Research On Cross-layer Design In CBTC Train Ground Communication Systems

As the center and brain of a rail transit system, the train control system controls the train’s moving speed and direction, and guarantees the safe distance between trains. In the premise of train operation safety, train control systems also improve the rail transit operation efficiency. With the development of communication and computer techniques, train control systems have radically changed. They are gradually developing from Track Based Train Control (TBTC) to Communication Based Train Control (CBTC).Train-ground communication systems are primarily designed to connect each com-ponent of CBTC systems, and its performance is essential to guarantee the high safety and efficiency of train operation. Due to the open frequency, open standard, easy instal-lation, low maintenance costs, and sufficient available commercial-off-the-shelf (COTS) equipment, IEEE802.11series based WLAN has been widely adopted as the urban rail transit CBTC train ground communication systems. However, IEEE802.11series stan-dard is not designed for train moving environments. Particularly, when a train moves away from the coverage of a WLAN access point (AP) and enters the coverage of an-other AP along the railway, a handoff procedure occurs. Both experiment and simulation results show that this handoff process may result in long communication latency. This long latency will have a considerable influence on the Quality of Service (QoS) of CBTC systems with high train density.Aiming to solve the long latency problem when abnormal handoff happens, in this thesis, we design more suitable communication stacks for train ground communication systems. With the proposed cross-layer design approach, we first map the stochastic channel state to the high layer QoS. The optimal handoff policy and communication pa-rameters adaption policy are obtained from stochastic control optimization. The optimal policy optimizes the train ground communication system performance, and improves the CBTC system QoS. Meanwhile, we apply the cross-layer design approach to urban transit passenger information system. Through mapping the stochastic channel state to the video service quality, the application layer parameters adaption policy and handoff policy are obtained by stochastic control optimization. The obtained policy can improve the video transmission quality. The innovations of the thesis are as follows:1). We design a CBTC train-ground communication system protocol stack based on MIMO-enabled WLANs. With the cross-layer design approach, the handoff decision and MIMO parameters adaption problem is modeled as a Semi-Markov Decision Process. The wireless channel is modeled as a finite-sate Markov chain, and the channel state transition probability matrix is derived from the data measured in real field test.2). Unlike the existing works, linear quadratic cost for the train controller in CBTC systems is considered as the performance measure in the handoff design. We take the control performance as the optimization objective in communication system. In MIMO-enabled WLANs based train ground communication system, with the control perfor-mance optimization objective, the tradeoff between MIMO diversity gain and multiplex-ing gain is optimized according to the train control performance.3). We design a seamless handoff train ground communication system based on SCTP and IEEE802.11p to provide high link availability in CBTC systems. With the cross-layer design approach, we formulate the handoff decision problem as a stochas-tic semi-Markov decision process. Maximizing the SCTP throughput and minimizing the handoff latency are the objectives in the SMDP model. Extensive simulation results are presented. Compared with the existing system based on IEEE802.11g and UDP, it is illustrated that the proposed system can significantly improve SCTP throughput and achieve seamless handoff in CBTC systems.4). As an extension to our cross-layer design application. We propose an urban rail Passenger Information System (PIS) train-ground video communication network based on IEEE802.11p, and fountain codes are used as the FEC scheme in application layer. With the cross-layer design approach, we formulate the handoff decision and application layer parameters adaption problem as a stochastic semi-Markov decision process. Min-imizing the end-to-end total distortion is the objective in our SMDP model. Extensive simulation results are presented. It is illustrated that the proposed optimization algorithm can significantly improve the end-to-end video quality in urban rail PIS systems.