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对于深空光PPM通信，基于EM定时误差估计原理，研究了一种基于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，论文提出一种将SCPPM译码输出的软信息转换为PPM各时隙期望值的方法。首先由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，然后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，之后再利用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，在时钟误差小于正负0.5个PPM时隙范围内，本文提出的方法能有效实现PPM时隙同步。
For deep space optical PPM communication, based on the principle of EM timing error estimation, a solution of optical PPM slot synchronization based on SCPPM code assistance is studied. Since the standard SCPPM decoding method can not obtain the expected value required by EM estimation, a method of converting the soft information output by SCPPM decoding into the expected value of each slot of PPM is proposed. Firstly, the hard decision is first performed by the SCPPM decoding output, and then the accumulator state is obtained through the accumulating module. Then, the next PPM mapping manner is selected according to the state, and then the probability and expectation of each PPM timeslot are obtained, and then the EM algorithm is used to predict the clock skew Value to get the best sampling point. The simulation results show that the proposed method can effectively achieve PPM slot synchronization for asynchronous sampled signals with 2 times PPM time slot frequency within the range of less than plus or minus 0.5 PPM time slot.