臺灣博碩士論文加值系統

直接序列展頻的技術現已廣泛地應用在商業行動通訊系統上。直接序列展頻系統的效能大大地仰賴在本地端與接收端展頻碼之快速且精準的同步。展頻碼的同步一般可分為兩個步驟，展頻碼獲取(粗調)及展頻碼追蹤(細調)，利用此兩種步驟去補償展頻碼之間的延遲偏差。在傳統的展頻碼同步實現上，大部份被提出的技術都是採用展頻碼的相關特性來完成。近年來，有人提出以適應性最小均方值演算法的方式來降低硬體複雜度及改善系統的效能，尤其是對於長的展頻碼而言。
本論文提出了一個新的同調適應性展頻碼同步系統。此系統是採用適應性限制最小均方值演算法並搭配最大權重值的方式來做展頻碼的獲取。藉著本論文採用之適應性限制最小均方演算法其權重值向量的統計值能能被推導出來並藉由平均獲取時間來評估系統的效能。理論分析及模擬的結果驗證了本論文所提出的方法在整數及非整數時間延遲上優越於使用傳統的最小均方值演算法。除此之外，在展頻碼獲取中不受訊雜比值及非整數時間延遲影響的門限設定值能被推導出來。
接著，採用本論文所提之適應性限制最小均方值演算法的技術並搭配直接延遲估測公式可應用在展頻碼的追蹤。藉由電腦的模擬驗證了本論文所提出之方法確實可以達到良好的延遲追蹤效能。相對於傳統的展頻碼追蹤迴路則是需要利用內插法來實現展頻碼追蹤，因此硬體複雜度能被降低。

The technique of direct sequence spread spectrum (DS/SS) has been widely used in commercial mobile communication systems. The efficiency of DS/SS system is highly dependent on the accurate and fast synchronization between the incoming and locally generated PN (pseudo-noise) codes. The code synchronization is processed in two steps, acquisition (coarse alignment) and tracking (fine alignment), to bring the delay offset between the two codes. Conventionally, for code synchronization, most of techniques were proposed based on the correlation property of PN codes. Recently, the different approach, by using the adaptive LMS filtering scheme, has been proposed to reduce the hardware complexity and to improve the performance of code synchronization, especially for a long PN code.
In this thesis, a new coherent adaptive code synchronization scheme is proposed, where the adaptive constrained LMS (CLMS) algorithm with the maximum tap-weight (MTW) test method is devised for code acquisition. The statistics of weight vector of the proposed CLMS scheme are derived to evaluate the performance, in terms of mean acquisition time (MAT). Analytical and simulation results verify that the proposed scheme for code acquisition outperforms the one using the conventional LMS filtering schemes, under the integer and non-integer time delay cases. Moreover, the setting of threshold value is derived for code acquisition, which is independent of the values of signal-to-noise ratio (SNR) and time delay.
Next, the CLMS scheme is proposed associated with the direct delay estimation (DDE) formula for code tracking. This approach does achieve a good delay-tracking performance, which is verified via computer simulation. Simultaneously, the hardware complexity can further be reduced due to that a code-tracking loop implemented by the interpolation method is not required.

Acknowledgement i
Abstract ii
Contents iii
List of Figures v
Chapter 1 Introduction 1
Chapter 2 Conventional Code Synchronization Technique in DS/SS Systems
2.1 Introduction 4
2.2 Signal Model and System Description 5
2.2.1 A Matched Filter Technique for PN Code Acquisition 7
2.2.2 Delay Lock Loop Technique for PN Code Tracking 9
Chapter 3 Adaptive Constrained Filtering Technique for Code Synchronization
in DS/SS Systems
3.1 Introduction 17
3.2 Conventional Adaptive LMS Filtering Algorithm for Code
Synchronization in DS/SS systems 18
3.2.1 Signal Model and System Description 18
3.2.2 Mean Square Error (MSE) Test Method for Code Acquisition 20
3.2.3 Maximum Tap Weight (MTW) Test Method for
Code Acquisition 23
3.3 Adaptive Constrained LMS Filtering Algorithm for Code Synchronization
in DS/SS Systems 27
3.3.1 Statistical Analysis for System Probabilities and
Mean Acquisition Time 28
3.3.2 Statistics of Weight Vector 28
3.3.3 System Probabilities and Mean Acquisition Time 37
3.3.4 Practicable Threshold Value Setting for Code Acquisition 38
3.3.5 Direct Delay Estimation (DDE) Formula for Code Tracking 42
3.4 Computer Simulation Results 43
Chapter 4 Conclusions 48
Appendix A 50
Appendix B 55
Appendix C 57
References 59

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