碼追蹤迴路 (code tracking loop) 廣泛地應用在許多通訊系統如: 衛星訊、行動通訊及全球定位系統 (GPS)。在展頻通訊中，其展頻波在傳送端和接受端必須同步。在同步的程序大致分為倆部份:碼獲得 (code acquisition) 和碼追蹤 (code tracking)。在此我們將討論碼追蹤的部份。在通訊及全球定位系統 (GPS) 中，由多路徑效應 (multipath fading) 所造成的範圍誤差 (ranging error) ，在Differential GPS中，通常是造成主要即時位置誤差的原因。在追蹤程序中，多路徑效應產生的誤差會扭曲鑑頻器特徵曲線，S曲線，而模擬結果也顯示在5 dB訊雜比下，多路徑誤差會使追蹤誤差達到0.3 切片時間，這會造成量測差距到100公尺左右的誤差。在此我們提供一個減少GPS ranging error的方法。我們考慮的環境是在多路徑延遲時間在一個切片時間以內。首先我們提出一個基於傅利葉轉換的路徑選取單元，以找出每條延遲路徑的延遲時間，並應用於提出的碼追蹤迴路中。有別於傳統非同調鎖延遲迴路 (Non-coherent Delay locked loop) ，我們在這裡運用通道多路徑叢集 (channel multipath diversity) 於鎖延遲迴路中，因此我們可以考慮到每條路徑造成的效應並語以消除。我們稱之為犛耙式碼追蹤迴路。 我們同時考慮同調與非同調的架構，並知道非同調架構較適合GPS的環境。在非同調犛耙式碼追蹤迴路中，我們運用多路徑干擾消除器 (multipath interference canceller) 來提昇其追蹤表現，其基本觀念是先重新產生多路徑干擾，並在接收端予以去除，被多路徑叢集接收後，再由非同調碼追蹤迴路追蹤相對的延遲時間。我們分析迴路的表現，並在模擬結果中證實所提出的碼追蹤迴路適用在多路徑的環境中。

In this thesis, we address the problem of GPS signal tracking in multipath environment. Code tracking loop is widely used in many communication system, such as mobile communication and Global Positioning System (GPS). In spread spectrum communication, the spreading waveform should be synchronized between the transmitter and receiver, and the synchronization procedure is divided into code acquisition and code tracking. Here the code tracking is the issue that we concerned. In GPS, multipath is a significant error source for all GPS users. In tracking procedure, the multipath error will distort the discriminator characteristics, S-curve. Simulations show that the multipath problem cause the tracking bias up to 0.3 Tc in 5 dB SNR, that cause about 100m distance ranging error. Here we propose an approach to mitigate the GPS multipath tracking error. We consider the unresolvable multipath, that is the multipath delay is smaller than 1 chip. First we propose a FFT-based path selection unit, which selects each path delay and the delay time is utilized in our proposed Delay lock loop (DLL). Different from the traditional DLL, we utilize channel multipath diversity in the tracking loop, thus we can consider each path’s effect and mitigate the multipath error. We named it Rake-like code tracking Loop. Both coherent case and noncoherent case are discussed, and we found that the noncoherent rake-like DLL is more suitable for GPS environment. And we apply a multipath interference canceller in noncoherent rake-like DLL to improve the tracking performance. It reproduce the multipath interference and removed it from the received signal, and then the multipath diversity and noncoherent discriminator is used to track to relative delay. We have analyzed the loop performance, and the numerical results show that the proposed tracking loop is able to operate in multipath environment and outperforms the traditional DLL.

Contents
Contents i
iii
List of Figures
Chapter 1. Introduction 1
Chapter 2. GPS overview. 3
2.1 GPS Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Space segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.1.2 Control segment . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.1.3 User segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
2.2 GPS signal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.1 Signal characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.2 C/A code generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.3 GPS data format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 GPS error source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Code synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
2.4.1 Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.4.2 Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Chapter 3. Multipath effect in code tracking loop 25
3.1 Multipath system and channel model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.2Analysis of multipath effect in noncoherent DLL . . . . . . . . . . . . . . . . . . . . 30
Chapter 4. Rake-like DLL with multipath delay estimation 37
4.1 System model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
4.2 FFT-based path selection . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .39
4.3 Coherent Rake-like DLL system analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.4 Noncoherent Rake-like DLL system analysis . . . . . . . . . . . . . . . . . . . . . . . 49
4.5 Envelope with subtraction tracking loop . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Chapter 5. Simulation results 61
Chapter 6.Conclusions 69
Appendix 71
Bibliography 75

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