说明：  首先介绍了LDPC码的校验矩阵和其因子表示方法，然后利用二分图对和积解码算法进行了详细的描述，最后给出了信度传播概率译码算法详细步骤，并对关键公式作了证明
(This paper,first introduces the check matrix and the factor graph of LDPC,then describes the sum-product algorithm by using the factor graph,and finally presents the detailed steps of the sum-product algorithm and gives a proof of certain important expressions. )