行動低軌道衛星通道特徵量測之實驗設計

Abstract

設計一個無線電通訊系統時，首先必需了解信號傳送時所經過的通道之特 徵。本論文針對行動-低軌道衛星通道特徵的量測提出一種可行的實驗系 統架構。這個系統的原理主要是根據Cox氏所提出的方法，利用偽亂碼的 特性來測量通道的脈衝響應。然而，由於低軌道衛星與地面的相對速度所 造成的嚴重都卜勒效應，我們需要對這樣的系統做一些修改。在信號同步 方面所使用的方法是類似Cahn提議之系統，其中我們主要考慮的問題是著 重於接收端的載波頻率獲取。為了能夠快速鎖定正確的載波頻率，我們使 用兩種不同的方法ぇ二次自動頻率控制迴路(Second-Order Automatic Frequency Control, AFC, Loop)え最小平方差估計法(Least-Squares Estimation Algorithm)。從模擬的結果可以得知，在接收端信號雜音比( carrier-to-noise ratio) 50dB-Hz的情況下，整個載波頻率獲取的過程 能夠在一秒之內完成，而且二次的最小平方差估計法會比AFC好。 In designing a wireless communication system it is essential to have information about the characteristic of the associated communication channel. This thesis studies a channel characteristic experiment using a low earth orbit (LEO) satellite to measure mobile-LEO-satellite channels. We proposed a system architecture based on the pseudo-noise (PN) sequence technique whose feasibility was first demonstrated by Cox. However, due to the high dynamic environment many modifications and improvements have to be made. The proposed process of synchronization is a noncoherent system similar to that suggested by Cahn. We also address major design concerns, in particular, the frequency acquisition and tracking issue in the ground receiver. Since the LEO satellite travels with a relatively high and fast varying speed with respect to mobile terminals on the ground, a second-order automatic frequency control (AFC) loop is needed for fast frequency synchronization. Our simulation results show that frequency acquisition can be accomplished within 1 second when the received carrier-to-noise ratio (CNR) is 50 dB-Hz. The effect of multipath fading is investigated as well.