Channel Estimation In OTFS High-Speed Mobile Communication Systems

Orthogonal frequency division multiplexing system modulation schemes have been widely used in modern wireless communication systems,but for time-varying channels,such as high-speed railway wireless communication channels,the large Doppler frequency shift and the rich multipath effect generate severe inter-subcarrier interference,resulting in the poor performance of conventional OFDM systems in this environment.OTFS is not only a new modulation method,but also a new type of mobile communication system,which,compared with OFDM,OTFS converts a doubly dispersive channel into a time-invariant two-dimensional channel in Delay-Doppler domain through two-dimensional transformation,which provides a more significant performance gain over existing modulation schemes in high-speed scenarios.This paper mainly focuses on the design of pilot sensing matrix and the estimation method of fast time-varying channel in OTFS system.Firstly,the input-output relationship between time domain and Delay-Doppler domain of OTFS system is analyzed respectively,which would be combined with compressed sensing theory to improve the design index of guide frequency sensing matrix and CS algorithm for CS channel estimation.The simulation result shows that the variance of the eigenvalues and the maximum eigenvalues are used as the design criteria of the pilot sensing matrix in the Delay-Doppler domain,and the resulting pilots have better pilot design stability.In time domain,PPM sequence channel estimation performance is optimal.Next,in order to reduce the resource occupation of channel estimation pilot,the data frame pattern is designed to be improved in SISO and MIMO systems respectively.For SISO system,based on pilot being framed independently,PRS-OMP algorithm is proposed.By considering the maximum delay and maximum Doppler frequency shift in doubly dispersive channel,the Delay-Doppler frequency shift and channel gain of each path are estimated by using compressive sensing principle.Aiming at MIMO system,the pilot pattern is designed by combining pilot and data into a frame,and applied it to 3D-SOMP channel estimation algorithm.Simulation results show that the PRS-OMP algorithm with phase rotation reduces the pilot occupancy rate,improves the accuracy and diversity order of channel estimation,and enhances the bit error rate performance of the system.The PRS-3D-SOMP algorithm further reduces the pilot occupancy rate,but some bit error rate performance is lost when removing the introduced interference.Finally,in view of the difficulty in estimating the channel state information of the physical path corresponding to the fractional Doppler channel in the OTFS system and the high computational complexity,an improved cross-correlation matched filter channel estimation algorithm is proposed in this paper.The algorithm firstly uses the method of combining pilot and data into a frame to obtain the estimation of equivalent channel.Then,the channel state information of each path is estimated by cross-correlation matched filtering.On this basis,windowing techniques are used in OTFS systems.The simulation results show that the method reduces the number of integer samples of the main flap of the window response and the level of the side flap,which effectively improves the autocorrelation of the Doppler response function of the equivalent channel and further reduces the interference of other symbols and noise on the estimated symbols.