针对无线光通信脉冲位置调制(PPM)与信道编码的结合应用，提出了一种多级编码调制方案，利用多级编码的多级标签结构，将PPM符号所对应的分组比特分拆到不同子码通道的码字中，同时结合PPM的信号集分割和极大似然检测解调方法，推导了方案的多阶段解调译码算法。在弱湍流大气条件下的仿真分析表明，PPM多级编码调制相对单级编码调制在误码率10-6下获得0.85 dB以上的增益，且可实现对不同重要程度信息段的不等差错保护。在分量码码型给定的条件下，按纠错能力逐级配置分量码并采用所推导的多阶段解调译码，可为系统提供更好差错性能，优于直接的并行译码方法。

Multilevel coded modulation (MLCM) scheme is proposed for the application of combination between pulse position modulation (PPM) and binary correct coding. The bits demodulated from one PPM symbol are divided into different subcode channels by using the multilevel labels structure of multilevel coding, and the multiple-step demodulation and decoding (MSDD) algorithm of the scheme is derived with signal set partitioning and maximum-likelihood detection of PPM. Simulation under weak turbulence shows that PPM MLCM gets 0.85 dB gain over single-level coded modulation under the bit-error rate (BER) of 10-6, and it can provide unequal error protection to in formation with different importance. When the subcode types are given, the system with the subcodes configuration in order of error corrective ability in levels, as well as MSDD being used, will have good error performance, much better than direct parallel decoding method.