Research On Application Of Protograph LDPC Codes In FSO Communication System Under Atmospheric Turbulence

Atmospheric interference is a major challenge to the development of FSO communication.The influence includes the absorption and scattering of atmospheric particles to the optical wave and the beam widening and intensity flicker effected by atmospheric turbulence.Channel coding can improve the ability of FSO communication system to resist atmospheric interference effectively.The optimized protograph LDPC codes have many advantages,such as low decoding threshold,low bit error rate,and minimum Hamming distance increasing with the code length.It has attracted a large number of researchers engaged in theoretical analysis,optimization design,technology application and other related work.This paper mainly studies the application of protograph LDPC codes in FSO communication system.This research has great significance to enhance FSO system's ability to resist atmospheric interference and improve the reliability.Based on insight into the FSO communication theory and application,this paper further analyzes the feasibility of applying the protograph LDPC codes to FSO communication to improve the communication quality.Then,the influence mechanism of atmosphere on FSO communication is analyzed and a mathematical model can be used to simulate the effect of atmospheric scattering,absorption and turbulence is discussed.This paper adopts the LR distribution to simulate the effect of atmospheric turbulence.The LR distribution can simulate varies atmospheric turbulent intensity by changing the correlation coefficient,and it is easy to analyze.Considering the problems in the expansion process of protograph LDPC codes,this paper optimizes the cyclic PEG algorithm,and proposes a design scheme for structuring the protograph LDPC codes based on AR4JA code.The grith restriction is introduced in the extension process to avoid the short cycles.Then the PEXIT algorithm and AWD function are used to analyze the decoding threshold and the minimum Hamming distance to evaluate the good performance of the coding.Considering the influence of modulation mode,atmospheric turbulence and background noise,this paper proposes a FSO communication system model to examine the feasibility of the proposed scheme.In the simulation test,we analyzed the BER under different turbulent intensity,code length and rate.The test results fully verified the feasibility of the coding scheme.