Abstract: Low density check code is a kind of line-codes basing on a sparse check Matrix.This kind of channel coding can achieve the near Shannon limit performance.And it has a quite simple decoding algorithm which introduces a iterative decoding method.Large preferences referring to LDPC are basing on binary modulation,such as BPSK.Comparied with multilevel modulation,such as MQAM orMPSK,BPSK using of bandwidth is inefficient.In future 3G+ mobile communications,the extensively adopted technique,OFDM, requires to combine with multilevel modulation. So,How to combine the channel coding,LDPC,with the mult-modulation OFDM is our primary work.

Key words: LDPC, OFDM, Probability-likelihood, Log-likelihood

摘要： 低密度校验码是一种基于稀疏校验矩阵的线性码，此码性能性能接近Shannon 限，并具有简单有效的迭代译码算法。大部分文献提到的LDPC 迭代译码算法均基于BPSK调制下，相对于MQAM/MPSK 调制，BPSK 调制频谱利用率太低。在未来的3G+高速无线通信中，最广泛运用的物理层技术OFDM必然采用频谱利用率较高的多电平调制。因此，如何把LDPC码用于多电平调制下的OFDM 是一个关键。

关键词: LDPC, OFDM, 概率似然比, 对数似然比