Channel Estimation And Equalization For Transform-Domain Modulation Based Multi-carrier System

Thanks to the flexible resource allocation and robustness in multi-path fading channel,orthogonal frequency division multiplexing(OFDM),has been widely used in the wireless communication systems,such as 4G/5G.Facing the new-generation wireless communication,such as vehicle to everything network,unmanned aerial vehicle and integration of air and earth,the high mobility of user terminals makes the transmission environment of wireless signal become much more complicated,which poses a challenge to physical-layer transmission technology.However,the high mobility induced high Doppler spread,will significantly destroy the orthogonality between subcarriers of OFDM.In order to realize high-performance demodulation,the channel frequency response(CFR)has to be accurately estimated,and further employed for channel equalization,thereby enlarging the pilot overhead and complexity.Thus,the aforementioned issues become the bottleneck in the application of OFDM for future high-mobility communication.To break the aforementioned bottleneck,recently,the transform-domain modulation based multi-carrier transmission has been concerned.Two typical transform-domain transmission schemes,namely time-delay(TD)domain based DFT-S-OFDM and delayDoppler(DD)domain based orthogonal time frequency space(OTFS),which utilize the channel sparsity in the delay and Doppler domains,are expected to decrease the overhead of channel estimation and complexity of equalization.This dissertation devotes to the channel estimation and equalization in the aforementioned transform-domain modulation based multi-carrier system.Specifically,the research contents are as follows:1.To tackcle the performance degradation in the high-mobility scenarios incurred by the current freqeuncy-domain channel estimation in the DFT-S-OFDM system,this dissertation proposes the time-domain channel estimation methods.The methods employ the basis expansion model to characterize the time variations of channel impulse response(CIR),and utilze the double orthogonality of Slepian sequences to achieve time-domain block interpoation.Compared with the current frequency-domain channel estimation,the proposed scheme can not only obtain accurate channel state information,but also significantly decrease the computational complexity and pilot overhead.In addition,different from the current frequency-domain channel equalization,this dissertation develop an LSQR-based TD-domain iterative channel equalization approach.The proposed scheme can realize fast convergence,and avoid the highly computation complexity involved by the TF-domain channel matrix inversion.2.To solve the problem that the current channel estimation schemes for the OTFS system can not accurately track the continuous-Doppler-spread channel,this dissertation proposes the low-dimensional subspace based channel estimation method.By leveraging the precise representation of Slepian sequences to continuous-Doppler-spread channel,this dissertation designs the Slepian sequences based transform-domain basis functions,and construct the low-dimensional subspace,so as to achieve the high-precision modeling of DD-domain channel responses in OTFS system.By leveraging the lowdimensional subspace channel modeling technology,the number of unknown responses in the scattering-abundant channel can be efficiently decreased.Also,the original underdetermined channel estimation task can be transformed to solve an overdetermined least square problem,which can significantly decreases the number of training pilots.3.Focusing on the drawbacks of exiting OTFS channel equalization methods for scattering-abundant environments,e.g.,extremely high computation complexity of the message passing and poor demodulation performance of linear equalization,this dissertation proposes the confidence interval based iterative detection scheme.By leveraging the sparsity of OTFS channel matrix,this dissertation constructs the LSMR based channel equalization,designs the confidence interval for detecting the transmitted data symbols,and proposes the linear-complexity interference reconstruction and cancellation method.Without of channel encoding/decoding,the proposed method can accurately detect the transmitted OTFS symbols.4.To solve the problems,i.e.,the high complexity of channel equalization and low efficiency of interference cancellation in multiple-input multiple-output(MIMO)OTFS systems,this dissertation proposes an efficient MIMO-OTFS data detection scheme.To tackcle the multi-dimensional interference in MIMO-OTFS system,which generates from space,delay and Doppler domain,this dissertation employs the idea of ”step-by-step interference separation,step-by-step combing detection”,and efficiently decompose the multidimensional interference.Also,by using the equal gain combining and maximum ratio combining technologies,the proposed scheme can efficiently extract the channel's multipath and multi-antenna diversities.As a result,it enables the MIMO-OTFS receiver to robustly demodulate multi-stream high-order quadrature amplitude modulation symbols,over high-mobility environments.This dissertation employs the transform-domain channel representation,as well as transform-domain modulation based multi-carrier transmission scheme.Taking full advantage of channel sparsity in the transform domain,this dissertation proposes the TD/DDdomain based channel estimation and equalization method,which provides the new ideas for the robust data transmission over high-mobility environments.