the order statistics of Weibull random variables to derive exact closed‐form expressions for the combined average signal‐to‐noise ratio (SNR) as well as amount of fading (AF) at a generalized selection combining (GSC) output over Weibull fading channels. We also use some simple AF‐based mappings between the fading parameters of the Weibull distribution and those of the Nakagami, Rice, and Hoyt distributions to obtain the approximate but accurate average SNR and AF of GSC over these types of channels. The mathematical equations are validated and illustrated by some numerical examples for scenarios of practical interest. Copyright © 2006 John Wiley & Sons, Ltd.     

SNR estimation in Nakagami-m fading with diversity combining and its application to turbo decoding

We propose an online signal-to-noise ratio (SNR) estimation scheme for Nakagami-m (1960) fading channels with L branch equal gain combining (EGC) diversity. We derive the SNR estimate based on the statistical ratio of certain observables over a block of data, and use the SNR estimates in the iterative decoding of turbo codes on Nakagami-m fading channels with L branch EGC diversity. We evaluate the turbo decoder performance using the SNR estimate under various fading and diversity scenarios (m = 0.5, 1, 5 and L = 1, 2, 3) and compare it with the performance using perfect knowledge of the SNR and the fade amplitudes.