RESEARCH Open Access

A CoMP soft handover scheme for LTE systems inhigh speed railwayWantuan Luo1,2*, Ruiqiang Zhang1 and Xuming Fang1

Abstract

With the development of high-speed railway and public growing demand on data traffic, people pay much moreattention to provide high data rate and high reliable services under high mobility circumstance. Due to the higherdata rate and lower system latency, long-term evolution (LTE) has been chosen as the next generation’s evolutionof railway mobile communication system by the International Union of Railways. However, there are still manyproblems to be solved in the high mobility applications of LTE, especially the higher handover failure probability,which seriously degrades the reliability of railway communication. This article proposes an optimized handoverscheme, in which the coordinated multiple point transmission technology and dual vehicle station coordinationmechanism are applied to improve the traditional hard handover performance of LTE. The scheme enables thehigh speed train to receive signals from both adjacent base stations and obtain diversity gain when it movesthrough the overlapping areas, so it improves the quality of the received signal and provides reliablecommunication between train and ground eNodeBs. Numerical analysis and simulation results show that theproposed scheme can decrease the outage probability remarkably during handover and guarantee the reliability oftrain to ground communication.

Keywords: LTE, soft handover, coordinate multiple point transmission (CoMP), high mobility, outage probability

1 IntroductionDue to the lower energy consumption, less environmen-tal pollution, larger transport capacity and more safety,railway transportation plays an important role for thedevelopment of country. Japanese Shinkansen, FrenchTGV, German ICE and China Railway have achievedremarkable successes. Nowadays, the development ofhigh-speed railway makes people’s lives more and moreconvenient. Meanwhile, it puts forward higher require-ments on high-2 speed railway communication services.The existing GSM for Railway (GSM-R) network ismainly based on the second-generation Global Systemfor Mobile Communications (GSM), and its data rate istoo low to meet the broadband mobile communicationaccess and other value-added service demands of pas-sengers. In order to provide broadband services andapplications for users not only at home but also on trip,long-term evolution (LTE) has been chosen as the next

generation’s evolution of railway mobile communicationsystem by International Union of Railways (UIC), whichsupports significant higher data rates and lower systemlatency. This article focuses on the applications of LTEtechnology in railway broadband wireless communica-tion network.Normally, the main problems caused by user’s high-

speed movement in cellular wireless communication sys-tem are over-frequent handover, Doppler shift and largepenetration loss, among which over-frequent handoverneeds to be paid special attention as it seriously affectsthe communication quality of service (QoS) and trafficreliability. Currently only traditional hard handoverscheme is supported in LTE, which encounters twochallenges under high speed movement circumstance.On the one hand, the handover delay caused by hardhandover is relatively large. The high-speed train passesthrough the overlapping areas so fast that the handoverprocedure can not be accomplished timely. On theother hand, the speed of MRS is so fast that it wouldmiss the optimal handover position, which degrades thehandover success probability. In order to overcome the

* Correspondence: Wantuan.luo@gmail.com1Institute of Mobile Communications, Southwest Jiaotong University,Chengdu, 610031, ChinaFull list of author information is available at the end of the article

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© 2012 Luo et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons AttributionLicense (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium,provided the original work is properly cited.

challenges mentioned above, the existing handoverscheme of LTE should be optimized to improve thehandover success probability in high-speed movementcircumstance.Currently, more and more researches focus on the

broadband communication access issues in railway com-munication. In [1], a fast handover algorithm suitablefor dedicated passenger line is devised by setting a newneighboring list. However, the optimization of systemparameters has limited improvement on the overall per-formance of system. A novel moving extended cell(MEC) concept is introduced in [2,3], which is based onuser centric virtual groups of adjacent cells that transmitthe same data to the user. The MEC concept utilizes amechanism for restructuring the virtual multi-cell areaaccording to the user’s mobility pattern. This handoverscheme is supposed to support high end-user mobilityin a 60GHz broadband pico-cellular radio-over- fibernetwork. Jeon and Sanghoon [4], Pabst et al. [5] haveintroduced relays to the process of handover in cellularnetworks in order to improve handover performance. In[6,7], multiple radios in devices are exploited to elimi-nate handover latency. In the proposed approach, multi-scan nodes rely on using their (potentially idle) secondwireless interface to opportunistically scan and pre-associate with alternative Access Points (APs) and even-tually seamlessly handover ongoing connections. Anovel handover scheme based on on-vehicle antennas isintroduced in [8], and the numerical analysis resultsshow that the proposed scheme can pre-trigger hand-over appropriately, which ensures a higher handoversuccess rate and has an improvement on systemthroughput.Coordinated multiple point transmission (CoMP)

transmission and reception allows geographically sepa-rated base stations to joint sending data to one terminaland joint receiving data from one terminal, by which theinter-cell interference could be reduced and the systemfrequency spectral efficiency would be improved. In [9],CoMP is introduced to solve inter-cell interferenceissues, and the simulation results show that the CoMPschemes achieve different gains in average sectorthroughput and 5% edge-user throughput gain as com-pared to that of conventional pre-coding scheme. Zhouand Wan [10] proposed an approach to improve thethroughput of systems applied CoMP by adjust Timeadvance (TA). A new handover scheme was designedfor CoMP scenarios in [11], and the handover model ofCoMP system was analyzed and the signaling transmis-sion procedure of CoMP cooperating sets handover wasdesigned. Unfortunately the system performance wasnot discussed.From the analysis above we can see that, there are few

researches on the application of base stations interaction

and multiple vehicle stations cooperation. In order totake full advantage of the multiple base stations coop-eration feature of CoMP systems under highspeed sce-narios, this article proposes a seamless soft handoverscheme based on CoMP, which allows the train toreceive signals from both adjacent base stations whenthe train travels through the overlapping areas. Thus,the handover failure rate is degraded and the reliabilityof train to ground communication is guaranteed.The rest parts of the article are arranged as follow:

Section 2 introduces the current handover scheme inLTE systems. Section 3 describes the proposed handoverscheme in detail. Section 4 analyzes the system perfor-mance. Section 5 illustrates the simulation results andthe improvement achieved by the proposal. And finallySection 6 concludes the whole article.

2 Conventional hard handover scheme in LTEsystemsThe current hard handover scheme in LTE systems isshown in Figure 1. The eNodeB is with the coverageradius R. The width of overlapping area is L. The verti-cal distance between a eNodeB and track is dmin. Thewhole railway mobile communication network has a fea-ture of linear coverage topology. The overall handoverprocedure could be described as follow.As the train moves into the overlapping area from cell

i, eNodeB i decides whether to handover or not accord-ing to the reported received signal strength indication(RSSI), reference signal received power (RSRP) or refer-ence signal received quality (RSRQ) measurement infor-mation by the train and the radio resource management(RRM) information, as shown in Figure 1a. Once thehandover is triggered, the train disconnects with eNo-deB i and tries to synchronize with the target eNodeB j.Then the Mobility Management Entity (MME) switchescommunication route and the previous station eNodeB ireleases both user plane resources and control planeresources when the handover procedure is completed, asshown in Figure 1b.As analyzed above, the current handover scheme in

LTE systems is a Break-Before-Make approach. Thescheme allows the train to receive signals from only onebase station at one time. It will cause a larger outage

Figure 1 Current hard handover procedure in LTE systems.

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probability and interrupt latency, which severely affectthe reliability of train to ground communication.

3 A CoMP based soft handover scheme in LTEsystems3.1 A co-channel deployment approach for railwaycommunicationIn order to achieve CoMP joint processing and trans-mission between the two adjacent eN- odeBs along therailway track, an interference-avoid co-channel deploy-ment approach is proposed in this article.As OFDM is used in LTE, the inter-cell interference is

the main source of interference. To avoid the possiblelarge inter-cell interference when train travels throughthe overlapping region, this article proposes a frequencyallocation approach for railway scenario: ignoring thereserved dedicated resources, we divided the whole fre-quency band into two parts which are called F1 and F2respectively, as shown in Figure 2. By doing so the pro-posed co-channel network approach is given in Figure 3:F1 is assigned to the up-direction trains (the trains aregoing to the metropolis) and F2 is assigned to thedown-direction trains (contrary to up-direction). Com-pared with the existing GSM-R network whose typicalfrequency reuse factor is 3, the proposed co-channeldeployment approach can not only maximize the spec-trum efficiency but also enable the interactions of adja-cent eNodeBs.

3.2 Dual on-vehicle stations collaborative schemeFor reliability and throughput, the transmission latencyis detrimental. In order to eliminate the transmissiondelay, this article proposes a dual on-vehicle stationscooperation scheme, which takes full advantage of thedistributed antennas transmission and the body length

of high speed train. The mobile relay stations (MRSs)controlled by central control station (CCS) are mountedin the front and the rear of the train. Figure 4 shows theschematic diagram of the scheme. Antennas 1 and 2belong to the front station and the rear station respec-tively. The uplink data of the users inside the train aregathered to the CCS by pico-base stations deployed on-vehicle, and then the front and the rear stations transmitthe gathered data to eNodeBs along the track under thecontrol of CCS. Meanwhile, the downlink data receivedby the two on-vehicle stations form eNodeBs along thetrack are gathered to the CCS, and then the CCS for-wards the collected data to the pico-base stations insidethe train. With the above procedure having been done,the communication between users and eNodeBs alongthe track can be successfully achieved.This scheme can solve the ‘processing capacity bottle-

neck’ problem caused by the conventional single on-vehicle station scheme. Moreover, a good diversity gainwould be obtained since the distance between Antennas1 and 2 is far away enough.

3.3 CoMP based soft handover schemeThis article proposes a seamless soft handover schemeutilizing CoMP joint processing and transmission tech-nology, which can significantly improve the handoverperformance when the train moves through the overlap-ping areas.As shown in Figure 5, as the front on-vehicle station

enters into the overlapping area, the source eNodeB iactivates the cooperative transmission set (CTS) com-posed of eNodeB i and eNodeB j. The two or moreeNodeBs communicating with MRSs simultaneously arecalled CTS. The CTS activation is based on the mea-surement information reported by the moving train andthe position information supplied by the communication

Figure 2 The frequency allocation approach.

Figure 3 Co-channel network approach for railwaycommunication.

Figure 4 Dual on-vehicle stations solution.

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based train control system (CBTC). Once the CTS isactivated, the source eNodeB i shares all the user planedata of users inside the train to the target eNodeB j bythe high-speed backhaul of LTE network. The two adja-cent eNodeBs both use the same frequency resource tocommunicate with the train. Signals from the eNodeBsin the cooperative set are in-phase superposed by pre-coding, which provides a diversity gain and power gain.It should be noted that CoMP CTS always contains twoeNodeBs in the linear coverage topology of high-speedrailway.As shown in Figure 6, eNodeB i and eNodeB j keep

the cooperative relation and communicate with the trainsimultaneously when the train body entirely enters intothe overlapping area. Both the front and the rear on-vehicle stations can receive signals from the two coop-erative eNodeBs, but the measurement information isonly forwarded to the source eNodeB i. The sourceeNodeB i decides whether to handover or not accordingto the measurement information reported by the movingtrain and the RRM information of the target eNodeB j,without interrupting the data transmission in user plane.The signal from the source eNodeB i drops gradually

as the train moves farther away from it. Once the signalstrength of target eNodeB j is larger than that of sourceeNodeB i for hdB, the handover is triggered. The signal-ing switch in control plane is a ‘break before make’ pro-cess, which is similar to current hard handover scheme.Meanwhile, both eNodeB i and eNodeB j transmit theuser plane data to users inside the train with the samefrequency resource by Physical Downlink Shared Chan-nel (PDSCH) without interrupting communications,which avoids the interrupt latency caused by currenthard handover in LTE systems. After the handover

procedure having been done, the new source eNodeB jkicks the former source eNodeB i off the cooperativeset, when the signal strength of eNodeB i drops to acertain threshold, as shown in Figure 7.Summarily, the optimized handover scheme based on

CoMP can significantly degrade the outage probabilityand improve the handover performance, by which thetrain can communicate with the two adjacent eNodeBsin the overlapping area. The scheme achieves a seamlesshandover performance as soft handover.

4 Performance analysisIn high-speed environments, the Doppler Effects wouldlead to irreducible bit error rate (BER) which is callederror floor [12,13]. However, according to technical spe-cifications (TS) of LTE [14,15], the procedure of trigger-ing handover contains three phases: the userequipments (UEs) measure the RSSI, RSRP or RSRQ,sent the measurement reports to source eNodeB, andthen the radio resource control (RRC) of source eNodeBdecides whether handover is triggered or not [16]. The3GPP evaluation documents [17] also point out that thehandover measurement and radio link failure (RLF) onlydepend on the RSSI, RSRP or RSRQ. Though the BERperformance would degrade the QoS, if the RSRPremains above a certain threshold for a fixed duration,the wireless link will be re-established and assured tocomplete the handover. At most of time, the high-speedtrain travels through the wide plain and viaduct, theline-of-sight (LOS) path experienced free-space loss onlybetween MRS and BSs is available and there are fewreflectors or scatterers. The major influence on wirelesschannel caused by relative motion between transmitterand receiver is Doppler shift instead of Doppler spread.Therefore in high-speed railway scenario, instead ofconsidering Doppler Effects which degrades BER, weonly need to consider Doppler shift which would impairhandover performance. In [18], Doppler shift in theoverlapping region of two neighboring eNodeBs (hand-over region) is almost unchanged, and can be compen-sated [19]. In this article we suppose that Train ControlInformation, such as train’s velocity and location, can beshared by the ground eNodeBs, and Doppler shift hasbeen compensated [20,21].

Figure 5 The target eNodeB joins the cooperative set.

Figure 6 Two adjacent eNodeBs serve the train simultaneouslyin the overlapping area. Figure 7 The former source eNodeB interrupts sending data.

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Without loss of generality, let the train be located at xaway from the eNodeB i, the received power in dBm indecibels can be expressed as

R(i, x) = Pt − PL(i, x) − A(i, x, σ ) (1)

where Pt is the transmit power in dBm, PL(i, x) is thepath loss between eNodeB i and location x, and A(i,x,s)is the shadow component at the location x, generallymodeled as a Gaussian random variable with mean zeroand standard deviation s. Typically, s is 6 or 8dBin [22].As the railway communication network is the linear cov-

erage topology, we consider a two-cell system. The shadowfading losses of the two adjacent eNodeBs are correlatedand can be expressed as follow according to [23].

Ai = aξ0 + bξi (2)

where a2 + b2 = 1. Note that i is the link number. ξ0is common to both A1 and A2, ξi represents the inde-pendent part between the two adjacent eNodeBs.Furthermore, both ξ0 and ξi are independent Gaussianrandom variables with mean zero and standard deviations. Meanwhile, because the tail of Gaussian distributionextends to infinity, a fade margin F dB is added to thetransmit power.

4.1 Outage in the CoMP handover schemeIn our proposal, the two adjacent eNodeBs communi-cate with the train simultaneously in the overlappingarea. Outage will happen if and only if both signals areof unacceptable quality. So the outage probability usingthe proposed handover scheme is

PCoMP handover = Pr[min(A(i, x, σ ),A(j, x, σ )) > F] (3)

where, A(i,x,s) and A(j,x,s) represent the shadow fad-ing losses of eNodeB i and eNodeB j respectively. F isthe fade margin and F = Pt - PL - Rs. PL is the path lossand Rs is the receiver sensitivity. By straightforwardmanipulation, the outage probability can be shown as

PCoMP handover =1

(2πσ )3/2

∞∫−∞

e−

t2

2σ 2

⎛⎜⎜⎜⎜⎜⎝

∞∫

F − atb

e−

ξ2i

2σ 2 dξi

∞∫

F − atb

e−

ξ2j

2σ 2 dξj

⎞⎟⎟⎟⎟⎟⎠dt

=1√2π

∞∫−∞

e−x2

2[�

(F − aσx

bσ

)]2

dx

=1√2π

∞∫−∞

e−x2

2[12erfc

(F − aσx√

2bσ

)]2

dx

(4)

where

�(x) =

∞∫x

1√2π

e−t2

2 dt, erfc(x) =2√π

∞∫x

e−t2dt (5)

4.2 Outage in current hard handover schemeIn the current handover scheme, the MRS can connect toonly one eNodeB at one time. For ideal case, the MRS isalways switched to the eNodeB with the best signal qual-ity. However, this may lead to the well-known ‘ping-pong’ effect around the cell boundary. In practical sys-tems, handover will be triggered on the condition thatthe received power of the source eNodeB is lower thanthat of the target eNodeB by hysteresis level h.In most of the existing researches, the outage prob-

ability in hard handover system is obtained with thehysteresis level h being assumed infinite so that it isimpossible to handover to the neighboring cell. Thisscenario is called an isolated cell. Let shadow fading lossA = ξ, where ξ is Gaussian random variable with meanzero and standard deviation s. An outage occurs whenthe fading component is larger than the fade margin,which is expressed as

Pisolated = Pr(ξ > F)

=1√2πσ

∞∫F

e−

ξ2

2σ 2 dξ

= �

(F

σ

)=12erfc

(F√2σ

)(6)

It is well known that the hysteresis is finite in practicalscenarios and varied according to different communica-tion scenario. In order to make a fair comparisonbetween conventional scheme and our proposal, a finitehysteresis level is assumed in this article. A handoveroccurs when the received power of the target eNodeB jis larger than that of the source eNodeB i by h dB, sothe handover probability can be expressed as

Px(i, j) = Pr[R(j, x) − R(i, x) ≥ h] (7)

When the train is located exactly at the midpoint ofoverlapping region, the outage probability is composedof three parts:

(1) If both signals from the two adjacent eNodeBscan’t be received by the train, outage will happen. Insuch case, the outage probability Pboth_outage can beexpressed as

Pboth outage = Pr[A(i, x, σ ) > F&A(j, x, σ ) > h] (8)

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(2) Before handover, if the signal strength of sourceeNodeB is unacceptable and that of target eNodeB isnot large enough to trigger handover, this isobviously an outage. Let εi be the event that thetrain is connecting to eNodeB i, and h be the hyster-esis level. The outage probability Pbefore_ HO can beexpressed as

Pbefore HO = Pr[εi;A(j, x, σ ) < F < A(i, x, σ )]

= Pr[A(i, x, σ ) − h < A(j, x, σ ) < F < A(i, x, σ )].

Pr[εi|A(i, x, σ ) − h < A(j, x, σ ) < F < A(i, x, σ )]

(9)

where

Pr[A(i, x, σ ) − h < A(j, x, σ ) < F < A(i, x, σ )]

=b2

(2π)3/2

∞∫−∞

e−t2

2

F + h − aσ t

bσ∫

F − aσ t

bσ

F − aσ t

bσ∫

x2−hbσ

e−(x1 − at)2 + (x2 − at)2

2b2 dx1dx2dt

(10)

In fact, the correlation of shadow fading losses woulddecrease with the increasing angle-of-arrival difference[24]. We assume that there is no site-to-site correla-tion (i.e, a = 0) as the angle-of-arrival difference isquite large. Thus, the above expression can be simpli-fied as

12π

F + hσ∫

Fσ

Fσ∫

x2−Fσ

e−x21 + x22

2 dx1dx2 (11)

(3) After handover, although the train has been suc-cessfully switched to the target eNodeB, outage willhappen if the signal strength of the target eNodeB istoo weak. It should be noted that the target eNodeBis determined by the direction to which the trainmoves towards, that is, the train can’t be switched tothe previous source eNodeB though the signalstrength of the novel source eNodeB is terrible.Thus, the outage probability Pafter_ HO can beexpressed as

Pafter HO = Pr[εj;A(i, x, σ ) < F < A(j, x, σ )]

= Pr[A(i, x, σ ) < F < A(j, x, σ )

· Pr[εj|A(i, x, σ ) < F < A(j, x, σ )]

(12)

where

Pr[A(i, x, σ ) < F < A(j, x, σ )] = Pr[aξ0 + bξi < F < aξ0 + bξi]

= P(

ξ0

σ= t

)· P

(ξi

σ<

F − aξ0bσ

)

· P(

ξj

σ>

F − aξ0bσ

)

=1

(π)3/2

∞∫−∞

e−t2

2

∞∫

F − aσ tbσ

F − aσ tbσ∫0

e−x21 + x22

2 dx1dx2dt

(13)

It should be noted that the value of second item in (9)is difficult to obtain analytically. Whether the train isconnecting to eNodeB i or not can not be determinedby a snapshot of the system. It is generally assumed tobe 1/2 in the midpoint of the overlapping region. SinceAj <Ai in (9), that is, the attenuation loss to eNodeB j issmaller, and the train has a higher chance to connect toeNodeB j. Following [24], this probability is chosen as0.6.From the analysis above, the overall outage probability

using hard handover scheme can be expressed as

Phard outage = Pboth outage + Pbefore HO + Pafter HO (14)

5 Simulation and analysisTo make a comparison between current scheme and ourproposal, the MATLAB simulation results and analysisare presented. Detailed simulation parameters are shownin Table 1 [22] As the train moves through the overlap-ping area, the received signal strength of the two adja-cent eNodeBs is shown in Figure 8. The red and greencurves represent the received signal strength of eNodeBi and that of eNodeB j respectively. Due to the influenceof path loss and shadow fading loss, both signals

Table 1 Simulation parameters [22]

Parameters Value

Channel bandwidth (BW) 10MHz

Antenna pattern Omni

Carrier frequency 2.5 Ghz

Pt 46dBm

Height of antenna: hat 1.5 m

Height of eNodeB: heNodeB 35 m

dmin 30 m

N0 -174 dBm/Hz

Cell radius (R) 3 km

Site-to-site distance 4.8 km

Path loss model ITU-R M.1225

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strength of eNodeB i and eNodeB j are extremely lowerat cell boundary than those in the interior of a cell,which would cause a higher handover failure probability.The outage comparison between the current handover

scheme in LTE systems and our proposal when s = 6and h = 6 dB is shown in Figure 9. From the figure, itcan be seen that our proposed hard handover estimationprovides a rather tight bound, compared with existingisolated cell estimation. Meanwhile, the CoMP softhandover scheme provides a significant improvement inoutage probability compared with that in current hardhandover scheme systems, which can be easily obtainedfrom the figure.Compared with Figures 9, 10 and 11, we can see the

outage probability comparison with different hysteresislevel and standard deviation of shadow fading loss.Additionally, from these figures, with the increasing of

hysteresis level and standard deviation of shadow fadingloss, both the current handover scheme in LTE systemsand our proposal experience a higher outage probability.

However, the proposals provide the better performanceas the hysteresis increases.In Figure 12, the outage probability using different

schemes is given according to distance. The green andthe blue curves represent outage probability using cur-rent hard handover scheme in LTE systems connectingto source eNodeB and target eNodeB, respectively. Thered curve shows outage probability using the proposal.From the figure, it could be seen that the proposal pro-vides a quite lower outage probability, especially in themidpoint of the overlapping area, where the receivedsignal strength from both eNodeBs is pretty low.In addition, Figure 13 provides a comparison of hand-

over success probability according to the location of thetrain. The red and green curves denote the handoversuccess probability of hard handover scheme and that ofthe proposal respectively. From the figure, it can beclearly seen that the proposal can significantly improvethe handover performance with the successful handoverprobability increasing from 89 to around 99%.

1.5 2 2.5 3 3.5−110

−105

−100

−95

−90

−85

−80

−75

−70

−65

The distance between train and source eNodeB (km)

Rec

eive

d si

gnal

str

engt

h (d

B)

Received signal strength from eNodeB iReceived signal strength from eNodeB j

Figure 8 Received signal strength according to train location.

6 7 8 9 10 11 12 13 14 15 160

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Out

age

Pro

babi

lity

Fade Margin (dB)

With CoMPP

isolated

Without CoMP

Figure 9 Outage comparison when s = 6 dB and h = 3 dB.

4 6 8 10 12 14 160

0.05

0.1

0.15

0.2

0.25

Out

age

Pro

babi

lity

Fade Margin (dB)

With CoMPWithout CoMP

Figure 10 Outage comparison when s = 8 dB and h = 3 dB.

4 6 8 10 12 14 160

0.05

0.1

0.15

0.2

0.25

Out

age

Pro

babi

lity

Fade Margin (dB)

With CoMPWithout CoMP

Figure 11 Outage comparison when s = 6 dB and h = 6 dB.

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6 ConclusionAs the evolution of railway mobile communication systemis confirmed by UIC, LTE technology becomes a potentialsolution for future railway systems. The railway communi-cation system has stringent requirements for wirelesscommunication availability and latency. However the fre-quent handover caused by high-speed movement will ser-iously affect the performance. This article proposes aCoMP based soft handover scheme for LTE system inhigh speed railway, meanwhile, a co-channel deploymentapproach for railway communication and dual on-vehiclestation coordination mechanism is proposed too. The pro-posal allows the train to receive signals of both adjacenteNodeBs, which significantly improves the handover per-formance and degrades the outage probability, as thesimulation results show. It is worth mentioning that sig-naling interactions in control plane and performance ana-lysis on MAC layer are the focuses in our following work.

AcknowledgementsThe work of the authors’ was supported partially by the 973 Program underthe Grant 2012CB316100, NSFC under the Grant 61071108, 61032002, theKey Program of Technological R&D of the Ministry of Railway under theGrant 2011X011-A, and the ZTE R&D Foundation.

Author details1Institute of Mobile Communications, Southwest Jiaotong University,Chengdu, 610031, China 2College of Physics and Electronic Engineering,Guangxi University for Nationalities, Nanning, 530006, China

Competing interestsThe authors declare that they have no competing interests.

Received: 6 February 2012 Accepted: 13 June 2012Published: 13 June 2012

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1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Distance(km)

Han

dove

r su

cces

s pr

obab

ility

Without CoMPWith CoMP

Figure 13 Comparison of handover success probability ofdifferent schemes.

1.8 2 2.2 2.4 2.6 2.8 30

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Out

age

Pro

babi

lity

Distance (Km))

Source eNodeB without CoMPTarger eNodeB without CoMPwith CoMP

Figure 12 Outage probability according to distance.

Luo et al. EURASIP Journal on Wireless Communications and Networking 2012, 2012:196http://jwcn.eurasipjournals.com/content/2012/1/196

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doi:10.1186/1687-1499-2012-196Cite this article as: Luo et al.: A CoMP soft handover scheme for LTEsystems in high speed railway. EURASIP Journal on WirelessCommunications and Networking 2012 2012:196.

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