中文摘要：

比较了LDPC码和RS码在PPM调制下的深空光通信系统中的性能，给出了在不同阶数PPM调制下，不同码长LDPC码的性能仿真结果，以及与RS码的比较结果。结果表明，随着PPM调制阶数增加，LDPC码性能会下降。分析了调制阶数影响LDPC码性能的原因。

英文摘要：

Performance of binary low-density parity check （LDPC） codes employing Q-ary pulse position modulation （PPM） and an avalanche photodiode （APD） is investigated in a deep space optical communications system in this paper. The performances of LDPC codes with various codeword length and various order of PPM modulation are given, and compared with Reed-Solomon （RS） codes. Simulations show that the performance of LDPC codes decreases rapidly as the order of modulation increases. In addition, the influence of the order of PPM upon the performance of LDPC codes is analyzed.