| 5G based LEO satellite communication is one of the most promising development directions of the future communication network.Due to the highspeed movement of LEO satellite,the Doppler frequency offset will be up to several hundred kHz,and the computational complexity of existing frequency offset estimation methods will be increased by tens of times.At the same time,with the satellite communication usually works in high frequency band,the path loss is large,and the SNR at the receiver is low,which leads to the larger estimation error of frequency offset.The large and time-varying CFO will greatly destroy the OFDM system and produce ICI.Meanwhile,the residual CFO caused by inaccurate frequency offset estimation and compensation will further affect the accuracy of uplink synchronization and the construction of communication link between satellite and UE.Therefore,how to compensate CFO and realize uplink synchronization has become a hot issue urgently to be solved in the LEO satellite communication network.To solve the above problems,this paper carried out the following research work:(1)Aiming at the LEO satellite communication scenario with low SNR and large time-varying CFO,a three-stage frequency offset estimation algorithm is proposed based on the SSB in 5G NR.In the pre-compensation stage,satellite ephemeris is utilized to compensate the large range of Doppler frequency offset to tens of kHz,which reduces the computational complexity.In the fine compensation stage,the PS S is used to estimate Doppler frequency offset and Doppler rate.In the Kalman filtering stage,the estimation results of Doppler frequency offset and Doppler rate are optimized by Kalman filter to improve the accuracy.Simulation results show that the three-stage frequency offset estimation algorithm can ensure that the MSE of frequency offset estimation is less than 103 and the MSE of Doppler rate estimation is less than 10’12 when SNR is-5dB of satellite channel.(2)Aiming at the co-exist of Doppler frequency offset and crystal frequency offset scenario where Doppler frequency offset is compensated negatively and crystal frequency offset is compensated positively,an algorithm for frequency offset estimation and separation is proposed based on SSB periodic transmission in 5G NR.The Doppler frequency offset is time-varying and the crystal frequency offset is fixed.Therefore,we use multiple SSBs in different frequency to separate the frequency offset,and the least square method is used to solve it.The simulation results show that the proposed algorithm can effectively realize the separation of Doppler frequency offset and crystal frequency offset,and the MSE of frequency offset estimation is less than 10-2 when SNR is-5dB.(3)Aiming at the uplink synchronization process with large residual CFO,a CFO robust uplink synchronization algorithm is verified based on 5 G NR PRACH preamble formats.Considering the characteristics of LEO satellite communication,the preamble format selection principle is designed,and the frequency-hopping structure is introduced.The CFO robust uplink synchronization algorithm uses differential processing to carry out preamble detection.The simulation results show that the CFO robust uplink synchronization algorithm has a good performance in the LEO satellite communication system based on 5G NR preamble formats,and the minimum SNR can be reduced by 1～2dB at the missed detection rate of 1%. |
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