Multiuser Frequency Offset Estimation And Compensation In LEO Satellite System For LTE

Compared with middle and high orbit satellite communication,LEO satellite communication has many advantages,such as lower orbit,shorter transmission distance,smaller path loss and smaller delay.Therefore,the LEO satellite has more development potential and prospects,and bas been paid increasing attention in recent years.However,LEO satellite communication also has some technical problems.For instance,the Doppler shift caused by the relative motion between the satellite and the mobile terminal reaches to tens of thousands of Hertz,which seriously affects the detection of the signal.Therefore,it is very important to study the uplink multi-user frequency offset estimation and compensation technology in LEO satellite system.Firstly,we introduce the channel model of LEO satellite communication system,and the LTE uplink transmission model.Then we analyze the effect of frequency offset on the system,which provides a solid theoretical foundation for the further study.Then,a frequency offset estimation algorithm based on the pilot pattern is proposed,namely PPC algorithm,in order to overcome the shortcomings of the current frequency offset estimation algorithm,which has low frequency offset estimation accuracy.The algorithm is based on a special mapping pilot,reducing the inter carrier interference and the multi-user interference of the pilot data,so the accuracy of the algorithm is improved.However,the interference between users still exists in the pilot data,which leads to the floor effect of PPC algorithm.Therefore,this paper proposes an algorithm called CR-IC-PPC algorithm to further improve the accuracy of frequency offset estimation,which reduces the multi-user interference by reserving pilot subcarriers at the transmitter and eliminating interference at the receiver.Simulation results show that the proposed PPC algorithm and CR-IC-PPC algorithm is able to reduce Mean Squared Error of normalized carrier frequency offset by about one to two orders of magnitude when the SNR is 20dB,compared with the existing DA algorithm.Finally,the frequency offset compensation algorithm is studied.Aiming at the shortcomings of the current FD-MUIC algorithm that the frequency offset compensation performance is poor when the frequency offset is large,that is because frequency offset compensation is performed only on the data of the subcarriers allocated to the user,an improved frequency offset compensation algorithm(IFD-MUIC)is proposed.The algorithm improves the performance by compensating the frequency offset of the data on all subcarriers after interference cancellation,which based on the cost of higher complexity.Therefore,this paper proposes an adaptive frequency offset compensation algorithm called AFD-MUIC algorithm,which can achieve the tradeoff between system performance and complexity by selecting the adaptive factorp.Simulation results show that the IFD-MUIC algorithm has a 5dB gain of SNR compared with the FD-MUIC algorithm,while the AFD-MUIC has a 4.8dB gain of SNR over FD-MUIC,when the normalized carrier frequency offset is 0.3 and the system's BER is 10-3.But the complexity of AFD-MUIC algorithm is reduced by about one order of magnitude relative to IFD-MUIC algorithm.