针对脉冲位置调制(PPM)与多级编码结合中的错误传播问题，利用多阶段译码原理的“链式规则”，提出一种基于迭代的解调译码方法，给出了8-PPM多级编码调制的硬判决迭代算法。仿真分析表明，迭代解调译码方法可以改善错误传播现象，降低系统的误码率；在相同迭代次数下，信道衰减越大，该译码方法获得的增益越明显。综合考虑性能改善效果及成本，M阶编码调制系统的迭代次数宜选M次以内。

To solve error propagation problem in combination of pulse position modulation (PPM) and multilevel coding, we propose an iterative demodulation and decoding method based on "chain rule" of multistage decoding principle and give the hard decision iterative algorithm of 8-PPM multilevel coded modulation. The simulation results show that the error propagation is improved and the bit error ratio (BER) is decreased effectively by the hard decision iterative method. The larger the channel attenuation is, the more obvious the gain gets with the same iterations. Considering both improvement and cost, the iterations should be less than M times for M-order coded modulation system.