Research On The Handover Of Communication Based Train Control Systems In Urban Rail Transit

Communication Based Train Control(CBTC) systems use the advanced communication and computer technologies to realize the bi-directional communications between the ground control system and the train, which can continuously control and monitor the train operation.CBTC usually adopts IEEE802.11 to realize the train-wayside communication. Due to the coverage of APs( Access Points) in the wireless LAN is limited, the handover will be more frequent with the increasing speed of train, which produces some obvious problems in handover process, such as delay, packet loss etc.. These problems will further affect the reliability of CBTC systems. Therefore, how to reduce the delay of handover and ensure the stable quality of the communication in handover process becomes more important.Firstly, the dissertation introduces the structure and function of CBTC system, the wireless LAN technology and handover process, and then analyzes the delay in the handover. Because of different handover scheme has the different trigger rules, the dissertation also analyzes several typical handover trigger rules.Secondly, the dissertation proposes the predictional two-channel asynchronous handover scheme on the basis of analyzing the disadvantages of the traditional handover schemes. On the one hand, the scheme considers the handover scanning delay, authentication delay and correlation delay, and the scanning delay is about 90% of the total delay, then the differential RSSI predication algorithm is proposed to reduce the handover delay. On the other hand, the scheme considers the prediction mechanism while it is possible to meet the requirements of minimum delay of data transmission. However, when the mobile sites handover frequently, more delay will occur, or even communications will interrupt, which affects the communication quality of the CBTC systems. Therefore, the two-channel asynchronous handover scheme has been proposed to ensure the communication quality of CBTC systems in the process of handover. Two-channel asynchronous handover scheme uses two separate wireless network interface to communicate with ground control system in a mobile site. By configuring different parameters of handover trigger to make the two network interfaces handover “early” or “late”, which can keep communication with at least one network interface when the mobile site crosses different service areas.Finally, the traditional handover scheme and the two-channel asynchronous handover scheme are simulated in terms of delay and throughput in this dissertation. The results show that the proposed handover scheme based on prediction can not only effectively reduce the handover delay, but also ensure the stable communication quality in the process of handover.