Coding Modulation Technology Research Of Ldpc Code

In communication system, channel encoding is an efficient way to improve reliability, and on the other hand, modulation is a necessary step to turn digital signal into signal which can transmit easier in the channel. Low-density parity-check (LDPC) code is one of the most important error correction code which is reintroduced in recent years, and the degree distribution of it has great influence to the performance of different modulation modes. LDPC code and modulation is combined in this paper, and by analyze the best LDPC code in different modulation mode, the performance of the system can be improved.It has been proved that irregular LDPC code has better performance than regular LDPC code, and for different modulation mode, there are some LDPC code with specific degree distribution can make the system has excellent performance. Because of this, the node expansion algorithm and modified progressive edge growth (PEG) algorithm which can construct LDPC code with random degree distribution is proposed in this paper. Meanwhile, based on the modified PEG algorithm, the method which can make quasi-cyclic LDPC code with random degree distribution is put forward.For different modulation mode, there are different binding method for LDPC code and modulation. For differential modulation which can overcome phase ambiguity, the iterative between differential decoding and LDPC decoding in the receiver is necessary in order to compensate the loss brought by differential encoding, while for common modulation mode, the iterative process in the receiver is not needed. Therefore, in order to adapt to the needs of different communication environment, the LDPC code and modulation combined system for different modulation mode is proposed in this paper.Extrinsic information transfer (EXIT) chart is a tool to analyze the convergence behavior of the iterative receiver. By using EXIT chart, the convergence behavior of LDPC code in different modulation mode can be predicted. In this paper, the degree distribution of LDPC code is optimized by EXIT chart in different modulation mode, which is the optimal degree distribution for different modulation. Then the LDPC code which has the optimal degree distribution can be applied to the combined system. Simulation results have proved that the obtained optimal degree distribution LDPC code for different modulation mode can improve system performance significantly.