| Natural disasters such as earthquakes have a great threat to the transportation order and operational safety of railways.Especially for high-speed railways,the impact of natural disasters is more prominent due to the high running speed and density.It is an important means to prevent or mitigate the impact of natural disasters on train running safety by releasing emergency disposition information quickly and reliably based on wireless communication,and triggering train emergency disposition measures by linkage.Natural disasters such as earthquakes usually have the characteristics of strong suddenness,wide coverage,great destructiveness and short duration.The accuracy and real-time of emergency disposition information transmission directly affect the effect of disaster prevention and mitigation.The wireless communication between trains and ground has the characteristics of open channel,high-speed movement of mobile terminal and vulnerability to external interference.Thus,it is difficult to improve the timeliness and accuracy of wireless data transmission.Based on the actual needs of emergency disposition information release,this paper studies the theory and method of reliable transmission of emergency disposition information applicable to China's railway,and solves the key problems of train real-time positioning and emergency disposition information transmission.The main innovations and contributions of this paper are as follows.(1)Aiming at the problems of large delay and insufficient bandwidth of existing railway wireless data communication,a software-defined railway wireless communication network architecture is proposed,which separates the control plane from the data plane,enables heterogeneous network interoperability among operators,provides user logical isolation using virtualization,and provides transparent data transmission for users.According to the characteristics and requirements of railway application services,three service modes are proposed,which include: link optimization,link aggregation and link redundancy.An improved intelligent water drop routing algorithm is proposed based on dynamic network topology model.(2)A wireless fingerprint positioning method based on railway wireless communication network is proposed.Firstly,high-speed trains are used to collect information such as the received levels,timing advance of wireless communication system and kilometer posts along the railway.The timing advance is mapped as the propagation loss of radio waves based on the path loss model;thus,the wireless fingerprints are generated and the database is established.Secondly,the signal space is dynamically divided according to the characteristics of the train trajectory,which can be used to constrain the wireless fingerprint search range.Then,a unified dimension wireless fingerprint similarity calculation model is established.Thirdly,the weighted k-nearest neighbor algorithm is used to search the wireless fingerprints matching with the train to be located in the wireless fingerprint database,and calculate the location of the train.Finally,since the train running can be regarded as uniform motion in a short time,the Kalman filtering algorithm can be used to further optimize train positioning.The test equipment is installed on the high-speed comprehensive inspection train and calculates the difference between the position obtained by wireless fingerprint positioning and actual position of the train.The results show that the average positioning error of the train is 77 m,which can meet the demand of emergency disposition information release.(3)This paper studies the optimization methods of emergency disposition information transmission in two modes of GSM-R packet-switched data and cell broadcast short message.In packet-switched data mode,the queuing model is used to build the radio link control layer delay model,and GPRS network operation modes such as radio link control,cell reselection,and radio resource management are proposed.Furthermore,transport layer protocol operation modes and algorithms such as timeout and retransmission,keep-alive timer,and delayed transmission are proposed.The influence of different operation modes on data transmission performance is analyzed.In cell broadcast short message mode,the mechanism and distribution characteristics of the transmission delay are theoretically analyzed,then the transmission delay is reduced by optimizing the signaling receiving manner of the mobile terminal.The simulation systems are established to test the key technical indicators of data transmission in packet-switched data mode and cell broadcast short message mode respectively.(4)In order to enable the railway mobile communication network to provide better services for emergency disposition information transmission in terms of carrying capacity,service quality and disaster tolerance capability,the wireless access network architecture and wireless coverage design are studied.Then,a centralized railway wireless access network deployment scheme and a wireless access scheme for on-board mobile terminal using mobile relay nodes are proposed.(5)In view of the link resource allocation problem in software-defined railway wireless communication network,a network resource scheduling model based on Markov decision process is proposed.Taking into account various factors such as the infrastructure network status and the user requirements,the model searches for the optimal link allocation with the optimization goal being to satisfy user demand and dynamically adapts to the user requirements for the network resources.Tests show that the proposed link resource allocation strategy enables flexible customization of user demand,and improves the network service quality and resources utilization efficiency.(6)Aiming at the problem that the emergency disposition information has strict requirements on the delay,success rate and other indexes of mobile data communication,a parallel redundancy protocol for railway wireless communication network is proposed.The protocol transmits multiple copies of the same packet through different paths.By introducing a redundancy adaptation layer into the traditional protocol model,multi-link data parallel redundant transmission is realized and transparent to the network layer and above.At the same time,according to the requirements of redundant data elimination in receiving entities,an improved Bloom filter is proposed.By expanding the bit array and using the countdown mechanism,the data set elements can be dynamically inserted,deleted and retrieved according to time.Theoretical analysis and test results show that the proposed protocol can improve the wireless transmission performance of emergency disposition information,and has the advantages of simple deployment,high reliability and stability. |
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