CDMA2000上行鏈路系統之犛耙式接收器在多路徑衰褪環境下的性能分析

Abstract

在本論文中，我們以cdma2000上行鏈路系統（Reverse Link）為架構，分析在以最大比率或相同增益結合的條件下，其系統性能的表現。而其個別的傳送通道經由不同的正交碼（Walsh Codes）而使其彼此間正交化，用來降低不同通道之間干擾的情形，其中之一的通道（Pilot Channel）被設計用來作為訊號的初步截取、時間追縱、犛耙式接收器（RAKE Receiver）同調相位參考的恢復、及量測其能量且適當的調整大小。此外，Hybrid Phase Shift Keying（HPSK）調變在 cdma2000中作為展頻技術已經被提出來規劃，它是利用複雜展頻的方法使得在零點交叉（Zero-Crossing）的信號過渡減到最少，如此功率放大器的使用就能更有效率。 經QPSK調變的訊號會通過以多路徑衰褪為模型的通道；而在接收器部份，我們採用同調的犛耙式接收器（RAKE Receiver）來收集從不同衰褪路徑所得到的訊號，在包含符號間干擾（ISI）及多個使用者干擾（MAI）的情況下，我們推導出犛耙式接收器的錯誤率表示式，同時我們採用兩階的Butterworth低通濾波器作為通道估計且補償因通道衰褪而造成的相位錯誤。分析與模擬的結果都顯示出以最大比率結合的接收器是最理想的，並且當結合的路徑增加時它也能更有效率的提高系統性能，然而在高訊號雜音比時，系統會因為符號間干擾（ISI）及多個使用者干擾（MAI）效應而使得錯誤率無法在降低。

The purpose of this thesis is to analyze the performance of Rake receivers for cdma2000 reverse link. The transmitted signal consists of multiple orthogonal signals, each coded with a distinct Walsh code. The pilot channel is used for initial acquisition, code/timing tracking, RAKE-receiver coherent reference recovery, and power control measurements. In addition, Hybrid Phase Shift Keying (HPSK) modulation scheme is proposed as the spreading technique to minimize the zero-crossing signal transition and obtains higher power efficiency. We consider frequency selective Rayleigh slowly fading channels. The receiver uses a second-order Butterworth low-pass filter to estimate the fading amplitude and the phase shift. Each finger has its own tracking loop and channel estimator. Both maximal ratio combining and equal gain combining rules are investigated. Our analysis takes into account the channel estimation error, the self-interference (intersymbol and inter-channel interference), multiple access interference (MAI) and the band-limiting effect. Numerical bit error rate (BER) performance of the RAKE receiver using MRC and EGC are obtained by both computer simulations and analytical formulae. As expected, MRC yields better performance and ISI and MAI are the primary limiting factors even if perfect channel estimate is available.