Research On Application Of LDPC And PPM Coding In Underwater Optical Signal Transmission

Underwater wireless optical communication occupies an extremely important position in the field of underwater wireless communication with its advantages of low loss window,strong anti-interference ability and high communication rate.However,the light beam is mainly affected by the random variability of ocean turbulence,absorption and scattering the oceanic channels.The resulted energy loss and channel fading will cause the bit error rate of the underwater optical signal transmission system to increase,and the transmission performance will be seriously deteriorated.Therefore,it is necessary to explore suitable underwater optical signal transmission methods and study channel coding and modulation techniques in underwater wireless optical communication systems.Under this background,a method of combining the application of LDPC code and PPM modulation is proposed in this paper,and the performance of LDPC and PPM coding in underwater optical signal transmission systems and the realization of system software and hardware are discussed theoretically and experimentally.The working principle of the underwater optical communication system is introduced in the thesis,including the influence mechanism of turbulence,absorption and scattering on the underwater wireless optical communication,and the lognormal distribution is adopted as the seawater turbulence channel model.By comparing several common modulation methods for underwater optical communications,it is known that PPM modulation can reduce the requirement of the average laser emission power.In terms of the coding gain and hardware implementation complexity comprehensively,the LDPC code is selected as the channel code of the underwater wireless optical communication system.Then the basic principle of LDPC code and the coding and decoding algorithm is presented in detail.By MATLAB,the system performance is simulated and analyzed in various conditions,such as different code length,code rate and iteration times.The simulation results show that,compared with the uncoded PPM system,the system with the combination of the LDPC code and PPM modulation with a code rate of 5/6 and a code length of 2304 can obtain a code gain of about 1.5d B in a weak turbulent channel;the system can effectively decrease the bit error rate and improve the transmission performance at the same signal-to-noise ratio.Based on theoretical research,a set of underwater wireless optical communication system using 532 nm laser and PIN photodiode is designed,and the application of LDPC and PPM joint coding scheme in FPGA system is discussed.Finally,in order to verify the error correction performance of the system combining LDPC code and PPM modulation,experiments are carried out in a water tank with different water environments due to bubbles,turbidity and turbulence caused by temperature changes.The experimental results show that,the system combining LDPC with PPM code has anti-channel interference capability,which is conducive to improve the performance of underwater optical signal transmission;the system obtains a lower bit error rate and stronger correction under the same transmission power error capability,under the condition of the same bit error rate,a code gain of about1.3d B can be obtained.