結合通道估計之複亂碼追蹤迴路在衰褪環境下的性能分析

Abstract

本篇論文探討直接序列展頻（DS/SS）寬頻分碼多工（CDMA）信號之數位同調複亂碼追蹤迴路（DDLL）在高斯雜訊通道（AWGN）以及複徑衰褪（Rayleigh Fading）環境下之性能分析。在複徑衰褪環境下，犛耙式接收器（RAKE Receiver）的每一個分支(finger)各用一獨立的追蹤迴路來負責亂碼同步。因為同調的要求，我們另需有通道估測線路來提供該路徑相位的資訊。我們應用了簡單的低通濾波器來估計衰褪信號、以便更正解展頻後的信號之相位。 我們所考慮的寬頻分碼多工系統為cdma2000上行鏈路系統（Reverse Link）。其中，各個上行通道經由不同的Walsh Code而正交化。我們除了可利用其中一參考通道（Pilot Channel）信號分析同調複亂碼追蹤迴路性能之外，也可運用其他載有資訊或控制信號之通道來改善追蹤的性能。分析與模擬的結果顯示無論是只用參考通道或加上其他的平行正交通道我們提出的同調複亂碼追蹤迴路都具體可行，而後者也的確可提升不少性能。此外，我們也發現，頻寬限制（Band-limitation）效應造成通道間不可忽略之干擾，尤其在高訊雜比（SNR）時，碼追蹤之性能提升更因而受限。

This thesis investigates the structures and performance of coherent bandlimited digital delay-locked loops (BL-DDLL) for complex-spread direct sequence spread spectrum (DS/SS) signals in either AWGN and Rayleigh fading channels. These loops use either the pilot channel alone or the pilot channel plus other data or control channels that are separated by orthogonal Walsh channelization codes. We consider a RAKE receiver with each RAKE finger suffers from independent Jakes flat Rayleigh fading. Tracking in the presence fading necessitates an auxiliary circuit to generate a channel estimate in real-time. We suggest a simple channel estimate derived from low-pass-filtering the de-spread pilot channel signal. The low pass filter is used to approximate the optimal Wiener filter for estimating the embedded low-pass fading process. The resulting loop actually performs code phase and carrier phase estimates simultaneously. Computer simulation is carried out to validate our analysis on the root-mean-squared (rms) tracking error (jitter) performance of various code tracking schemes. Numerical results also point out the limitation imposed by the self-interference and demonstrate that impressive performance improvement is attained by using non-pilot channel signal in a decision-directed manner.