Research On Massive MIMO Beamforming Algorithms In High Speed Rail Scenarios

With the rapid development of high-speed railways,the needs of passengers on trains for mobile communications have gradually changed from voice calls to video telephony and network multimedia.The existing Global System for Mobile Communications for Railway(GSM-R)has inherent narrow-band characteristics that can not meet the needs of people for data traffic services.In order to cope with the rapid growth of data traffic services,Massive MIMO technology has attracted widespread attention.Different from the traditional MIMO technology,massive MIMO can further improve the data transmission rate and link reliability by configuring more antenna arrays at the base station.The application of Massive MIMO can not be separated from beamforming technology.Through the combination of Massive MIMO and beamforming technology,the energy of the beam is more concentrated and the performance of the system is further improved.Therefore,this paper has conducted in-depth research on the massive MIMO beamforming technology in the high-speed rail scenario.The specific work is as follows:First of all,this paper deeply analyzes massive MIMO technology and beamforming technology.According to the environmental characteristics of high-speed rail communication scenario,the system model of Massive MIMO and the channel model of high-speed rail communication scenario are introduced.The basic theory of beamforming technology and the application of beamforming technology are described.Secondly,this paper proposes a beamforming method for Massive MIMO uplink in high-speed rail scenarios.This method does not need channel status information(CSI),uses position information to assist the method to generate the weight vector of the beam,and introduces the concept of effective beamforming probability to ensure the reliability of the transmission link.Considering the influence of positioning error on the method in this paper,the optimal beamwidth is selected based on the effective beamforming probability threshold to maximize the beam directionality and improve the system performance.In order to further reduce the computational complexity of beamforming,the fan-shaped area covered by the beam on the base station side is divided into several equal width sub sections,and only the real-time change of the beam width in one sub section needs to be calculated.Simulation results show that the scheme can still maintain good system performance under the condition of low complexity.Finally,this paper studies the beamforming method of downlink in high-speed rail scene,and proposes an adaptive beamforming scheme based on position information.In this method,the flexibility of the system and the complexity of the system are improved by properly adjusting the beam direction and beam width.When the beam width transmitted by the base station cannot meet the communication requirements,the beam width shall be adjusted first to avoid frequent adjustment of beam direction to track the running train.When the beam width reaches the limit of its adjustment range,the beam direction shall be adjusted to align the beam with the train.Due to the poor performance at the edge of beam coverage,it is necessary to adjust the beam direction before the train arrives at the position of beam direction adjustment.The set signal-to-noise ratio(SNR)threshold can be used to calculate the position where the beam direction needs to be adjusted next time.By comparing the SNR of the current and next adjusted beam direction positions,it is decided whether to adjust the beam direction in advance.Through simulation,it is found that the performance of this scheme system is better than the traditional beam switching scheme and the performance is close to the real-time beam tracking scheme.