中文摘要：

LDPC码字具有优异的性能，在空间通信中得到广泛应用．为进一步降低LDPC码构造及编码的复杂度，给出了一种高性能、低复杂度的QC—LDPC码构造方法．设计了扩展近似下三角阵（externApproximateLowerTriangular，eALT）结构的全局矩阵，通过增加双对角阵结构全局矩阵的列重，降低差错平底（errorfloor）．为降低传统循环移位系数选择的复杂度，提出了一种基于Zig-Zag的移位系数设计方法，采用数学公式计算循环移位系数，无需计算机搜索即可完全消除长度为4的短环．给出了所构造码字线性编码的实现过程．仿真结果表明，所提构造方法在保证线性编码复杂度的前提下，增大了码字间最小距离，降低了差错平底，提高了码字性能；采用结构化的方法设计循环移位系数，无需计算机搜索即可消除4环，所构造的码字与CCSDS标准中的码字在性能相近的情况下，降低了实现的复杂度．

英文摘要：

A class of QC-LDPC codes with high error performance and low complexity was proposed. Firstly, a global matrix M with extern Approximate Lower Triangular （eALT） method was designed. Each 1 in the global matrix M was replaced by a permutation matrix, and each 0 was substituted by a null matrix to reduce the number of small stopping set and error floor. Then, In order to lower the complexity of searching cyclic shift coefficient, a Zig-Zag based method was put forward, which uses a given formula to generate coefficient and does not need search by computer. At last, the linear encoding process was given with the proposed QC-LDPC. Simulation result showsthat the proposed method can reduce the number of small stopping set, increase the minimum distance of LDPC codes, and achieve very low error floor with linear encoding complexity, where the cyclic shift coefficient was generated by structured method.