automatic train control over lte: design and performance

IEEE Communications Magazine • October 2015102 0163-6804/15/$25.00 © 2015 IEEE

The authors are withKorea Railroad ResearchInstitute.

ABSTRACT

Due to technical advances in train control andwireless communications, unmanned train opera-tion has gained in popularity of late. On theother hand, any errors involved in managing theQoS of train control traffic will cause negativeconsequences such as possible loss of life. Opera-tors therefore naturally wish to scrutinize thespecifications so that the wireless communica-tions system is capable of guaranteeing the QoSof the train control traffic. In this article, we pro-pose a feasible QoS management scheme fortrain control traffic based on the methodologyused in a conventional LTE system. Based on theproposed scheme, we evaluate the feasibility ofthe LTE system using a testbed built in a com-mercial railway region. The key issues to supportthe train control services by the LTE system arethe design of a QoS policy based on analyzingthe characteristics of the train control traffic andthe appropriate adjustment of the cell parame-ters during the cell planning and optimizationprocedures in order to resolve any network issuesthat may cause problems with data pause.

INTRODUCTIONTrain control systems (TCSs) are responsible forall kinds of instructions for controlling train ser-vices at the wayside and train side. TCSs havebeen studied and developed to automaticallyguarantee safety according to the availability ofvarious control technologies. By the end of 1930,locomotive engineers were operating trains bywatching the signal lights on the sides of tracksand then making decisions manually in a low-speed environment. As the operational speed oftrains and the number of operating trainsincreased, there was an unmanageable risk ofcollisions caused by human error. To addressthis, additional safety measures such as automat-ic train stop using beacons or balises weredeployed. This ensured that locomotive engi-neers could not exceed the maximum allowedspeed. After 1980, guaranteeing safety in high-speed environments became of paramountimportance, and many other supplementarydevices were introduced to control trains auto-matically. Automatic train control decides theproper speed based on:

• Information provided by ground controlthrough a track circuit or a loop cable

• The current status of the train• The trackside environment and weather

conditions.More recent research on TCSs has been

aimed at extending automatic train control inorder to achieve the goal of unmanned opera-tions [1, 2]. To this end, the most significantchange was to apply wireless and informationtechnologies to TCSs. Traditional railway com-munications systems, such as track circuits andtransponders, have critical problems in terms ofmaintenance. To solve the problems, the com-munication-based TCS was proposed formetropolitan railways. In this TCS, informationtransfer within the track region was achievedthrough wireless LANs. For high-speed railways,the European Train Control System (ETCS),which uses the Global System for Mobile (GSM)system for wireless communications betweenconventional trackside devices, was commercial-ized in Europe in 2004. Recently, the KoreaRadio-Based Train Control System (KRTCS)project established in 2010 was completed forthe use in all kinds of railway environmentsincluding metropolitan and high-speed railways.

According to this technical trend, TCSs havebecome closely aligned with a wireless railwaycommunications system. The most widely knownsystem is GSM-Railway (GSM-R), which is cur-rently used in conjunction with ETCS in theEuropean commercial field [3]. Moreover, rail-way services have gradually become more tech-nologically advanced, and the demand for datahas continued to increase. In this circumstance,many started to expect Long Term Evolution(LTE) to provide all kinds of railway servicesincluding voice communication, push-to-talk,multimedia-based supervision, and maintenancedata transfer as well as train control. Conse-quently, considerable research efforts have beendevoted to LTE as the next generation of a rail-way communications system [4–9]. Specifically,the feasibility of LTE as railway communicationsin a system aspect was validated in [4–6], andvarious algorithms and protocols were proposedin [7–9] to bring performance improvement ofrailway communications.

Since the fundamental concept of spectrumusage in LTE is quite different from that in GSM,

FUTURE RAILWAY COMMUNICATIONS

Juyeop Kim, Sang Won Choi, Yong-Soo Song, Yong-Ki Yoon, and Yong-Kyu Kim

Automatic Train Control over LTE:Design and Performance Evaluation

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IEEE Communications Magazine • October 2015 103

it is necessary to consider additional technicalissues for providing a train control service basedon LTE. GSM, mainly for narrowband communi-cations, partitions the spectrum into several chan-nels and allocates a channel to each service type.In contrast, LTE uses the entire wideband spec-trum for all kinds of traffic, and data packets ofvarious service types can be mixed in the spec-trum. One of the critical issues in LTE involvesguaranteeing quality of service (QoS) for eachservice. Especially, train control traffic includesvital information, and violation of the QoS fortraffic will cause a problem for train service,which may threaten human safety. Therefore,LTE must be particularly concerned with guaran-teeing the QoS of train control services. Conven-tional research including [10, 11] has consideredQoS issues of voice and data services in LTE, butthere is little research dealing with those of a TCSin the aspect of network management.

The goal of this article is to show the feasibil-ity of LTE to serve TCSs in a practical environ-ment. We aim to provide the parameter designof an LTE-based railway communications systemfor guaranteeing the QoS of train control ser-vices. Based on the architecture for QoS man-agement in LTE, we provide a procedure fordesigning QoS parameters for TCSs. We thenprovide a verification procedure to validatewhether the train control services work in practi-cal scenarios based on a testbed built in a com-mercial railway region, and present performanceevaluation results obtained from the testbed.

TRAIN CONTROL SYSTEM MODELIn this article, we assume that KRTCS is used as aTCS. Figure 1 shows the structure of the waysideand onboard systems in KRTCS. It is composedmainly of the following three components: auto-matic train supervision (ATS), automatic trainoperation (ATO), and automatic train protection(ATP). ATS is used to supervise the overall statusof the train services at the wayside and performremote control. ATO is used for the overall oper-ation control in trains. ATO controls the trainspeed and train stops at regular positions at a sta-tion, issues commands to open and close thedoors, and controls supplementary devices in sta-tions. In addition, wayside and onboard ATPs per-form real-time train positioning and decide movingauthority so that a safe distance between consecu-tive trains can be maintained.

To accomplish its own mission regarding traincontrol services, each of the components inKRTCS communicates with its correspondingcomponent through wired and wireless links overIP. Specifically, ATS communicates with ATO toobtain a train status report and issue commandsfor various operations. Wayside and onboardATPs communicate with each other to share thetrain status and perform various ATP operationsregarding train movement. It is noted that thedata communications between wayside and trainside must pass through the air interface, which iscovered by an LTE system in this article.

ATP traffic should be managed carefullybecause ATP plays a significant role in the safemovement of trains. The main mission of ATP isto move a train safely in a forward direction.

Specifically, based on the train positioning reportfrom the onboard ATP, the wayside ATP orATS issues a moving authority, indicating a mov-ing range in which the train is allowed to pro-ceed, to the onboard ATP. The train shouldspeed down or trigger the brake if it is about toreach the edge of the moving range. In addition,a wayside ATP can cover a limited area of a spe-cific region, and handover is therefore per-formed between wayside ATPs. This procedurecauses a train to be served seamlessly when thetrain crosses the coverage boundary of neighbor-ing wayside ATPs. To accomplish their missions,onboard and wayside ATPs generate traffic andtransfer to each other. Otherwise, problems willoccur in performing their operation.

ANALYSIS OF KRTCS TRAFFICTo define QoS parameters suitable for KRTCS,it is necessary to analyze the characteristics ofKRTCS traffic. Traffic from ATS and ATO areusually bursty at specific moments, such as onstation entry. However, these types of traffic aregenerated rather infrequently and occur whenthe train is in the region of a station, in whichthe signal received from an LTE base station isin good condition. On the other hand, waysideand onboard ATPs generate their traffic contin-uously and periodically while the train is movingover the railway. Thus, ATP traffic can be gener-ated anywhere in the region along the track andat any moment that the received signal environ-ment is not be suitable for wireless communica-tions due to geographical variations. Due to thefact that ATP traffic is the bottleneck in terms ofguaranteeing QoS, it is important to analyze thecharacteristics of ATP traffic for grasping theQoS requirement of KRTCS.

In general, the amount of ATP traffic gener-ated by a single train is maximally 50 kb/s for thedownlink and 20 kb/s for the uplink. Becauserailway operators should make use of dualization

Figure 1. Overall structure of KRTCS.

Po "VFU V

Ground control Ilro station Daebul station

ATS

Onboard LTE UE

Onboard ATP/ATO Onboard ATP/ATO

Onboard LTE UE

Wayside ATP Wayside ATPWayside

Railwaycommunications

Network

Trainside

LTE

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IEEE Communications Magazine • October 2015104

on wireless sections to improve link availability,it is necessary to take into consideration the factthat twice the ATP traffic indeed occurs in theradio access layer. In addition, when handoverbetween wayside ATPs takes place, onboard andwayside ATPs generate duplicate traffic. There-fore, the total amount of ATP traffic is up to200 kb/s for the downlink and 80 kb/s for theuplink.

Similar to a voice service, data generated bywayside and onboard ATPs has a real-time prop-erty, and must be transferred to the other sidewithin a specific time. This implies that delay isan important QoS parameter for ATP traffic justas it is for voice traffic. According to Annex C in[2], the maximum allowable transfer delays oftrain-to-wayside data and wayside-to-train data is0.5–2 s. Based on the requirement, it would bereasonable to set the QoS parameter of thepacket delay budget to 500 ms.

In the aspect of the theoretical cell capacity,it is sufficient for an LTE system to guaranteethe above QoS requirements. However, it stillmatters that the LTE system guarantees therequirements strictly at any time. Because of itsvital functionalities, ATP does not allow the vio-lation of QoS at any point. This indicates that aservice drop event should not occur throughoutvast regions of service, including in tunnels andunder bridges. Since it is desirable that packetloss be rare for ATP traffic, a higher prioritymust be allocated to the transmission of ATPtraffic. From this aspect, QoS management forATP should be differentiated from that for avoice service. Specifically, a voice service allowsa certain amount of data pause in the aspect ofservice continuity, whereas ATP does not allow ashort data pause.

QOS GUARANTEED FRAMEWORKDESIGN FOR KRTCS

PRELIMINARIES ON THE QOS CLASS IDENTIFIER

Since KRTCS traffic has unique characteris-tics and differs considerably from voice, video,or background data traffic, it is necessary todefine a new set of QoS parameters dedicated tothe KRTCS traffic. In general, the LTE standardprovides a guideline for QoS parameters for effi-cient QoS management. The LTE standarddefines typical QoS parameters and provides setsof predefined values of the QoS parameters forfrequently used services. These predefinitionsrelax the complexity of implementation, andallow the functional entities of an LTE system tobe optimized to guarantee the QoS based on theQoS parameters. In other words, it is hard toapply new and customized QoS parameters ormodify the value of a QoS parameter in practice.For QoS management of KRTCS in a practicalsense, we utilize the present status of the feasi-ble QoS parameters in the LTE system.

Typical QoS parameters dealt with in LTEare resource type, priority, packet delay budget,and packet loss rate. Resource type indicateswhether the minimal bit rate is guaranteed ornot. The bearers with guaranteed bit rate havethe minimal bandwidth such that the system isguaranteed in any network condition. Priorityindicates the priority level for a bearer, and isapplied during bearer establishment or modifica-tion. Packet delay budget is the upper bound onthe end-to-end delay in the LTE system, and thisaffects scheduling operation in a base station.Packet loss rate means the maximal rate of pack-et loss at the link layer level.

In fact, the most important factor in QoS isthe QoS class identifier (QCI), which representsthe set of QoS parameter values, and is com-monly used throughout the LTE system.Detailed QoS parameters are given in Table 1,which is defined in [12]. In Table 1, it can bededucted that QCIs 1 and 5 are suitable forKRTCS from their strict QoS parameters.Specifically, QCI 5 aims to transfer traffic that issensitive to data loss. Thus, the packet loss rateof QCI 5 is set to be extremely low, and the pri-ority of QCI 5 is set to be the highest. Due tothose characteristics, QCI 5 is usually applied tobearers for application-level signaling messagesin a commercial LTE system, such as call setupmessages for voice services. However, QCI 5does not guarantee a minimum bit rate per bear-er, which is needed for providing a stable datarate to KRTCS traffic. On the other hand, QCI1 is suitable for voice data. QCI 1 allows a cer-tain amount of data loss, and requires a guaran-tee of a minimum bit rate per bearer. QCI 1 isusually applied to voice data bearers in a com-mercial LTE system. This QCI, however, allowspacket loss in abnormal situations such as net-work congestion, which is not desirable forKRTCS services.

A FEASIBLE QOS MANAGEMENT SCHEMEThe present status of the QCI table in LTEreveals that no QCI is perfectly suited to KRTCStraffic. An alternative way to support KRTCS

Table 1. QoS class identifier table.

QCI Resourcetype Priority

Packetdelaybudget

Packetlossrate

Railway services

1

Guaranteedbit bate(GBR)

2 100 ms 10–2RailVoice dedicated,ATPControl,ATOControl dedicated

2 4 150 ms 10–3

3 3 50 ms 10–3

4 5 300 ms 10–6

5

Non-GBR

1 100 ms 10–6 ATPControl, ATOControl default

6 6 300 ms 10–6

7 7 100 ms 10–3

8 8

300 ms 10–6

9 9 Internet default, RailVoice default



traffic without modifying the QCI table is to usea mixture of QCIs 1 and 5. This can be achievedby making a KRTCS session have two data bear-ers of QCIs 1 and 5. The data bearer of QCI 1carries most of the KRTCS traffic in a normalscenario. The data bearer of QCI 5 carries somesignificant messages, which require extremelylow loss rate, or takes the role of the data bearerof QCI 1 when congestion or frequent data lossoccurs.

Essentially, this method is based on a timesharing technique [13] in the information-theo-retic sense. It is widely used to achieve a broadrange of rate pairs given two achievable fixedrate pairs. The technical philosophy can fulfillthe QoS requirements of KRTCS traffic in termsof delay, packet loss, and minimum data rate. Inaddition, this method uses network resourcesefficiently, because the data bearer of QCI 5,which has severe QoS parameters and is expect-ed to use significant network resources, will beminimally utilized for transferring KRTCS traf-fic.

QOS FRAMEWORK DESIGNFigure 2 provides the architectural view forguaranteeing QoS in an LTE system [14, 15]. Asshown in [14], it is composed of an applicationserver, a policy and charging rules function(PCRF), a packet data network gateway (P-GW), an eNodeB, and a user equipment (UE).The application server corresponds to ATS orwayside ATP. The PCRF contains sets of QoSparameters for each service and condition, anddecides the QoS policy by forwarding the QoSparameters to the P-GW. The P-GW has therole of a gateway to the application servers,which are identified by an access point name(APN) in the LTE system. The P-GW leads tothe management of data bearers according tothe given QoS parameters. In addition, the

eNodeB performs actual operations with respectto data packet transfer, such as scheduling, radioresource management, and mobility manage-ment, according to the QoS parameters of thedata bearer [15].

The part of the control plane in Fig. 2 showsthe procedure for starting a KRTCS session andconfiguring an Evolved Packet System (EPS)bearer, which is a data bearer between a UE anda P-GW. We allocated two independent APNs,ATOControl and ATPControl, to the ATS andwayside ATPs, respectively. During the train’spower-on and attach procedure, a default EPSbearer of QCI 5 was configured for each APN.This default EPS bearer usually carries configu-ration and management messages of a KRTCSsession. Also, this bearer can be fully utilized totransfer ATS or ATP traffic in case of networkcongestion.

Based on the status of the KRTCS session,PCRF plays an autonomous role in the dynamicdecision of the QoS policy [12]. When the trainservice starts, the ATS and the wayside ATPgenerate sessions with ATO and ATP on thetrain side, respectively, and notify the PCRF.The PCRF then gives the QoS policy to the P-GW so that the P-GW configures a dedicatedEPS bearer of QCI 1 for each APN. TheATS/ATO pair and the wayside ATP/onboardATP pair then start exchanging their data witheach other through the dedicated EPS bearers.The dedicated EPS bearer can guarantee a mini-mum data rate of KRTCS traffic for each traineven in case of network congestion. In addition,network congestion may already occur beforestarting a KRTCS session, and there may beinsufficient network resources to configure anew EPS bearer. In this case, the P-GW canattempt a preemption, in which existing EPSbearers of the lowest priority are released, andnew EPS bearers are alternatively configured for

Figure 2. Architecture for guaranteeing QoS in an LTE system.

UE

Control plane

Data planeBearer mapping

Ap

plic

atio

n(O

ATP

)

Filte

ring

(TFT

/SD

F te

mpl

ate)

Scheduling

Default EPS bearer

Dedicated EPS bearer

eNB PDN PCRFP-GW

PDN connection (EPS session)

APN (atpcontrol)UE IP address

ATP session start

Provisioning QoS policy

Dedicated EPS bearer context activation

Default EPS bearer context activation (attach)

RRC connectionestablishment

Session startindication

IP flow 2(WATP2)

IP flow 1(WATP1)

SDF1 QoSpolicy

SDF2 QoSpolicy

The LTE standard

defines typical QoS

parameters and

provides sets of pre-

defined values of the

QoS parameters for

frequently used

services. These

pre-definitions relax

complexity of imple-

mentation, and allow

the functional enti-

ties of an LTE system

to be optimized to

guarantee the QoS

based on the QoS

parameters.



the new KRTCS session. In addition, we set twomore APNs, VoIP and internet, to provide voiceand video services, respectively. For voice ser-vice, a default EPS bearer of QCI 9 is used forsignaling, and a dedicated EPS bearer of QCI 1is used for carrying voice data. The dedicatedEPS bearer is configured at the moment the UEstarts a voice session. It is estimated that allocat-ing QCI 1 to a voice bearer would not affect theKRTCS data transfer significantly, because theamount of voice traffic is negligible in railwayenvironments. On the other hand, since theamount of video traffic can be sufficiently largeto cause congestion, we let the default EPS bear-er of QCI 9 be configured for internet. This con-figuration enables the rate of video services perAPN to be controlled.

PERFORMANCE OF KRTCS OVER LTETESTBED DEPLOYMENT

To carry out a field test in a practical railwayenvironment, we built an LTE testbed in con-junction with a KRTCS testbed on a commercialrailway. The testbed for KRTCS was built on theDaebul line in Mokpo, which is in southwesternSouth Korea. As shown in Fig. 3, the test sitewas 12 km long, and included a long (2.2 km)tunnel and a bridge. At this test site, five tempo-rary stations were constructed to verify the oper-ations in stations. With respect to a test train, wemodified a commercial light train to be undercontrol of onboard ATP and ATO. ATS wasplaced at the Ilro station, and wayside ATPswere deployed at the Ilro and Daebul stations,located at the end of the test site, so that inter-ATP handover could occur between the two sta-tions. In addition, various supplementary devices,such as train closed circuit TVs (CCTVs) and

screen doors, were deployed to assess the detailsof the unmanned operations.

For the LTE system, we used Samsung LTERelease 8 network devices with frequency bandof 2.6 GHz. For the eNodeB deployment, weused 10 radio units and two digital units to coverthe whole test site and connected them throughbackhaul. To extend the coverage to the tunneland bridge regions, we positioned radio units atthe ends of the tunnel and the bridge facingtoward the middle of the region. The core net-work devices, including a P-GW and a PCRF,were placed at the Ilro station. For an onboardLTE UE, we used a GCT GDM7240 as an LTEmodem chip. There were two onboard LTE UEsin the train for backup operation, and eachonboard LTE UE was connected to a set ofonboard ATP and ATO. Further details aboutsystem parameters in terms of the LTE testbedare described in Fig. 3.

To meet the QoS requirements of KRTCSacross the whole railway service region, celldeployment was done systematically. Specifically,we endeavored to reflect the characteristics ofKRTCS during cell deployment and optimiza-tion. Since the KRTCS service was affected bycontinuous data pause, we adjusted the cellparameters to minimize the occurrence andduration of data pause. For example, we reducedradio resource control (RRC) timer T310, whichcorresponds to an out-of-synchronization timer,to trigger cell selection quickly in that case. Also,RRC timers T300 and T311 are reduced suchthat the onboard terminal could quickly give upthe on-going random access or connection re-establishment procedure followed by attempttingcell selection again. Unlike the commercial field,above setting is valid in a railway environment. Itis because the received signal is strong in the

Figure 3. Cell deployment in the Daebul testbed.

RU #2

RU #2

RU #3

RU #3

RU #4

RU #5 RU #5

RU #6

RU #6

RU #7

RU #7

RU #10

RU #10

RU #8RU #8

RU #9

RU #9 DU #2

RU #1

RU #1DU #10.129 km

RU #4

Ilro St.

Daebul St.

Tunnel(2242 m)

Bridge(3314 m)

2.243 km 4.515 km 4.909 km

9.573 km

RU: radio unit; DU: digital unit

Samsung network devices(eNodeB and core) GCT GDM7240

10.214 km

3GPP LTE Specification

Frequency/bandwidth

FFT size

Subcarrier spacing

Duplex mode

Maximum transmit power

Maximum speed of train

Half power beam width (HPBW)

Array gain

Antenna type

MIMO

Release 8

DL: 2670–2675 MHz; UL: 2550–2555 MHz

512

15 kHz

FDD

23 dBm (onboard terminal), 43 dBm (eNodeB)

80 km/h

H-plane: 65° ± 5°; E-plane: 7° ± 2°

> 17.5 dBi

Sector antenna

2 TX, 2 RX

11.174 km

4.485 km

6.199 km

0.372 km 1.472 km 1.843 kmTo carry out a field

test in a practical

railway environment,

we built an LTE

testbed in conjunc-

tion with a KRTCS

testbed on a com-

mercial railway. The

testbed for KRTCS

was built on the

Daebul line in

Mokpo, which is in

southwestern South

Korea.



track region and it takes short time to finish thecell selection or random access procedure formost cases.

PERFORMANCE EVALUATIONWe evaluated 51 KRTCS test cases reflectingvarious unmanned operation scenarios thatcould occur on a commercial railway. The repre-sentative test cases include followings:• Check if a train reports its position to way-

side and gets a moving authority from way-side in time.

• Verify if a train is at the right position with-in a range of ±500mm with a probability of100 percent, and within range of ±250mmwith a probability of 90 percent when thetrain stops at a station.

• Validate whether inter-ATP handover pro-cedure has been successfully completedwithout any loss of control signaling when atrain crosses the boundary region.

• Verify whether the distance between twosuccessive trains at a speed of 40 km/h ismaintained or not.During the test, we allowed the train to gen-

erate various background traffic such as video orvoice data for railway operation and to transfer

via the LTE system. This was for checkingwhether the QoS requirements of KRTCS couldbe satisfied preferentially while other serviceswere ongoing.

The verdicts were passed for all the test caseswith no service drop event. Figure 4 shows thesnapshot of KRTCS traffic observed by Wire-shark during the test. WATP1 and WATP2 indi-cate the wayside ATP in Daebul and the waysideATP in Ilro, respectively. The graph in the upperpart of Fig. 4 shows that the majority of theKRTCS traffic was ATP messages related tomoving authority and train status reporting. Inaddition, the average throughput caused by theKRTCS traffic is much smaller than the capacityof an LTE cell, but the KRTCS traffic was con-tinuously generated throughout the test. Itimplies that the LTE system should care forguaranteeing QoS of KRTCS traffic at any timeregardless of where the train is and how theother traffic from CCTVs and voice services aregenerated. The graph in the lower part of Fig. 4depicts the trace of packet sequence in time.This result ensures that the packet sequenceincreases continuously most of the time, and lessthan 1 packet was lost per second at some inter-val. This reveals that the LTE system serves

Figure 4. Observation of KRTCS traffic: a) observing KRTCS traffic generation; b) packet sequence trace.

WATP1 (192.168.12.31)WATP2 (192.168.11.32)OATP (192.168.31.42)ATS (172.16.1.12)ATO (192.168.31.44)

WATP1WATP2

400380360340320300280260240220200180160140120100806040200

2100

KRTC

S pa

cket

seq

uenc

eTh

roug

hput

(bi

t/s)

2400 2700 3000 3300 3600 3900 4200 4500Time (s)

(a)

(b)Time (s)

4800 5100 5400 5700 6000

2100 2400 2700 3000 3300 3600 3900 4200 4500 4800 5100 5400 5700 6000

300

250

200

150

100

50

0

–50



KRTCS traffic without causing significant packetloss and service drop during the test.

Figure 5 shows more details about the perfor-mance of data transfer between wayside ATPand onboard ATP in terms of transfer delay. Inview of end-to-end delay, we checked round-triptime (RTT). We measured transmission time ofa request message from an onboard ATP andreception time of the corresponding responsemessage at the onboard ATP. The result showsthat RTT was about 75 ms on average and wasless than 100–110 ms with 99 percent probability.This means that the end-to-end delay betweenthe onboard ATP and the wayside ATP was typi-cally 37.5 ms and mostly less than 55 ms. Thisresult implies that the LTE system with our pro-posed scheme can fulfill the requirement ofpacket delay budget derived earlier. In addition,in view of jitter, we measured inter-arrival timesof ATP messages, which are transmitted with aperiod of 600 ms. The result shows that theinter-arrival time was 610 ms on average and wasless than about 620 ms with 99 percent probabil-ity. It is expected that the periodic messagesgenerated by one side of ATP can be stablytransferred to the other side of ATP through theLTE system so that periodicity of the ATP mes-sages can be preserved at the reception side.

The performance results reveal that our pro-posed scheme can make the LTE system fulfill

the QoS requirements of KRTCS traffic. It showsthe feasibility that the traffic from ATS, ATO,and ATP can constantly proceed in the aspects oftransfer delay and experience rare loss in theLTE system. Also, it shows the feasibility of theLTE system guaranteeing the QoS regardless ofthe train’s operation and position. It is remark-able that the LTE system in the testbed guaran-tees QoS even in the region of problematicenvironments such as tunnel and bridge sections.Hence, we conclude that the LTE system is notonly qualified to serve KRTCS, but also can be agood candidate for serving other kinds of TCSs.

CONCLUDING REMARKSMany railway operators have recently stated apreference for unmanned TCSs for efficient man-agement, but at the same time they are concernedabout safety issues. In particular, errors in manag-ing the QoS of the train control traffic coulddirectly bring about a loss of human life. Conse-quently, operators wish to assess TCSs carefullyand ensure that the wireless communications sys-tem is capable of guaranteeing the QoS of traincontrol traffic. One of the key points in this articleis to design a QoS policy based on analysis of thecharacteristics of the train control traffic. Accord-ing to the traffic analysis, it is required to guaran-tee a minimum bit rate and low latency for each

Figure 5. The performance results in terms of delay and jitter. Top: end-to-end delay; bottom: inter-arrival time.

OATP – WATP

CDF

CDF

CDF

CDF

OATP – WATP

WATP1 & WATP2WATP1WATP2

WATP1 & WATP2WATP1WATP2

Round trip time (s) Round trip time (s)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1.00

0.99

0.98

0.97

0.96

0.95

0.94

0.93

0.92

0.91

0.90

1.000.990.980.970.960.950.940.930.920.910.900.890.880.870.860.850.840.830.820.810.80

0.05 0.06 0.07 0.08 0.09 0.10 0.11

Jitter (s)0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.080 0.085 0.090 0.095 0.100 0.105 0.110

Jitter (s)0.60 0.61 0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 0.70

It is remarkable that

the LTE system in the

test-bed guarantees

the QoS even in the

region of problemat-

ic environments such

as tunnel and bridge

sections. Hence, we

conclude that the

LTE system is not

only qualified to

serve KRTCS but also

can be a good can-

didate for serving

other kinds of TCSs.



KRTCS session. This critical issue was settled byallocating the two different functional EPS bearerswith time sharing. Our proposed scheme is vali-dated by the performance evaluation in terms ofdelay and jitter. The other key point is to adjustthe cell parameters appropriately during cell plan-ning and optimization procedures in order toresolve any network issues that may cause datapause. The performance in terms of packet losswas shown, while our testbed did not allow signifi-cant data pause and caused no service drop. Con-sidering these key points will assure operators ofthe safety of unmanned train control over LTE.

ACKNOWLEDGMENTThis research was supported by a grant from theR&D Program of the Korea Railroad ResearchInstitute, Republic of Korea.

REFERENCES[1] R. D. Pascoe and T. N. Eichorn, “What Is Communica-

tion-Based Train Control?,” IEEE Vehic. Tech. Mag., vol.4, no. 4, Dec. 2009, pp 16–21.

[2] IEEE 1474.1-2004, “Standard for Communication-BasedTrain Control(CBTC) Performance and FunctionalRequirements,” 2005.

[3] UIC CODE 951 v15.3.0, “EIRENE System RequirementsSpecification,” 2012.

[4] R. lvarez and J. Romn, “ETCS L2 and CBTC over LTE —Convergence of the Radio Layer in Advanced Train Con-trol Systems,” IRSE Australasia, Oct. 2013, pp. 1–12.

[5] A. Sniady and J. Soler, “LTE for Railways: Impact on Per-formance of ETCS Railway Signaling,” IEEE Vehic. Tech.Mag., vol. 9, no. 2, June 2014, pp. 69–75.

[6] J. Calle-Sanchez et al., “Long Term Evolution in HighSpeed Railway Environments: Feasibility and Chal-lenges,” Bell Labs Tech. J., vol. 18, no. 2, Aug. 2013,pp. 237–53.

[7] M. Cheng and X. Fang, “Location Information-AssistedOpportunistic Beamforming in LTE System for High-Speed Railway,” EURASIP J. Wireless Commun. andNet., July 2012, pp. 1–7.

[8] H. Gao et al., “A QoS-Guaranteed Resource SchedulingAlgorithm in High-Speed Mobile Convergence Network,”Proc. IEEE WCNC Wksp. ‘13, Apr. 2013, pp. 45–50.

[9] J. Wang, H. Zhu, and N. J. Gomes, “Distributed Anten-na Systems for Mobile Communications in High SpeedTrains,” IEEE JSAC, vol. 30, no. 4, May 2012.

[10] M. Alasti et al., “Quality of Service in WiMAX and LTENetworks,” IEEE Commun. Mag., vol. 48, no. 5, May2010, pp. 104–11.

[11] F. Capozzi et al., “Downlink Packet Scheduling in LTECellular Networks: Key Design Issues and a Survey,”IEEE Commun. Surveys and Tutorials, vol. 15, no. 2,June 2013, pp. 678–700.

[12] 3GPP TS 23.203 v9.14.0, “Technical SpecificationGroup Services and System Aspects; Policy and Charg-ing Control Architecture,” 2014.

[14] T. M. Cover and J. A. Thomas, Elements of InformationTheory, Wiley, 2012.

[15] 3GPP TS 23.401 v9.16.0, “Technical Specification GroupServices and System Aspects; General Packet Radio Ser-vice (GPRS) Enhancements for Evolved Universal Terres-trial Radio Access Network (E-UTRAN) Access,” 2014.

[15] 3GPP TS 36.300 v9.10.0, “Technical SpecificationGroup Radio Access Network; Evolved Universal Terres-trial Radio Access (E-UTRA) and Evolved Universal Ter-restrial Radio Access Network (E-UTRAN); OverallDescription; Stage 2,” 2012.

BIOGRAPHIESJUYEOP KIM ([email protected]) is a senior researcher in theICT Convergence Team at Korea Railroad Research Institute(KRRI). He received his M.S. and Ph.D. in electrical engi-neering and computer science from Korea Advanced Insti-tute of Science and Technology (KAIST) in 2010. His currentresearch interests are railway communications systems,group communications, and mission-critical communica-tions.

SANG WON CHOI ([email protected]) received his M.S. andPh.D. in electrical engineering and computer science fromKAIST in 2004 and 2010, respectively. He is currently asenior researcher in the ICT Convergence Research Team ofKRRI. His research interests include mission-critical commu-nications, mobile communication, communication signalprocessing, and multi-user information theory. He was therecipient of a Silver Prize at the Samsung Humantech PaperContest in 2010.

YONG-SOO SONG ([email protected]) received his Master’sdegree in electrical engineering from Yonsei University in2004. He has been with KRRI since 2004. He is workingtoward his Ph.D in electrical engineering from Yonsei Uni-versity. His current research interests are in cell planningand handover in LTE railways.

YONG-KI YOON ([email protected]) received his M.Sc. degreein electrical engineering from Chungbuk National Universi-ty, Republic of Korea, in 1996. He is currently a principalresearcher in the metropolitan railroad system researchcenter at KRRI. His current research interests are in commu-nication based train control system, train position trackingmethod.

YONG-KYU KIM ([email protected]) received his M.S. in elec-tronic engineering from Dankook University, Korea, in1987, and his D.E.A. and Ph.D. in automatic and digitalsignal processing from Institute National Polytechnique deLorraine, France, in 1993 and 1997, respectively. He is cur-rently an executive researcher inthe ICT Convergence Teamat KRRI. His research interests are in automatic train con-trol, communication-based train control, and driverlesstrain operation.

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automatic train control over lte: design and performance

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