碼同步之相量獲取方法

Abstract

「碼同步」在展頻通訊系統中是一項重要且值得探討的議題，而傳統的作法是在時域上做序列比對。 藉著快速傅立葉轉換(FFT)，我們可以將碼序列轉變為相量，接著利用逆傅立葉轉換(IFFT)，即可找到輸入序列和本地序列的相位差。當碼序列長度增加時，所耗費的運算量也隨之增加。 本論文中，我們提出一個新的「碼同步」方法，它不需作逆傅立葉轉換以減少運算量。這個方法主要利用複數相量之間的相位關係，藉此可以擷取所需的相位差。在分析複數相量的統計特性之後，我們提出一個簡單的方法求相位差。接著， 我們進一步提出一個進階的方法以提升系統的準確度。最後，我們利用電腦模擬驗證所提的理論，其中分析的重點在於相位變異數的變化以及估測的準確度。模擬的結果顯示，所提方法可減少許多運算量並在高SNR的環境下可以獲致良好的結果。

Code synchronization is a critical issue in spread spectrum communication systems. The traditional method to achieve code synchronization is via serial search in the time domain. By performing Fast-Fourier-Transform (FFT), the code sequence can be projected to the phasor domain. Then, code phase between local sequence and input sequence can be found via Inverse-Fast-Fourier Transform (IFFT). However, as the code length increases, the computation increases considerably. In this thesis, we propose a new method for code synchronization in the phasor domain without IFFT. The method is based on the phase relationship of complex phasors, whereby we can extract code phase via phasor phases. We analyze the statistical property of complex phasors and design a simple method to extract the code phase. Moreover, we further design an improved method to enhance the accuracy under noisy condition. Computer simulation is performed to verify the proposed scheme, which focuses on variance reduction and estimation accuracy. It can be demonstrated that the proposed method works well without much computation while acceptable accuracy is achievable in high SNR environment.