對於無線通訊系統之服務，直接序列展頻分碼多重接取技術為主要的多工技術之一。展頻通訊系統在通道裡利用較大的頻寬技術來傳送信號。在DS-CDMA系統中，由於多重接取架構而導致其產生不可避免的多重接取干擾 (MAI)，使得系統的效能下降。且系統在頻率選擇衰減環境下，可應用耙式 (RAKE) 接收機來抑制已存在的符碼間的干擾 (ISI) 及多重接取干擾。由於在實際無線通訊環境裡，有可能會發生好幾個不同的通訊系統在同一個區域同一時間傳輸。所以在本論文，我們針對一個遭受到窄頻干擾的DS-CDMA系統來作探討，而這干擾是由於其他通訊系統的加入而產生的。一般而言，系統在穩定的狀態下，若突然受到窄頻干擾信號的干擾，必定會導致系統效能大幅下降。在上述的這種情況下，傳統的適應性耙式接收機也許無法快速地應付這種同時存在變動劇烈的窄頻干擾、符碼間干擾及多重接取干擾之環境。
大家都知道在適應性演算法中滑動視窗之線性最小平方值遞迴式適應性演算法非常適用在變動劇烈的環境。本論文的主要核心概念即在極小/極大法則下，提出一個滑動視窗常模最小平方值遞迴式演算法來處理系統在多路徑通道裡受到窄頻干擾等問題。為了降低系統實現的複雜度，我們利用廣義旁波帶消除器 (GSC) 的架構來執行提出來的演算法，即滑動視窗常模廣義旁波帶消除器-遞迴式最小平方值演算法。本文所提出來的滑動視窗常模最小平方值遞迴式演算法便是用來抑制窄頻干擾的效應。此方法在展頻碼不匹配及窄頻干擾信號同時干擾的環境下，有較快的收斂速度與較好的系統效能表現。在本文中的最後，我們也將此演法算延伸應用到時空直接序列分碼多重接取耙式接收機 (space-time DS-CDMA RAKE receiver)。最後，由電腦擬摸可以得知，本文提出來的演算法架構比起傳統的常模最小平方值遞迴式演算法及最小平方值輸出演算法提供更優越的系統效能及通訊品質。

The code division multiple access (CDMA) system implemented by the direct-sequence (DS) spread spectrum (SS) technique is one of the most promising multiplexing technologies for wireless communications services. The SS communication adopts a technique of using much wider bandwidth necessary to transmit the information over the channel. In the DS-CDMA system, due to the inherent structure interference, referred to as the multiple access interference (MAI), the system performance might degrade. Next, for DS-CDMA systems over frequency-selective fading channels, the effect of inter symbol interference (ISI) will exist, such that a multiuser RAKE receiver has to be employed to combat the ISI as well as MAI. Since, in practical wireless communication environment, there may have several communication systems operated in the same area at the same time. In this thesis, we consider the environment of DS-CDMA systems, where the asynchronous narrow band interference (NBI) due to other systems is joined suddenly to the CDMA system. In general, when a system works in a stable state with adaptive detectors, a suddenly joined NBI signal will cause the system performance to be crash down. Under such circumstance, the existing conventional adaptive RAKE detectors may not be able to track well for the rapidly sudden changing NBI associated with the problems of ISI and MAI.
It is known that the adaptive filtering algorithms, based on the sliding window linear constrained recursive least squares (SW LC-RLS), is very attractive to a violent changing environment. The main concern of this thesis is to propose a novel sliding window constant modulus RLS (SW CM-RLS) algorithm, based on the Min/max criterion, to deal with the NBI for DS-CDMA system over multipath channels. For simplicity and having less system complexity the generalized side-lobe canceller (GSC) structure is employed, and is referred to as the SW CM-GSC-RLS algorithm. The aim of the SW CM-GSC-RLS algorithm is used to alleviate the effect of NBI. It has the advantages of having faster convergence property and tracking ability, and can be applied to the environment in which the NBI is suddenly joined to the system under the effect of channel mismatch to achieve desired performance. At the end of this thesis, we extend the idea of the proposed algorithm to the space-time DS-CDMA RAKE receiver, in which the adaptive beamformer with temporal domain DS-CDMA receiver is employed. Via computer simulation results, we show that our new proposed schemes outperform the conventional CM GSC-RLS algorithm as well as the GSC-RLS algorithm (the so-called LCMV approach), in terms of mean square error of estimating channel impulse response, output signal to interference plus noise ratio and bit-error-rate.

Acknowledgements i

Abstract .ii

Contents iii

List of Figures and Tables iv

Chapter 1 Introduction 1

Chapter 2 Conventional Adaptive DS-CDMA System with
Adaptive Constrained Approaches 4
2.1 Introduction 4
2.2 Concept of Discrete-time DS-CDMA System Model 6
2.3 Generalized Side-lobe Canceller - Recursive Least Square
(GSC-RLS) Algorithm Based on Min/max Criterion 11
2.4 Constant Modulus GSC-RLS Algorithm
Based on Min/max Criterion 14

Chapter 3 Adaptive DS-CDMA Receivers with Sliding Window
Constant Modulus GSC-RLS algorithm 20
3.1 Introduction 20
3.2 Sliding Window Constant Modulus GSC-RLS Algorithm
with Min/max criterion 21
3.3 Space-Time DS-CDMA RAKE Receiver
with the Proposed Algorithm… 27

Chapter 4 Computer Simulation Results and Conclusions 33
4.1 Computer Simulation Results 33
4.2 Conclusions 42
References 45

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