Channel Modeling And System Optimization Of Train-Ground Communications In CBTC Systems

ABSTRACT:Communication-based train control systems of urban rail transit adopt the train ground communication to achieve the high capacity, continues and bidirectional ex-change between trains and the control center, which can keep the train operation efficient and reliable.Due to the characteristics of urban rail transit, most of the lines are deployed in underground tunnels, on viaducts and in cities. For tunnels and cities, the propaga-tion medium of the train ground communication system is generally selected as the free space or leaky coaxial cable; while for viaducts, leaky waveguide is often applied as the medium, which can supply high stability and anti-interference. For CBTC systems, the train ground communication is designed according to the experience, which is lack of the support of the available channel model. As a result, there may be some problems with the train ground communication system, including the handoff performance and the performance at the edge of the AP cell, which can cause the performance decrease of the train ground communication system. Therefore, CBTC QoS will be affected.In the thesis, the CBTC train ground communication based on WLAN is treated as the research object, and the effects on CBTC QoS caused by the decrease of the train-ground communication performance are quantified. The system analysis on the performance of different propagation mediums is performed, and the finite state Markov channel (FSMC) model with multiple matrices is build. The cognitive control is applied, which can integrate the wireless communication with the CBTC operation service based on the FSMC with multiple matrices. With reinforcement learning, the optimized hand-off policy and the communication parameters adaption policy are obtained to increase the performance of the CBTC train ground communication, which can further promote CBTC QoS.The innovations of the thesis are as follows:1).Based on the characteristics of the urban rail transit operation, the indicators of CBTC QoS are summarized and concluded. According to802.11protocols, the perfor-mance of the train ground communication system is calculated, which builds the basis to quantify the effects of CBTC QoS caused by the train ground communication.2) We propose the finite state Markov channel model with multiple matrices for tun- nels and leaky waveguide which can provide high accuracy for CBTC wireless channels. Based on the precise train location in CBTC systems, the wireless coverage is divided many intervals. The LLOYD-MAX non-uniform quantization and AIC are adopted to partition the channel states for each interval, and the corresponding state transition ma-trix can be obtained.3).We propose the channel model for the near field of leaky waveguide based on the specialty of CBTC scenarios, and the method to measure the radiation ability of leaky waveguide near field is also developed. The large scale fading of leaky waveguide near field is linear, and the slope is approximated by the transmission loss of leaky waveguide. The distribution of the small scale fading is determined to be Log-normal, which shows the stability of leaky waveguide through the statistical results of Log-normal distribution parameters.4).We apply the cognitive control into CBTC systems, which can treat wireless com-munication and CBTC operation together as a whole. Based on the FSMC with multiple matrices, the information gap is obtained through the calculation of the train-ground com-munication system performance. According to principles of cognitive control, the cost function is got which shows the train operation efficiency, energy consumption and in-formation gap. Considering the reinforcement learning, the optimized communication parameters adaption policy and handoff policy are selected for each communication cy-cle. Therefore, the performance of the train ground communication system is promoted, which can increase CBTC QoS. Through extensive simulations, it is illustrated that the effects on CBTC QoS caused by the train ground communication is decreased when the cognitive control is applied.