基于混沌扩频的多用户应急通信系统模型设计与仿真分析

针对应急通信中信道噪声干扰严重影响通信质量的实际问题,研究了应急通信系统信道噪声干扰的来源与本质特征,提出了应急通信信道模型;论述了混沌扩频应急通信系统干扰噪声抑制原理,设计了多用户混沌扩频应急通信系统仿真模型,并结合典型信道干扰噪声环境对其BER进行了仿真与分析。仿真实验结果表明,基于混沌扩频的应急通信系统能够有效抑制强噪声干扰,显著改善应急通信系统误码性能。     

具有减少码间干扰的多用户混沌非相干通信系统

该文用混沌载波信号代替传统载波信号,设计了一个多用户通信系统。混沌序列具有良好的相关的特性,但是不足以克服多用户通信中普遍存在的不同用户信号之间的相互干扰。利用沃尔什正交函数系优秀的正交特性,把混沌振荡器产生的混沌序列和沃尔什矩阵行向量组合,生成正交的基函数,进而产生正交的时间序列。对应不同的符号、不同的用户,会有不同的正交序列与其对应。理论推导证明改进的混沌多用户非相干通信系统可以极大地减少不同用户间信号的干扰,计算机仿真了系统的误码率性能,表明系统的性能可以得到很大提高。     

基于混沌系统的正交小波变换盲均衡算法

针对常数模盲均衡算法(CMA)收敛速度慢、稳态误差大和局部收敛的缺点,将正交小波变换与混沌通信理论相结合,提出了一种基于混沌通信系统的正交小波变换盲均衡算法(CS-WT-CMA)。该算法充分利用了混沌映射的伪随机性、遍历性、相关性以及无限宽带功率谱等特点,将混沌调制系统用于产生宽带混沌信号,使用混沌信号作为载波,在调制的同时直接对发射信号进行扩频,从而降低了信道输入信号的自相关性,更好地抑制了码间干扰和多径衰落;同时对均衡器输入信号进行正交小波变换,并作能量归一化处理,降低了信号的自相关性,从而有效地加快了收敛速度。水声信道仿真结果表明,与正交小波变换盲均衡算法(WT-CMA)相比,该算法收敛速度更快、稳态误差更小。     

一种新的混沌扩频序列优选算法

混沌序列因其相关性好、对初值敏感、保密性强等特性成为直序码分多址(DS/CDMA)系统中性能较好的一种扩频码。为了提高DS/CDMA系统保密性、降低多址干扰,改进混沌序列的传统优选算法,研究混沌互相关优选,提出一种新的混沌扩频序列互相关优选算法,对所得的优选序列进行性能仿真比较。结果表明,该算法能提供较大的系统容量且抗多址干扰能力强。     

基于三元组的扩频码构造及其性能分析

为提高扩频通信的可靠性和安全性,提出一类基于三元组密钥流的扩频码构造方法。针对传统扩频码周期短和混沌扩频码算法随机性弱等不足,将周期不重复、初值敏感和算法随机的三元组随机序列进行同态均匀映射或正交变换,以获取同态三元组扩频码或正交三元组扩频码;通过实验验证了该类扩频码具有良好的平衡度、自相关和互相关等特性指标;将该类三元组扩频码应用于蒙特卡罗直接扩频通信模型和DS-CDMA多用户通信系统模型进行仿真测试,仿真结果表明,该类扩频码相比传统扩频码、混沌扩频码更适合扩频通信需要。     

复合混沌码的直扩系统优化及抗干扰性能研究

在复杂电磁环境中,直接序列扩频作为现代通信系统最常用的通信体制,会受到诸多人为和非人为干扰。针对直扩系统抗干扰性能进行研究,提出了基于复合映射产生混沌二值序列,并通过平衡性和相关特性对序列进行筛选优化。以优选后的复合混沌扩频码应用于直扩系统,对系统设计仿真。仿真结果表明,该复合混沌码直扩系统在不同信噪比和干扰环境下,抗干扰性能优于其它扩频码,系统误码率降低,误码率增益提高5dB。     

多用户混沌扩频通信系统的设计与仿真

为了研究基于混沌扩频序列的多用户扩频通信系统性能,文章分析了多用户混沌扩频通信系统原理,在Matlab/Simulink平台上,建立了多用户混沌扩频通信系统动态可视化仿真模型,讨论了信号发生器、混沌序列发生器、积分器等主要模块的参数设置,研究了基于Chebyshev混沌映射扩频序列的扩频通信系统中用户数及信道噪声对扩频系统可靠性的影响,比较了混沌序列与m序列扩频通信系统性能.通过Simulink动态仿真结果表明,该系统具有较好的抗噪声和抗多址干扰的能力,且Chebyshev映射产生的混沌序列要明显好于m序列的系统性能.     

基于改进混沌扩频序列的多用户混沌扩频通信系统的研究与仿真

提出一种改进混沌扩频序列,并将其应用在多用户混沌扩频通信系统中,给出了多用户混沌扩频通信系统的理论推导和SIMULINK仿真模型。对该系统进行了详细的性能分析,包括信噪比、用户数、改进混沌扩频序列的分形参数和初始值等各自对系统误码率的影响,并比较了改进混沌扩频序列与传统混沌扩频序列的系统误码率随信噪比和用户数的变化情况。结果表明改进混沌扩频序列要好于传统混沌扩频序列的系统误码率,具有较好的抗噪声和抗多址干扰的能力。     

基于logistic序列的混沌超宽带通信抗干扰性能分析

本文采用改进型Logistic映射迭代产生的混沌序列作为跳时码,建立CTH-PPM混沌超宽带信号模型,并在多径信道下,对该信号在单频正弦干扰、部分频带干扰和窄带干扰下进行了性能仿真.由仿真结果可知,因混沌序列码元丰富及优良的相关特性和白噪声统计特性,系统性能得到了改善,抗干扰能力增强.     

基于混沌的DS-UWB通信系统的设计与SDR实现

依据混沌序列的白噪声统计特性、较理想的自相关和互相关特性以及产生简单、码元丰富、抗干扰能力强等特性,提出了基于混沌扩频序列的DS-UWB通信系统方案,并针对混沌DS-UWB通信系统的特点对同步和安全性能做出了改进,最后给出了一种基于软件无线电的实现方案,仿真结果表明本系统有较大的用户容量和较低的误码率。     

Performance of relay-aided downlink DS-CDMA system using transmitter preprocessing based on feedback information

In this contribution, we investigate the performance of relay-aided downlink (DL) direct sequence code division multiple access (DS-CDMA) system with multi-user transmitter preprocessing (MUTP) based on vector quantized channel impulse responses (VQ-CIRs). Specifically, the CIRs are estimated with the aid of training-sequence based estimation technique at each of the relays. These estimated CIRs are then VQ and the magnitudes and phases are fed back to the base station (BS) through feedback channels, that conflict noise and fading. At the BS, the CIRs recovered using a linear detector are then exploited to formulate the preprocessing matrix to mitigate the DL multi-user interference at the relays. Our study shows that the attainable bit-error-rate (BER) degrades when noise and fading contaminated VQ-CIRs are invoked to realize the preprocessing matrix. Nevertheless, the resultant BER performance of the MUTP based on VQ-CIRs acquired via ideal feedback remains close to that achieved with perfect CIRs assumption.     

多用户载波索引差分混沌移位键控通信系统及性能分析

现有载波索引差分混沌移位键控（DCSK）通信系统较传统DCSK系统能耗更低且数据传输速率更高,但其无法应用于多用户通信环境。提出一种多用户载波索引差分混沌移位键控系统,为系统用户分配预定义的私有子载波发送混沌参考信号,所有用户共享剩余子载波发送数据信号以提高子载波利用率,结合公共子载波的索引值与DCSK系统共同传送用户的数据比特,并在加性高斯白噪声和多径Rayleigh衰落信道中计算得到系统的近似误码率。实验结果表明,与MU OFDM-DCSK、HCS-DCSK等多用户系统相比,该系统的误码率更低,在不占用过多带宽的情况下,具有更高的频谱效率。     

衰落信道下SCMA性能仿真研究

本文在一种上行SCMA（稀疏码分多址）系统模型的基础上,研究了衰落信道下SCMA通信系统的误码性能。首先比较了AWGN（高斯）信道以及衰落信道下SCMA和SC-FDMA（单载波频分多址）两种系统的性能;接着分析了快衰落信道、慢衰落信道、平坦衰落信道和频率选择性衰落信道下SCMA系统性能。仿真结果表明,与传统正交多址SC-FDMA相比,衰落信道下SCMA在用户连接数和误码性能上有一定优势。与AWGN信道相比,衰落信道使得SCMA系统误码性能迅速下降。在瑞利平坦衰落信道下,信道衰落的快慢变化导致SCMA系统误码性能最大产生约5dB损失;在频率选择性衰落信道下,多径时延扩散致使SCMA系统误码性能最差。     

不同信道条件下MIMO空时编码技术的特性分析

为了分析多输入多输出（ MIMO）空时编码技术在不同信道条件下的性能优劣,研究了无线信道的衰落统计特性,对比了在Rayleigh信道、Rician信道和Nakagami信道条件下,正交频分复用空时块编码（ STBC-OFDM）、垂直空时分层码（ V-BLAST）及空时格型码（ STTC）的系统误码率。通过仿真分析结果表明：STBC-OFDM系统在衰减指数为5的Nakagami信道条件,系统误码率最低,Rician信道次之,Rayleigh信道最高,而且当系统误码率为10－3时,其所需的信噪比（ SNR）分别为7,9,11 dB;但对于V-BLAST和STTC系统,随着信噪比的增加,不同的信道条件对系统误码率的影响各有优劣。     

On the effect of Multiple Access Interference in a Space Time Spreading Time Hopping PPM UWB system

This study investigates the effect of multiple sources using STS across a Time-Hopping-PPM Modulation that is over an UWB Saleh-Valenzuela channel with each source using two transmit antennas. We model and analyze this system, showing that the presence of such MAI results in a significant degradation in BER performance. Simulation results show that for strong signal strength of interfering MAI signals degradation in BER performance is observed for the cases where we have five and 10 distinct MAI sources present when using Walsh-Hadamard thirty two chip spreading sequences. Using simulation, we investigate the use of other sets of orthogonal spreading sequences with improved cross correlation properties and find that for some sequences a significant improvement is seen compared to Walsh-Hadamard sequences when they are similarly misaligned. We show that MAI degrades the BER performance of such systems and that selection of improved cross correlation spreading code sequence sets improves BER.     

Adaptive rate coding using convolutional codes for asynchronous code division multiple access communications over slowly fading channels

This paper presents a method of code rate adaptation using punctured convolutional codes for direct sequence spread spectrum communication systems over slowly fading channels. A blind channel estimation technique is used to estimate the nature of the multi-user channel at the detector (before the decoder). The path gains obtained from the channel estimation technique are used to adapt the code rates. Punctured codes derived from a specific rate 1/2 (M = 4) mother code are used to provide error protection corresponding to the actual channel state. The upper and lower bounds on the bit error probability and the upper bound on the error event probability are derived for hard-decision and soft-decision decoding over Rayleigh and Rician fading channels. The throughput gains obtained using the adaptive scheme and the performances of the punctured codes are studied.     

Multiple level orthogonal codes and their application in MC-CDMA systems

This paper proposes the use of multiple level orthogonal (MLO) codes in multicarrier CDMA (MC-CDMA) systems. It is extremely challenging to improve system error probability performance of MC-CDMA systems with binary spreading codes in multi-user conditions, since attainable-diversity performance is severely degraged by multi-user interference (MUI) in frequency-selective fading channel conditions, even with the use of optimum multi-user detection (MUD) methods. MLO codes are shown to improve system error probability performance in heavily-loaded or fully-loaded systems, in comparison to binary codes. Some widely used MLO code generation methods are summarized, and a new generation method is also provided. The performance advantage of MLO codes over binary codes is analyzed by treating the spreading process in MC-CDMA as a coding process and via analysis of pair-wise sequence error probability. Rules for choosing desirable MLO codes for multi-user MC-CDMA are also given. Numerical results show that MLO codes can provide a substantial performance improvement in fully-loaded systems. For example, for a K = 4 user system with spreading gain L = 4, our system can obtain a diversity order of 3, whereas the binary code system diversity order is only slightly larger than 1. The MLO code application provides a new way to compensate for multi-user interference (MUI) and makes MC-CDMA more attractive for future high data-rate transmission systems.     

Throughput analysis of STS-based CDMA system with variable spreading factor in non-frequency selective Rayleigh fading channel

The throughput enhancement of space-time spreading (STS)-based code division multiple access (CDMA) system is investigated in this paper. Variable spreading factor (VSF) is utilized to improve the data throughput of the system in non-frequency selective Rayleigh fading channel. In this contribution, an analytical approach is proposed to compute a new expression for the bit error rate (BER) performance of the STS-based CDMA system against pre-despreading signal-to-noise-ratio (SNR) for different values of spreading factor. The other contribution of the paper is deriving a new analytical expression for the throughput enhancement of the VSF STS-based CDMA system in non-frequency selective Rayleigh fading channel. Simulation and analytical results demonstrate that using VSF method in STS-based CDMA system improves the throughput of the system by keeping the BER performance at a target level.     

Throughput improvement of STS-based MC DS-CDMA system with adaptive modulation

The throughput enhancement of space-time spreading (STS)-based multicarrier direct sequence code division multiple access (MC DS-CDMA) system is investigated in this paper. Adaptive modulation is performed to improve the data throughput of the system in Rayleigh fading channel. In this contribution, an analytical approach is proposed to compute a new expression for the bit error rate (BER) performance of the STS-based MC DS-CDMA system for QAM data. The other contribution of the paper is deriving a new analytical expression for the throughput enhancement and the BER performance of the adaptive modulated STS-based MC DS-CDMA system over Rayleigh fading channel. The novel expressions presented in this paper bring out the facility of evaluation of the proposed system with no need for more calculations and simulations.     

适合酉空时调制的多进制Turbo码及其MAP译码算法

酉空时调制技术可以在发射和接收两端均无信道状态信息的环境中实现无线通信,适合快速Rayleigh平衰落信道,然而它的一个本质问题是只有在高信噪比条件下才会有理想的差错性能.现有的比特级处理方案在改善系统可靠性的同时带来了过大的计算量、存储量和译码时延的问题.文中提出一种适用于酉空时调制的多进制Turbo码方案,同时给出其相应的酉空时符号级多进制MAP译码算法.仿真表明:相比于现有方案,文中的算法可以降低系统组成及处理的复杂度,同时又保持良好的差错性能.另外,文中从酉空时调制的概念出发,首次给出了信噪比过低时结合纠错编码的情况下发射分集性能恶化的定性分析.