利用自适应滤波器实现扩频码的捕获

使用自适应滤波器对扩频信号进行扩频码捕获,不仅理论上是可行的,而且此方法还能够实现快速捕获,同时还可以对输入信号内的多种干扰信号起到抑制的作用。自适应滤波器法结构简单,易于实现;同时跟踪采用与捕获同一个支路,不需要额外硬件实现,从而节省硬件资源。自适应滤波器法扩频码捕获时,只需期望扩频码的周期与长度,并不期望码型与干扰信号的任何信息。在稳态时,此方法还具有对多径信号分集接收的潜力。     

全数字线性调频信号产生及失真自适应接收机技术研究

对流层散射信道是一种随机多径信道,在该信道上传输数字信息时,首先要克服多径时散引起的码间干扰和快衰落导致的通信性能恶化。本文阐述了分离多径技术方案抗码间干扰、抗快衰落以及失真自适应接收的机理,研究了线性调频信号多径分集调制解调器的数字化实现,给出了系统性能测试的结果,积极有效地解决了散射信道存在的主要问题。     

一种新的抗远近效应的盲DS-CDMA接收机

本文研究多径频率选择性衰落信道下直接序列扩频码分多址(DS-CDMA)信号的处理问题,提出了一种新型的盲接收机.该接收机不需预知多址干扰用户的扩频码,不需预知信道参数,只需已知期望用户的扩频码和粗略的定时,就可以完成用户的盲检测.同时,该接收机通过降维自适应去相关滤波和基于判决指导的自适应多径合并,来获得低成本和良好的抗衰落、抗远近效应性能.仿真结果验证了本文提出的接收机的优良性能.     

多径衰落信道中M进制直接序列扩频系统RAKE接收机性能分析

本文分析了存在码片定时误差和载波相位估计误差情况下,采用BPSK调制的M进制正交码直接序列扩频信号在多径衰落信道下的RAKE接收机性能.分别讨论了不使用分集和采用选择式合并、等增益合并和最大比合并三种分集合并策略时接收信号的误码率,并推导出相应的近似计算方法.文章最后在此基础上分析了不同的分集合并办法、码片定时误差和载波相位误差对系统性能的影响.     

基于DMF捕获系统频率选择性信道下捕获性能的分析

对基于数字匹配滤波器(DMF)捕获系统在频率选择性信道下的捕获性能进行了深入讨论，利用状态转移图推导出单次驻留判决方式时平均捕获时间的表达式，对平均捕获时间与多径分量的关系进行了数字分析。得出的主要结论是：对于多径分量为非衰落信号时，多径分量之间的能量差别越大，系统捕获性能越好，多径分量的能量相等时，系统捕获性能最差，且随着可以分离的多径路数的增加下降；对于多径分量为慢衰落信号时，多径信道的捕获性能好于单径信道(非频率选择性衰落信道)，且多径分量之间的能量差别越大，系统捕获性能越差，当多径分量的能量相等时，捕获性能最好，这与非衰落信道的情形相反。因此在频率选择性衰落信道中，采用分集接收可以改善系统的捕获性能。     

级联正交/PN码DS-CDMA系统在Nakagami信道下的性能

本文分析了级联正交/长PN码的互相关和自相关特性 .分析的结果用来计算在这种新的扩频方案下直接扩频系统在多用户和多径衰落下的系统性能.多径衰落信道为Nakagami 信道.接收端采用最大比率分集接收机.     

失真自适应接收机技术及信号形式选择

对流层散射信道是一种典型的多径衰落信道,多径传播造成了信号的深衰落和多径扩散。而多径扩散现象的存在,会使速率高、码元窄、频谱宽的信号遭受频率选择性衰落的影响,造成传输中的信号码元之间产生码间干扰,严重影响信息传输质量。在此提出了一种基于失真自适应接收技术和3/4占空半余弦信号形式的数字接收机方案,能够自适应地跟踪失真信号的变化而进行最佳相干接收,使系统具有稳定的通信性能。     

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.     

随机M进制正交码混合DS—SFH CDMA扩频通信系统性能分析

本文研究了随机ＭＦ是制正交码混合ＤＳ－ＳＦＨ扩频码分多址信号经过多径瑞利频率非选择性衰落信道，在非相干ＲＡＫＥ接收机中采用最大输出信噪比选择或最大输出选择接收时系统的性能；给出了两种分集接收情况下差错概率表示式。数值计算研究了分集数，Ｍ值，多用户干扰对系统性能的影响，并且对这两种分集接收情况下系统性能进行了比较。     

通信系统中多径衰落信道传输性能的改进

通信系统中多径衰落信道传输性能的改进是当前比较热门的研究课题.我国的通信系统中多径衰落信道传输性能还存在严重的劣势,还存在着诸多有待解决的问题.在实用瑞利衰落信道下,存在加性高斯白噪声,对直接序列扩频通信系统抑制码片内、外多径干扰的能力进行理论分析与计算机仿真.在多径衰落信道较强的条件下,与提高扩频增益相比,增加扩频码的码长对于提高移动通信系统的抗多径衰落性能更加有效,但是扩频增益并非越高越好,需合理选取;证实扩频通信系统在多径瑞利衰落信道中传输时能有效地克服多径衰落.本文简要分析了瑞利衰落信道模型,并深入的研究和探讨了扩频通信系统抗多径干扰性能.     

Performance analysis for an adaptive filter code-tracking techniquein direct-sequence spread-spectrum systems

This paper investigates tracking of direct-sequence spread-spectrum (DS/SS) signals based on an adaptive filtering technique. It is shown that a previously proposed hardware for code acquisition is also capable of code-tracking, and, hence, by performing both acquisition and tracking with the same circuitry, a significant simplification in the overall DS/SS receiver structure is gained. Analytical results show that the proposed scheme has a good tracking performance, as measured by the hold-in time and the false alarm penalty time, and is less sensitive to variations in the signal-to-noise ratio (SNR) compared to conventional delay-locked loops (DLLs). Moreover, simulation results show that the proposed adaptive filter tracking scheme has a smaller residual tracking error than that produced by a conventional maximum-likelihood estimator (MLE)     

Acquisition of PN code with adaptive threshold for DS/SScommunications

An adaptive acquisition scheme of PN sequences is presented for DS/SS communications. The basic idea of the scheme is to apply the radar CFAR technique to the acquisition of PN sequences. The detection performance is analysed and an example of adaptive thresholding for multipath Rayleigh fading channels is given     

基于神经网络的方法获取DS／SS信号的PN码

提出了两种基于神经网络(NN)获取直接序列扩频(DS/SS)信号的伪随机码(Pseudo RandomCode)的方法。这两种方法充分利用了神经网络无监督的自适应学习方法来提取DS/SS信号的协方差矩阵主特征向量,并可以进行并行运算,因此有较快的运算速度。同时文章对这两种算法进行了性能比较,试验结果和理论分析表明,这两种方法可以在较低的输入信噪比的条件下较好地估计DS/SS信号的PN码。     

Robust adaptive recovery of spread-spectrum signals with short data records

The problem under consideration is the adaptive reception of a multipath direct-sequence spread-spectrum (SS) signal in the presence of unknown correlated SS interference and additive impulsive noise. An SS receiver structure is proposed that consists of a vector of adaptive chip-based Hampel nonlinearities followed by an adaptive auxiliary-vector linear tap-weight filter. The nonlinear receiver front end adapts itself to the unknown prevailing noise environment providing robust performance over a wide range of underlying noise distributions. The adaptive auxiliary-vector linear tap-weight filter allows rapid SS interference suppression with a limited data record. Numerical and simulation studies under finite-data-record system adaptation show significant improvement in bit-error-rate performance over the conventional linear minimum variance-distortionless-response (MVDR) SS receiver or conventional MVDR filtering preceded by vector adaptive chip-based nonlinear processing.     

A beamspace-time interference cancelling CDMA receiver for sectoredcommunications in a multipath environment

A beamspace-time (BT) receiver is proposed for interference suppression and multipath diversity reception in sectored wireless code division multiple access communications. The scheme involves two stages. First, a set of adaptive space-time diversity processors, in the form of beamformer-correlator pairs, is constructed which provides effective suppression of unwanted interference and reception of signals from a prescribed space-time region. Second, the output data obtained by these processors are maximum ratio combined to capture the signal multipath components coherently. The proposed BT receiver is blind in that no training signal is required. The only information required is the signature sequence, timing and a rough estimate of the angle of arrival of the signal for selecting the sector of interest     

Evaluation of a soft synchronization technique for DS/SS signals

Soft synchronization of direct-sequence spread-spectrum (DS/SS) signals introduces the capability of despreading with a continuously improving estimate of the spreading code. The soft synchronization technique presented and demonstrated in this paper exploits the eigenstructure of a frequency-channelized DS/SS signal to estimate the code. The estimate improves steadily as more data is collected, resulting in an improved estimate of the message signal. This allows commencement of despreading at an early stage when the code-estimate is still imperfect. Under infinite-time average assumptions a perfect code estimate can be obtained when the signal is received in arbitrary levels of white background noise. This paper demonstrates the synchronization performance of the technique through simulations under finite time-average conditions for environments with fading, multipath, and multi-user interference. The soft synchronization capability is demonstrated. Results show that the technique can remove frequency offsets on the received signal that are integer multiples of the code repeat rate, and that its performance is not degraded by multipath     

Space-diversity reception for v.h.f. mobile radio

Work on aerial diversity reception to combat multipath fading is described. A 3-channel adaptive array has been built around a conventional v.h.f.?a.m. receiver, and been modified to measure signal amplitudes and relative phases. Experimental results show the effects of typical multipath environments, and the improvement obtained with an adaptive array.     

Multitone spread spectrum multiple access communications system ina multipath Rician fading channel

The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated