在直序展頻系統中一種基於快速傅立葉轉換之PRN碼捕獲方法

Abstract

大多數的直序分碼多重存取系統接收機，主要利用多重時段搜尋的方式以捕獲 Pseudo Random code(PRN 碼)。本論文提出一種基於快速傅立葉轉換之PRN 碼捕獲方法以應用於cdma2000 系統中。我們藉由快速傅立葉轉換來減少相關性函數的計算量，並提供一個快速的捕獲結果。此方法基於非同調四相位鍵移解調器，並折疊本地的PRN碼以減少運算量。快速傅立葉轉換可一次運算整個相關性函數，接著選擇最大相關性峰值當作結果，其處理時間決定於接收的訊號長度以及運算單元的速度。我們利用多路徑衰減通道模擬非同調四相位鍵移解調器，同調差分碼片偵測器以及本地折疊仿製共三種方法的效能，結果顯示出此通道環境，對於我們所提出的本地折疊仿製方法造成很大的效能影響。不過，藉由增加碼序列的長度，其效能仍然可以改善。

Most DS-CDMA receivers use multiple dwell time search schemes for pseudo random codeacquisition. In this thesis, we propose a FFT-based code acquisition method to reduce the correlation computation by FFT algorithm so as to provide fast acquisition in cdma2000 systems. This method is based on the noncoherent QPSK demodulator scheme and utilizes the foldinglocal replica method to further reduce the calculation. The FFT-based code acquisition schemes make decision by choosing the maximum correlation peak, whose processing time depends on the incoming data length and the arithmetic unit. We present the FFT-based noncoherent QPSK demodulator, chip differential coherent detector and the folding replica method in multipath channel environment. Numerical results show that the multipath channel may seriously degrade the performance of the proposed methods, but the performance can be improved if a longer code sequence is employed.