针对深空光脉冲位置调制(PPM)通信，基于最大期望(EM)定时误差估计原理，研究了一种基于串行级联脉冲位置调制(SCPPM)码辅助的光PPM时隙同步的解决方案。由于标准的SCPPM译码方法无法获得EM估计时所需的期望值，因此提出一种将SCPPM译码输出的软信息转换为PPM各时隙期望值的方法。由SCPPM译码输出先进行硬判决，再通过累加模块获得累加器状态；根据该状态选择下一个PPM映射方式，进而获得各PPM时隙的概率及期望；最后利用EM算法预测时钟偏差值从而获得最佳采样点。仿真结果表明，对于2倍PPM时隙频率的异步采样信号，当时钟误差在-0.5～0.5个PPM时隙范围内时，此方法能有效实现PPM时隙同步。

Aiming at deep space optical pulse position modulation (PPM) communications, based on the principle of expectation-maximization (EM) timing error estimation, a new solution of optical PPM slot synchronization based on serial concatenated pulse position modulation (SCPPM) code is studied. Due to that the expected value of EM estimation can not be got by the standard SCPPM decoding method, a method to convert the soft information output from SCPPM decoder to the expected value of each PPM slot is proposed. Firstly, the hard decision of output from the SCPPM decoder is made, and the accumulator state through accumulation module is got. Then, a mapping method according to the former accumulator state is chosen, and the probability and expectation of each PPM slot is obtained. Finally, EM algorithm is used to predict the clock offset and then to obtain the best sampling point. According to the simulation results, for the asynchronous sampling signal with two times PPM slot frequency, the proposed method can effectively achieve the PPM slot synchronization under the condition of the clock error among －0.5~0.5 PPM slots.