The performance of low-density parity-check code (LDPC) based on protopraph expansion greatly depends on the expansion method. A new QC-LDPC construction method was proposed. Two main steps in the parity check matrix construction include the multi-edges elimination of protograph and quasi-cycle expansion. The first step make local girth maximum ?under the constraint of edge permutation. In the second step, optimum shift values of the permutation matrices were obtained by searching the closed loops within the specified length and comparing the length and approximate cycle extrinsic message degree (ACE) of cycles, aiming at reducing small cycles with low connectivity which was harmful to the code performance. For protograph without multi-edges, the only step was the quasi-cycle expansion. As proved by simulation, the constructed QC-LDPC has excellent performance in both decoding threshold and error floor aspects.%基于原模图构造的低密度奇偶校验码(LDPC)性能很大程度上取决于扩展算法.为此,提出了一种构造准循环低密度奇偶校验码(QC-LDPC)的新方法.所述算法经过两步扩展得到QC-LDPC:第一步是原模图去重边,在边置换条件的约束下,使扩展所得矩阵局部围长最大化；第二步进行准循环扩展,通过计算机搜索得到规定长度内的所有闭环路径,比较环长和近似环路外信息度得到置换矩阵的最优偏移量,目的是剔除连通性差的短环对码性能的负面影响.对于不存在重边的原模图,则直接进行准循环扩展.仿真结果表明,利用该方法构造的QC-LDPC在译码门限和误码平层两方面都具有优异的性能.
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