Optimized Handover Schemes For The High Speed Train-Ground Wireless Communication System

With the development of railway modernization, lots of railway lines for passenger transportation have been built. Due to the increasing speed of train, more and more people choose to travel by high speed train, which brings remarkable challenges for the train-ground wireless communication system. Handover process is one of the greatest influences for the system performance, so we should improve the handover performance, decrease the handover outage rate and latency. Normally, there are two main types of train-ground wireless communication systems. One is the GSM for Railway (GSM-R), which is used to transmit the information of high speed train operation and control, and the other is the next generation wireless communication system, which can provide the broadband services for passengers on the train to meet the increasing requirement of communication.This thesis proposes two optimized schemes for above two types of train-ground wireless communication systems respectively. The major research contents and contributions are described as follows.Firstly, the characteristics of high speed train-ground wireless communication systems both in home and abroad have been analyzed. The system architecture and handover process for the GSM-R and the next generation wireless communication system are discussed in detail. The key points of penetration for the handover optimization in the train-ground wireless communication system have been outlined.Secondly, the thesis proposes an onboard dual-antenna seamless handover scheme in GSM-R redundant network. The improved scheme takes full advantages of the intercross base station redundant network structure and combines the different BSs in GSM-R two layer redundant network to a virtual cell serving for the train. Both of the front and rear of the train are deployed with an antenna respectively, which are used for a bridge of communication between the train and the BS on the ground. Onboard dual-antenna can access different BS in the virtual cell. When the train travels along the cells and doesn't handover to the neighboring BS, the dual-antenna could acquire diversity reception performance. When the train executes the handover process, the dual-antenna and virtual cell communicate collaboratively and implement handover seamlessly. The proposed scheme can improve the handover performance remarkably and guarantee transmission of train control information safety.Finally, the paper proposes a modified strategy based on 802.16 next generation wireless communication system to satisfy the communication requirements of passengers on board. The proposed scheme takes mobile relay station (MRS) for handover process. MRS should be installed in the middle of overlay region. On the one hand, the MRS can provide better signal quality to cover the overlay region when the train handovers to the neighboring BS. On the other hand, due to the feature of handover in the MRS, when the communication link between the train and the BS switches from one-hop link to the two-hop with the MRS forwarding, the MRS handovers to the neighboring BS and the train doesn't involve in the handover process. After the MRS completes handover, the connection identity of users should be updated and the handover process between the neighboring BSs should be completed. The proposed scheme can improve the signal quality, reduce the handover outage rate, trigger handover process and complete some signaling interactions earlier than the traditional schemes. Some signaling interactions could be possibly omitted. The handover time consumed can be decreased dramatically, so that the handover performance is improved and the passengers could enjoy a better communication quality.