Research On The Handover Strategies Of Train-to-Wayside Communication For CBTC Systems

In recent years,with the rapid development of urban rail transit,the imbalance between supply and demand in large cities,which is caused by the continuous expansion of urbanscale and the growing urban population,has been effectively solved.Safe,efficient and green transport is the eternal theme of construction and development for urban rail transit.Communications Based Train Control(CBTC)is of vital importance to ensure the safe operation of urban rail transit.Using modern wireless communication technologies,CBTC systems closely combine wayside eqquipment and trains,and form a complete closed-loop control process to ensure the safe and efficient operation of trains.The railway lines of urban rail transit are usually distributed in tunnels,on the ground or viaducts,and the media of train-to-wayside communication of CBTC systems adopts free space,leaky cables,leaky waveguides,etc.,in which the wireless channel is complex and there are signal fading and interference phenomena.Moreover,in the peak period of operation for urban rail transit,the safe tracking interval of trains can reach up to 90 s or even smaller,which puts forward much higher requirements of safe train protection and efficient operation for urban rail transit.During the process of train running,handover frequently occurs,which will result in increased transmission delay or data packet loss and reduce the performance of train-to-wayside communication for CBTC systems.If the real-time information of control and display for CBTC systems can not be delivered reliably,the service braking or emergency braking of trains beyond train schedules will be triggered,which will restrict the stability of train running and the operation efficiency of urban rail transit.The relevant research shows that handover delay is the most critical factor affecting the train-to-wayside communication of CBTC systems and the total number of packet loss caused by handover is much larger than that caused by normal wireless transmission.Therefore,how to make rational use of communication resources to optimize the handover strategies for the improvement of quality of service(QoS)and availability of CBTC systems has become the main problem of CBTC systems.In the dissertation,the train-to-wayside communication of CBTC systems is taken as the research object,and the close relationship between train-to-wayside communication and train control is also considered.In terms of the mainstream applied technology of wireless local area networks(WLAN),the recent applied technology of Long Term Evolution for Railway(LTE-R)and the visible light communication(VLC)technology,the handover strategies of train-to-wayside communication has been deeply analyzed and optimized by game theory,which can make full use of communication resources and improve the performance of train-to-wayside communication networks to meet the requirements of safe and efficient operation of trains.The main innovations of the dissertation are as follows:(1)According to the characteristics of lines and operation of urban rail transit,a model of combining train-to-wayside communication and train control is established,and the influence of train speed on the handover of train-to-wayside communication is also analyzed.Combining with the practical engineering application in the field,a handover algorithm of frequency combination in the environment of WLAN has been proposed.The number of detected frequencies is used to get the information of the target access points(AP)in advance for smooth handover.Furthermore,the constraint relationship between the train speed and the hysteresis parameters of handover is deduced.The hysteresis parameters are dynamically adjusted to meet the requirements of train-to-wayside communication when a train is running at different speeds.(2)Aiming at the handover problem of train-to-wayside communication of CBTC systems in the environment of WLAN,a train handover algorithm based on cooperative diversity technology has been proposed.A Stackelberg game model with competition mechanism is used to take the bandwidth resources of WLAN,bandwidth requirements of train,cooperation diversity,parameter setting and other factors into consideration.Then,a mathematical model of handover is established when a train is running through two continuous APs.The parameters of price and revenue are introduced for each AP,and the strategy of mobile nodes is updated by iterative learning method,which maximize the effectiveness of all participants in the network.The simulation results show that the proposed game strategy can maximize the network revenue,realize the rational allocation of network resources,and effectively improve the performance of CBTC systems.(3)In order to solve the handover problem of CBTC systems in the coexistence environment of WLAN and LTE,a vertical handover algorithm based on price mechanism is proposed.The relationship between trains and base stations is considered by auction theory,and a mathematical model of train preference and base station preference is established.The available data transmission rate,the power allocation of base station and the currency cost are used as the handover criteria to evaluate the processing ability of candidate nodes and detect signal strength for dynamically choosing the optimal network.The simulation results show that the proposed game strategy can maximize the total revenue of both auction parties and adapt to the dynamic changes of train-to-wayside communication networks.(4)As for the handover problem of train-to-wayside communication of CBTC systems in the environment of VLC,an image-based handover algorithm is proposed.Due to the inherent shortcomings of WLAN and LTE,the deep research and application of VLC has attracted much attention,which can provide some ideas for the network convergence of train-to-wayside communication and the interoperability of CBTC systems.After the analysis of the handover process of train and the model of VLC,a probability algorithm based on distance is proposed to determine the handover time of train in order to maximize the signal quality and achieve a higher successful handover rate.The simulation results show that the proposed algorithm has better effect in improving signal quality and successful handover rate and is much more suitable for the operation scenarios of tunnuls for urban rail transit.