Research On Uplink Performance Of Mm Wave Massive MIMO System With Digital/Analog Hybrid Structure

The millimeter wave(mm Wave) massive multiple input multiple output(MIMO)technology relies on the abundant available mm Wave frequency band resources and the great spatial freedom brought by massive antenna arrays,which can significantly improve the spectral efficiency and energy efficiency of the system,and reduce the transmission delay.Therefore,mm Wave massive MIMO technology has become one of the key technologies for the fifth generation mobile communication system(5G).Since each antenna is connected to an independent radio frequency(RF)chain,the hardware cost and power consumption of the mm Wave massive MIMO system are high.The digital/analog hybrid beamforming structure introduces a phase shifter network between the antenna and the RF chain,and performs beamforming in the digital and analog domains,which can significantly reduce the number of required RF chains,thereby reducing hardware costs and power consumption.According to the different connection modes of the antenna and the RF chain,the digital/analog hybrid beamforming is divided into two structures: fully-connected structure and sub-connected structure.The latter requires fewer phase shifters and has lower complexity.In addition,due to the high frequency of the mm Wave,the path loss is large,leading to low diffraction and penetration ability.Therefore,the mm Wave signal is easily blocked by obstacles and has a small coverage area.The intelligent reflecting surface(IRS)based on electromagnetic metamaterials can reshape the wireless channel environment,and realize analog beamforming by adjusting the reflection coefficients,which can greatly improve the mm Wave coverage performance.This paper studies the optimal sub-connected structure in the mm Wave massive MIMO system,and compares the beamforming performance based on the phase shifter network and the IRS.The specific research contents are as follows.Firstly,consider the phase shifter network based digital/analog hybrid beamforming structure and study the system performance of the sub-connected structure.Using the channel characteristics of mm Wave and Jensen's inequality,the asymptotically closed expression of the spectral efficiency is derived,and the influence of the distance between adjacent antennas and sub-connected structure parameters on the spectral efficiency is analyzed.The research results show that when the distance between adjacent antennas is equal to an even multiple of the half-wavelength,the spectral efficiency of the localized sub-connected structure is the largest;when the distance is equal to an odd multiple of the half-wavelength,the interleaved sub-connected structure has the highest spectral efficiency,in which the antenna sub-array is interleaved by one antenna.Secondly,the IRS based digital/analog hybrid beamforming scheme is studied,and the expression of the spectral efficiency is derived and compared with the phase shifter network based digital/analog beamforming structure.The research results show that the mm Wave massive MIMO system assisted by the IRS can provide users with reliable communication services when the signal is blocked by obstacles;the spectral efficiency of the transmission scheme based on the IRS increases with the increase of the reflecting units.When the number of phase shifters and reflecting units are the same,the structure based on IRS has obvious advantages in the single-user single-RF link scenario;in the multi-user multi-RF link scenario,when the number of antennas is the same,the structure based on IRS has better performance.When the number of base station antennas based on the phase shifter network is large,the structure based on the IRS has better performance under low signal to noise ratio(SNR),while the structure based on the phase shifter network has better performance under high SNR.The research of this paper is useful for the design of digital/analog hybrid beamforming structure.