| The fifth generation(5G)communication system has been widely used.Additionally,worldwide investigation and study of the sixth generation(6G)communication technology have started.Wide coverage and high transmission capacity are features of the low earth orbit(LEO)satellite communication system.It is a crucial component of the 6G network’s global reach.LEO satellite channel modeling is the basis of satellite communication system design and performance evaluation.However,the high speed and complex relative motion between LEO satellite and user equipment(UE)bring many challenges to channel modeling and simulation.The key technical challenges are high dynamic time-varying satellite channel characteristic modeling and nonlinear characterization,shadow fading and doppler power spectrum simulation of complex space-time coupled satellite channels,and transmission performance optimization of long delay satellite channels.Aiming at these problems,this paper deeply studies the key technologies,such as the timevarying non-stationary channel model of LEO satellite-to-ground link,the high-precision simulation method of shadow fading and doppler power spectrum,and the optimization of hybrid automatic repeat requests mechanism of LEO satellite communication system.Meanwhile,theoretical research and experiments have been carried out.The main contents and innovations of this paper are as follows:1.This paper proposes a time-varying non-stationary channel model of LEO satellite-to-ground link based on a double spherical sector structure.It includes characterization of free space loss,shadow fading,small-scale fading,and nonlinear effects of spaceborne radio frequency(RF)devices.Aiming at the multi-peak and asymmetric characteristics of shadow fading probability density distribution,the mapping relationship between dynamic distribution parameters and time-varying communication elevation angle was established based on the gaussian mixture distribution fitting method.A description method of multipath distribution in the spherical coronal active scattering region is proposed to realize the modeling of multipath evolution during satellite overhead.A method of hierarchical decoupling and modeling error compensation is proposed to solve the problem of coupling characterization of nonlinear effects of spaceborne RF devices.The accuracy of the theoretical channel model is verified by comparing the temporal auto-correlation function(TACF)and doppler power spectral density(PSD)between the theoretical model and the simulation data.The level crossing rate(LCR)and average fade duration(AFD)of the theoretical model are verified by using the measured data to ensure the availability of the channel model.Simulation results show that the gain of normalized mean square error(NMSE)and adjacent channel error power ratio(ACEPR)is about 0.67dB and 2.24dB compared with the linear cascade method.2.In view of the high-precision simulation problem of shadow fading under the coupling effect of time-varying communication elevation angle and space environment change,a method of shadow fading correlation space recognition based on semi-variogram function is proposed.The simulation interval of dynamic shadow fading is segmented by the correlation spatial angle range.And then a shadow fading simulation method based on spatial correlation is proposed.The accurate simulation of shadow fading in complex relative motion between satellite and UE is realized.Experimental results show that the simulation accuracy of the spatial correlation simulation method is about 47.24%higher than that of the temporal interpolation method.In response to the high-precision simulation problem of doppler PSD under the influence of a strong line of sight(LOS)path and non-uniform angle of arrival distribution,the realization principle of the typical random angle of arrival simulation method is analyzed.A time-varying asymmetric doppler power spectrum model of the LEO satellite is established.To provide an accurate simulation of an asymmetric doppler power spectrum with a strong LOS path,a simulation method based on time-varying Laplacian distribution is then presented.The experimental results show that the simulation accuracy of the time-varying Laplacian distribution method of exact doppler spread is improved by 30.04%,23.3%,and 31.41%compared with the method of exact doppler spread(MEDS),random method of exact doppler spread(RMEDS),method of exact doppler spread-set partitioning(MEDS-SP),respectively.3.To address the problem of transmission performance degradation caused by the long round-trip time(RTT)of the LEO satellite-to-ground link,a channel state adaptive code block group hybrid automatic repeat requests(ACBG-HARQ)mechanism is proposed.The influence of timevarying RTT on the number of parallel processes in HARQ is analyzed.And a high precision error probability fitting model of code block group is established.Then,the time-varying channel state,receiver memory state,and code block group retransmission strategy are abstracted as the Markov decision process(MDP).Based on the fuzzy Q-learning algorithm,the optimal modulation and coding scheme(MCS)selection and adaptive code block group segmentation are implemented.The simulation results show that compared with the explicit buffer overflow notification HARQ(EONHARQ)and the variable rate HARQ(VR-HARQ),the spectral efficiency of the ACBG-HARQ mechanism is improved by about 12.5%and 22.6%,respectively.4.The LEO satellite channel emulator is developed and the experimental platform for LEO satellite channel modeling and HARQ mechanism optimization is built.The time-varying non-stationary channel model of the LEO satellite-to-ground link and the HARQ mechanism optimization technology for the long RTT satellite communication system is comprehensively evaluated and experimentally verified.Experimental results show that the channel model theoretical values,channel emulator output values,and measured values have high consistency.And for the LEO satellite-to-ground link,the system throughput of the ACBG-HARQ mechanism is about 15.6%higher than that of the existing EON-HARQ mechanism. |
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