用於亂碼同步的偵測法之探討

Abstract

雖然亂碼信號的獲得， 可藉由亂碼匹配濾波器 (PN matched filter) 達 到。 但在亂碼週期長或速率高的時， 硬體的複雜度、 速度以 及成本之限制，使得 亂碼匹配濾波器不易實現。接機率比值偵測法 (sequenti應是理想且可行的選擇。最佳之串接偵測法，在文獻已被証明 者，僅限於一種稱為串接機率比值偵測法 (sequential probability ratiotest, SPRT)。 此法之證明利用到了獨立以及相同機率分佈取樣之 特性。況且，串接偵測法不易分析以及數值最佳化。事實上，到目前為止 並沒有任何有關非同調串接偵測法之完全分析。相反的，多區間偵測法較 易於分析及最佳化，我們的數值結果顯示，相較於單區間偵測法，此法能 減少約一半的平均獲得時間，這樣的改善幅度與串接偵測法相當接近。本 論文探討數種亂碼獲得之多區間偵測法。我們推導了多區間偵測法的獲得 時間之機率分佈以及其簡化公式。另外，我們也推導了同調以及非同調偵 測器在加成白高斯雜訊以及非頻率選擇性衰減環境下，使用多區間偵測法 之工作特性。對於獲得時間之分佈及平均值之數值分析結果包括了分析以 及模擬結果之比較。我們也探討了多重進接干擾對偵測器性能之影響。另 一個探討之課題為在資料調變下之亂碼獲得。我們提出了一種稱為部份匹 配的新偵測法，這方法較傳統之非同調結合法佳，且在硬體實現上又不會 太複雜。 Although pseudonoise (PN) signal acquisition can be readily accomplished by employing PN matched filters. Hardware complexity, speed and costlimitations might preclude such an implementation for long and/orchip rate PN codes. Multiple dwell (MD) or sequential method is thus anattractive alternative under the said circumstances. The optimum propertyof sequential detectors is proved for only a very limited subclass called sequential probability ratio test (SPRT) in which independent and identicaldistributed (i.i.d.) samples are assumed. Furthermore, the analysis andoptimization of sequential detectors are difficult to perform. In fact,no complete analysis of noncoherent sequential detectors can be found inthe literature. On the other hand, the multiple dwell detectors are much easierto analyze and optimize, and, as our numerical results demonstrate,can bring mean acquisition time improvement of the same order as that achievable by a sequential detector. This thesis examines various multiple dwell detection schemes for PN signalacquisitions. We derive the probability distribution of the acquisition time and a simpleapproximation for a general class of MD detectors. The operating charac- teristics (OC) of various coherent andnoncoherent MD detectors under both AWGNand frequency non-selective fading are provided. Numerical results forcomparing analytic formulae and simulation predictions are given. We alsoexamine the effect of multiple access interference (MAI) on the detector per-formance. Another issue studied is the problem of acquisition in the presenceof data modulation. A new method called partially matched (PM) detectoris proposed. This method outperforms the conventional noncoherent combining(NC) detector while requires a complexity compatible to that of an NCdetector.