Design Of High-Mobility Support Adaptive Handover Mechanism

In cellular wireless communication system, some problems are caused by users' high-speed movement, such as over-frequent handover, Doppler frequency shift, and channel fast variation. For the wireless communication of high-speed railway, handover success rate critically impacts on the traffic safety. In order to ensure the system robustness and effectiveness, it requires shorter handover time and higher handover success rate, which cannot be achieved by the most current wireless communication systems. Therefore it has important significance to design new handover scheme to meet demand of the high-speed railway communication.Distributed wireless communication system and relay technology have been mainly studied in this thesis for high-speed railway communication. The author's main research work and contributions are as follows:Firstly, distributed antenna technology, as one of the important next-generation wireless communication technologies, has aroused extensive attention. The technology has been applied in high-speed movement environment. Such system is characterized by using a large number of distributed antenna units along the railway to provide high-quality, seamless coverage of radio signals. On the one hand, different trains choose different antenna units to get access. The spatial diversity improves system capacity and Quality of Service (QoS). On the other hand, mobile trains take flexible handover manner to ensure reliable high-quality communications link when they move across two adjacent cells. This thesis proposes a novel handover scheme based on on-vehicle dual-antenna for high-speed railway distributed antenna system (DAS). On-board antennas which collaborate each other, are mounted on the top of high-speed train (the one is at the front-end and the other is at the rear-end). The proposed scheme utilizes distributed transceivers and centralized processing technology. The numerical analysis and simulation results show that the novel scheme can pre-trigger handover appropriately, guarantee the higher handover success rate, and increase the system throughput by around 50%. In addition, the scheme is feasible and easy to be implemented.Thirdly, at present, the wireless communication system for high-speed rail is GSM-R (GSM for Railway) system in China. This thesis proposes an enhanced handover scheme with shared-RS (shared-Relay Station) in GSM-R network to improve system handover performance. In the scheme, shared-RS improves the target BS signal quality in overlapping region, and the handover can be triggered in advance so that there is much longer distance for completing handover process when the train is traversing through the overlapping region. The analysis and simulation results show that the scheme can trigger handover appropriately in advance about 500m, eliminate ping-pong effect, reduce handover failure probability to 10-6, and thereby guarantee the higher successful handover probability.