Computer simulation of Rayleigh fading channels in MC-CDMA systems

For Multi-Carrier-Code Division Multiple Access (MC-CDMA) systems,it is usually as- sumed that the fading of the subcarriers is frequency non-selective and independent of each other. This paper shows that the two assumptions are incompatible.In fact,the MC-CDMA signals at each subcarrier undergo fading that are highly correlated.Based on this observation,this paper develops a simulation algorithm for Rayleigh fading channels via frequency-domain correlation function,which incorporates the Doppler effect simultaneously.Numerical examples are presented to demonstrate the effectiveness of the new algorithm,with the conclusion that the independence assumption of subcar- rier fading overrates the system performance.     

改进的多载波CDMA系统性能分析

多载波CDMA技术能够提供大的系统容量、对抗瑞利衰落和消除符号间干扰等优点,成为具有发展前景的调制技术之一。本文基于各用户服务指标(QoS)的不同,提出了一种自适应子载波分配的改进的多载波CDMA系统。通过在衰落信道下的仿真,结果表明:采用本文提出的自适应子载波分配算法的改进MC—CDMA系统较之传统的MC-CDMA系统的误码率性能有了较大程度的改善。     

Performance of Up-link MC-CDMA System with Frequency Offset and Imperfect Channel Estimation Simultaneously over Nakagami Fading Channels

In this paper, effects of carrier frequency offset on performance of uplink MC-CDMA (Multi-Carrier Code Division Multiple Access) system in Nakagami fading channel are investigated through the theoretical analysis and Monte Carlo computer simulations. Both perfect maximal-ratio combining (MRC) and equal gain combining (EGC) receivers are analyzed; the impact of imperfect channel fading estimation on the performance of MRC is also explored. The performance of MC-CDMA system is also compared with that of the conventional single-carrier DS-CDMA system. Our results indicate that the performance of MC-CDMA system is sensitive to even small values of carrier frequency offset and that the performance of MC-CDMA system improves as number of subcarriers increases. In perfect channel fading estimation, the overall performance of MRC is superior to EGC. However, when imperfect or inaccurate channel fading estimation exists, which leads to serious performance degradation, EGC becomes superior to MRC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluation of Nakagami fading behaviour based on measurements in urban scenarios

It is well known that the short-term fading conditions of the received envelope in wireless communications channels can be modelled by means of the Nakagami distribution. The value of the m parameter, also called shape factor, indicates the severity of multipath fading, and is a measure of channel quality, making its estimation necessary in many applications. In this letter, a range of possible values of the m parameter based on the analysis of a measurements campaign in urban scenarios is presented.     

选择式合并方法在Nakagami信道上的性能分析

本文根据最大输出电平准则，对选择式合并在Nakagami信道上的性能作了比以往理论分析更符合实际的评价，导出了误码率的闭式解，并与最大信噪比准则时作了比较。结果表明，采用最大输出电平准则时系统的性能稍有恶化，但实现的复杂度却大为降低。     

MQAM和MPSK在Nakagami衰落信道中的BER性能

基于AWGN信道中MQAM和MPSK的BER性能的近似准确分析结果，本文导出并分析了MQAM和MPSK在Nakagami-m衰落信道中采用或不采用MRC分集时的BER性能。分析结果表明，在AWGN信道、瑞利衰落信道甚至莱斯衰落信道下，现有采用或不采用MRC分集的系统性能分析结果均可以作为本文分析结果的特例。     

MC-CDMA调制在散射通信信道下的性能分析

为提高对流层散射通信系统容量和抗干扰能力,对散射信道下的多载波扩频方案进行研究.基于所建立的Rayleigh 分布对流层散射信道模型,分析了频域扩频多载波技术MC-CDMA的系统性能,对典型散射信道下的系统误码率进行了分析与仿真.结果表明:MC-CDMA可有效提升散射通信系统容量,单用户时系统误码率性能较好,随着用户数和散射信道多普勒频移的增大,系统误码率随之增大,需采用多址干扰控制和子载波干扰消除技术来改善系统误码率性能.     

Nakagami衰落信道下的蜂窝系统性能分析

文中考虑多小区蜂窝系统中小区边缘用户受到来自其他小区间同信道干扰,分析了在Nakagami-m衰落信道下,用户和基站均为单天线时的多小区蜂窝系统性能,以获得干扰信号衰落参数m和干扰基站发射功率对系统性能的影响。采用概率密度函数（PDF）分析法推导了系统中断概率和平均误符号率（ASER）的闭合表达式,并进一步通过数值仿真来分析同信道干扰对系统性能的影响。仿真结果表明,系统的中断概率和ASER的解析曲线均与仿真结果一致,系统性能随着干扰信号衰落参数m和干扰功率的增加而降低。     

基于分形的多径衰落信道估计

该文研究了多径衰落的分形性质,提出了一种利用分形维数和小波重构来改进最小均方误差方法的新型多径衰落信道估计算法。该算法改进了衰落信道参数估计的准确度,并消除了判决方法的错误传递性。仿真结果表明该算法能较准确地估计出多径衰落信道的参数,显著提高快衰落条件下接收机的误码性能。     

带有时偏的MC-CDMA上行链路的盲信道估计

为了能在带有时偏的MC-CDMA上行链路中估计信道，提出了一种新的信道估计方法。该方法能消除时偏对信道估计产生的影响，从而无需系统同步就能准确估计出信道。另外，由于扩频序列的信息对接收端是已知的，因此该信道估计方法可以利用扩频码的信息来完成盲估计。最后的仿真结果显示，该信道估计方法有良好的性能。     

MC-CDMA系统中基于子空间的半盲信道估计

该文提出了MC-CDMA系统中一种基于子空间的半盲信道估计方法。该方法结合MC-CDMA系统信号模型的特点,通过初始信道估计来解决收敛速度慢的问题;通过部分用户的导频符号以及预均衡技术来估计模糊因子;通过迭代算法来降低计算复杂度。这些措施使得半盲信道估计方法的实用化成为可能。计算机仿真验证了该方法的有效性。     

Statistical properties of the delay spread in a two path mobile radio with Nakagami fading channel

In this paper we study the statistical properties of the rms delay spread in a two path mobile radio environment, where each path is subject to Nakagami fading. We assume a typical urban power delay profile (exponential decay) for each path. Closed form equations for the mean of the rms delay spread are derived for different values of the m parameter of the Nakagami distribution. Monte Carlo simulations are also generated and compared with the analytical results.     

A new ML detection algorithm for Orthogonal Multicode system in Nakagami fading channel

Based on the Maximum-Likelihood （ML） criterion, this paper proposes a novel noncoherent detection algorithm for Orthogonal Multicode （OM） system in Nakagami fading channel. Some theoretical analysis and simulation results are presented. It is shown that the proposed ML algorithm is at least 0.7 dB better than the conventional Matched-Filter （MF） algorithm for uncoded systems, in both non-fading and fading channels. For the consideration of practical application, it is further simplified in complexity. Compared with the original ML algorithm, the simplified ML algorithm can provide significant reduction in complexity with small degradation in performance.     

室内多载波CDMA系统在深度衰落环境下的能量受控均衡策略

多载波CDMA(MC-CDMA)是将多载波调制OFDM与DS-CDMA结合起来的多址访问技术。如何从接收到的信号恢复出发送信号，涉及到频率分集接收和均衡。本文提出的能量受控均衡技术，在多用户的环境下能较好地恢复扩频码的正交性且避免信道深度衰落的影响，而且，实验表明该方法对于不同的信道状况均能很好适应。     

一种新的快衰落信道频域跟踪算法

该文研究了第三代移动通信系统中用户移动速度在150km/h以上时,无线信道的快衰落特性,并且在3GPP系统的协议框架下提出了一种新的基于频域的信道跟踪方法(FBP),利用快衰落信道频域上的相对稳定性,解决了由于信道参数变化的速度超过自适应算法收敛速度,而造成的在时域上无法进行信道跟踪的问题,文章给出了该算法的模拟结果,并与另一种对衰落信道的参数进行基于小波的信道跟踪算法(WBP)进行了比较,该文提出的算法具有更好的性能和更低的算法复杂度。     

Nakagami信道的中断容量分析

本文讨论了Nakagami-m衰落信道系统的中断容量性能,在发射端未知信道信息,接收端的信道估计存在误差时,给出了计算中断容量上界和下界的表达式,它们是接收端的估计误差和信道参数的函数,仿真结果表明随着估计误差的增大中断容量的上界和下界同时降低,但是中断容量的下界随着信道参数的增大而增大,上界随着信道参数的增大而减小。     

MC-CDMA系统盲空时干扰抑制

该文研究在频率选择性瑞利衰落信道中MC-CDMA系统的盲空时干扰抑制。考虑对应于子载波的衰落系数是信道冲激响应的离散傅里叶变换,通过研究多径信号频域码空间和数据矢量空间,采用噪声子空间技术进行盲信道估计。为了抑制多址干扰(MAI),提出一种基于投影的辅助矢量(PAV)算法,用前一级滤波矢量的输出重构最大比合并(MRC)滤波矢量,将重构滤波矢量投射到由基本滤波矢量和前几级辅助矢量张成子空间上的正交投影作为辅助矢量,将前一级滤波矢量和新产生辅助矢量线性合并得到新的滤波矢量。仿真结果验证了该算法的有效性。     

Performance comparison of MRC and EGC on a MC-CDMA system with synchronization errors over fading channels

This work derives the average bit error rate (BER) of the uplink and downlink multicarrier code division multiple access (MC-CDMA) systems using maximum ratio combining (MRC) and equal gain combining (EGC) with synchronization errors over fading channels. The derived equation can simultaneously incorporate the parameters of the fading channel and all of the synchronization errors, including frequency offset, carrier phase jitter, and timing jitter. Numerical results indicate that those two combining schemes on the uplink and downlink MC-CDMA systems are degraded by all of the normalized synchronization errors over 10⁻². The comparison outcomes between MRC and EGC reveal that the MRC generally outperforms EGC in the uplink MC-CDMA system. However, EGC achieves better performance when the number of users is small, the normalized synchronization errors are low and the signal to noise ratio (SNR) is high. In the downlink system, EGC mainly outperforms MRC when the SNR and the number of users are gradually increased and the normalized synchronization errors are low. Therefore, the selection of MRC or EGC depends on the SNR, the synchronization errors and the number of users in uplink and downlink MC-CDMA systems.     

Rayleigh衰落信道M元多码MC-CDMA系统的性能分析

针对扩频通信中的M元多码系统,结合OFDM技术特点,提出了一种新的M元多码MC-CDMA系统.在分析系统基本原理基础上,建立了系统信号模型,对系统在Rayleigh衰落信道情况下误码性能进行了研究,推导出误码率公式,对误码性能进行了仿真.在相同传输条件下,与传统的M元多码扩频系统和MC-CDMA系统进行了对比,结果表明M元多码MC-CDMA系统在误码性能和信息传输能力上都有一定的提高.     

Performance of RAKE receivers in Nakagami fading channel witharbitrary fading parameters

A new expression for the bit error rate of RAKE receivers with either coherent or noncoherent/differentially coherent binary demodulation schemes in a Nakagami fading channel is derived. The analysis assumes an arbitrary number of diversity branches with arbitrary fading parameters