用於偽亂碼獲得的窄頻干擾抑制濾波器之設計

Abstract

由於微電子技術的進步，以往軍事系統專用的展頻通訊(spread spectrum communication)技術已經逐漸廣泛地被使用在無線電通訊系統 。雖然展頻信號具有抑制干擾(interference rejection)、抗複徑衰褪 (multipath fading)以及低竊取機率等能力，然而一般的展頻通訊系統接 收器仍然無法有效抑制比較強的干擾雜訊。因此，為增強系統的性能，其 他的抑制干擾的信號處理技術便有必要。本篇論文旨在研究直接序列展 頻(direct seqence spread spectrum)通訊系統中，用來增強偽亂碼獲 得(psuedo-noise code acquisition)能力的窄頻干擾抑制濾波器之設計 。我們探討兩種不同架構的接收器。第一種為同調接收器(coherent receiver)，我們考慮解調指向式卡門濾波器 (decision-directed Kalman filter)以及其相關的最小均方(least mean square)濾波器， 第 二種是非同調接收器(noncoherent receiver)，我們使用複數形式的卡門 濾波器、最小均方濾波器和最小均四方(least mean fourth)濾波器。無 論是在理想的獲取(perfect acquisition)或是在不完全獲取(imperfect acquisition)的情況下，電腦模擬的結果都顯示窄頻干擾抑制濾波器可相 當有效的提昇系統性能。有關設計這些抑制窄頻干擾濾波器可能面臨之考 量，以及各種干擾抑制 濾波器之比較， 都是本文討論的主要議題。 Spread spectrum techniques have many applications, including interference rejection, multipath suppression, multiple access, low probability of intercept, and accurate ranging. Of all the potential applications, the ability of a spread spectrum system to withstand interference, both intentional and unintentional, is probably its greatest asset. Nevertheless, any spread spectrum receiver can only suppress a given amount of interference; if the level of interference becomes too great, the system will not function properly. Interference rejection techniques must therefore be used to sustain the system performance requirements. In this thesis, interference rejection filters are employed in both coherent mode and noncoherent mode to suppress narrowband interference. We consider both perfect acquisition and imperfect acquisition cases. In the coherent case, a nonlinear techniques, decision- directed Kalman (DDK) filter and a related nonlinear least mean square (LMS) filter, are introduced to perform interference rejection. For the noncoherent case, we consider complex Kalman filter, complex LMS algorithm and complex least mean fourth (LMF) algorithm. It is shown that the presence of the rejection filter can significantly improve the performance in both acquisition mode and tracking mode. Comparsions of various rejection filters are made and design considerations are addressed.