跳頻衛星系統同步微調之反干擾設計

Abstract

在衛星通訊系統中，因為由地面上不同用戶到衛星之間距離的不同，傳輸延遲的時間也不同。通常我們會以衛星上的時間當做位準，要求地面用戶與之同步。若該系統採用跳頻多工，（frequency - hopped multiple access），則衛星轉頻器將會估計出各用戶所發射的信號因時間延遲效應所造成的誤差時間，進而下傳給地面用戶；用戶接收到此修正信號後，藉它來詷整發射器的跳頻器的時間位準，使得所有由地面發射的信號到達衛星後能與衛星上解跳頻器（dehopper）的時間位準一致。一般來講這種跳頻同步的過程分成粗調及微調兩個步驟，這里我們只探討同步微調（fine - time synchronization）這一部分。在這類微調的方法中，有一種稱為F2的估測法，其性能要比其他的方法來的好。這方法主要是利用一個早遲濾波器（early - late filter）的技術來處理接收進來的探針信號（probe signal）以估計出誤差時間值。 本論文旨在研究軍事衛星通訊的環境中，當敵方送出一干擾信號（jamming signal）企圖擾亂我方的通訊時，F2法在此狀況下的性能。同時我們針對F2法做一些改進：利用估計雜訊的變異數（variance of noise）大上並以此值作為在估計誤差時間過程的權數（weight），以求在最少的跳頻週期（hopping period）裡估計出準確的同步誤差時間。我們將分別比較理想的估計器（ideal estimator）、實用的估計器（imperfect estimator）和原來的F2法在有干擾環境下的性能。由模擬結果可看出實用的估計器在有干擾環境下遠比F2法為佳。同時當用來估計雜訊的變異數的取樣數夠大時，實用的估計器的性能可接近理想的估計器。

In a frequency - hopped multiple access (FHMA) satellite communication system, the hopping clocks of the ground users must be synchronous with that of the transponder. This FH clock synchronization process is usually divided into two parts, namely, the coarse clock synchronization and fine - time synchronization. This thesis deals exclusively with the fine - time synchronization process. Because of the long round - trip delay between the ground users and the satellite, correct estimate of the clock timing error is made aboard the satellite and sent via the downlink to the ground users to adjust their carrier hop transition times. After a few closed - loop iterations, a ground user's signal arriving at the satellite receiver will be synchronous with the hop transition time of the dehopper. To carry out such a fine - time synchronization, a ground user would send a probe signal for a number of hop periods. Several estimation algorithms using the probe signal had been proposed. Among them, the so - called method F2,. which is based on an early - late filter technique to process the probe reponse, was shown to render the best overall performance. Since the satellite FHMA system is designed for military applications, it is necessary to examine the anti - jam capability of such a system. Unfortunately, the F2 estimator is far from robust against jamming. We suggest practical solutions to save this estimator.Performance analysis of various modified estimators is presented. Numerical examples using either analytical results or computer simulations are provided to study the behaviors of the proposed new estimators under various jamming threats. It is shown that, with proper design, fine - time synchronization using probe signals can be accomplished even in the presence of jamming.