现有DVB-S2标准中LDPC码的设计未考虑其长码编解码器的实现复杂性,也未给出其缩短码的设计.本文在现有DVB-S2标准中LDPC码的设计框架下,提出具有无短环和低码重码的缩短码的设计.本文推广DVB-S2的LDPC码,将其变换为缩短的LDPC码.码长大于4 000的小幅度缩短的LDPC码可无4环和低码重码,但码长小于4 000大幅度缩短的LDPC码存在4环和低码重码.本文对大幅度缩短的LDPC码进行两方面的修改:一是修改部分子矩阵的列重,以解决4环问题;二是采用802.16e中的准双对角线子矩阵替代DVB-S2标准中LDPC码使用的双对角线子矩阵.在AWGN信道下仿真结果表明,改进后的LDPC码性能比改进前的误码率性能有较大提高.%The LDPC design in the present DVB-S2 Standard has not considered the implementation complexity of the codec of their long codes and has not included design of their short codes. The design of the shortened codes without girth-4 and lower weights is proposed in the paper under the frame of the DVB-S2 Standard. The paper transforms the DVB-S2 LDPC codes to the ones of short lengths. When the code length is shortened to be larger than 4 000, no girth-4 and codes of low weights exist;when the code length is shortened to be smaller than 4 000, girth-4s and codes of low weights exist. The DVB-S2 LDPC codes are revised in the following two respects: to reduce the column weights of parts of the parity check matrix; to replace the bi-diagonal matrix with the quasi-bi-diagonal matrix that is used in the IEEE 802.16e LDPC codes. The results of simulation in the AWGN channels show that the BER performance of the improved LDPC codes is much better than that of the former codes.
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