Analysis And Research Of Train-ground Communication Handoff Performance For CBTC System Based On WLAN

Train-ground communication as key technology in communication-based train control system (CBTC), carries the secure transmission of train control information. The wireless local area networks (WLAN) technology is used in subway CBTC system train-ground communication, whose performance impacts on the traffic safety, which has always been the focus of research. Due to the limitation of transmission power and the influence of the subway environment, wireless access point (AP) must be arranged reasonable intervals. So it ensures normal train-ground communication by handoff when the train is crossing the cell. Because CBTC system uses IEEE 802.11g standard, the working frequency is 2.4GHz industrial scientific medical (ISM) frequency. Train-ground normal communication will be disturbed by radio waves of other work in the same or similar frequency, it can even cause the interruption of communication in the WLAN handoff. Therefore, the analysis of communication handoff performance of CBTC system under the interference has certain practical significance for the system safe and reliable operation.Firstly, the thesis describes the structure of CBTC system and handoff mechanism. Handoff mechanism used extend service set (ESS) network for CBTC system based on WLAN is expounded in detail, which includes trigger, scanning, certification, reassociation. And it analyzes access mechanisms of 802.11 standard medium access control (MAC), WLAN authentication and delay produced at each stage in WLAN handoff.Secondly, the AP layout of subway CBTC system is designed combined with shadow fading, small-scale fading, other parameters affecting the communication distance and radio wave propagation large-scale fading model. The model is identified by the characteristics of the wireless channel in the subway. The rationality of design is proved combining with actual AP layout interval in Shenyang subway Line 2. The interference of subway environmental is analyzed for train-ground communication, and inter-channel interference factor is introduced to lay the foundation for the next simulation.Thirdly, the radio wave propagation model and directional antenna model of CBTC system are established through OPNET simulation platform. And EAP-TLS authentication module are added. The established models form the network model of CBTC system train-ground communication. The traffic arrangement and configuration of operations are completed based on the WLAN node model coming with OPNET.Finally, the thesis constructs train-ground communications network topology of CBTC based on WLAN, and respectively analyzes the handoff communication performance under the non-interference, on-board interference and ground interference. Simulation results show that interference seriously impacts the handoff communication performance for subway CBTC system WLAN handoff. It can improve switching performance and guarantee the security of transmission information by using encryption and authentication methods. And the suggestions and improving measures are provided for the practical engineering network.