The Analysis And Simulation Of 802.11 Interference In CBTC

With the deployment of technologies, Communication Based Train Control (CBTC) system has been widely applied in rail transportation. The bidirectional train-way communication is one of its key technologies. One main application is taking the use of wireless communication of IEEE 802.11.But because of its character of radio and working frequency on ISM band, the interference is unavoidable. Hence, the research of 802.11 interference in CBTC system, and the system performance under interference, is meaningful to analyze the system.In the thesis, firstly, combined with IEEE 802.11, the topology of CBTC system, the path of the data stream, and the characteristic of the wireless channel are expatiated. Whereas the powerful function of module and convenient simulation of wireless channel, the tool of OPNET is chosen. By the analysis of the radio wave transmission and its large scale and small scale path loss in open space, it gets an transmission mode fitted for the radio environment of rail traffic.Secondly, it specifies the several interferences that may effect on the system. In the case that multiple trains associate with and communicate to the same AP, it may lead the problem of medium contention. The medium access delay is analyzed combined with the mechanism in 802.11. In the case that exist other users with the technology of wireless local area network and Bluetooth, there is interference. The effect of the overlapping frequency is analyzed in frequency domain. And methods of computing the interference effect are summarized, which are the foundation of the simulation.Then according to the situation of the line and network of CBTC system, the simulation scenarios are set up in OPNET. Some modules should be created or modified first. The main task is simulating the radio transmission model in programming based the theory by the pipeline stage. Besides, the basic service set (BSS), the trajectory of the mobile node, the communication application, and the statistics should be set. On the preparative, different scenarios are set up, including simple network configuration of CBTC system without interference, scenario with inter-system interference, scenario with outside interference on the ground or on board, and more complex scenarios added train roaming. Results from different scenarios are compared to show the performance of the system. Some improving advices are put forward for the bad situations, which is good for application. The work of this thesis centralizes radio interference analysis. There are still many challengeable problems, for instance, the applicability of the protocol in OPNET, and the communication performance of the system with multiply trains. They are deserved close study.