Time And Frequency Synchronization For 5G Integrated Leo Satellite Mobile Communication System

The integration of low earth orbits(LEO)satellite mobile communication system and the terrestrial 5th generation(5G)mobile communication system can give full play to their advantages,and realize the service target of seamless coverage and handover in global range.In future communication system,the integration of 5G air interface technology and LEO satellite mobile communication system will enable the satellite communication to adopt the advanced terrestrial communication technologies to improve the communication performance.Besides,the compatible design of air interface can make the satellite communication enjoy the mature terrestrial communication industry chain,thus greatly reducing the design and manufacturing costs of multi-mode satellite terminals.However,the significant difference between satellite-ground link and terrestrial communication environment sets many challenges to the integration of air interface technology.Considering the large carrier frequency offset(CFO)caused by the oscillator frequency error and the Doppler shift in LEO satellite mobile communication system,we investigate the applicability of 5G air interface signal in satellite-ground link.Based on the study of the channel model and Doppler shift characteristics in satellite communication system,we focus on investigating the time-frequency synchronization technology and solving the synchronization problems under large frequency offset.First,the channel model of satellite communication system and the characteristics of Doppler shift in satellite-ground link are studied.After reviewing several classical satellite channel models,a series of channel characteristics such as the probability of line of sight(LOS)and propagation path loss are analyzed,which is based on the latest space-ground integration channel model proposed by 3GPP for non-terrestrial networks(NTN).Focusing on the analysis of Doppler shift characteristics,a fast method of calculating Doppler shift based on geometric operation is presented,which can avoid the large number of matrix operations existing in the traditional ephemeris-based method,thus having a low computational complexity.The analysis of Doppler shift characteristics in LEO satellite communication shows that the maximum Doppler frequency offset in satellite-ground links can be several times of the subcarrier spacing(SCS)defined in 5G new radio(NR).Therefore,in satellite-ground links with large Doppler frequency offset,the traditional time-frequency synchronization methods can not achieve accurate time-frequency synchronization with the synchronization sequence specified by NR,which calls for exploring the time-frequency synchronization method supporting large Doppler frequency offset.Second,we investigate the time and frequency synchronization in the downlink transmission for LEO satellite mobile communication system.On the basis of the synchronization signal model with the primary synchronization signals(PSS)defined in NR,we propose the PSS algorithm and the cyclic prefix(CP)assisted algorithm.The PSS algorithm transforms the traditional time-frequency two-dimensional search optimization problem into two one-dimensional search problems.Based on the finite amplitude characteristic of Doppler shift caused by the visibility window in satellite communication,the PSS algorithm further transforms the optimization problem into the problem of maximizing the approximation of the objective function,and then utilizes the modulated discrete prolate spheroidal sequences(DPSS)to accomplish the eigen-decomposition of the involved high-dimensional matrix,which can reduce the computational complexity of the estimation.Furthermore,based on the proposed PSS algorithm,the CP-assisted algorithm utilizes the structure of CP in multiple OFDM symbols as auxiliary information to improve the estimation performance.To avoid the one-dimensional exhaustive search involved in the frequency offset estimation of the PSS scheme,the CPassisted algorithm estimates the CFO using the proposed PSS scheme on the basis of the estimated fractional frequency offset(FFO)using CP.Combinating the above two algorithms,the DPSS assisted algorithm is proposed,which uses the PSS algorithm to estimate the timing offset and the CP-assisted algorithm to estimate frequency offset respectively.Other than initial synchronization,we also propose an algorithm that can be used in the tracking synchronization,which is obtained by simplifying the proposed DPSS assisted algorithm and uses PSS for timing tracking and CP for frequency tracking respectively.The numerical simulations indicate that the proposed algorithms can achieve accurate time-frequency synchronization under large Doppler shift,and the achieved initial synchronization can be will maintained by the proposed algorithm in tracking synchronization during the overtopping procedure of satellite.Finally,we investigate the time and frequency synchronization considering the oscillator frequency error in LEO satellite mobile communication system.Based on the research of random access procedure of NR,a closed-loop time-frequency synchronization method considering oscillator frequency error is proposed,which utilizes the closed-loop mechanism of random access procedure.After the downlink time-frequency synchronization and the random access procedure between the user equipment(UE)and the satellite,the proposed synchronization method can estimate the frequency offset caused by Doppler shift and oscillation frequency error respectively,and then make the corresponding compensation.In order to realize the uplink synchronization involved in the closed-loop synchronization method,a time-frequency estimation algorithm based on the DPSS assisted algorithm is proposed,which uses the random access signal specified by NR.The simulation results show that the proposed uplink synchronization algorithm can achieve accurate time-frequency synchronization under large CFO.Considering the performance of the proposed downlink synchronization algorithms,it can be concluded that,for the communication scenario where both Doppler shift and the CFO caused by oscillator frequency error exist,the closed-loop time-frequency synchronization method proposed in this thesis can achieve accurate time-frequency estimation and compensate the frequency offset correctly.