Research On OFDM Based OTFS Technology

With the continuous breakthrough of the operating mileage and speed of high-speed train,high-speed train is gradually becoming one of the best choices for long-distance travel,and people’s communication demand for high data rates in high-speed scenarios is gradually becoming more prominent.High-speed train is one of the typical high-speed scenarios,which not only has significant Doppler effect and rapid time-varying,but also has the unique feature of linear coverage along the track.As the fifthGeneration(5G)New Radio(NR)mobile communication is currently the most widely used mobile communication system,orthogonal frequency division multiplexing(OFDM),the multi-carrier modulation technology adopted by 5G NR,is urgently in need to solve the problems brought by the high-speed train scenario.OFDM in NR adopts a larger sub-carrier spacing design to combat Doppler effect.However,the larger subcarrier spacing corresponds to the shorter cyclic prefix,the ability to resist multipath effects will decrease.As a result,OFDM cannot meet the requirements under the time-and frequency-selective channels with high delay and high Doppler frequency offset,and the future wireless communication system needs the emergence of new multi-carrier modulation technology.Recently,a new two-dimensional modulation technique called orthogonal time frequency space(OTFS)has been proposed for time-and frequency-selective wireless channels,which aims to convert time-varying multipath fading channels into the discrete,sparse and distinguishable static channels in the delay-Doppler(D-D)domain,so that each transmitted symbol has a consistent and relatively constant channel gain.Different from conventional OFDM,OTFS modulates information symbols in the D-D domain,and then spreads them onto the timefrequency(T-F)domain through the two-dimensional orthogonal basis transform functions.Firstly,we analyze the general model of the OTFS system,and then deduces D-D domain equivalent channel matrix under ideal waveforms based on the OTFS technology,using the property that the D-D domain channel matrix is a block circulant matrix to simplify the D-D domain linear receiver.Then an improved time-frequency domain linear receiver is proposed for the OTFS system which is modulated by the rectangular waveform.The proposed receiver uses minimum mean square error equalizer in the T-F domain considering inter-carrier interference,and implements the equaivalent channel normalization in the D-D domain.As simulation results show,the proposed receiver can achieve a lower bit error rate for the uncoded system,and also a lower block error rate for the coded OTFS system using low density parity check code in 5G systems,with the complexity is almost the same as that of the conventional linear receiver in the T-F domain.Finally,we design the OFDM based OTFS system numerologies according to NR numerologies,compares the performance of T-F/D-D domain receivers under different cyclic prefix addition methods and different waveform assumptions.After the designation of OFDM Based OTFS system,we make a comprehensive evaluation of coded OFDM and OTFS’s different receivers with perfect channel state information,including whether the channel is fading,channel delay,Doppler frequency offset,channel gain,code rate,then summarize and compare different receivers respectively In addition,we also compare the performances of OTFS linear receivers and OFDM with imperfect channel estimation,the result shows that OTFS still has significant advantages compared with OFDM in practical applications.