Research On Wireless Channel Modeling And Link Simulator In CBTC

CBTC (Communication Based Train Control) adopts WLAN (Wireless Local Area Network), which is based on IEEE802.11internet protocol suite, to realize train-ground two-way communication. As the specific characteristics of urban railway system, its lines mainly distribute in open space, elevated bridge and tunnel. Wireless free wave is adopted to transmit information in tunnel and open space, while leaky waveguide is used to preclude the influence of interference in elevated bridge. There are much reflection, dispersion and refraction which makes the wireless propagation environments complex, meanwhile no general channel model for leaky waveguide to conduct theoretical research.Therefore, this paper completes the link simulation of IEEE802.11g, and establishes the channel models of open space, runnel and leaky waveguide respectively; then combine these two parts together to simulate and analyze the transmission performance of different operating environments, such as open space and tunnel, and different transmission mediums, such as free wave and leaky waveguide.The specific research contents in this paper are as follows.(1)Above all, this paper models three channels which are open space, tunnel and leaky waveguide. Open space can be approximated multipath Rayleigh fading channel; the tunnel model is established based on geometrical optics model, waveguide model and Poisson summation formular; received electric-field strength of arbitrary postions above leaky waveguide is calculated based on equivalent magnetic dipole method, and then leaky waveguide model is obtained.(2)Next, with the basis of Matlab and Simulink, this paper completes the CBTC train-ground communication link simulator based on IEEE802.11g, which includes sending, channels and receiving. Channels consist of open space, tunnel and leaky waveguide; receiving adopts least square method to realize channel estimation.(3)After that, this paper verifies the availabilities of channel models and link simulator. The method of comparing calculated received power with measured values or their statistical properties are used to realize verification; the method of comparing simulated bit error rate/block error rate with theoretical values or existed simulated results in VC++are adopts to realize verification.(4)Finally, lead the verified channel models into link simulator, and obtain link transmission characteristics of different operating environments and transmission mediums.The consequences show that:the block error rate of leaky waveguide is smaller than that of free wave in tunnel or open space; when free wave is used to transmit information, the block error rate is smaller in open space than that in tunnel. Simultaneously, this paper can also provide guiding advices for actual use of transmission mediums.