Research On Optical Signal Propagation And Digital Coherent Combination Technique For Satellite Laser Communication

Compared with microwave communication,satellite laser communication has smaller terminal size,weight and power consumption,and has the advantages of large communication capacity,rich spectrum resources,good directivity and strong confidentiality,so it can make up for the shortage of microwave communication.At present,the receiver in laser communication generally needs to couple the free space signal into the optical fiber,and then leverage mature optical fiber communication devices to improve the system performance.However,due to the influence of atmospheric turbulence effect,beam divergence angle and platform vibration,the coupling efficiency of the optical signal into a single-mode optical fiber in satellite communication is low and difficult to maintain stabilit y,which greatly limits the performance of the laser communication system.Recent studies show that the combination of multi aperture coherent optical receiver and digital coherent combining technology can effectively increase the receiving area of optical signal,improve the signal-to-noise ratio of received signal,and effectively suppress the fluctuation of signal optical power caused by atmospheric turbulence effect.This thesis focuses on the propogation model of optical signal and digital coherent combining technique.The specific research contents are as follows: 1)For inter satellite communication,the estimation model of the link transmission loss is established,and the influence of satellite platform vibration on optical fiber coupling efficiency is studied,which provides guidance for link power budget analysis and parameter design.2)Aiming at satellite to ground communication,the atmospheric propagation numerical model based on multi-layer phase screens and the fiber coupling numerical model are established.The optical field distribution at the receiving end,the fluctuation of fiber coupling efficiency,and the dynamic changes of the optical signal to noise ratios(OSNRs)of the input signal and the output OSNR of the multi aperture receiver after ideal maximum ratio coherent combination are simulated when the aperture spacing is greater than the atmospheric coherent length.The numerical simulation results are verified by the theoretical statistical model.The simulation system lays the foundation for the optimal design of the system.3)Considering the phase alignment error exiting in the practical digital coherent combination,a statistical model is established to describe the relationship between the number of symbols used by the phase alignment algorithm,the phase error,the input OSNRs,and the output OSNR obtained with equal ratio and maximum ratio coherent combining.Based on this model,the phase alignment algorithm is optimized to minimize its computational complexity while guaranteeing the expected output averaged OSNR.Finally,the accuracy of the model is verified by a large number of Monte Carlo simulations and also by experimental results.