Reserarch On Precoding In Millimeter Wave Massive MIMO Systems

Precoder at the transmitter and equalizer at the receiver are corresponding.The precoding technique was originally proposed to simplify the receiver.The existing mobile communication working frequency bands are mainly concentrated below 6GHz,which makes the spectrum resources very crowded,while the millimeter wave frequency band(30GHz-300GHz)is rich in available spectrum resources,and its signals have shorter wavelengths,which can significantly reduce the spacing between antennas and is beneficial to antenna deployment,it is combined with Massive Multiple Input and Multiple Output(Massive MIMO)system to provide better beamforming gain and multiplexing gain.This paper mainly uses Rayleigh flat fading channel and millimeter wave channel application scenarios to study Massive MIMO digital precoding and hybrid precoding,mainly including the following five aspects of research work.1.The channel capacity and precoding process of Massive MIMO system are described in detail,typical linear and nonlinear precoding schemes are introduced,and the simulation verifies that linear precoding can obtain good spectral efficiency in Massive MIMO system.The simulation results show that for Rayleigh flat fading channel,the channel state information is very important for precoding.When there is imperfect channel state information,the system performance is significantly reduced.2.The iterative method is used to invert the matrix in digital precoding to reduce the computational complexity.In Millimeter-wave Massive Multiple Input and Multiple Output(MM-MIMO)systems,simple linear precoding can achieve the theoretically optimal performance compared to complex nonlinear precoding.Since matrix inversion is computationally complex in high-dimensional digital precoding,increasing the efficiency of the matrix inversion algorithm will significantly reduce the implementation complexity of digital precoding.From the Zero Forcing/Regularization Zero Forcing(ZF/RZF)precoding matrix,it can be seen that the ZF/RZF precoding matrix requires the generalized inverse matrix of the channel,and frequent matrix inversion will bring aboutO(N_t~3)computational complexity,especially in large-scale antenna systems.In order to reduce complexity,the special structure that requires an inverse matrix is used,and the iterative method is applied to digital precoding to achieve matrix inversion.The simulation results show that,compared with the direct inversion of the precoding matrix,the computational efficiency of the iterative inversion method can be improved by more than 70%.3.By optimizing the Maximum Ratio Transmission(MRT)precoding matrix,a closed form expression describing the spectral efficiency,outage probability and outage capacity of the system is obtained in Rayleigh flat fading channel.This paper describes the system performance of MRT precoding combined with ZF and MRT precoding,the Improved Maximum Ratio Transmission(Improved-MRT)precoding is obtained by enhancing the channel gain of the desired signal.The simulation results show that:(1)The probability density of the interference signal of the MRT and Improved-MRT precoding is the same.For the matrix inversion in the Improved-MRT precoding,iterative inversion can be used to reduce the calculation complexity of the system.(2)The Improved-MRT precoding can obtain better spectral efficiency and outage probability.(3)On the MATLAB platform,Improved-MRT precoding compared with MRT precoding,under the same conditions,can serve 5 to 15 more users at the same time,and the outage capacity is better than MRT precoding.4.The hybrid precoding algorithms of typical fully-connected and partially-connected structures are implemented and the spectral efficiency of different algorithms is analyzed.For the hybrid precoding of fully-connected structures,this paper mainly introduces the hybrid precoding algorithm based on Orthogonal Matching Pursuit(OMP),for the hybrid precoding of partially-connected structures,this paper mainly introduces the hybrid precoding algorithm based on Successive Interference Cancellation(SIC).The hybrid precoding algorithm,full digital precoding algorithm and analog precoding algorithm are compared.Simulation results show that:(1)OMP-based fully-connected precoding algorithm can be very close to full digital precoding when the number of Radio Frequency(RF)links is relatively large,and its complexity is relatively high;(2)The performance of SIC-based partially-connected precoding algorithm will be lost,but its complexity is low.With the increase of RF link,its spectral efficiency will also increase.5.For the hybrid precoding structures in MM-MIMO systems,the spectral efficiency of hybridly-connected structure is superior to that of the partially-connected structure,but it is inferior to that of the fully-connected structure.In order to improve the spectral efficiency of hybridly-connected hybrid precoding,the optimal hybrid precoding matrix under the ideal conditions was firstly obtained by using the principle of SIC.Secondly,the optimal hybrid precoding matrix was decomposed into the digital precoding matrix and the analog precoding matrix by using the gradient descent theory.Finally,considering the constant modulus constraint condition of the analog precoding matrix,the digital and analog precoding matrices were optimized by using the alternating minimization method aim to maximize the spectral efficiency.Due to the hybridly-connected structure,the proposed hybrid precoding design algorithm is significantly superior to the partially-connected and fully-connected hybrid precoding in terms of the system energy efficiency.Meanwhile,the algorithm does not increase any hardware complexity and only increases a small amount of computation of the hybridly-connected hybrid precoding.The simulation results exhibit that the proposed algorithm can improve the system spectral efficiency of the hybridly-connected hybrid precoding,and the upgrade of spectral efficiency is more significant especially in the conditions that the number of RF links is greater than the number of data streams.Since the sub-blocks are not necessary to satisfy orthogonality conditions,the proposed algorithm is more suitable for practical application than the existing hybridly-connected hybrid precoding.