Transmission Characteristics Of OAM Wireless Optical Communication In Weak Atmospheric Turbulence

Outdoor wireless optical communication(WOC),also known as free space optical(FSO),has attracted great attention due to its advantages of large transmission bandwidth,no spectrum permission and good security performance.In recent years,researchers have found that compared with the traditional degrees of freedom such as amplitude,frequency and polarization,orbital angular momentum(OAM)provides another new degree of freedom and has infinite modes in theory,which further improves the channel capacity and spectral efficiency.Therefore,the OAM-FSO communication has aroused widespread concern.However,atmospheric turbulence would cause the energy fading and shifting,that is,the channel attenuation of OAM eigenstates and the crosstalk between different OAM modes,resulting in the performance degradation.Therefore,it is vital to explore the relationship between the statistical characteristics of OAM channel and turbulence strength,as well as the transmission scheme to mitigate the impact of turbulence in FSO communication system.In this work,based on the vortex beam transmission model in atmosphere,the statistical model of OAM attenuation and crosstalk is built,then the numerical calculation and simulation are carried out,which confirming its effectiveness.Furthermore,an adaptive scheme of mitigating the influence of atmospheric turbulence is proposed.The specific work is as follows:1.In weak turbulence,a statistical model,describing the attenuation and crosstalk,is proposed.In this model,the probability density function(PDF)parameter of OAM factor is a function of turbulence intensity as an independent variable,and the mixed exponential generalized gamma(EGG)distribution function is used as the unified function to describe the statistical characteristics of self-channel fading and interference between different OAM modes.In addition,the corresponding algorithm to solve the parameters of the statistical model is given,and its effectiveness is verified.Then,the quadrature phase shift keying(QPSK)modulation OAM multiplex FSO communication system is presented,and the bit error rate of the channel is calculated,which confirming the effectiveness of the statistical model.2.Based on Gerchberg-Saxton algorithm,an adaptive transmission scheme for Laguerre Gaussian(LG)beam in weak turbulence is proposed.In this scheme,the probe beam is used to obtain the distortion information induced by turbulence,and the light field distribution of at transmitter and receiver are regarded as the input to the improved adaptive algorithm.The phase mask distribution of LG light can be obtained effectively.The simulation results show that the OAM modes of probe beam and transmission beam are closer,the correction performance is better.And the topological charge is smaller,the correction performance is better.Furthermore,the FSO communication system of OAM keying(SK)is taken as an example.The simulation results show that the transmission scheme can effectively improve the performance of the communication system,and when the turbulence strength is stronger,the algorithm performs better.This paper provides a reference for the study of OAM-FSO communication transmission characteristics in weak turbulence.