| As the most advanced wireless mobile communication specification,the 5th generation(5G)new radio(NR)mainly meets three application scenarios,With the introduction of advanced technology,the specification has become the most powerful provider of terrestrial communications infrastructure and digital pipelines.In spite of the special radio frequency technology,5G NR communication specification is still unable to achieve global 5G ubiquitous coverage,such as the areas of open seas and high altitudes.Satellite communication has become an important part of the global communication network of the future due to its advantages of easy setup,flexible networking,wide coverage,and freed from environmental constraints.Among different types of satellite networks,low earth orbit(LEO)satellites operate in low orbits,which have the advantages of low transmission loss,short communication delay,and high reliability.Therefore,the integration of satellite communication systems and terrestrial communication systems to jointly build an air-space-ground integrated network with seamless coverage across the entire area to meet the needs of users for a variety of business services is an important direction for the future communication development.However,the significant differences between the satellite-to-earth link and the ground communication environment have brought many challenges to the NR.In particular,due to the relatively large relative velocity and acceleration between the terminal and the satellite,it will cause severe time-varying Doppler frequency shift characteristics,which increases the difficulty of compensating the frequency offset of the signal at the receiving end.Therefore,this thesis focuses on the carrier synchronization technology in the high dynamic environment in the 5G-based low-orbit satellite communication system.The specific work includes:1.Based on 5G NR downlink primary synchronization signal(PSS)frequency offset estimation and the use of Doppler frequency shift characteristics combined with ML algorithm to estimate the frequency offset change rateIn order to realize fractional frequency offset(FFO)estimation and integer frequency offset(IFO)estimation,the article uses 5G NR downlink PSS.First,the conjugate symmetry characteristic of PSS is utilized in the time domain to perform FFO estimation,and the estimated FFO is employed to compensate the system frequency offset.Then the compensated PSS is converted from time domain to frequency domain by fast Fourier transform and multiplied by the PSS converted from the transmitter to frequency domain,and then the IFO is estimated according to its cyclic shift characteristics in frequency domain.To estimate the rate of change of Doppler frequency offset,the characteristics of Doppler frequency shift is first utilized to obtain the maximum value of the rate of change of Doppler frequency deviation in a single pass of the LEO satellite,and the maximum value obtained from the simulated result and its corresponding inverse act as the evaluation range of the rate of change of Doppler frequency offset.Then the maximum likelihood(ML)algorithm is adopted to find the value of the specific Doppler frequency deviation change rate.In the Doppler frequency offset algorithm,the additional system overhead is not required with the low complexity.At the same time,it can estimate the frequency offset with a large range,which can effectively solve the large frequency offset estimation problem in the LEO satellite mobile communication system.The simulation results show that the Doppler frequency offset and the mean square error(MSE)of the Doppler frequency offset change rate have been improved to a certain extent compared with the comparison algorithm.2.The Second Order Difference(SOD)algorithm is combined with the frame structure in 5G NR to estimate the frequency offset change rateBased on the SOD algorithm and combined with the frame structure in 5G NR,an improved Doppler frequency offset change rate estimation algorithm is proposed.This algorithm uses the PSS in the synchronization signal and PBCH block(SSB)to perform differential multiplication.Compared with the original SOD algorithm using Cyclic Prefix(CP)in orthogonal frequency division multiplexing(OFDM)symbols,the newly proposed improved algorithm using PSS in SSB can increase the number of sampling points and the distance between sampling points to reduce the effect of noise,thereby improve the performance of Doppler frequency deviation rate of change estimation.The simulation results show that the proposed algorithm can effectively solve the problem of the Doppler frequency deviation change rate caused by the large acceleration of the LEO satellite mobile communication system. |
PDF Full Text Request