Research On Polar Coding In Orbital Angular Momentum Underwater Wireless Optical Communication System

The optical communication based on Orbital Angular Momentum（OAM）can realize high speed information transmission,which provides a new solution for the growth and expansion of underwater wireless optical communication（UWOC）.However,affected by the ocean turbulence and other factors,the OAM wireless optical signal will have intensity flicker,phase distortion and beam expansion,resulting in the reliability and communication range of the communication system is limited.Therefore,how to build a stable and reliable communication link in the UWOC system is an important issue to improve the system reliability and enhance the communication range.In this paper,a polarization coding scheme based on Monte Carlo construction method is designed according to the characteristics of ocean turbulent channels,and Polar codes are applied to the underwater OAM wireless optical communication system for the first time to suppress the ocean turbulence effect.The research in this paper is as follows:（1）The ocean turbulence channel model is constructed based on the power spectrum inversion method,and the spatial distribution and spiral spectrum distribution of the Laguerre-Gaussian（LG）beam-carrying OAM in the ocean turbulence channel after transmission under different channel parameters are analyzed by numerical simulation,which provides a reference for the mode selection of OAM in polar coding.（2）An underwater OAM wireless optical communication simulation system is built to compare the performance of the OAM recognition scheme at the receiver side using the OAM recognition scheme based on orthogonal characteristics and the OAM recognition scheme based on the Convolutional Neural Networks（CNN）model.In the weak turbulence environment,the accuracy of the OAM recognition scheme based on CNN model is improved by about 16.23%compared with the OAM recognition scheme based on orthogonal characteristics.（3）The code word construction method of Polar code based on Monte Carlo method is presented.The bit error rate of underwater OAM wireless optical communication system with polarization coding under different OAM modes is studied through simulation experiments,and the coding gain provided by different decoding algorithms of Polar code is compared.Bit error rate of underwater OAM wireless optical communication system using Low Density Parity Check（LDPC）code and Polar code.The experimental results show that Polar codes can effectively suppress the ocean turbulence effect,and the BER of the underwater OAM wireless optical communication system reaches 10^-6 in the weak turbulence environment when the transmission distance is50m;in the strong turbulence environment,the BER of the underwater OAM wireless optical communication system reaches 10^-4.In strong turbulent environment,the Successive Cancellation List（SCL）decoding algorithm and the Cyclic Redundancy Check Aided Successive Cancellation List（CA-SCL）decoding algorithm provide 5d B and 6.9d B more coding gain,respectively,than the Successive Cancellation（SC）decoding algorithm when the system BER reaches the Forward Error Correction（FEC）tolerance.When the code length is 1024 bits,the coding efficiency is 0.5 and the system BER reaches the FEC tolerance,Polar codes can provide 3.55d B more coding gain compared to LDPC codes.