Research On Handover Technology In LTE-R For High-Speed Railway Environment

In recent years,high-speed railways(HSRs)are being developed rapidly in China,which are gaining increasing popularity for inter-city transport due to their advantages including low energy consumption,high punctuality rate,high transport efficiency,added comfort,security,etc.HSRs improve the quality of service(QoS)and customer satisfaction on the trip,create enormous social and economic benefits.However,the existing Global System for Mobile Communications for Railway(GSM-R)is unable to support the requirements under high mobility scenarios,such as significantly huge data traffic,low end-to-end delay,high reliability and security.Thus,the International Union of Railway(UIC)has approved Long-Term Evolution for Railway(LTE-R)to replace GSM-R as the next-generation railway communication system,which can support the communication requirements in high mobility scenarios.Handover is a key technology which plays an important role in the LTE-R,which can ensure the continuous connection of the communication when a train passes through a cell and enhance the validity and reliability of the system.Compared with the handover schemes in public networks,the special operating environments of HSRs and the high mobility of the trains result in higher requirements for the handover.Many problems remain unsolved,such as a high probability of handover failure,frequent handovers,a large system overhead caused by group handovers,etc.Therefore,it is of vital significance to improve the handover performance and guarantee communication quality and service demand.Through the review of the related literatures and analysis,the handover technology in the HSR scenarios is studied in this paper.The research and innovation points are as follows:(1)A speed adaptive handover scheme based on beamforming technology is proposed.The beamforming technique is used in the overlapping area of evolved NodeBs(eNBs)along the railway.When the high-speed train enters the overlapping area,the antenna of the eNBs changes its working mode to beamforming mode,and improves the received signal strength of the train by adjusting the beamforming gain in different areas of the overlapping area.At the same time,a dynamic adjustment algorithm of handover trigger threshold is proposed,which can adaptively adjusts the handover trigger threshold according to the position and speed of the train.Simulation results show that the proposed handover scheme can improve the handover trigger probability and guarantee the handover success probability.(2)A fast dual-link handover scheme based on the control/user plane split network is proposed.The control plane and the user plane information of the HSR mobile communication system are decoupled and separated.The control plane with high reliability of transmission is carried by the macro eNBs in the low frequency band,and the user plane information is supported by the small eNBs in the high frequency band.When the train crossing the macro cell,the handover between the macro eNBs needs to be switched in respective control plane and user plane.In order to ensure the good handover performance,a fast dual-link handover scheme is proposed.Two antennas are set up respectively at the front and the rear of the train to ensure the handover success probability and improve reusability and stability of the system.At the same time,in order to reduce the handover delay to achieve fast handover,the bi-casting is used to optimize the handover signaling flow.Finally,the performances of different handover schemes are simulated and compared.Simulation results show that under this network architecture,the proposed scheme can effectively improve the handover success probability,reduce the outage probability and ensure the communication reliability.