寬頻CDMA系統在高速移動環境下之效能分析

Abstract

隨著高速行動通訊服務的需求增加時，第三代行動通訊系統應用於高速火車上之研究將變得是不可或缺的。本篇論文主要是探討寬頻CDMA 系統在高速移動環境下之傳輸效能。由於考慮載波同步及通道估測的效應而導致系統效能因都卜勒過度偏移(over Doppler shifting)的問題而降低。為了改善系統效能，我們提出一種稱為都卜勒偏移補償(Doppler shift compensation)的方法來克服因為載波同步後所造成的都卜勒過度偏移的問題。在本論文中，我們就所提出的方法及傳統線性內差的通道估測方法推導出各別的通道估測的均方差;並且證明出當K 因子很大時，我們所提出的方法將有極大的改善。基於所提出的接收機架構並且考慮了載波同步及通道估測的效應下，我們去分析了寬頻CDMA 系統在高都卜勒衰落通道下的系統效能。模擬結果也顯示出我們所提出的方法比傳統的方法更能改善系統效能且提高通訊品質。此外，我們也展示出不論都卜勒暫態(Doppler transient)效應有多大，鎖相迴路都能去鎖定耙式(RAKE)接收機的等一個finger。然而，當都卜勒的改變率很大時，此時耙式接收機如果結合更多的fingers 將會導致系統效能的降低。

As the demand of wireless communication services for high-speed mobile stations is increasing, the researches on the wireless mobile communication systems such as 3G WCDMA system utilized for high-speed trains (HST) will become more indispensable. In this thesis, we investigate the transmission performance for WCDMA system under the high-speed mobile rural area environments. Carrier synchronization and channel estimation are usually imperfect and the system performances are degraded due to the over Doppler shifting (ODS) problem. In order to improve the system performance, we propose a method, called the Doppler shift compensation (DSC), to overcome the ODS problem. We derive the mean square error (MSE) of the channel estimation by the conventional linear interpolation method and by our proposed approach. We found that our proposed approach has a tremendous improvement when the K factor is large. Based on the proposed receiver structure, we analyze the system performance for WCDMA system and compare it with that obtained by the conventional receiver in various high-Doppler multipath fading channels, where we have taken into account the effect of carrier recovery and imperfect channel estimation. Besides, we also demonstrate that no matter how large the Doppler transient is, the PLL can track the first finger well. However, when the change rate of Doppler frequency is very large, combining more fingers degrades the system performance for both the conventional and our proposed method.