摘要
目前，扩频技术不仅在军事通信中被大量采用，而且还广泛运用于民用系统，这是由于扩频技术具有许多优越性，包括多址能力、隐蔽性好、抗多径干扰等优点，使其特别适合于无线移动通信环境。它最突出的优点是当扩频增益足够大时，扩频系统具有良好的抗干扰的能力。然而，扩频系统自身的干扰抑制能力总是有限的。在实际的扩频通信中存在着大量的干扰因素，若扩频增益不够大或扩频技术主要用于实现多址时，若不采用其他途径对窄带干扰和宽带干扰进行抑制，将会大大影响系统的性能，导致系统容量大幅度减少。
可见在扩频通信系统中，干扰抑制是一个很重要的问题，影响着整个系统的性能。所以必须在接收机中采用专门的干扰抑制技术。本文即是研究在直接序列扩频(简称直扩)系统中的干扰抑制问题，由于针对窄带干扰的抑制技术比较成熟，在本文中，只重点针对宽带干扰进行研究。由于干扰和信道的特性都是动态变化的，为了应用于实际的通信环境中，就必须采用自适应的接收机结构。
近年来，周期维纳滤波理论在信号接收问题中得到较大的发展。周期维纳滤波是基于信号的周期平稳特性的，可用来设计单信道的自适应频移(FRESH，FREquency-Shift)滤波器。FRESH滤波器利用了周期平稳信号的谱相关特性，从而达到抑制干扰与噪声的目的。由于大多数数字通信信号都具有周期平稳特性，FRESH滤波器能够在通信接收机中提供良好的干扰抑制性能。
本文首先对直扩通信系统中的多种干扰抑制技术进行了简要的概述。然后详细介绍了周期平稳信号的基础理论和周期维纳滤波理论，在这个基础上，讨论了FRESH滤波器是如何运用于宽带干扰情况下的，以及它是如何利用周期平稳信号的谱相关特性来抑制干扰的。根据FRESH滤波器的原理和结构，本文编写了采用自适应方案的FRESH算法，并建立直扩系统模块，进行仿真。仿真结果表明，在有强干扰的情况下，利用信号谱相关特性的FRESH滤波器具有良好的宽带干扰抑制能力。
Currently, Spread-Spectrum (SS) technique has been widely applied both in military and in civilian communications for its many advantages, which make it suitable for the environments of wireless communication. The best advantage is its outstanding ability to anti-jam if the processing gain is large enough. However, its ability of interference suppression is always limited. The system performance is dramatically degraded and its capacity is largely reduced if the processing gain is not sufficiently large and no other methods can be adopted to suppress strong interference.
Therefore, interference suppression is a crucial issue in SS systems. It is necessary to apply the special suppression techniques in new detection structures. The suppression techniques to narrowband interference are very mature in DSSS systems. So, in this thesis we concentrate on wideband interference suppression. Secondly, since both interference and channel characteristics are dynamically changed in real environments, adaptive receiver construction must be applied.
Recently the theory of cyclic Wiener filtering, based on the cyclostationarity assumption for the signals, developed greatly in signal reception program and can be applied to design single-channel adaptive frequency-shift (FRESH) filters. The FRESH filter exploits both temporal and spectral correlation properties to remove interferences and noises from the desired signal. As most digital communication signals are cyclostationary, FRESH filter can perform excellent of suppression interference in a communication receiver.
This thesis firstly introduces the basic theories of both cyclostationarity and cyclic Wiener filtering. Then, it investigates how a FRESH filter removes interference by exploiting the cyclostationarity of signals including baud rate related and carrier related cyclostationarity. Finally an adaptive FRESH algorithm is programmed and a model of DSSS system is built and simulated, based on the principle and structure of FRESH filter. The results will prove that, in case of
strong interference, FRESH filter can display good ability to wideband interference rejection.
引文
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