Channel Estimation In Millimeter Wave Multi-user Massive MIMO System

The future wireless communication technology can overcome the shortage of spectrum resources by using millimeter wave of high frequency band to provide a large amount of available bandwidth for users.In general,millimeter-wave communication technology with high propagation loss needs to be combined with Massive Multiple-Input multioutput(Massive MIMO)technology to play a greater advantage.However,in highly dynamic mobile scenarios,the communication service quality of millimeter wave Massive MIMO system is greatly reduced due to Doppler shift,which cannot meet the normal communication requirements.Orthogonal Time Frequency Space(OTFS)is a modulation scheme that multiplexes data symbols in the delay-Doppler domain.It has the ability to obtain a total diversity in the orthogonal time frequency space.MIMOOTFS can achieve higher spectral efficiency and robustness in millimeter wave MIMO channels with high Doppler.Therefore,this thesis mainly studies the Channel estimation algorithm in the highly dynamic scenario of future wireless communication,and uses the obtained Channel State Information(CSI)to improve the communication service quality of millimeter wave MIMO-OTFS system.In view of the disadvantage that the estimation scheme of delayDoppler-angle domain sparse channel in millimeter wave MIMO-OTFS system fails to accurately calculate the Angle domain support set,in this thesis,a 3D Structured Adaptive Support Detection(3D-SASD)algorithm is designed which can accurately estimate the channel.By using the special structure of the millimeter wave channel in the Angle domain,the algorithm adaptively calculates the length of the non-zero burst in the Angle domain on each main path according to the noise,and obtains more accurate detection results of the support set.Finally,the non-zero value of the three-dimensional sparse channel is estimated by the matching tracking scheme including pruning.The simulation results show that the 3D-SASD algorithm can get accurate support set in high dynamic scenarios,which has obvious advantages in low signal-to-noise ratio environment,and is closer to the system performance in the case of ideal channel.Aiming at the high complexity of channel estimation algorithms in millimeter wave MIMO-OTFS systems,a 3D-Multi-Step Conjugate Gradient Pursuit(3D-MSCGP)channel estimation algorithm is designed in this thesis.The algorithm improves the conjugate gradient tracking algorithm.On the one hand,a limited number of front search directions are used to update the search direction,which improves the search freedom and convergence speed of the algorithm.On the other hand,the threedimensional structural sparsity of the channel is used for atom selection.By analyzing the characteristics of the high frequency band of the millimeter wave system,the mathematical model of the user speed and the non-zero block size of the Doppler domain is established to simplify the steps of atom selection and further reduce the complexity of the algorithm.Simulation results show that 3D-MSCGP algorithm can obtain relatively complete CSI with short running time in high dynamic scenarios,which meets the communication requirements of high reliability millimeter wave system with low time delay.