Orthogonal Time Frequency Space Waveform For High-Mobility Scenarios

Future wireless communication networks are envisioned to include diverse applications and scenarios,where the high-mobility scenario is an important component.The conventional waveform techniques in the time-frequency domain,e.g.,orthogonal frequency division multiplexing(OFDM),can mitigate the inter-symbol interference(ISI)caused by the time dispersion.However,the frequency dispersion caused due to the Doppler frequency offsets(DFOs)in the high-mobility scenarios results in significant inter-carrier interference(ICI),which severely degrades the performance of OFDM systems.Therefore,it is very important to explore the new waveform techniques to guarantee the quality of service(QoS)of high-mobility communications.Orthogonal time frequency space(OTFS)is a new waveform technique designed in the delay-Doppler domain.A key hallmark of OTFS is that it can effectively convert a doubly dispersive channel into an almost non-fading channel in the delay-Doppler domain,and consequently all symbols in a frame experience a relatively stable channel gain,which makes it very promising to solve the effect of the DFOs for future wireless communications with high-mobility.Nevertheless,as a new technique,the current research on OTFS is still in the preliminary stage,and further analysis needs to be carried out.Motivated by the above observations,in this thesis,we study the OTFS waveform and the transmission method in high-mobility scenarios.The main contributions of the thesis can be summarized as follows:1.One critical issue for OTFS is the very high complexity of equalizers.This thesis proposes low complexity linear equalizers for OTFS by exploiting the two-dimensional(2D)fast Fourier transform(FFT),and consequently significantly reduce the complexity of the equalizer.More specifically,the doubly block circulant feature of the OTFS channel in the delay-Doppler domain is revealed first.Then,we show that the doubly block circulant channel matrix can be diagonalized by 2D DFT/IDFT matrix,which further inspires us to obtain the output of the conventional linear equalizers by 2D FFT/IFFT operation without conventional full matrix inversion or multiplication.As a beneficial result,compared with other existing linear equalizers for OTFS,the proposed linear equalizers in the thesis enjoy a much lower complexity without any performance loss.2.For the high-mobility scenarios with rich reflectors,the multipaths between the transceivers result in a large number of DFOs,which seriously degrade the performance of the wireless communication systems.This thesis proposes an OTFS based message passing(MP)-maximum ratio combining(MRC)iterative receiver with multi-antennas to deal with the challenges of DFOs.More specifically,we show that the multiple DFOs associated with multipaths can be separated with the high-spatial resolution provided by multi-antennas.Meanwhile,combined with the design of the OTFS receiver,the effect of the residual DFOs can be mitigated.Next,a joint MP-MRC iterative detection algorithm for OTFS is further proposed to gain an excellent multi-antenna diversity and also improve the convergence performance of the iteration.Besides,empowered by the enhanced channel sparsity after the beamforming network,the MP-MRC algorithm can be conducted at a low computational complexity.3.The discussions of the applications for OTFS are relatively few and mainly focus on the terrestrial communications.This thesis proposes the performance evaluation of OTFS in marine high-mobility communication scenarios.Considering the low accuracy of the conventional path loss model in marine communication system modeling,we propose a revised marine path loss model exploiting machine learning.Based on it,aiming at the problem of the sparse DFOs in marine UAV high-mobility communications,the OTFS waveform is adopted to cope with the challenges,and the system performance is simulated and evaluated.