Research On Key Technologies For Free-space Laser Communication

Free-space laser communication technology combines large capacity of optical communication systems with the flexibility of wireless technologies,which is of great significance for deep space exploration,satellite navigation,earth observation,and applications supporting 5G+ cellular network upgrades.Therefore,the research on key technologies for free-space laser communication has great research value and engineering application value.This dissertation focuses on long-distance and ultra-long-distance laser communication links,comprehensively studying coherent laser integrated communication/ranging system and modeling of atmospheric turbulent channel fading.The contents of this dissertation are listed as follows:Firstly,for high-dynamic ultra-long-distance and long-distance laser communication links,coherent detection with high sensitivity is applied in this dissertation.An integrated communication/ranging algorithm with feedback-homodyne detection is proposed,which is based on digital signal processing.As demonstrated by the MATLAB simulation and real-time FPGA implementation,for asynchronous clock sources with subtle dynamic frequency offset and sufficient stability,the fractional symbol ranging algorithm based on timing offset compensation has been proven to improve the accuracy of ranging results to millimeter-level at Gbps communication rate.Meanwhile,the parallel FFT frequency offset estimation algorithm is proposed for tracking the highspeed wavelength shift caused by the dynamic Doppler frequency shift and insufficient laser stability.It's verified that the algorithm can handle dynamic wavelength offsets up to 300 MHz/s at Gbps communication rate,and residual offset meets the processing requirements of subsequent phase noise compensation in the receiver,which greatly improves the performance of the receiver.Secondly,for satellite-to-ground or ground laser links,atmospheric turbulence will greatly affect the quality of received signals.This dissertation presents a step-by-step propagation method for simulating the refraction and diffraction characteristics of the turbulent atmosphere.The Fresnel integral is used to characterize the diffraction effect,while the random phase screen simulates the refraction characteristics of the atmosphere.The simulation results of fading irradiance and phase of the received signal under different turbulence intensities are obtained.Then,the statistical coherence factor of the simulated receiving signal is calculated,which proves the step-by-step propagation method can effectively simulate the beam propagation.In addition,the random phase screen simulating atmospheric turbulence is generated by the Fourier transform method with subharmonic low-frequency compensation.For verification,the statistical characteristics of the random phase screen generated by simulation is calculated,and the results are consistent with the corresponding theory.The establishment of this step-by-step propagation model can help simulation based on previous data collection and statistical models under laboratory conditions,helping quantitative analysis of atmospheric turbulence,which is of great importance for the reliability evaluation of Free-space laser communication systems.