The Research And Application Of LDPC Codes Decoding Algorithm

LDPC codes, a kind of linear block codes approaching to Shannon limit, have become one of focus in channel coding areas due to their outstanding performance of bit error rate even in the environment of extremely low signal to noise ratio. LDPC codes have been identified as key technology of the fourth-generation mobile communication system, which have good application prospect in high-speed broadband wireless communication in the future.This paper mainly includes:First, based on analyzing the basic theory of LDPC codes, such as Tanner graph, construction methods and encoding methods. The impact of codes length, column weight and iteration times on BER performance of LDPC codes is demonstrated by computer simulation and theoretical analysis.Then, aiming at the problem of LLR-BP algorithm based on Tanh-rule and Gallager- approach, which has decoding computation complexity due to the exist of hyperbolic tangent function when check nodes in the process of updating, the improved decoding algorithm of LLR-BP algorithm based Tanh-rule and Gallager-approach is proposed, which can reduce computation complexity by linear fitting to hyperbolic tangent function. The theoretical analysis and simulation results demonstrate that the improved decoding algorithms have superiorities about computation complexity and performance, comparing to the standard LLR-BP algorithm.At last, OFDM system simulation model based on LDPC codes is constructed, and modified improved decoding algorithm of LLR-BP algorithm based on Tanh-rule is applied to the OFDM system based on LDPC codes. Based on the IEEE 802.11a standard, in the AWGN channels, the performance of OFDM system based on LDPC codes is simulated elementarily.The simulation results show that LDPC codes can improve OFDM system performance in a certain extent. In the modulation mode of 2PSK, QPSK and 16QAM, when LDPC codes adopt (Multi-Level Modulated) improved decoding algorithm of LLR-BP algorithm based Tanh-rule, the performance of OFDM system based on LDPC codes is superior to the performance of system when LDPC codes adopt (Multi-Level Modulated) decoding algorithm of LLR-BP algorithm based Tanh-rule.