Study On Application Of LDPC Code In Underwater Acoustic Communication

Error-correcting codes are widely applied in many fields, such as mobile communications, satellite communications, optical communications and disk recorders. Low-density parity-check (LDPC) codes belong to a family of error-correcting codes, approaching to Shannon limit. LDPC as a channel code is expected to be prosperous in underwater acoustic communication. In the dissertation, the theoretical analysis, the numerical simulation and the experimental work are utilized to study the transmission characteristics of binary LDPC codes, nonbinary LDPC codes, LDPC codes in MIMO system and LDPC Codes based on high order modulation.This thesis has been supported by the Fundamental Research Funds for the Central Universities "Study on application of LDPC Code in Underwater Acoustic Communication" (HEUCFL20101114) of Harbin Engineering University.The main contents of this thesis are as follows:1. According to the constitution element of LDPC codes, it can be usually divided into binary LDPC codes and nonbinary LDPC codes. The basic principle, encoding and decoding method of the binary LDPC codes are investigated, then the simulations are carried out to show the transmission characteristic of the binary LDPC codes in the underwater acoustic communication, the effects of the iterations times and the code length on the decoding performance are analyzed elaborately, which provides the reference for the determination of LDPC's parameters. Being comparison with the convolution codes and Turbo codes, the transmission of LDPC codes in underwater acoustic channels is feasible. Nonbinary LDPC codes have also been attracted much attention in recent years. Nonbinary elements can reduce the number of short cycles in the check matrix, consequently nonbinary LDPC codes is superior to binary LDPC codes. However, to my knowledge, no relevant reports have been published on the solution of transfer matrix between nonbinary LDPC codes and binary LDPC codes. In this dissertation the linear algebra is used to prove the uniqueness of transfer matrix, subsequently the solution is presented. This solution is still applied to the transformation of Gray maps between the nonbinary and binary elements. As a main obstacle in nonbinary decoding algorithm, the message transmission from the check node to the variable node is studied, and simulated results are utilized to successfully recognize the decoding performance. Finally, nonbinary LDPC codes are applied in the underwater acoustic channel and the results indicate that they are ideally suitable for applications in the complex sound-speed distribution.2. The design method of degree distribution is proposed based on the simulated annealing algorithm and EXIT (Extrinsic Information Transfer Chart). The simulated annealing algorithm is a non-derivative optimization algorithm, which is fast and accurate in the aspect of combination optimization. Meanwhile, the match degree between the variable node curve and the check node curve of the EXIT is treated as the objective function of the simulated annealing algorithm optimization. The results reveal that there is a good agreement between the match degree and the density evolution and the proposed method is not only superior but also reduces the optimization time of the degree distribution parameter.3. When the LDPC codes are applied into the MIMO system and the factor graph represents the MIMO channel, a joint detection and decoding algorithm of LDPC codes and space-time block codes in MIMO system is proposed. Iterating the soft information of LDPC-MIMO can improve the reliability of channel information and the decoding performance. The simplified algorithm of soft information extraction is applied into LDPC codes system with higher-order modulation, which simplifies the two-dimensional distance into the one-dimensional calculation. The results show that the soft demodulation obtains more coding gain than the hard one, then the capacity of communication system is improved and the error rate is reduced. At last, Lake trials of the LDPC codes are performed, and the transmission performance of the data and image information is tested. The conclusion can be drawn that the LDPC codes are feasible and reliable in practical underwater acoustic applications.