全球衛星定位系統在高動態環境中之訊號獲取

Abstract

高動態環境下GPS信號的獲得，對系統設計者而言，其主要困難在於亂碼 相位及都卜勒頻率的不確定範圍頗大信號又有數據調變。加上差分都卜勒 並不可忽略使得偵測時間受到限制。因此我們的問題就成了如何在短時間 內同時估計到接收信號正確的亂碼相位及都卜勒頻率。本論文提出了一種 後微分檢測式積分法(post differential detectionintegration)，可以 同時解決數據調變(data modulation)與都卜勒位移(Doppler shift)並能 有效降低雜訊的影響。這種偵測法並可利用快速傅立葉轉換(FFT)來節省 硬體複雜度。我們探討了三種信號獲得系統的架構:(1)完全平行式搜尋( fully parallel search)(2)兩段式平行搜尋( two dwell parallel search)(3)混合式平行串聯搜尋(hybrid parallel-serial search)， 並 將 所提出的新型檢測法與早先提出的平方律結合檢測法 (square-law combining detector)應用到這三種系統上。我們討論了平均訊號獲取時 間與硬體複雜度之間的關係並在固定的載波-雜訊密度比(carrier to noise density ratio)下，對系統參數作了最佳化以決定最快的訊號獲取 時間。從計算出來的數值的結果可以得知，在硬體複雜度及信號獲取時間 的雙重考量下後微分檢測式積分法的性能會比平方律結合檢測法好。 GPS signal acquisition in high dynamic environments requires fast identification of the incoming signal's frequency and PN code phase offsets that lies in very large uncertaintyregion. The presence of data modulation makes the acquisition problem even more difficultto deal with. This thesis proposes a new detection scheme suitable for acquiring the GPS signal in such an environment. Coherent complex integration of the received waveform withthe local PN coded signal is performed. The integration outputs are differentially detected andthe detected samples are accumulated, the magnitude of the resulting sum is then used as thetest variable for deciding if the current frequency/code phase offsets are the correct estimates.This detector like the conventional square-law combining detector can beimplemented by FFTs. The operation characteristic of the proposed detector is analyzed and the analytic results are shown to be consistent with computer simulations. Numerical results show that, compared with the square-law combining detector, our new detector is more robust against thermal noise and Doppler shift but, in a few cases, also more sensitive to the presence of data modulation.Both the new and the conventional detectors are applied to three code acquisition systems and the associated performances are compared. These three systems are (i) the fully parallel system, (ii) the two-dwell parallel system, and (iii) the (two-dwell) parallel-serial system.The latter two systems have two phases: the coarse acquisition phase and the fine acquisitionphase. The first phase uses larger frequency bin width and code phase step size, poorer frequency resolution results because it employs a shorter coherent integration time. The second phase searches the one chosen by thefirst phase plus a small neighboring region. First-order (mean), second-order (standard deviation) and complete statistics (pdf and cdf) of the code acquisition time for the latter two systems (those for the first system are trivial and are omitted) are obtained. Tradeoff between hardware complexity and performance is discussed. System parameters are optimizedand performance are compared. When the cost of hardware is not considered,the fully parallel architecture yields the fastest mean acquisition time performance. For both parallel and parallel-serial systems, the mean acquisition time is a linear function of the system complexity (the product of the numbers of the detectors and FFT' s). With a reasonable hardware constraint, the proposed scheme does provide performance that is superior to that of the systems using conventional square-law combining detector.Our calculation also show that the new detector serves our application well--it yieldsexcellent acquisition performance with an affordable complexity.