中文摘要：

针对提高光纤通信系统中LDPC编译码方案频谱利用率的问题，本文研究了光纤通信系统中具有较高频谱效率的LDPC高阶调制方案，重点以16QAM调制的LDPC为例分析了基于卡方分布的光纤信道中软解调对数似然比的理论推导，提出并推导了其简化算法，设计了16QAM-LDPC的光纤通信系统方案。仿真结果表明，采用16QAM软解调译码算法能带来较好的系统性能，当码长2304，码率为5／6的QC—IX）PC码在时，对应的Q=9．1dB。其相应的简化算法在0．2dB的性能损失代价下，可以具有较低的译码复杂度。

英文摘要：

To improve the spectrum utilization of LDPC coding scheme in optical fiber channel, a new LDPC scheme with high order modulation was studied in this paper. The research mainly concentrated on 16QAM-LDPC code, deduced the soft LLR algorithm in optical channel based on chi-square distribution, and further obtained the simplified algorithm. At last, we simulated the proposed LDPC scheme in MATLAB. The experimental results show that, the proposed scheme can provide a good performance with high spectrum utilization. When using codes with length~2304 and rate=5/6, we can achieve a Q=9. ldB in .While the corresponding simplified algorithm can have a low decoding complexity at the cost of performance loss of 0.2riB.