Research On Hybrid Precoding Of Millimeter Wave Massive MIMO System

With the improvement of mobile communication quality,many terminals access wireless network,which increases the mobile data traffic.Meanwhile,terminal users also need better network experience.In order to meet the above communication requirements,it is necessary to improve the capacity of network and data transmission rate,and reduce delay of communication.The millimeter wave communication has a wide bandwidth,which can provide an effective solution to the shortage of spectrum resources faced by the current communication and improve the data transmission rate.At the same time,the short wavelength of millimeter wave enables the integration of massive MIMO antenna arrays in a small device at the base station.Spectrum utilization and system capacity are improved by using precoding techniques associated with massive MIMO to direct signal radiation in the vicinity of the target user.The precoding technology is crucial to improve system performance,so this paper conducts in-depth research on the precoding technology in millimeter wave massive MIMO system,and the specific contents are as follows:Firstly,the characteristics of millimeter wave signal propagation and the mathematical structure of channel are introduced.Then,according to the characteristics of massive MIMO technology and the analysis of its channel capacity,the influence of precoding on system performance is introduced.In millimeter wave massive MIMO systems,hybrid digital and analog precoding is generally used to reduce RF chain and energy consumption.Hybrid precoding systems are generally divided into fully-connected and sub-connected.The composition of fully connected structure requires more phase shifters.Although it can achieve higher spectral efficiency,the difficulty of hardware implementation and system power consumption will be greatly improved.Although sub-connected reduces the number of phase shifters,its spectral efficiency is low.Therefore,this paper makes an in-depth study on hybrid precoding scheme based on hybrid-connected.In the hybrid-connected systems structure,The Singular Value Decomposition(SVD)has high complexity in calculating the optimal unconstrained hybrid precoding of each submatrix.Therefore,a low complexity hybrid precoding algorithm based on Projection Approximate Subspace Tracking(PAST)is proposed to estimate the optimal unconstrained hybrid precoding of submatrices,thus avoiding the high-complexity SVD process in this paper.Simulation results show that the proposed algorithm is close to SVD-based hybrid precoding in spectral efficiency,but with the increase of transmitting antennas,the complexity and time consuming of the PAST algorithm are much lower than SVD.In order to further improve the system performance of hybrid precoding algorithm,this paper designs analog and digital precoding matrices based on hybrid-connected and PAST algorithm using the idea of alternate optimization.The proposed hybrid precoding scheme aims at maximizing the subrate of each submatrix and adds unitary matrix constraints to the digital precoding of the submatrix.The optimization of analog precoding is not to directly use the optimal unconstrained mixed precoding phase information,but to take this phase information as the initial value,based on which further iterative search is conducted.Simulation results show that the AO-based hybrid precoding scheme is superior to the based-SVD in system performance,and has a higher system performance improvement than the hybrid precoding algorithm based on PAST.