| High-speed railway mobile communication and passenger wireless access requirements are showing a trend of broadband,diversification,and dataization,which pose higher challenges for high-speed rail mobile communication systems in terms of large capacity and low power consumption.In order to meet the high-reliability and large-capacity requirements of the next-generation wireless communication system of the high-speed railway,millimeter-wave beamforming technology and large-scale antenna technology have outstanding advantages in improving the system’s spectral efficiency and energy efficiency,and can be used as an effective solution for improving the performance of high-speed railway wireless communication systems.Based on this,this paper mainly focuses on the research of millimeter-wave large-scale antennas and beamforming technologies in high-speed railway scenarios.Firstly,in the case of how to package a large number of antenna elements in a limited area in high-speed rail,we study the impact of large-scale antenna arrays with different arrangements on the performance of hybrid precoding systems.Considering uniform linear arrays,uniform rectangular planar arrays,uniform hexagonal planar arrays,and uniform circular planar arrays,the corresponding vectors of the four antenna arrays are analyzed,and the corresponding antenna arrays are searched by the orthogonal matching tracking algorithm for different array corresponding vectors.Simulation results show that the higher the concentration of a uniform planar array antenna,the higher the spectral efficiency of the system and the energy efficiency of the receiver.For the spectral efficiency of the system and the energy efficiency of the receiver,the performance of the planar antenna array is better than the linear array.Secondly,in order to enhance the beamforming gain of the millimeter-wave large-scale multiple-input multiple-output system and effectively reduce the hardware cost of the trackside base station phase shifter,a hybrid precoding design scheme based on a discrete phase shifter is proposed.With the goal of optimizing the spectral efficiency,this scheme analyzes the response vector of the antenna array,considers the design of the spatial characteristics of the millimeter wave channel as a spatial sparse reconstruction problem,and uses the principle of discrete orthogonal matching pursuit to obtain the digital precoding and discretization of analog precoding.The simulation results show that the performance of the phase shifter with the accuracy of 3 bits in the proposed discrete hybrid precoding scheme is close to the performance gain of the full-precision phase shifter.When the phase shifter has an accuracy of 4 bits,the optimal energy efficiency can be obtained,which greatly reduces the energy consumption of the base station beside the track.Finally,in order to further improve the energy efficiency of railside base stations,the millimeter-wave spectrum and energy efficiency of hybrid precoding with low accuracy of analog-to-digital converters(ADCs)and digital-to-analog is studied.First,considering the quantization noise of ADCs/DACs,an approximate expression of spectral efficiency is derived,and a two-stage alternating minimization optimization scheme is proposed to obtain the optimal hybrid precoding matrix.Simulation results verify that the proposed hybrid precoding scheme has almost the same spectral efficiency as full digital precoding.Compared with full digital precoding,the proposed hybrid precoding scheme can achieve a better compromise between spectral efficiency and energy efficiency.In addition,the improvement of ADC/DAC accuracy can effectively improve the spectral efficiency performance of the system,and ADCs/DACs with 4-bit accuracy achieve the best energy efficiency for hybrid precoding systems. |
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