Research On Channel Estimation In Millimeter Wave Massive MIMO Systems

As the key technology of 5G,millimeter wave communication is of great significance to the development of wireless communication.The characteristics of high frequency and short wavelength of millimeter wave communication make it possible to configure massive antenna array at the transmitter and receiver.In millimeter wave multiple-input multiple-output(MIMO)system,beamforming technology can improve the array gain and compensate the serious path loss problem of millimeter wave communication.In traditional MIMO system,each antenna is connected to a radio frequency link,which has high hardware cost and system energy consumption.Therefore,the hybrid beamforming structure is used as an alternative to the digital structure in millimeter wave massive MIMO system.At the same time,the discrete lens array is combined to form the beamspace structure.By using the sparsity of the beamspace channel,it is very important to reduce the number of radio frequency links through effective beam selection to improve the system performance.Beam selection technology needs to obtain channel state information(CSI)of large dimension channel,and accurate CSI acquisition depends on channel estimation.In the case of limited radio frequency links,channel estimation of massive MIMO system based on lens antenna array is facing challenges.Therefore,this thesis mainly studies the efficient channel estimation schemes of narrowband and wideband millimeter wave massive MIMO system in beamspace.Firstly,according to the typical millimeter wave MIMO system structure,the system structure of single antenna multi-user narrowband millimeter wave massive MIMO under lens antenna array is introduced.By using the sparsity of beamspace channel,the performance of classical maximal magnitude and interference-aware beam selection schemes is analyzed by simulation.Then,ant colony optimization(ACO)beam selection scheme is introduced into the millimeter wave system.On this basis,a joint channel estimation scheme of support detection(SD)based on ACO beam selection is proposed,and the traditional switched beam selection network is improved to an adaptive selection network based on 1 bit phase shifter.In the process of data transmission,the adaptive selection network realizes the traditional beam selection function through configuration.In the process of channel estimation,the network acts as an analog combiner to receive the uplink signal.At the same time,the system performance is analyzed in terms of time complexity,beam selection model,estimation error and achievable sum-rate.The ACO-SD joint estimation scheme can realize reliable estimation of large dimension channel with lower pilot overhead and computational complexity.The simulation results show that ACO-SD scheme can achieve near optimal performance by using less radio frequency links than full-digital system.Compared to the channel estimation scheme based on traditional compressive sensing and classical beam selection,it can obtain higher system sum-rate,especially in the case of low uplink signal-to-noise-ratio.After,the improved ACO-SD channel estimation scheme is applied to the narrowband millimeter wave system for multi-antenna users,and the scheme still has a good performance in the multiantenna users system.The above scheme is extended to the wideband millimeter wave massive MIMO beamspace structure for single antenna and multi-user.Aiming at the inaccurate channel estimation caused by the assumption of common support in the wideband millimeter wave system,a successive support detection(SSD)based on ACO is proposed.This channel estimation scheme has lower time complexity and pilot cost than the traditional orthogonal matching pursuit and synchronous orthogonal matching pursuit channel estimation schemes based on common support.The performance of the wideband millimeter wave system under the perfect CSI and different channel estimation schemes are compared by simulation.The simulation results show that the ACO-SSD channel estimation scheme has higher estimation accuracy than the traditional orthogonal matching tracking channel estimation scheme.Especially in the case of low signal-to-noise-ratio of uplink,the advantage of the improved scheme is more obvious.It is better than the channel estimation scheme based on synchronous orthogonal matching tracking,which can achieve the system performance close to perfect CSI.At the same time,the channel estimation scheme of wideband millimeter wave system for single antenna users is improved,which is applied to the multi-antenna users wideband millimeter wave system.The simulation verifies that the improved scheme has smaller estimation error than the traditional channel estimation scheme.