Research On Handover Mechanism With High Reliability Under High-Mobility Scenario

The moving speed of high speed train keeps increasing from200km/h to350km/h, even up to500km/h in the near future. The rising speed provides much convenience to passengers, but on the other hand, it also brings great challenges to the current wireless communication. Firstly, the Doppler shift and fast fading channel characteristics caused by high speed lead to a great threat to the high speed data transmission. Especially for TDD system, the uplink frequency offset is twice of the same case in downlink, which may conduce that the received signals is difficult to decode. Meanwhile, in handover region, the train will receive signals from two adjacent base stations, which are of the same energy, but with a jumping from positive to negative in frequency offset. Secondly, the increase in speed will result in frequent handover, which will require a higher performance on mobility management, especially fast handover control. Finally, as the travelers on board are geographically located relatively concentrated, group handover will occur when the train goes through the cell edge, which is also a problem rarely faced in traditional communication systems.LTE is a paradigmatic technology merging mobile communication and broadband wireless access technology, and is the mainstream of the future broadband mobile communication standards, with higher channel capacity, greater bandwidth efficiency, stronger anti-interference ability and better transmission performance, which can meet the demand of high capacity, high bandwidth, high speed and high spectrum utilization for the next generation railway communication system. International Union of Railway (UIC) also plans to evolute GSM-R to LTE-R in the next few years, to ensure that GSM-R can follow the pace of the development of communication technology. However, the current LTE network only adopts hard handover, and need to forward data through the interface between eNodeB during handover, which will result in large handover latency. As a result, the handover performance in LTE under high mobility is remarkably poor. Therefore, the handover mechanism of current LTE system cannot meet the need of high mobility.In this paper, a CoMP and bi-casting based soft handover scheme for high speed railway (HSR) is proposed, which is based on LTE network architecture, and without any change to the existing network and the train. In the scheme, CoMP is used on air interface and bi-casting is used on S1interface. With the binding triggering of CoMP and bi-casting, only one suit signaling is needed and the system complexity is much simplified. Meanwhile, as much handover related information is stored in corresponding entity during the join process of CoMP and bi-casting, the handover process is much simplified and converted into update of serving eNB. It can be seen from theoretical analysis and simulation results, compared with traditional handover scheme, the handover delay is reduced more than67%, the handover outage probability is decreased to less than0.05%, the handover success rate is improved by about20%, and the system throughput in handover area is increased by30%with zero interruption in the user plane.On the other hand, almost all the existing research on the handover mechanism under high speed rail scenario use vehicle relay to avoid group handover, but the implementation of vehicular relay need to get support of the railway ministry, and it is difficult to apply it to public communication sevices on vehicle considering the particularity of special commication of railway. Therefore, research on group handover under high speed rail scenario is with urgency and importance. In this paper, the restraining factors of group handover performance in LTE/LTE-A network are analysed from the aspects of handover signaling process and channel resource allocation, and the possible solution to group handover is proposed, and the simulation of the optimized mechanism introducing RRC timer information into the process of signaling resource allocation scheme during handover is presented.