Research On Millimeter Wave Beamforming Methods For High Speed Railway Communications

With the rapid development of high-speed railways(HSR),the demand for data transmission rates and reliable mobile services in high mobility scenarios is increasing.Consequently,the HSR communications have higher service requirements for data rates,bandwidth and quality of service(Qo S).The combination of millimeter-wave(mm Wave)and beamforming technology based on large-scale antenna arrays can achieve ultra-high data transmission rates.When applied to HSR communications,it could effectively improve the system performance and communication quality,which is a promising trend in the development of HSR.Therefore,the mm Wave beamforming methods for HSR communications are addressed in this study.The main work is arranged as follows.First of all,we analyze the related technology of HSR mm Wave communication,and it focuses on the propagation characteristics of mm Wave in the atmosphere,including the path loss model and the attenuation of mm Wave by oxygen,water vapor and rain.In mm Wave communications,massive multiple-input and multiple-output(MIMO)technology and beamforming technology are required to enhance the directional performance of mm Waves and compensate for the mm Wave transmission losses.Therefore,we investigate the traditional MIMO,massive MIMO and beamforming technology.The basic principle of uniform linear antenna array(ULAA),antenna pattern and calculation of half power beam width are analyzed as well.Additionnally,the problems of massive MIMO technology and mm Wave beamforming technology in HSR scenarios are discussed.Secondly,an mm Wave-based adaptive multi-beamforming method for HSR train-to-ground communications is proposed in this study.In the proposed scheme,it is assumed that a mobile relay(MR)is installed on the top of each carriage of the train.In order to enhance the throughput performance and reduce the number of beam-switching,the base station(BS)uses mm Waves to form multiple beams with different beam width to transmit signals simultaneously.Additionally,the outage probability of the system can be decreased through the signal transmission of multiple beams.However,the longer the distance between the train and BS,the more serious the inter beam interference(IBI)caused by multiple beams will be.To solve this problem,an adaptive beam activation technology is adopted to adjust the serving beams in real time,so as to keep the system throughput at the optimal level.Simulation results suggest that the proposed scheme can evidently improve the throughput performance of the system and obtain a lower communication outage probability.Finally,aiming at the stability and reliability of data transmission in HSR wireless communication system,we propose a location fairness based mm Wave beamforming method under the dual network redundant coverage structure of interleaved sites.In this method,a target data rate is predefined as the reference rate,through which an adaptive searching algorithm is adopted to calculate the optimal beam width and beam boundary points.The performance gap between the given target and the average data rate within the coverage of each transmitting beam should be minimized.Thus,the instantaneous data date fluctuates near the target rate and thereby the violent fluctuation of data transmission during the operation of the train is decreased.The simulation experiments prove that the proposed method can effectively improve the stability of data transmission in HSR wireless communication system.