展頻信號同步技術之探討

Abstract

由於展頻信號具有抑制干擾、抗複徑衰褪以及低攔截機率等優點，展頻通訊技術已廣泛地被使用在各種軍用及民用的通訊系統中。和傳統通訊系統不同的是，展頻信號的接收需要有額外一層的同步，即碼同步 (code synchronization)。這一層的同步通常是整個接收過程的第一步也是最關鍵性的一步。本論文旨在研究兩種展頻信號之碼同步技術：跳頻系統之同步微調 (fine-time synchronization) 與直接序列展頻系統之亂碼獲得 (PN code acquisition)。 本論文的第一部份探討在多調式干擾 (multitone jamming) 與複徑瑞雷衰褪通道 (multipath Rayleigh fading channel) 環境下對同步微調造成的影響。我們使用的同步微調方法是利用早-遲濾波器 (early-late filter) 技術來處理接收到的探針信號 (probe signal)，然後用二階動量 (second-order moment) 來估計出誤差時間值。第二部份則分析在多重進接 (multiple access) 之環境下亂碼獲得之性能。我們使用非同調 (noncoherent) PN匹配濾波器 (matched filter) 之獲得方式，這種方式包含兩個模態：搜尋模態 (search mode) 與確認 (verification mode) 模態。在加成性白高斯雜訊通道 (AWGN channels) 與關聯性衰褪通道 (correlated fading channels) 之下，我們各別導出偵測機率 (detection probability) 與誤警機率 (false alarm probability) 之表示式，計算最低平均獲得時間 (mean acquisition time)，並就各系統參數對獲得時間的影響進行分析與討論。

Although spread-spectrum (SS) techniques enjoy many advantages, they also need extra dimension of code synchronization. This thesis investigates two code synchronization issues in SS communications, namely, fine-time synchronization for frequency-hopped (FH) systems and code acquisition for direct-sequence (DS) multiple access (MA) systems. The first part deals with the fine-time synchronization problem. We present second-order moment estimation techniques for mitigating the effects of multitone jamming or multipath Rayleigh fading. For the latter case, the expressions of the timing error are derived for channels characterized by a path-delay vector and a path-strength distribution vector. The second part of the thesis contains our analysis of code acquisition subsystem performance in an MA environment. The acquisition system uses a noncoherent PN matched-filter (MF) for the search mode and a coincidence detector (CD) for the verification mode. The probabilities of the detection and false alarm are derived and the mean acquisition time are evaluated. Both additive white Gaussian noise (AWGN) and correlated fading channels are considered. We also provide a solution to the acquisition of complex spreading codes.