全球衛星導航系統信號之數位載波及亂碼追蹤

Abstract

以軟體方式實現的GPS接收機，為了降低計算負擔與儲存空間，並達成即時性的需求，往往會採用很少位元的類比數位轉換器(ADC)，甚至僅採用單一位元的類比數位轉換器(ADC)。然而伴隨此少位元ADC而來的是接收性能的損失，本文探討少位元ADC因素所導致的性能損失，並提出一因應的載波相位補償結構做法，稱為後相關器相位補償(PCPC)結構，以改善此性能損失。 另外，在載波相位鑑別器方面，一般最常使用的是正切的反函數(arctangent)，主要是基於arctangent對於類比訊號可獲致最大訊雜比(SNR)。然而在採用低位元ADC的情況下，其量化誤差(quantization error)也是不可忽略的因素。 針對此，本文提出一新型的載波相位鑑別器，稱為NB-DPD，此鑑別器經模擬分析與實驗驗證：在高訊雜比的環境下，其性能優於arctangent；而在低訊雜比的環境下，則其性能與arctangent是不相上下的，但在計算負擔上則優於arctangent。本文並推導與分析此鑑別器的統計特性。 由於NB-DPD具備高效率與高精度的特性，且在數位技術不斷發展的趨勢中，是GPS接收機非常適用的載波相位鑑別器。

When implementing a software-GPS-receiver (SGR), few bits analog-to-digital converter (ADC) are usually selected to reduce the required computation and storage. In this thesis, the performance degradation induced by few bits ADC is discussed and an improved structure called post-correlator phase compensation (PCPC) is proposed. The significant feature of the PCPC structure is the improvement of correlator output magnitude because it prevents the quantization effect on carrier phase ambiguity. The improvement becomes more critical when one-bit ADC is used.

On the other hand, in a SGR, the carrier synchronization typically adopts arctangent or its approximations as the phase discriminator because it maximizes SNR for analog signal cases. However, for few-bits ADC, this method greatly reduces the accuracy of phase estimate because of the significant quantization error. In this thesis, a novel phase discriminator, called NB-DPD, is proposed for one-bit ADC SGR. The statistical properties of the NB-DPD estimator are provided. The analytical results are verified by computer simulation. In addition, the NB-DPD is equivalent to DPD[1] in noiseless case and thus inherits high accuracy properties of DPD in high SNR cases. The NB-DPD performs generally better than the DPD in noisy cases. The lower the SNR is, the greater the improvement. Moreover, the NB-DPD works almost as well as arctangent-phase discriminator (APD) in low SNR environments and thus can be applied to GNSS receivers.

Finally, the NB-DPD is implemented in a one-bit software-defined GPS receiver using PCPC structure. The experimental results demonstrate the feasibility of the proposed scheme.