Research On Channel Eatimation Method Of Atmospheric Laser Communication SISO/MIMO System

The adaptive transmission system uses the coherence time of the atmospheric channel to estimate the channel state information and adjusts the transmission parameters of the transmitter in time according to the channel conditions,which improves the system throughput and spectrum efficiency without sacrificing the bit error rate,and effectively suppresses the influence of turbulent fading.Therefore,the estimation of atmospheric turbulence channel parameters is the key factor to realize adaptive transmission.In this paper,for the atmospheric laser Single Input Single Output(SISO)system and Multiple Input Multiple Output(MIMO)system,respectively,the atmospheric turbulence channel fading parameters and channel transmission matrix estimation methods are studied.The channel estimation performance of the atmospheric laser SISO system is verified based on the measured data of different weather.The main content includes as follows:1.The influence of atmospheric absorption,atmospheric scattering and atmospheric turbulence on atmospheric laser communication is explained.Three kinds of atmospheric turbulence Gamma-Lognormal,Lognormal-Rician and Gamma-Gamma composite distribution channel models are introduced,and the advantages and disadvantages of atmospheric channel parameter estimation based on different composite distribution turbulence models are compared and analyzed.2.The Gamma-Gamma turbulence distribution channel is proposed as a composite distribution model,the maximum likelihood method is used to estimate the turbulent channel fading parameters,and the maximum likelihood estimation expression based on the Expectation Maximization(EM)calculation of unknown parameters is derived.The Newton-Raphson(N-R)and Generalized moment method(GMM)channel parameter estimation methods are studied.Based on the Gamma-Gamma channel simulation data of atmospheric turbulence,the estimated mean square error of three estimation algorithms,EM,N-R,and GMM,is compared and analyzed.And using Cramer-Rao Bound(CRB)to evaluate the effectiveness of the three parameter estimation methods.Aiming at the experimental measurement data under different weather conditions,the performance of the three estimation algorithms is compared by estimating the mean square error and the fitting degree.Atmospheric turbulence channel parameter estimation results show that the EM estimation algorithm based on the Gamma-Gamma composite distribution model can obtain the best channel fading parameter estimation performance.3.The channel fading parameters and transmission matrix of the atmospheric laser MIMO system are estimated.The performance of using EM algorithm and N-R algorithm to estimate MIMO channel parameters is compared and analyzed.The simulation results show that as the number of apertures at the transceiver end of the MIMO system increases,the mean square error of the estimated parameters of the two estimation methods increases.When the number of sample points is the same,compared with the N-R algorithm,the mean square error of the estimated parameters of the EM algorithm is closer to the CRB,and the estimation performance obtained is better.Then,the estimation method of MIMO channel transmission matrix in Free Space Optical Communications(FSO)is studied.The expressions of Singular Value Decomposition(SVD)iteration,improved SVD iteration,and maximum likelihood estimation algorithm to estimate the MIMO channel transmission matrix H are derived.The effects of training sequence length,signal-to-noise ratio at the receiving end and the number of transmit and receive apertures on the performance of MIMO channel estimation are analyzed.Simulation experiments show that a longer training sequence,a large signal-to-noise ratio and a small number of apertures can improve the estimation accuracy of the channel matrix H.Among the three channel estimation algorithms,the improved iterative SVD algorithm can obtain the best estimation performance.