GPS L5接收系統中結合碼擷取及頻率估測演算法研究

Abstract

在DS-CDMA 系統裡, 碼擷取的效能深受頻率偏移量影響，至今已有多種方法被提出來解決此類問題。其中傳統的2維（two-dimensional）循序搜尋或利用大量的離散傅立葉轉換（discrete Fourier transform：DFT）運算的方法不是需要較長的擷取時間，就是需要複雜度較高的運算。在GPS L5接收系統裡,所使用的碼切片率為10.23 MHz；它高出現有的L1訊號系統甚多，而較大的切片率即表示有較多的碼相位將可能被搜尋，因此傳統的碼擷取方法的效能將相對的變得更低。在本篇論文中,我們提出了一個可以同時進行碼擷取與頻率偏移估測的新方法，此新的演算法首先使用部分匹配濾波器（partial matched filter）來擷取碼相位，接著應用Luise and Reggiannini估測器來估計頻率偏移量。同時，我們亦對擷取效能進行理論分析，再依據分析結果我們就能夠推導出使系統擷取錯誤率降至最低的最佳臨界值。模擬的結果顯示擷取效能和DFT方法相近，但頻率偏移估測較準確。而所提的演算架構運算複雜度較低，更是適合未來硬體的實現。

In DS-CDMA systems, it is known that the efficiency of code acquisition will be greatly affected if the frequency offset is present. Many methods have been proposed to solve the problem. Conventional methods such as the two-dimensional search or the DFT approach either require long acquisition time or high computational complexity. In the GPS L5 system, the chip rate is 10.23 MHz it is much higher than the existing L1 system. The larger chip rate indicates that more code phases need to be searched. The efficiency of conventional code acquisition methods will degrade further more. In the thesis, we propose a new method that simultaneously acquire code phase and estimate the frequency offset. The new algorithm first uses the partial matched filter method to acquire code phase. Taking advantage of the special matched filter structure, we then apply the Luise and Reggiannini estimator to estimate the frequency offset. We also provide theoretical analysis for the acquisition performance. Based on the results, we are able to derive the optimal threshold minimizing the probability of acquisition error. Simulations show that the acquisition performance is similar to that of the DFT approach and the frequency-offset estimate is more accurate. The computational complexity of the proposed algorithm is low and suitable for real-world implementation.