Simulation Analysis On Wireless Lan Handoff Performance In Cbtc

Recently, IEEE 802.11 based Wireless LAN has been widely used as bi-directional train ground communication in Communication Based Train Control (CBTC) system. Due to the transmit power regulation and ground construction effect, the coverage area of a single AP is limited. The distance between adjacent APs is often 200 meters in CBTC. In order to guarantee the reliable and continuous communication between train and ground, when a train leaves an AP's coverage area, it has to handoff and re-association with another one. Both field and laboratory tests illustrate that most of the communication interruptions are caused by handoffs.According to the IEEE 802.11 Standard, the paper first introduces the Wireless LAN physical topology. It then describes the handoff procedures in the WLAN Based train ground communication and analyzes the delay caused by every handoff phase. The handoff procedures include:Scanning, Authentication, Re-association.Next, with network simulation tool OPNET, this paper constructs a simulation network for WLAN based CBTC train ground communication system. The train control application model and train mobility model is established based on real train operation principles. With the three layer models in OPNET, the paper realized the network model, protocol model, directional antenna model and revised physical layer fading model in the CBTC train ground communication system. The handoff algorithm is revised based on the original MAC layer process model.With the established train ground communication simulation model, this paper simulates the handoff performance under different scenarios. The simulation results show that:higher or lower handoff threshold both have negative effect on handoff performance, it shall be set according to real network; The handoff time doe not change too much when the train speed is less than 60km/h; The handoff performance degrades dramatically when the train speed is more than 110km/h, and the wireless equipment does not work any more; The received signal power suddenly drops when the train go in to the back of the directional antenna, which might increase the handoff time; the encryption improves the system safety, but degrades handoff performance.Last, the conclusion and future work is presented.