一般而言，展頻碼同步包含兩個步驟，獲取（粗調）與追蹤（細調），來估測兩個碼之間的延遲之抵補值(延遲差值)。近來在直接序列分碼多重存取系統中許多利用適應性地共同調整展頻碼獲取與智慧型天線之權重係數的一個聯合適應性程序來提升接受信號對雜訊干擾比及減少平均獲取時間的機置被提出。因此對於平均獲取時間的表現而言門檻值的設定就扮演一個很重要的角色了。接收信號對雜訊干擾比通常是會改變的，因此若使用固定的門檻值的獲取演算法就可能會導致不可接受的表現。為了改善上述的問題，在本篇論文中，我們提出一種新的可以用於可加性高斯白雜訊及多路徑衰減信道環境中使用在聯合中波束合成直接序列分碼多重存取接收機及適應性展頻碼獲取門檻值機置。其中最基本的創意是利用估測智慧型天線輸出的平均信號功率來得到一個在每個觀測區間的參考門檻值，由此可以得到常數錯誤警告率。為了計算在兩條路徑緩慢衰減通道的平均獲取時間，我們將推導出系統的機率值。數值分析及模擬結果將以輸出信號對雜訊干擾比，檢測機率，以及平均獲取時間來與固定值門檻值方法來比較，來證明我們提出的適應性門檻值機置確實可以有較佳的效能表現。

In general, PN code synchronization consists of two steps: PN code acquisition (coarse alignment) and PN code tracking (fine alignment), to estimate the delay offset between received and locally generated codes. Recently, the schemes with a joint adaptive process of PN code acquisition and the weight coefficients of smart antenna have been proposed for improving the received signal-to-interference-plus-noise ratio (SINR) and simultaneously achieving better mean-acquisition-time (MAT) performance in direct-sequence code-division multiple access (DS-CDMA) systems. In which, the setting of the threshold plays an important role on the MAT performance. Often, the received SINR is varying, using the fixed threshold acquisition algorithms may result in undesirable performance. To improve the above problem, in this thesis, a new adaptive threshold scheme is devised in a joint adaptive code acquisition and beam-forming DS-CDMA receiver for code acquisition under a fading multipath and additive white Gaussian-noise (AWGN) channels. The basic idea of this new adaptive threshold scheme is to estimate the averaged output power of smart antenna to scale a reference threshold for each observation interval, such that it can approximately achieve a constant false alarm rate (CFAR) criteria. The system probabilities of the proposed scheme are derived for evaluating MAT under a slowly fading two-paths channels. Numerical analyses and simulation results demonstrate that the proposed adaptive threshold scheme does achieve better performance, in terms of the output SINR, the detection probability and the MAT, compared to a fixed threshold method.

Acknowledgement...i
Abstract...ii
Contents...iii
List of Figures...v
Chapter 1 Introduction...1
Chapter 2 Adaptive PN Code Acquisition Using Smart Antennas for DS-CDMA Systems...6
2.1 Introduction...6
2.2 Joint adaptive LMS Filtering Scheme for Code Acquisition...8
2.2.1 Signal Model...9
2.2.2 Joint adaptive Acquisition System Description...11
2.2.3 Joint adaptation algorithms...12
2.2.4 Mean and Variance of Optimal Weights...18
2.3 Two kinds of Decision Device...20
2.3.1 Weight Vector Square Norm (WVSN) Test Method...20
2.3.2 Traditional Mean Square Error (MSE) Test Method...21
2.4 Acquisition System Probabilities and Mean Acquisition Time...22
2.4.1 WVSN Test Method...22
2.4.2 The Traditional MSE Test Method...23
2.5 Simulations...24
2.5.1 Numerical Analyses and Simulations for PDF of WVSN and MSE Test Methods...24
2.5.2 Numerical Analyses and Simulations for System Probabilities and Mean Acquisition Time of WVSN and Traditional MSE Test Methods ...26
Chapter 3 Adaptive Threshold Scheme and Performance Under Mobile Fading Environment
3.1 Introduction...37
3.2 Adaptive threshold scheme...38
3.3 System performance under different environment...40
3.3.1 System performance under AWGN environment...40
3.3.2 System performance under mobile slow fading environment...41
3.4 Simulations...43
Chapter 4 Conclusions ...56
References...58
Appendix A...61
Appendix B...71
Appendix C...76

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