Analysis And Optimization Of IEEE802.11g-based Network Performance Of Train-ground Communication In CBTC

Communication-based train control(CBTC) uses wireless networks, such as wireless local area networks(WLANs), to transmit trains status and control commands. CBTC systems have stringent requirements for train-ground communications, as the train control system depends on the accurate, timely and reliable data over the communication links. However, since WLANs are not originally designed for high mobility environment, random packet delay and losses are inevitable in WLAN-based CBTC systems, which could result in unnecessary traction, brake or even emergency brake of trains, affecting the performance of train, losses of line capacity and passenger satisfaction.In the thesis, we study the situation that delay and packet losses happen in the course of data transmission in IEEE802.11g-based CBTC systems. Firstly, analyze the reasons for random delay from both random transmission errors and random access at MAC layer. The packet drops caused by handovers, which has a drop rate depending on the handover time, the AP coverage range, and the overlapping coverage area between APs. Secondly, the train control system which vehicle-ground communication appears simultaneously random delay and packet losses in CBTC systems is modeled as a network control system(NCS).Stability conditions of the network control system are given and demonstrate. Finally, the results of the simulation examples show that the network control system can maintain the stability of the system.In the case of the data packet dropout, according to the model established by the network control system with data drops, to estimate the losing data packets contained the status of train by adopt estimated strategies. Analyze effects of packet drops on the performances of the trains’ control system. Finally, the improved scheme of train control system with data packets dropout is proposed.