LTE-U based Train to Train Communication System in CBTC ...

Research ArticleLTE-U based Train to Train Communication System in CBTC:System Desin and Reliability Analysis

Hao Liang ,1 Hongli Zhao ,2 Shuo Wang ,1 and Yong Zhang 1

1School of Electronics and Information, Beijing Jiaotong University, 100044 Beijing, China2State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, 100044 Beijing, China

Correspondence should be addressed to Hongli Zhao; [email protected]

Received 23 August 2020; Revised 8 December 2020; Accepted 14 December 2020; Published 31 December 2020

Academic Editor: Changqing Luo

Copyright © 2020 Hao Liang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Communication-Based Train Control (CBTC) system is a critical signal system to ensure rail transit’s safe operation. Comparedwith the train-ground CBTC system, the train control system based on train-to-train (T2T) communication has the advantagesof fast response speed, simple structure, and low operating cost. As the core part of the train control system based on T2Tcommunication, the reliability of the data communication system (DCS) is of great significance to ensure the train’s safe andefficient operation. According to the T2T communication system requirements, this paper adopts Long-Term Evolution-Unlicensed (LTE-U) technology to design the DCS structure and establishes the reliability model of the communication systembased on Deterministic and Stochastic Petri Nets (DSPN). Based on testing the real line’s communication performanceparameters, the DSPN model is simulated and solved by π-tool, and the reliability index of the system is obtained. The researchresults show that the LTE-U-based T2T communication system designed in this paper meets the train control system’s needs forcommunication transmission. This paper’s reliability evaluation method can complete the reliability modeling of train controlDCS based on T2T communication. The research in this paper will provide a strong practical and theoretical basis for thedesign and optimization of train control DCS based on T2T communication.

1. Introduction

Today, the CBTC system has been widely used in urban railtransit systems [1]. The existing CBTC system is technicallyvery mature. However, there are still certain shortcomings:the system structure is involved, which leads to too manyinterfaces between subsystems; the information transmissionprocess is cumbersome, and the on-board equipment cannotdirectly obtain relevant information, which limits theincrease in train speed; the system consists of more equip-ment, resulting in higher system construction and operatingcosts, which is not conducive to the large-scale developmentof urban rail transit [2].

As an improvement of the train control system based ontrain-to-ground communication, the train control systembased on T2T communication has many advantages: simplestructure, short transmission time, strong system flexibility,and low operating cost. Therefore, train control systemsbased on T2T communication have become an important

development direction. Alstom implemented a streamlinedCBTC system on line 1 of Lille, France, and first proposed atrain control system based on T2T communication. Thereis no operating line of the train control system based onT2T communication in China, but there have been manyrelated studies. Lin and others introduced T2T communica-tion technology based on the existing train control systemto achieve train collision protection, designed the overallarchitecture of the system, and analyzed the simplified sys-tem through the reliability block diagram [3]. Guan et al.conducted a series of detections on the T2T communicationchannel and investigated the influence of train operation onthe communication channel [4]. Liu compared the new traincontrol system with the traditional CBTC system in terms ofstructure, performance, maintenance, and backup mode andobtained the system [5]. These research results provide basictheoretical knowledge for the design of the train control DCSbased on T2T communication. However, the existingresearch focuses on the performance of the communication

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system, and there has not yet been comprehensive systematicresearch on the reliability of T2T communication.

The train control DCS based on T2T communicationreduces the system coupling. It simplifies the train communi-cation process, thereby significantly reducing the DCS com-plexity and improving the system’s efficiency. Also, thewireless communication technology used in the DCS has amore significant impact on the data transmission processbetween train control subsystems related to the quality ofcommunication. Compared with wireless local area network(WLAN) technology, LTE technology based on the 1.8GHzlicensed frequency band has strong anti-interference ability,complete mobility management function, high transmissionrate, and low system delay. Now, it is successfully applied tourban rail transit [6]. However, with the advancement ofsmart urban rail applications, the contradiction betweeninsufficient authorized spectrum resources and urban raildata transmission requirements is becoming more danger-ous. As a supplementary technology to LTE, LTE-U aggre-gates authorized frequency band and unlicensed frequencyband resources and has many advantages in licensed fre-quency bands. Simultaneously, the use of unlicensed fre-quency band resources can effectively alleviate the pressureof tight spectrum resources to meet the future developmentneeds of urban rail transit.

LTE-U technology was first proposed at the 3GPPRAN62 summit in 2013, and 3GPP introduced it as a criticaltechnology in LTE R13 [7]. Qing analyzed the technicaladvantages of LTE-U in applying urban rail transit throughthe professional research of LTE-U [8]. Bai researched theapplication of LTE-U in the urban rail transit vehicle-ground communication system, and its wireless resourcescheduling management and mobility management algo-rithms [9]. At present, the research on the LTE-U DCS ofurban rail transit is in its infancy, so it is of considerable sig-nificance to study the design and reliability of train DCSbased on LTE-U.

The reliability of the DCS is of great significance forensuring the safe and efficient operation of trains. The exist-ing set of sophisticated research methods for the reliabilityresearch of the train control DCS based on train-to-groundcommunication is not entirely suitable for the reliabilityresearch of the train control system based on T2T communi-cation. Besides, the existing model parameters in the reliabil-ity research of the DCS are almost all hypothetical, and thesystem performance parameters are not obtained from thereal train control DCS. The main contributions of this paperare as follows:

(i) According to the requirements of the train controlsystem based on T2T communication, using LTE-U technology, a train control DCS based on T2Tcommunication is designed

(ii) According to the technical characteristics of LTE-U,and the unique needs of train control, the reliabilityof train control DCS based on T2T communicationis fully defined. In order to evaluate the reliabilityof the system, this paper focuses on the analysis

of typical communication scenarios for T2Tcommunication

(iii) Use DSPN to model T2T communication scenariosduring train communication. Through the commu-nication performance parameter test in the real lineenvironment, the reliability of the T2T communica-tion scene is obtained by using the π-tool simulation,and the influence of the communication limit timeon the reliability of the communication system isstudied

The rest of this paper is organized as follows. Section 2introduces the design of the T2T communication systembased on LTE-U. Section 3 proposes the reliability definitionof train control DCS and analyzes and models typical scenar-ios. In Section 4, reliable performance parameters are obtainedby testing the communication link of the actual line. Section 5examines the reliability of the train control DCS based onthe exact line performance parameters. Section 6 summa-rizes the paper.

2. The Design of Train Control DCS

2.1. LTE-UWireless Data Communication Technology. In theearly days, the DCS of CBTC used WLAN technology basedon the IEEE802.11 series protocols to carry out two-wayinformation transmission between train and ground [10].Although the data wireless communication system based onWLAN can meet traditional urban rail transit requirementsfor availability, reliability, and safety, it can no longer meetthe rapid development of urban rail transit operationrequirements in the future. Its limitations mainly includelimited operating speed and coverage. The distance is short,the link design is complicated, and wireless interference isdangerous.Whenmultiple services are concurrent, it is impos-sible to schedule resources according to the priority [11].

LTE-M is an LTE system based on LTE wireless commu-nication technology and customized according to the busi-ness needs of urban rail transit [12]. The LTE-M systemfully considers the reliability and real-time requirements ofurban rail transit business and combines the advantages ofthe LTE network to form a systematic and standardized solu-tion, which makes up for the disadvantage of WLAN tech-nology carrying CBTC service, and provides innovativetechnical support for the safe operation of urban rail transit.However, at present, LTE-M systems occupy the proprietaryfrequency band of 1785MHz-1805MHz [13] and are onlyallowed to use 20MHz bandwidth resources at most. At thesame time, CBTC services adopt dual-network redundancymode, and their single-network services usually need5MHz bandwidth, so integrated bearer service networksoften use 15MHz bandwidth resources at most. In addition,if the frequency transfer method is used to solve the interfer-ence problem of the public-private network, the wirelessresources available in the urban rail transit system will befurther reduced, and the comprehensive carrying servicenetwork is easy to block the network capacity. Reduce theperformance of the communication system. Reduce the

2 Wireless Communications and Mobile Computing

performance of the communication system. Therefore, newtechnology is needed to solve the problem of lack of spectrumresources to meet the growing needs of urban rail transit.

LTE-U aggregates authorized band and unlicensed bandresources through carrier aggregation (CA) technology andintroduces it into urban rail transit DCS to supplement thelimited spectrum resources of the LTE-M system. While theauthorized band provides reliable and stable transmission,an unlicensed band provides high throughput transmissionto meet the future development needs of urban rail transit.LTE-U inherits many advanced technologies of the LTEsystem and introduces its unique channel sharing strategy.LTE-U is the same as LTE-M in network architecture. Still,the frequency band of the RF work unit is different, includingEvolved Packet Core (EPC), Base Band Unit (BBU), RadioRemote Unit (RRU), and User Equipment (UE).

At present, there are mainly three bands of unauthorizedspectrum available for LTE-U, namely, the 2.4GHz group forthe industry, science, and medicine, the 5GHz band for unli-censed international information facilities, and the newlyproposed 28~60GHz millimeter-wave band [14]. Therefore,in the application of urban rail, LTE-U works in the 5GHzgroup, which will have abundant unauthorized spectrumresources and have a good application prospect. In order torealize the integrated service bearer with a high transmissionrate and real-time requirements in the high-speed operationscenario, LTE technology is introduced into the 5.8GHzopen frequency band, that is, LTE-U can achieve full high-speed data transmission of nonsecure information withhigher security and environmental adaptability based onmore precious frequency resources and higher spectrumefficiency [15].

As a new technology supported by 3GPP and MulteFirealliance, LTE-U technology has obvious technical advantagesin urban rail transit applications, such as excellent mobilityand handoff performance, small transmission delay, stronganti-interference ability, large coverage radius, good QoSguarantee, high security, and high reliability. Therefore, theapplication prospect of LTE-U technology in urban rail tran-sit is broad.

2.2. Train Control System Based on T2T Communication. Thetrain control system based on T2T communication simplifiesthe structure of train control system based on train-groundcommunication, reduces the ground equipment Zone Con-troller (ZC) and Computer Interlocking (CI), integrates itsfunctions into on-board equipment, and adds TrainManage-ment Unit (TMU) and Object Controller (OC). The basicstructure is shown in Figure 1.

In the process of train operation, the train runs accordingto the operation plan issued by the Automatic Train Supervi-sion (ATS). When it is about to enter the next section of theline, the train actively sends a communication request to theTMU of the section, registers the identity information, andqueries other train information in the line. TMU stores theinformation on the car and responds to the query of the train.According to the identity information obtained, the trainestablishes the communication between train and train andrequests the location information of other trains. The train

uses the speed measurement and positioning equipmentand the transponder to determine the position of the trainon the line, map it to the electronic map, and judge the logicalrelationship between the front and rear trains.

The on-board equipment calculates the endpoint of theMovement Authority (MA) through the interlocking modulein the Vehicle On-Board Controller (VOBC) according tothe driving plan, the status of the signal equipment besidethe track, and the position information of the front train onthe line. The Automatic Train Protection (ATP) module willcalculate the train speed protection curve based on the MA,line information, temporary speed limit, and other informa-tion and update it in real time. Automatic Train Operation(ATO) completes the functions of train traction, speedadjustment, and automatic return under the supervision ofATP. At the same time, the train will check the status of theassociated signal equipment in the route and request the con-trol of the trackside equipment from the OC. OC responds tothe query, returns the state of the device, and transfers con-trol rights. When the train is about to clear from the line sec-tion, the train actively disconnects the communicationconnection with other trains on the line and cancels the traininformation in the TMU.

2.3. The Design of the T2T Communication System Based onLTE-U. Based on the network structure of the LTE-U com-munication system, combined with the structure of the traincontrol system based on T2T communication, a train controlDCS based on T2T communication using LTE-U communi-cation technology is designed. The system structure is shownin Figure 2. ATS interconnects with the core networkthrough the backbone network, and the core network is con-nected with the base station on the line. Trains within the sig-nal coverage of the base station can communicate with ATSthrough wireless communication.

The specific functions of the network elements in theLTE-U system are as follows:

(1) Core network EPC: EPC is a full IP packet core net-work, and all services are switched in the PacketSwitch (PS) domain. EPC core network includesMobility Management Entity (MME), Serving Gate-way (SG), Packet Data Node Gateway (PGW), HomeSubscriber Server (HSS), and other network ele-ments. The primary function of MME is to controlsignaling transmission, authenticate and page the ter-minal, and realize mobility management such ashandover and roaming. SGW routes and forwardspacket data in the system. The functions of PGWinclude checking, filtering and filtering data, andinterworking with networks outside the LTE system.HSS stores and manages user data and authenticationdata, including location information of mobile users

(2) Wireless access network E-UTRAN: wireless accessnetwork E-UTRAN is composed of a plurality ofevolved NodeB (eNodeB). The communication net-work is connected wirelessly by eNodeB users. eNo-deB has the functions of routing, establishing or

3Wireless Communications and Mobile Computing

releasing wireless links, mobility management,scheduling and allocation of wireless resources,bearer control, etc.

(3) Terminal: train installs train access unit (TAU) as theservice data transmission equipment to realize wire-less communication. The TAU is connected withthe functional modules on the train to send the infor-mation that the train needs to upload through theantenna, and then, the information is transmitted tothe ground equipment or other trains. At the sametime, the TAU receives the information forwardedby the base station through the antenna and sends itto the train equipment after processing

3. Reliability Definition and Scenario Models ofTrain Control DCS

3.1. Data Transmission Flows of Train Control DCS Based onT2T Communication. Because the structure of train control

DCS based on LTE-U technology is different from that basedon WLAN technology, the data communication flow is dif-ferent from the previous communication system, so it isnecessary to analyze the information transmission flowbetween train and train. As shown in Figure 3, the solid lineis the data flow, and the dashed line is the signaling flow.

Curve 1 is the signaling flow channel after the train iselectrified. When the train starts, TAU searches for cell sig-nals, selects the cell, and triggers the random access process,ensuring communication links between terminals and net-works. TAU sends a connection establishment request signalto the base station, allocating the radio resources for it.Simultaneously, through the S1-MME port, the base stationsends the attachment request to the MME. After completingthe TAU authentication, the MME sends the attached signalto the base station, and the base station sends the TAU capa-bility information to the MME. Finally, the base station sendsa security activation message, and after TAU activation, itsends configuration completion and attachment completionmessages to the base station and MME.

ATS HSS PGW

EPC

E-UTARN

Terminal

MA

VOBCVOBC TAU TAU

SGWMME

Backbone

eNodeB

Figure 2: Train control DCS based on T2T communication.

Control centreATS

Station localATS

Station localATS

Database storageunit

Backbone network

Basestation

Digitalmap

TMU

MA

ATP/ATOZC

Digitalmap MA

ATP/ATOZC

Digitalmap MA

ATP/ATOZCVOBC VOBC VOBC OC

Figure 1: Basic structure diagram of the train control system based on T2T communication.


Curve 2 is the data flow channel of train communicationunder different base stations. The transmitting terminalsends messages to the base station through the empty port,and the base station forwards the messages to the SGWthrough the S1-U port. After the SGW inquiry, the messagesare forwarded to the base station in the cell where thereceiving terminal TAU is located and then to the receivingterminal TAU.

When the train reaches the overlap of base stationsignals, the service area has to be changed. Curve 3 is thechannel of signaling flow based on X2 port switching. First,the source station sends measurement control down toTAU, which feeds back measurement reports. The sourcebase station decides to switch based on the measurementreport and then sends the switching request to the destina-tion base station. After receiving the acknowledgment signal,the source base station sends configuration information toTAU. After the TAU configuration is completed, the sourcebase station begins switching and sends the connectionsuccess information to the destination base station. The basestation requests MME to update the data channel. TAUreleases the cell resources under the source base station tocomplete the switch between stations.

3.2. Reliability Definition of Train Control DCS. The reliabil-ity of train control DCS affects the probability of a successfulexchange of train information, so it is vital to use reasonablemethods to evaluate its reliability. Based on the communica-tion performance parameters, this paper defines the reliabil-ity of DCS from two links.

The first link is the reliability of data transmission. Thereliability of data transmission is defined as the probabilitythat the available information reaches the receiving endwithin a limited time. Both transmission delay and continu-ous packet loss affect the reliability of data transmission.According to the definition of reliability, as shown inFigure 4, (a) the packet delay t1 is greater than the limit timeT ; then, the transmission of packet 1 is unreliable. Otherwise,the transfer of packet 2 is reliable; (b) when a continuouspacket loss occurs, the duration of no data reception at thereceiving end can be calculated according to the packet send-ing rate. If the term is greater than T , the data transmission isnot reliable at this time.

The second link is the reliability of the communicationscenario. During the train operation, communication pro-cesses such as data retransmission, switching between sta-tions, and interrupt reconnection are required. Therefore, itis stipulated that the train needs to complete the communica-tion process within the communication limit time. After thecommunication limit time is exceeded, the train will be inan unsafe operation state, and the reliability of the communi-cation scenario is the probability that the train is in a safestate.

Since the existing reliability studies are mostly based onassumed parameters, in order to make the data communica-tion system reliability research results more in line with theactual situation, the parameters of the model in this paperare derived from actual communication performance testresults or the requirements of the communication system in

the urban rail transit communication specification, and theanalysis results have higher practical significance.

3.3. Typical Scenario Analysis of T2T Communication. In thetrain control system based on T2T communication, the com-munication between trains is the primary demand, amongwhich the communication scene of redundancy train accessunit and roaming switching communication scene are typicalcommunication scenes in the process of train operation.

3.3.1. Communication Scenarios of Redundancy Train AccessUnits. T2T communication is an important communicationlink in the process of train operation. However, the channelenvironment changes caused by geographical location,weather conditions, and other factors will reduce the commu-nication quality, affect the real time and availability of the datareceived by the train, and then affect the regular operation ofthe train and the passenger’s riding experience. In the existingresearch, it is a standard method to improve communicationreliability using equipment redundancy. Therefore, the com-munication scenario of the redundant train access unit is typ-ical in the T2T communication scenario.

As shown in Figure 5, two sets of train access units andon-board antennas are, respectively, configured at the headand tail ends of the train, and the two sets of communicationequipment work independently and do not interfere witheach other. Therefore, only one set of train access units cansuccessfully receive reliable data within the specified time toensure the train’s regular operation. After receiving the infor-mation, the two sets of train access units process the informa-tion and obtain the available information required by on-board equipment, including train position information andobstacle information. Simultaneously, if the trackside equip-ment status and route information are obtained, the logicmodule of on-board equipment can calculate according tothe relevant information to obtain the train operation per-mission terminal point. The emergency braking curve andautomatic driving curve are calculated by the ATP andATO logic module of on-board equipment, and the train isin normal operation.

3.3.2. Communication Scenarios of On-Board TerminalHandoff. Train cross-line operation is an inevitable require-ment for the development of urban rail transit. It is necessary

HSSPGW

TAU TAU TAU TAU TAU

SGWMME

eNodeB

Line 1Line 2Line 3

Figure 3: Signaling transmission flow and data transmission flow.


to ensure that the train can achieve on-board terminal roam-ing, realize interconnection, and save train resources. Whenthe train is in the process of roaming switching, other trainson the line still need to communicate with each other. Theroaming switching process has a more significant impact onthe communication delay.

As shown in Figure 6, on the existing operation line,one set of communication equipment is used on the frontline due to the construction planning. In contrast, anotherset of communication equipment is used on the follow-upline. When the train runs to the junction of two sets ofcore network equipment, it needs to carry out roamingswitching.

As shown in Figure 7, when the train is roaming switch-ing, the information sent by the ground equipment andother trains will be forwarded to the destination core net-work through the source core network and then to the basestation of the switching train. After the roaming switching iscompleted, the train will obtain the information of theground equipment and other trains on the line through theforwarding of the destination core network, which willincrease large transmission delay. When the train crosses aline, the running interval of the line will change, and thetrain speed will be limited. To improve operation efficiency,it is necessary to quickly adjust the relevant parameters oftrain operation, so it is necessary to ensure the communica-tion between trains.

3.4. DSPN Models of T2T Communication Scenarios

3.4.1. Deterministic and Stochastic Petri Net. The traincontrol data DCS is a complex dynamic system, and its datacommunication has large randomness and high correlationwith time. Therefore, this paper uses Deterministic andStochastic Petri Net (DSPN) to model the train controlDCS based on its data transmission characteristics.

A system of DSPN can be defined as a decimal group: Σ= ðS, T , I,O,H, Y , K , λ,W,M0Þ. The store holds tokens,representing a network element of the system or a step inthe process. In a DSPN, the maximum number of tokens thata store can hold can be set, the number of elements in the setK for that store. Therefore, the state of the Petri net can bedefined by the number of tokens in each store, as

m = # p1ð Þ, # p2ð Þ,⋯, # pkð Þð Þ, ð1Þ

wherem is also called the identification vector of the Petrinet. Each element is the number of tokens in the store p, andit must satisfy MðpÞ ≤ KðpÞ. The set of m is the set of systemstates.

In Petri nets, the triggering of changes represents eventsin the actual system, causing the system to transition fromone state to another. Moreover, in stochastic Petri nets, tran-sitions have implementation delays, and implementationdelays are associated with a particular distribution, and theset of transition implementation rates is λ. In DSPN, accord-ing to different implementation delays, there are threechanges:

(1) The implementation delay is zero. Such a transition iscalled an immediate transition, indicating a control-ling role or a logical choice that does not requireprocessing time

(2) The implementation delay is a random variable. Thiskind of vicissitude is one kind of time vicissitude,which is called exponential vicissitude. It indicatesthat the event takes a certain amount of time, andthe distribution of time follows an exponential distri-bution: FðtÞ = 1 − e−λtλ > 0

(3) The implementation delay is a definite value. Thiskind of transition is called fixed-time transition,which means the implementation delay of the transi-tion corresponds to the processing time in the realsystem

13 24567

13 24567t2t1

Time T

(a) Unreliable transmission caused by delay

Received packet

Lost packet

Time T

(b) Unreliable transmission caused by packet loss

Figure 4: The definition of data transmission reliability.

ATS HSS

MA terminus

TAUTAUTAUTAU

PGW SGWMME

Backbone

eNodeB

Figure 5: Communication scenario of redundancy train accessunits.


Besides, the Petri net visual simulation tool π-tool is usedto establish a communication model and simulate [16–18].The π-tool can realize the establishment of many kinds ofPetri net models. It is a perfect modeling and simulation tooland has many advantages:

(i) Model building: the implementation rate of transi-tions in the software can be set to a variety of com-mon distributions. For immediate transitions, theweight and priority of the transitions can be set.Ability to build structured models

(ii) Model analysis: the Petri net model can be deadlockedto detect system resource allocation and consumption.The reachability graph can be automatically generatedaccording to the model to determine the state spaceand state transition of the system. Using Monte Carlosimulation analysis, the Petri net model with anytransition can be analyzed to obtain the occupancyrate of the store, and a certain transition can bespecifically observed to determine the time when itis first triggered

In the π-tool graphical interface, each element of theDSPN is graphically represented, as shown in Table 1.

3.4.2. Models of Communication Scenarios for RedundancyTrain Access Units. The model is shown in Figure 8. First,the train is in the typical running state run, and then triggerstransition GenMsg to start data transmission. At the sametime, triggering transition trans, timer timer is started. Thistimer is the communication limit time for the train to com-plete an individual link. After that, the token is transferredto the store train0, which means that the information istransmitted to another train, triggering the transition eNo-deB, and the token is transferred to the store train, whichmeans that the data is transmitted to the base station. At thistime, there are two situations. The train is in the process ofswitching between stations or interrupting the reconnectionprocess. The transition handover indicates that the train is

performing the switching process. The stores HOX2-TAU1.Msg and HOX2TAU2.Msg are the switching processesrequired by the two train access units. The transitioninterrupt means that the train’s reconnection process isperformed, and the stores AttachTAU1.connection andAttachTAU2.connection are the reconnection processrequired by the two train access units. This paper considersthe most unfavorable situation: the handover and reconnec-tion processes need to start from the initial signaling.

In this communication scenario, if both train access unitsneed to switch between stations, and in the case where onetrain access unit switches successfully, the switching time ofthe other train access unit is no longer limited. The methodfor judging whether the reconnection of the two train accessunits is successful is the same as the handover. Only one ofthe train access units completes the reconnection processwithin the communication limit can be regarded as a success-ful reconnection. Taking the first train access unit success-fully switching or reconnecting as an example, theswitching or reconnecting submodel transfers the token tostore success. At this time, through the limiting effect of thetest arc, stores AttachTUA2.fail and HOX2TAU2.fail willobtain the cause the token generated by the transition fail-TAU2 that triggers the second train access unit to switch orreconnect the token in the submodel, and the model can startthe next information transmission. However, if the timer hasreached the communication limit time during the handoveror reconnection, it is judged that the handover or reconnec-tion is unsuccessful. At this time, all tokens in the submodelare cleared, and the system state changes from normal todanger until the next time the train successfully completesthe handover or reconnection within the specified time; thesystem status changes to normal.

3.4.3. Models of Communication Scenarios of On-BoardTerminal Handoff. Figure 9 shows the DSPN model. This isthe top-level model. There are three bottom-level submodelsunder the top-level model, namely, the HOS1, Transmission,and TAUpd submodel. When the train runs, it generatesinformation that needs to be sent to other trains, that is, thetoken is transferred from the store run to Msg. At this time,the transition trans is triggered, and the token is transferredto the timer timer, which limits the time of the communicationprocess. At the same time, a token is transferred to the storetrain0 and data transmission begins. The transition eNodeBtrigger indicates that the data has been transmitted to the basestation. In the most unfavorable situation, another train needsto complete all interactive processes of roaming switching.

First, the train performs the cross-core network switchingprocess, and the token is transferred to the submodel HOS1through the associated store HOS1.handover, and the storePN.HOsucces representing the completion of the switch inthe submodel is associated with the HOsuccess store in thetop model. After the submodel process is completed, thetoken is transferred toHOsuccess. Afterwards, the data is for-warded. The data is forwarded from the source core networkto another core network. The store Transmission.data isassociated with the store data in the submodel Transmission.In the submodel, after completing the process of data

PGWSGWMME PGW SGW

MMEATS

HSS

MA terminus

TAU TAU

Backbone

Figure 6: Train roaming communication scenario.


PGWSGWMME PGW SGW

MMEATSHSS

TAUTAUTAUTAU

Backbone

Figure 7: Data forwarding process.

Table 1: Graphic elements and their meanings of DSPN.

Element Symbol Meaning

StoreThe store in the hierarchical model

The store associated with the previous or next level

Token Resources in the store and their quantity

Transition

Exponential transition

Fixed time transition

Immediate transition

Normal arc Directed arcs, input and output functions between locations and transitions

Suppression arcRestrictions on transition, when there are a number of tokens corresponding to the arc weight in the store

onnected to the transition use prohibition arc, the transition prohibition triggers

Test arcRestrictions on the transition, when there are a number of tokens corresponding to the arc weight in the store

connected to the transition test arc, the transition is allowed to trigger

Next trans Run

GenMsg

Msg

Trans

Train 0

Train

eNodeB

Interrupt

Handover

AttachTAU2. connetion

AttachTAU1. connetion

HOX2TAU2.Msg

HOX2TAU1.Msg

Timer

HOX2TAU1.fail

AttachTAU1.fail

HOX2TAU2.fail

AttachTAU2.fail

Failtrain0

Retrans

NormalSuctrans

DangerSuccess

FaitransFail

Timeout

sucTAU1

sucTAU2

SuccessTAU1

SuccessTAU2

AttachTAU2.fail

AttachTAU1.fail

failTAU2 HOX2TAU2.fail

failTAU1 HOX2TAU1.fail

Figure 8: Models of communication scenarios for redundancy train access unit.


transmission and retransmission, the token enters the storetranssuccess in the top model through the associated storePN.transsuccess, indicating that the data forwarding processhas been completed. Finally, by triggering the transitionTAUstart, the tracking area update process begins. Similarly,the token enters the store TAU in the TAUpd submodelthrough the associated store TAUpd.TAU. After the tokencompletes the tracking area update process in the signalinginteraction sequence in the submodel, the token is trans-ferred to the TAUsuccess store in the top model through theassociated store PN.TAUsuccess. If the cross-core networkhandover process, data forwarding process, and tracking areaupdate process are completed within the communicationlimit time, the next round of information transmission canbegin. However, if the train fails to complete the roaminghandover process when the timer reaches the communica-tion limit time, indicating that the cross-core network hand-over fails, the top model triggers the timerout transition tocause orders to appear in the associated stores HO21.fail,TAUpd.fail, and Transmission.fail of the three submodelsCard and then empty the token in the submodel. At the sametime, the system state changes from normal to danger untilthe next successful roaming switching process; the systemstate changes to normal.

4. Communication Performance Test of TrainControl Data Link

The transmission performance of the train control data linkis very critical and affects the reliability of communication.

The existing reliability studies are mostly based on assumedperformance parameters. Therefore, to make the researchresults of DCS reliability more in line with the actual situa-tion, the model parameters in this paper need to be derivedfrom actual communication performance test results orurban rail transit communications, the requirements for thecommunication system in the specification; such analysisresults have higher practical significance. To determine thecommunication quality of the wireless link and obtain thetransmission performance of the link during train operation,this paper is based on the actual operating line of Beijingmetro line 15 to test the communication performanceparameters of the link. The test section of the operating lineis the 2 stations 1 section upstream section (including thestation line) from Maquanying to Sunhe, a total of 3.3 km,of which the 2.3 km section east of Maquanying Station isan underground tunnel, which is immediately adjacent tothe viaduct, with a length of 1 kilometer to Sunhe Station,as shown in Figure 10.

The LTE-U system test in the test section of line 15mainly includes three parts: communication room centerequipment (EPC/switch, CCTV/PIS ground equipment),trackside base station equipment (8 sets), and on-boardequipment (TAU/switch). Test networking is shown inFigure 11. Among them, base station 1 to base station 5 arecovered by antennas with a distance of 400~450m, one sideof base station 6 is covered by an antenna, a slotted wave-guide covers the other side, and base station 7 and base sta-tion 8 are covered by the slotted waveguide.

First, test the data transmission delay and handover delaywhen the train is running. During the train operation, thetrain is within the signal coverage of a base station for a spec-ified period. At this stage, the data transmission delay is onlydetermined by the network environment and channel qual-ity. When the train travels within the signal overlap rangeof the two base stations, the handover mechanism, handoverbetween stations is required, and handover delay occurs.When the train is running on the line, seven handovers occurin a single journey, and seven handover delay data can beobtained. To obtain more handover delay data, the train runsback and forth between stations, and the handover delay ismeasured multiple times.

Figure 12 shows the delay and handover delay resultsobtained in a one-way test. At certain moments, the timedelay will be significantly increased. The test time in thefigure can be divided into eight stages, with a higher delayas the demarcation point. Each stage indicates that the trainis within the signal coverage of an individual base station,corresponding to eight stages. For the signal coverage of eachbase station, a higher delay indicates that the train is at thejunction of the base station’s signal coverage. Therefore, thereare seven handover delays in the figure. In the subsequentanalysis, the delay and handover delay in all test data mustbe separately counted.

In order to avoid mutual influence when measuring thedelay and packet loss at the same time, the delay and packetloss tests are carried out separately. To ensure sufficient datavolume, the train runs multiple times to test and obtainpacket loss data. If the software operation is interrupted, it

NormalFail transSuc trans

Danger

Success Next time Run

GenMsg

RetransFailMsg

TransTimerTimer out

HOS1.fail

TAUpd.fail

Transmission.fail

Failtrain0

Achieve

Train0

eNodeB

HOS1.handover

HOsuccess

Msgtrans

Transmission.data

Transsuccess

TAUstart

TAUpd.TAU

TAUsuccess

Figure 9: Models of communication scenarios of on-board terminalhandoff.


will affect the accuracy of the test results. Therefore, discardthis set of data and perform it again to ensure the accuracyof the measured data. In the actual test, the packet loss ratetest result is 0.497%. Figure 13 shows the test result of contin-uous packet loss.

5. The Reliability Analysis of Train Control DCS

5.1. Reliability Analysis of Communication Scenarios forRedundant Train Access Units. This paper uses the MonteCarlo simulation method, and according to the reliabilitydefinition of the communication process in 3.1, under the

condition that the parameter value of the fixed transitiontimeout is different, the model is simulated. In practice, theparameter value of timeout represents the time limit of thesignaling process performed on the train, from which thereliability under different communication time limits can beanalyzed. The simulation obtains the reliability when thetrain is switched or reconnected separately under differenttimeout parameters.

According to the simulation results, as the timeoutparameter value becomes more extensive, the communicationscenario’s reliability gradually increases. When the parametervalue is 990ms, the reliability of the communication scenario

Starting station

Terminal station

Total length3.3 km

2.3 km

Citee Taoyuna club

1.9 km

Figure 10: The test section of Beijing metro line 15.

Control center PIS/CCTVGround equipment Core network Simulation test terminal

(IxChariot)

Trackside

MaquanyingStation

Router

SunheStation

AirNode1 AirNode2 AirNode3 AirNode4 AirNode5 AirNode6 AirNode7 AirNode8

Tunnel section Elevated coverage

Waveguide

Basestation

Electricbridge

Directionalantenna

Train Train internal antenna

Train bottom antenna

TAU TAUPIS/ CCTVvehicle equipment

Train internal antenna

Train bottom antenna

Figure 11: 5.8G system network structure diagram of line 15.


for train switching reaches 99.9928%, while when the param-eter value is 1050ms, the reliability of the communicationscenario for train reconnection can reach 99.9902%. Simulta-neously, the results show that the reconnection processreduces train communication’s reliability to a certain extent,

affecting the communication between the train and othertrains.

As a comparison, consider the reliability of the com-munication scenario when the train has only one trainaccess unit. According to the simulation results, when the

0.10

0.05

Seco

nds

0.000:00:00 0:01:00 0:02:00 0:03:00

Elapsed time (h:mm:ss)

0:04:00 0:05:00 0:06:00

Figure 12: Test results for transmission delay in the real environment.

70

60

50

40

30

20

10

01 2-3 4-5 6-10 11+

Number of consecutive lost packets (a)

Perc

enta

ge (%

)

Figure 13: Test results for consecutive lost datagrams in the real environment.


communication limit time is short, whether it is a switchingprocess or a reconnection process, the train access unit’s reli-ability is greatly improved when the train access unit isredundant. As it becomes larger, the reliability improvementrate gradually decreases.

Without considering the cable transmission delay andretransmission, the time for handover and reconnection pro-cess is related to the wireless transmission delay, and its timeconforms to the Gamma distribution, so it can be solved byits density distribution function to complete the wirelessunder different communication limit time, the probabilityof transmission. The density function is shown in

f tð Þ = tα−1λαe−λt

α − 1ð Þ! , t > 0, ð2Þ

where α is the number of signaling that needs to be transmit-ted using the air interface during the signaling interaction.

In many cases, the reliability of the communication sce-nario is shown in Figure 14. It can be obtained that the trainaccess unit has better communication reliability when thereis redundancy, and its reliability can reach 99% when thecommunication limit time is 960ms, and the reliabilityexceeds 99.99% when the communication limit time exceeds

1050ms. In the case of nonredundant train access units,the communication limit time needs to be 1230ms whenthe reliability reaches 99%, and when it exceeds 1710ms,the reliability can exceed 99.99%.

5.2. Reliability Analysis of Communication Scenarios for On-Board Terminal Handoff. According to the definition of thecommunication process’s reliability, the probability of thetrain being in the normal state when the parameter value ofthe transition timeout is different is studied. Therefore, theparameter value of the timeout transition in the top modelcan be set to a series of values. Under different parametervalues, the Monte Carlo method simulates the model toobtain the probability that the train is in the normal state.According to the simulation results, the higher the com-munication process threshold, the greater the reliabilityof the communication. When the communication limittime is 1140ms, the reliability exceeds 99%, and whenthe communication limit time is 1300ms, the reliabilityof the train can reach 99.99%. Simultaneously, with theincrease of the communication limit time, the improvedrange of reliability gradually decreases, indicating that theinfluence of the communication limit time on reliabilitygradually decreases.

100

90

80

70

60

50

40

30

20

10

0

Communication limit time (ms)

Best state switching of communication link

Optimal reconnection of communication linkSingle TAU switching processSingle TAU reconnection process

Redundancy reconnection processRedundancy switching process

Relia

bilit

y (%

)

400 500 600 700 800 900 1000

Figure 14: The reliability of communication scenarios of redundancy train access units.


Without considering the delay and retransmission ofcable transmission between ground equipment, the time forthe roaming switching communication process conforms tothe Gamma distribution, and its density function is Equation(2). Figure 15 shows the reliability of the communication sce-nario. It can be obtained that when the communication linkis in the best state and the communication threshold is lessthan 1100ms, the reliability of the communication scenariois higher than the reliability when the link has interference.After the communication limit time gradually increases, thedifference in reliability gradually decreases.

6. Conclusion

This paper combines the train control DCS based on T2Tcommunication with the existing communication reliabilityanalysis methods to study the reliability of communicationscenarios. Based on the structure and principle of the traincontrol system based on T2T communication, the basicstructure of the train control DCS based on LTE-U commu-nication technology is studied. The reliability of train controlDCS is defined. Simultaneously, two critical communicationscenarios of redundancy train access unit and on-board ter-minal handoff are extracted for the T2T communication sce-nario, and the communication process is analyzed in detail.Use the π-tool to build a DSPN model of the scene. Besides,the actual line’s communication performance is tested toprovide more accurate performance parameters for themodel. By setting different communication limit times, thesimulation obtained the reliability of crucial communicationscenarios. The results show that, combined with the reliabil-

ity evaluation method proposed in this paper, DSPN can bet-ter realize the communication system models. The reliabilityresearch of the DCS in this paper provides an essential refer-ence for the design, implementation, and optimization of thetrain control DCS based on T2T communication.

Data Availability

The data used to support the findings of this study are ownedby a company.

Conflicts of Interest

The authors declare that there is no conflict of interestregarding the publication of this paper.

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Relia

bilit

y (%

)

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Disturbed communication linkDisturbed communication link

Figure 15: The reliability of a communication scenario for on-board terminal handoff.


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