High Performance Equalization Algorithms Of The OTFS System

One of the targets of the fifth generation mobile communication system is to achieve reliable communication in highly mobile scenarios.However,the Doppler spread caused by high speed movement will lead to the inter-carrier interference(ICI),which will destroy the orthogonality of orthogonal frequency division multiplexing(OFDM),the traditional multicarrier modulation technique,and lead to the decline of system performance.As a new multi-carrier modulation technique,the Orthogonal Time Frequency Space(OTFS)modulation technique can make the transmission symbols experience close to the constant channel gain in the in the high mobile scenarios,are robust to the Doppler,shows significant advantages in the high time-varying channels environment.In addition,the OTFS system is capable of obtaining the full diversity gain of time and frequency with the appropriate equalization algorithm.Therefore,it is of great significance to study the high performance equalization algorithms of the OTFS system.This paper mainly studies the high performance equalization algorithms of the OTFS system.The OTFS system is set in two scenarios of non-coded and coded,and the high performance equalization algorithms for these two scenarios are proposed.The main work and innovation points of this paper are as following:1.For the non-coded OTFS system,a novel two-stage equalization algorithm based on the sliding window-assisted minimum mean square error(SW-MMSE)equalization is proposed.This two-stage equalization algorithm improves the performance of OTFS receiver by applying the SW-MMSE equalizer in the time-frequency domain and the delay-doppler domain equalizer(DDE)in the delay-Doppler domain to equalize the received signal,respectively.More specifically,the SW-MMSE equalization utilizes the distribution characteristics of ICI in the timefrequency domain to simplify the channel response matrix,and then equalizes the received signals according to the new simplified channel response matrix.Then,the DDE further eliminates the inter-symbol interference caused by the residual interference of the previous equalizer.The result shows that the proposed two-stage equalization algorithm has a performance gain of about 3dB compared with the traditional minimum mean square error equalizer at a bit error rate of 10-4.2.For the coded OTFS system,the Low Density Parity Check(LDPC)decoding algorithm and the equalization algorithm are jointly designed,and a LDPC-based OTFS iterative equalization algorithm is proposed.In this paper,we combine the factor graph of the message passing equalization algorithm which is proposed for the OTFS system with that of the decoding algorithm of LDPC to give a global factor graph,and derive the message update algorithm of each node based on this global factor graph.Simulation results show that the proposed algorithm can improve the performance of the OTFS system while considering the complexity of the equalization algorithm.