Research On Beamforming Methods For Beam Switching In High Speed Railway Communications

The boom in high-speed railway has brought with it challenges for high-speed railway communications.Passengers on board trains need high quality network access,for example mobile working,watching HD video,online gaming,video calls and much more.To meet the demand for high quality communication and data services,it is important to replace the current GSM-R(Global System for Mobile communications for Railway,GSM-R)with a next generation dedicated railway communication system with higher system capacity and data transfer rates.The reason why millimeter waves attract people’s attention is that it has a large amount of unused bandwidth resources and supports higher data rates.At the same time,the short wavelength of millimeter waves and therefore its small antenna size allows space on the base station to deploy large scale Multi-Input Multi-Output(MIMO)antenna arrays.In high speed railway communication systems,millimeter wave and massive MIMO can be combined and beamforming techniques are used to concentrate the energy of the transmitted signal in a particular direction,thus increasing the system capacity and the reliability of the communication.Therefore,this thesis provides an in-depth study of beamforming techniques in high speed railway communications.The main work is as follows.Firstly,the relevant technologies in high-speed railway communications are analyzed.For millimeter wave technology,the propagation characteristics of millimeter wave technology are analyzed,as well as the problems of applying millimeter wave technology in high-speed railway communications.For large-scale MIMO technology,the capacity of traditional MIMO systems is analyzed,as well as the characteristics of applying large-scale MIMO in high-speed rail communications.For beamforming technology,the principle of beamforming is introduced and the characteristics of beamforming technology in high-speed railway communications are analyzed.Secondly,in order to address the problem of frequent beam switching in high-speed railway communication systems,a switching-based beamwidth optimisation method for high-speed railway communications is proposed in this thesis.In the method,firstly,the base station generates as wide a beam as possible to extend the distance of beam switching interval when the communication quality at the current switching point is satisfied.As the train moves,beam switching is required when the train is not within the coverage of the current service beam,otherwise the beam width is adjusted so that the ratio of the system data transmission rate to the probability of beam switching is maximised.Simulations show that this method outperforms beam-switching.Finally,the base station can form multiple beams simultaneously to serve multiple mobile relays to increase system capacity in high-speed railway communication systems.However,as trains move from the center of the cell to the edge of the cell,the interference between multiple beams becomes greater and the number of activations of multiple beams needs to be continuously varied to ensure the performance of the system.Dynamically changing the number of beam activations in real time is computationally intensive and affects beam selection in real time,especially in high-speed moving train communication systems.Therefore,a low-complexity beam selection method is proposed in this thesis.By observing the variation pattern of optimal multi-beam selection in high-speed railway communication systems,the operating states of the beams are first divided into two categories,one for activating all beams and the other for activating only one beam.The angular resolution between the beams is calculated to determine the beam switching point.The train runs from the center of the cell to the edge of the cell,and all beams are activated to serve all mobile relays before the train reaches the switching point;only one optimal beam is activated to serve one mobile relay when the train reaches the switching point.Simulations show that this method can maintain the high throughput of the high-speed railway communication system while significantly reducing the complexity of the multi-beam selection implementation.