Research On The Characteristics Of Atmospheric Channel And Signal Processing Technology In Satellite-ground Coherent Laser Communication

Advancing remote sensing techniques put forward higher requirements for information transmission between satellite and earth.Among the developed fast information transmission techniques in space,satellite-ground coherent laser communication becomes the research focus for its advantages of high receiving sensitivity,high capacity and strong anti-jamming ability,etc.However,signal light of satellite-ground coherent laser communication would easily suffer from influences from atmospheric channel,thus making its performance get worse.The thesis analyzes influences from atmospheric channel in satellite-ground coherent laser communication and focuses on signal processing techniques to overcome influences above.The topic of the thesis is from the National Natural Science Foundation of China(no.61571446).The main contents of the thesis include 4 parts:(1)Polarization analysis of the received light under the influence of atmospheric channel.Firstly,Rayleigh multi-scattering model is constructed to calculate Stokes vectors of the received light.It could be obtained from numerical simulations that only about 1%~2% loss of coherent detection efficiency occurs caused by Rayleigh scattering,which has low effect on the performance of the communication system;Subsequently,the changing model of polarization influenced by Mie scattering is analyzed based on the theories of Mie scattering and Stokes vector model.Monte carlo simulation is taken to calculate the depolarization ratio of the received light after propagating through atmosphere with different optical depths.The research findings show that the depolarization ratio of the received light has a one-to-one match with the optical depth of clouds;Finnaly,the polarization changing characteristics of circularly/elliptically polarized light affected by atmospheric turbulence is analyzed based on Gauss-Schell model(GSM)light beam and coherent optical theory.Furtherly,expressions to calculate received light's polarization parameters,including degree of polarization,length of major polarization axis,length of minor polarization axis and polarization ellipticity are derived,and then,the total coherent detection efficiency of the received light after polarization diversity receiving is obtained.The derived model to calculate coherent detection efficiency could provide theoretical basis for the design of polarization control system at the receiver.(2)Wavefront aberration influenced by atmospheric channel is accurately modeled,based on which,wavefront correction techniques are proposed for the heterodyne/homodyne detection and self-differential detection system.Firstly,the phase screen modeling method based on sub-bands division and multirate sampling is proposed,through which low frequency power compensation is achieved.Compared with the existing low frequency compensation methods based on subharmonics,computational complexity of the proposed method decreases about 2 orders of magnitude,thus could it be able to accurately model wavefront aberration in real time;Then,the method based on wavelet fractal interpolation is proposed to raise the resolution of wavefront correction in heterodyne/homodyne detection system,while it could also remove the noise introduced by the wavefront corrector.The proposed method could obtain a SNR gain of about 2~3 dB compared with the wavefront correction method based on minimum variance estimation,and it has low computional complexity;At last,the free-space self-differential detection system is designed to achieve wavefront correction on satellite,while its coherent detection efficiency and output SNR are calculated.On the basis,the phase retrieval method based on K-means clustering algorithm is proposed to correct the residual wavefront aberration after self-differential detection.The method could also overcome influences from noises of the receiver,thus it could improve the symbol error performance of the communication link.(3)The multipath effect in atmospheric channel of thick clouds is analyzed,based on which the technique of channel equalization is proposed to deal with the phase modulation signal.Firstly,multi-scattering theory is used to analyze the influence mechanism of multipath effect and calculate the average pulse broadening time.Moreover,eye pictures are simulated to analyze the loss of communication quality;A multirate communication scheme is designed based on detecting depolarization ratio,however its performance is greatly limited under the condition of adverse multipath effect;The channel equalization method based on modified constant model algorithm(CMA)is put forward to further restrain the multipath effect.The proposed method could be applied to the self-differential detection system to process its output signal.It outperforms the existing channel equalization methods in convergence speed and steady-state error.(4)The application characteristics of Turbo code and LDPC code in satellite-ground coherent laser communication are analyzed.Furthermore,the interleaving scheme with high throughput is designed to cope with the effect of time-varying atmospheric channel.It could correct concentrated burst errors when used combined with error-correcting code,thus greatly improving bit error performance of the communication system under circumstances of medium turbulence and strong turbulence.The research findings have guiding significance in the design of satellite-ground coherent laser communication system and provide theoretical basis for high-speed information transmission in space.