Research On LPDC Code And MPPM Coding Technology In Laser Acoustic Communication

There are two transmission channels with completely different characteristics in air-sea cross-domain communication,namely the atmospheric channel and the underwater channel.The channel environment is extremely complex,and it is difficult for common communication methods to adapt to the characteristics of the two channels at the same time.Laser-induced acoustic communication uses the mechanism of converting laser signals into acoustic signals underwater,fully utilizes the advantages of atmospheric optical communication and underwater acoustic communication,and perfectly adapts to the characteristics of air-sea cross-domain channels.However,in the process of signal transmission,the signal will be affected by the characteristics of the channel,resulting in a decrease in transmission performance.Choosing an appropriate modulation scheme and channel coding can effectively improve the transmission performance of the communication system.In the signal conversion process of laser-induced acoustic communication,the amplitude of the signal will be unstable.Using multi-pulse position modulation(MPPM)can effectively retain the information carried by the signal,improve the reliability of the communication system,and reduce the laser average transmit power.LDPC codes in channel coding have better performance than Turbo codes and polar codes in long code applications,and the hardware implementation complexity is lower.Therefore,this paper proposes an experimental scheme for a laser-induced acoustic communication system jointly coded and modulated by LDPC codes and MPPM.The main research contents are as follows:(1)Use MATLAB to simulate the performance of the transmission rate,transmission bandwidth and bit error rate of different PPM modulation schemes,and determine the modulation scheme of 3-13 MPPM through the simulation results;for the decoding performance of most existing LDPC decoding algorithms For the unideal problem,an adaptive minimum sum decoding algorithm(SSMS)based on the second smallest value is proposed.The simulation results show that the SSMS decoding algorithm can obtain an additional gain of 0.3～0.5d B compared with other algorithms at the same bit error rate.(2)The communication system hardware implementation scheme of joint coding and modulation of LDPC code and MPPM is proposed,the corresponding hardware circuit is designed according to the channel characteristics,and the waveform simulation and function verification of the hardware implementation scheme are carried out.The simulation results prove the feasibility of the scheme.(3)A hardware implementation experimental test platform for laser-induced acoustic communication system was built based on theoretical research and simulation analysis.The platform consisted of Nd: YAG solid-state laser,hydrophone,and FPGA.The combined coding and modulation scheme of laser-induced acoustic communication was tested under laboratory conditions to conduct transmission performance tests.The results indicate that the LDPC code and MPPM joint coding and modulation communication system outperforms the communication scheme without LDPC code coding,with significantly improved performance and lower bit error rate.