Discussion and implementation of LDPC and PPM joint coding in laser underwater data transmission

Aiming at the characteristics of signal fading caused by the intensity fluctuation and the off-axis drift of the underwater wireless optical communication, a method of laser-based trans-media transmission from atmosphere to underwater employing low density parity check (LDPC) and pulse position modulation (PPM) joint coding is proposed. The encoded frame structure is studied. Furthermore, by simulation, the performance of LDPC and PPM joint coding in Gaussian channel and weak turbulence channel is analyzed and compared with that of the uncoded PPM system. A practical communication system based on Field Programmable Gate Array (FPGA) is established and relevant lab tests are carried out. The simulation results show that the joint coding of LDPC and PPM can significantly improve the system error performance under these two channels, and the performance of Gaussian channel is better than that of weak turbulence channel. It is verified by the tank experiment that the system combining LDPC and PPM can obtain 1.5 dB code gain compared to the uncoded PPM system at the same bit error rate, which fully demonstrates the anti-interference ability of the system combining LDPC with PPM in data transmission.