討論了PPM序列的符號同步原理；在分析隨機PPM序列的精確符號同步概率上限的基礎上，給出了一種在PPM序列中插入同步信號來實現PPM精確符號同步的方案及其數字邏輯電路實現。該方案通過改變PPM調制序列結構，按照一定的間隔在PPM序列中插入脈沖時隙肩并肩的2個相鄰符號作為同步符號，通過對同步符號的檢測來提取同步信號。理論及仿真實驗表明，該方案能實現PPM精確符號同步。
關 鍵 詞 無線光通信; PPM序列; 脈沖位置調制; 符號同步

Abstract One of the most important issues of pulse position modulation (PPM) for optical wireless communications is the receiver symbol synchronization subsystem, in this paper, the basic principle of symbol synchronization for PPM sequence is discussed, and the upper limit of exact synchronization probability for random PPM sequence is analyzed. To solve this problem a new scheme of inserting synchronous back-to-back PPM symbols to the PPM sequence at intervals of certain symbols is presented, the synchronization signal is obtained by searching for the synchronous symbols, and the corresponding digital logic circuit is presented too. Theory and experiment results show that the exact PPM symbol synchronization can be obtained by this scheme with more credibility compared to the scheme proposed by Ref.[8].
Key words optical wireless communications; PPM sequence; pulse position modulation; symbol synchronization