| LDPC（Low Density Parity Check）is one of the best performing coding methods in the world today.Quasi cyclic-LDPC（QC-LDPC）codes don’t has long code length,and it also has excellent performance and can well continue the randomness of LDPC codes.OFDM（Orthogonal Frequency Division Multiplexing）technology has the characteristics of more efficient utilization of limited Frequency band resources and effective resistance to some interference in the transmission process,and it is the important technologies in communication technology.Combining these two technologies is beneficial to play the advantages of both,but some problems will also arise in the face of special application scenarios.The environment of underwater acoustic channel is very complicated.The underwater acoustic channel is often accompanied by a large amount of environmental noise and very serious multipath effects and other adverse factors that affect information transmission,will make the BER（Bit Error Ratio）become bad.The research shows that the combination of channel coding technology and OFDM technology can be applied to the underwater communication system,it can make system batter.Its performance analysis is a key issue that is generally concerned in the current underwater acoustic communication field.This paper introduces the principle of LDPC code,including the construction of the check matrix and code encoding methods,at the same time,the Bit Flip decoding method and the Belief Propagation decoding method are introduced,and the quasi-cyclic LDPC code is introduced,and the simulation analysis is carried out.Through simulation diagram,we can see that when the SNR（Signal-Noise Ratio）value is 4,the code length is 1296,and the QC-LDPC code rate is 1/2 and iterations is5,BER is close to 10-5,and BER is worse when the bit rate is 2/3,BER near 10-3,the BER is worse when the bit rate is 3/4,BER is 2×10-2,and when the bit rate is 5/6,BER is the worst.When the SNR is 4.5,BER of QC-LDPC code with 1296 code length and 3/4 bit rate is 2×10-2 in iterations is 5,when iterations is 20,BER is 2×10-3,when iterations is 50,the BER is 9×10-4.At the same time,two different code lengths of 1944 and 1296 were compared in the simulation.When the code rate was 3/4 and the SNR was 4.5 and iterations is 5,the BER of the 1944 code length was 10-2 and the BER of the 1296 code length was 2×10-2.Then,the QC-LDPC code and OFDM technology are combined on the MATLAB platform to simulate and analyze its performance.Finally,the UWA（Underwater Acoustic）channel and several main factors affecting the performance of the UW communication system are analyzed.Combining QC-LDPC+OFDM in the underwater acoustic channel model for simulation analysis and verification.The simulation results show that QC-LDPC+OFDM system can improve the transmission performance of underwater communication system,and the performance in underwater acoustic channel will become better with the increase of code length or the decrease of code rate and the increase of iteration times. |
