Research On Technology Of MIMO Coherent Satellite-to-Ground Optical Communication Link

The optical communication link between satellite and ground has low power consumption,high transmission rate,great security and compact equipment.In recent years,great progress has been achieved in this technology in both theory research and practice engineering.A number of new methods are emerging.In spite of the advantages and the progress,the satellite-to-ground optical link has not been used widely in commerce.Most of the links are established for experiment purpose.The greatest challenge for the link is derived from the atmosphere channel.This dissertation aims at suppressing the atmosphere channel fading by improving the ground station structure and the signal processing algorithm.The work of this dissertation can be divided into two parts.One is analyzing the performances of the communication link with different number and diameters of receiving apertures and various channel parameters of atmosphere channel.The other one is boosting the signal processing method to improve the coherent communication system performance with multiple apertures.The contributions of this dissertation are as follows.(1)The performances of the communication link with different number and diameters of receiving apertures and various channel parameters of atmosphere channel are analyzed.To alleviate the signal fluctuation,two important schemes can be used.One is adopting a large receiving aperture,and the other is employing a receiving array consisting of multiple apertures and multiple transmitting lasers(MIMO).The effects on communication system made by the number and area of the receiving apertures,the wavelength and number of the lasers and the signal combining methods are analyzed based on the point receiving model and aperture averaging model.The SIMO and MISO systems are discussed too.Before analyzing the MIMO system with aperture averaging,the bit error rate,outage rate and average capacity of SISO link based on aperture averaging are investigated.(2)The scintillation curves with various aperture diameters are simulated.The data collected in the satellite-to-ground link is analyzed and the scintillation of the signal received by the apertures with different diameters is simulated based on the mathematic model.(3)The channel estimation algorithm based on high order statistics is improved.The atmosphere channel can be considered as slow fading.The channel state can be estimated precisely by the channel estimation algorithm based on high order statistics with high SNR.Once the SNR become low,the great estimation errors appear.This algorithm is improved and the more precise channel state is obtained with low SNR atmosphere channel.(4)The phase difference estimation algorithm based on Kalman filter is designed.The signals from the same laser in the satellite and received by various apertures should be combined and the phase difference among the signals should be estimated and compensated.A algorithm based on Kalman filter is designed to estimate the phase difference and the simulation results show the advantages of this new algorithm.(5)The Viterbi-Veterbi algorithm is combined with Kalman Filter and the phase offset estimation accuracy is improved.Coherent receiving can improve the sensibility and reliability of the communication system.In order to realize coherent receiving,the phase offset between the local laser and the carrier wave should be estimated and compensated.A new phase offset estimation algorithm based on Viterbi-Viterbi algorithm and Kalman filter is given.The estimation error of the new algorithm is smaller than the traditional method especially when the SNR is low.And this new algorithm can make nearly perfect effect on MIMO system.