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Handover Mechanism for Device-to-DeviceCommunication

Ho-Yuan Chen, Mei-Ju Shih, Hung-Yu Wei∗

Graduate Institute of Communication Engineering, National Taiwan University, Taiwan∗hywei@cc.ee.ntu.edu.tw

Abstract—Device-to-Device (D2D) communication is aproximity-based technique used by Long Term Evolution-Advanced (LTE-A) systems. When a ProSe-enabled UE in D2Dcommunication moves across a cell boundary, seamless handoveris expected. However, LTE-A does not specify D2D handoverprocedures, and the current LTE-A handover procedures cannotsupport the ProSe service continuity requirements [1]. Therefore,an efficient D2D handover mechanism is necessary to meet therequirements. In this paper, a D2D handover mechanism, whichconsiders the signal quality of D2D pairs, is proposed. Theproposed mechanism includes a Joint Handover procedure anda Half Handover procedure based on a D2D handover decisionmethod. Simulation of Urban Mobility (SUMO) [2] is adoptedfor the mobility model and performance evaluation simulations.The properties of this mechanism satisfy the ProSe servicecontinuity requirements while decreasing the D2D HO failurerate as well as reducing the amount of information exchangedbetween the source eNB and the target eNB.

Index Terms—device-to-device, ProSe, handover, service con-tinuity, mobility, SUMO.

I. INTRODUCTION

D2D communication in Long Term Evolution-Advanced(LTE-A) systems has been a hot issue nowadays, for thegreat demand increases in the wireless communication system.D2D communication is selected as a promising technologyto realize Public Safety (PS) and for commercial usage.However, implementation of D2D communication faces anumber of challenges. With regard to PS scenarios, distributedD2D communication should be made possible. A distributedsynchronization method is proposed to achieve D2D discoveryand synchronization at the same time [3]. To reach collisionfree in a short time, a distributed feedbackless resource allo-cation scheme for D2D broadcast communication is proposed[4]. In this work, we handle another important issue: themobility management problem caused by D2D communicationhandover. Nevertheless, little research is available on handoverof D2D communication. A D2D-aware handover (D-A HO)solution is proposed to permit a UE to postpone the handoverto a target eNB until signal quality of the source eNB becomeslower than a threshold [5]. When the signal quality of the targeteNB is able to meet the D2D HO conditions for both UEs,both UEs will handover to the target eNB simultaneously. Theprior work does not account for the signal quality of the D2Dpairs nor does it discuss mobility in different directions.

The third generation partnership project (3GPP) agrees thatD2D discovery and communication will become one of thenew features to be studied during 3GPP Rel-12 and Rel-13 under the LTE ProSe study item [6]. According to thescope of Rel-13 [7], the ProSe continuity is considered asan important feature for D2D communication enhancement.

Fig. 1. After UE2 does LTE handover, ProSe service continuity is interrupted

Device mobility, which affects performance of ProSe servicecontinuity, is one of the factors deserved to be considered.When the ProSe-enabled UE in D2D communication movesacross the cell boundary, seamless handover is expectedso asto provide PS service continuity. How to provide the ProSeservice continuity is challenging.

II. PROBLEM STATEMENT

D2D handover problem occurs because legacy LTE systemcannot support D2D handover. When D2D pairs perform LTEHO, several drawbacks might emerge, such as latency, extraresource wasting, extra signaling exchange, and interruptedD2D link. For example, as shown in Fig. 1, the ProSe-enabledUEs perform ongoing D2D communication in the same celland move forward the same direction at first. Then the UE2

might be handed over to its neighboring cell. The D2D com-munication link may be interrupted because the UE2 performsLTE handover. After LTE HO, cross-cell D2D communicationis re-established. When the UE1 is handed over to the sameneighboring cell, the cross-cell D2D communication link maybe interrupted again. Afterwards, D2D communication is re-established in the same neighboring cell. As a result, how toprovide more reliable D2D communication and maintain theProSe service continuity support is an important direction.

III. PROPOSED SCHEME

A. Joint Handover ProcedureConsidering the social behavior of the ProSe-enabled UEs,

all the ProSe-enabled UEs in ongoing D2D communication

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may enter the neighboring cells sooner or later. Since theProSe-enabled UEs are often in close proximity to each other,it might be useful for the network to consider the jointhandover to another cell so as to provide the ProSe servicecontinuity. In Fig. 2, we further discuss the details of the JointHandover procedure from four perspectives: Measurementscheme (step 1 - step 2), X2-interface scheme (step 3 - step6), joint handover scheme (step 7 - step 11), and path switchscheme (step 12 - step 18).

Fig. 2. Joint Handover procedure

1) Measurement scheme: As illustrated in Fig. 2 (Step 1 -Step 2), the eNB first transmits a network layer (L3) signal-ing, Measurement Control, through an RRC (Radio ResourceControl) Connection Reconfiguration message when RRCconnection is established. Measurement Control indicates therequired information of measurement, including MeasurementID, Measurement Object, Reporting Configuration, Measure-ment Gap, etc. The source eNB sends Measurement Controlto the ProSe UEs by Multicast Control Channel (MCCH).The reason is that the ProSe UEs use the same context andmessage. Then the UE reports the Measurement Reports tothe eNB periodically or by events through Dedicated ControlChannel (DCCH).

• D2D Measurement ID: Measurement ID is used toidentify Measurement Objects, to which the UE shouldmeasure its signal strength. The UE uses Measurement

ID to measure the RSRP of the target eNBs. RegardingD2D communication handover, the ProSe UE measuresnot only signal to the target eNB, but also signal amongits ongoing D2D communication ProSe UEs. That is,D2D pair or D2D group should be assigned a new IDto measure the D2D communication signal among eachother.

• Measurement Object: Measurement Object, specified byMeasurement ID, provides information about E-UTRAnetworks to be measured by a UE, such as frequencychannel number, Physical Cell ID (PCI) of the cells tobe measured. In LTE handover, a UE will not measurethe signal strength to a cell belonging to different PublicLand Mobile Network (PLMN). That is, the UE alwaysmeasures the signal strength to eNBs within the samePLMN.

2) X2-interface scheme: As illustrated in Fig 2 (Step 3- Step 6), the source eNB requests a handover by sendinga Handover Request message to the target eNB. Throughthis message, the source eNB delivers the stored contextinformation of ProSe UEs.

• Handover Request message: This message is deliveredby the source eNB to the target eNB. The informationincluded in the message is as follows [8]:

– UE Context information: the ProSe UE contextstored at the source eNB.∗ D2D bearer: the information of ongoing D2D

communication resource [9] [10].∗ E-RAB to be setup: E-RAB (EUTRAN Radio Ac-

cess Bearer) of the ProSe UE information storedat the source eNB.

∗ UE Security Capability: security algorithms sup-ported by the ProSe UE (encryption and integrityalgorithm).

– D2D handover type: the source eNB decides the D2Dhandover type, based on the D2D Handover Decisionmethod.

• Admission Control: Upon receiving the Handover Re-quest message, the target eNB begins handover prepara-tion to ensure seamless service provision for the ProSeUE. Using AS (Access Stratum) security keys, the targeteNB can communicate securely with the ProSe UEs overthe radio link when the UE accesses. Next, the targeteNB, based on the information of E-RAB to be setupand D2D bearer, checks if the same QoS (Quality-of-Service) provided by the source eNB is available at thetarget eNB as well. If available, it establishes an uplink(UL) S1 bearer connecting to service gateway (S-GW) byusing the information of UL S1 bearer and D2D resourcereservation stored at the source eNB. The target eNBnotifies the ProSe Function that D2D is performing han-dover and the ProSe Function authenticates identificationof the ProSe UEs. On the basis of the information of theE-RAB, D2D bearer and QoS, the target eNB reservesRRC resources for the UE to use over the radio link andallocates Cell Radio Network Temporary Identifier (C-RNTI).

• Handover Request Acknowledge message: This mes-sage is delivered by the target eNB to the source eNB

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if the target eNB has successfully completed resourceallocation.

• Handover Preparation Failure message: This messageis delivered by the target eNB to the source eNB ifresource allocation at the target eNB fails.

3) Joint handover scheme: As illustrated in Fig. 2 (Step7 - Step 11), once the source eNB completes the handoverpreparation with the target eNB, it commands the ProSe UEto perform a handover by sending a Joint Handover Commandmessage. The UE detaches from the source eNB and accessesto the target eNB. The target eNB becomes capable of sendingand receiving packets once the UE has successfully accessedto it.

After receiving the Joint Handover Command message, theProSe UEs obtain C-RNTIs, dedicated random access channel(RACH) Preambles and Target Data Radio Bearer (DRB) IDfor the target cell. The ProSe UEs detect the synchronizationsignal from the target eNB. Once synchronized with the targeteNB, the ProSe UEs initiate non-contention based randomaccess. The target eNB sends the ProSe UEs the information oftiming alignment and D2D Grant [11]. The ProSe UEs sendthe target eNB a Handover Confirm message. Now, the UEcan send/receive packets to/from the target eNB and use D2Dresources of the target eNB to perform D2D communication.The D2D seamless handover has completed.

4) Path switch scheme: As shown in Fig. 2 (Step 12 -Step 18), once the ProSe UE completes its radio access tothe target eNB successfully, the bearer path of the ProSeUE is connected to the target eNB. The target eNB informsEvolved Packet Core (EPC) and sends a Path Switch Requestmessage to Mobility Management Entity (MME), so that theEvolved Packet System (EPS) bearer path can be modifiedaccordingly. MME requests S-GW for S1 bearer modification.Upon request, the S-GW establishes a downlink (DL) S1bearer that connects to the target eNB. Then it stops sendingDL packets to the source eNB, and begins to send them tothe target eNB through the newly established DL bearer. TheMME informs the target eNB that the DL S1 bearer path hasbeen modified. The target eNB sends the source eNB a UEContext Release message, allowing the source eNB to releasethe D2D resource.

B. Half Handover ProcedureIn some cases, Joint Handover procedure cannot perform

because the ProSe UEs in ongoing D2D communication arenot in close proximity to each other, or because one of theProSe UEs may be handed over to its neighboring cell. In viewof this, we proposed a Half Handover procedure to maintainservice continuity. In Half Handover procedure, one of theProSe UEs can be handed over to the target eNB, and the otherstill in the source eNB. Measurement scheme (step 1 - step2) and X2-interface scheme (step 3 - step 6) are the same asthe Joint Handover procedure. In Fig. 3, we discuss the detailsof the Half Handover procedure from two perspectives, halfhandover scheme (step 7 - step 15) and path switch scheme(step 16 - step 22).

1) Half handover scheme: As illustrated in Fig. 3 (Step7 - Step 15), once the source eNB completes the handoverpreparation with the target eNB, it orders the UE to per-form a handover by sending a Half Handover Command

Fig. 3. Half Handover procedure

message. A Half Handover Command message contains C-RNTI, dedicated RACH Preamble and DRB ID to be used atthe target cell. The ProSe UE will perform Half Handoverwhen it receives a Half Handover Command specified forit (e.g., UE1). If the ProSe UE receives a Half HandoverCommand not specified to it (e.g., UE2), it will store the HalfHandover Command message and wait for next trigger. TheUE1 detects the synchronization signal from the target eNB.Once synchronized with the target eNB, the UE1 initiatesnon-contention based random access. The target eNB sendsthe UE1 the information of timing alignment and D2D Grant.In the meanwhile, D2D communication still continues usingD2D resources of the source eNB. The UE2 waits to triggera handover to the target eNB. However, the target eNB thatthe UE2 measures should be the same as the eNB to whichthe UE1 hands over. When the UE2 satisfies the handovercondition, it uses the stored Half Handover Command messageto conduct the same procedure as the UE1. Now, the ProSeUEs can send/receive packets to/from the target eNB anduses the D2D resources of the target eNB to perform D2Dcommunication. The D2D seamless handover has completed.

HoTimer is a timer which starts counting when the UE2

receives a Half Handover Command. When HoTimer timesout, it will cause Half Handover failure. Half Handover failurehas two reasons: 1. the UE2 may not cross the serving cell

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during HoTimer and 2. the signal strength of D2D pair isbelow the D2D threshold and this phenomenon continueslonger than the duration of HoTimer, which means the ProSeUEs may move away from each other. The ProSe UEs performmode switch (from D2D/cellular mode to cellular/D2D mode)[12]. The reason is the signal strength of D2D pair will degradeas time passes by.

2) Path switch: If the source eNB performs Half Handoversuccessfully, the path switch scheme is the same as JointHandover procedure. Nevertheless, if not, the ProSe UEsperform mode switch and D2D communication is interrupted.The UE1 will performs path switch to the target eNB alone.

C. D2D Handover Decision MethodThe handover decision method, as illustrated in Fig. 4, is

a basic but effective handover algorithm consisting of severalvariables, Handover Margin (HOM), Time to Trigger (TTT)timer, LTE threshold (LTETh), D2D threshold (D2DTh) andTime to Trigger of D2D (TTTD). These variables assist inmaking handover decisions, such as Joint Handover procedure,Half Handover procedure, or no handover.

Fig. 4. D2D handover decision method

HOM is a constant variable that represents a threshold of thedifference between the received signal strength to the sourceeNB and the received signal strength to the target eNBs. Thereceived signal strength is called reference signal receivingpower (RSRP) in an LTE system. HOM ensures the target eNBas the most appropriate eNB for the ProSe UE to camp on. ATTT value is the time interval that is required to satisfy HOMcondition [13]. A handover action can only be performed afterthe TTT condition has been satisfied. The ProSe UEs can usedifferent values of TTT. Both HOM and TTT are used forreducing unnecessary handovers, called “Ping-Pong effect”.When the ProSe UE is experiencing the Ping-Pong effect, itwill perform a handover from the source eNB to the target eNBand back again during a short period of time. In such a case,the required signaling exchanges and resources increase, whichin turn decreases the system throughput and increases the datatraffic delay caused by buffering the incoming traffic at thetarget eNB during each handover. Therefore, it is essential to

prevent unnecessary handovers. A handover is triggered whenthe triggering conditions, Eq. (1) and Eq. (2), are both satisfied.

RSRPT > RSRPS +HOM (1)

HO Trigger > TTT (2)

,where RSRPT and RSRPS are the RSRP received from thetarget eNB and the source eNB, respectively and HOTriggeris the handover trigger timer which starts counting when Eq.(1) gets satisfied [13].

LTETh is a constant variable that represents whether thesource eNB can provide basic services to the ProSe UEs. Ifthe RSRP from the source eNB is greater than LTETh, thesource eNB can provide the services to the ProSe UEs. Incontrast, if the RSRP from the source eNB is less than theLTETh, the source eNB cannot provide the services to theProSe UEs. D2D signal is the radio signal strength betweenProSe UEs. We propose D2DTh to determine the radio signalstrength of D2D quality. D2D Trigger is a D2D signal qualitytimer which starts counting when Eq. (3) gets satisfied. A Timeto Trigger of D2D (TTTD) value is the time interval that isrequired to satisfy Eq. (3). The eNB makes the D2D handoverdecision based on the condition in Eq. (1), (2), (3) and (4)illustrated in Fig. 4.

D2D signal > D2DTh (3)

D2D Trigger > TTTD (4)

Based on our D2D handover decision method, we list allcombinations of signal quality, as shown in Table I. Signalquality to the source eNB is in the “S” column. A “+” indicatesthat the UE can receive basic services from the source eNB.In contrast, a “-” indicates that the source eNB cannot provideservices to the ProSe UE. Signal quality to the target eNB is inthe “T” column. A “+” indicates that the triggering conditionsin Eq. (1) and (2) are both satisfied. In the D2D column, a “+”indicates the condition that signal strength in the D2D pairis greater than D2DTh continues longer than TTTD, whichmeans that the ProSe UEs may be in close proximity. On theother hand, a “-” in the D2D column indicates the conditionthat the signal strength in the D2D pair is higher than D2DTh

cannot continue a period of TTTD; this means that the ProSeUEs may be moving away from each other. The HO types areindicated in the type column, i.e., Joint, Half, and no HO type.

TABLE ID2D DECISION TRUTH TABLE

UEA UEB D2D type UEA UEB D2D typeS T S T S T S T+ + + - none No - + + + + Joint+ - + + none No - + + + - Half+ + - + + Joint + + - + - Half+ + + + none No + + - - none No+ - + - none No + - - + none Half+ - - - none No - + + - none Half- + - + none Joint - + - - none Half- - + + none No - - + - none No- - - + none Half - - - - none No

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Road

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TABLE IIPARAMETER SETTINGS

Parameter SettingNetwork layout Hexagonal grid, 19 sitesInter-Site distance 500 mNumber of D2D pairs 120Carrier frequency 2 GHzMacro BS downlink power 46 dBmD2D transmission power 23 dBmMinimum association RSRP -112 dBmfor D2D communicationCellular path loss and fading Macro Urban [6]D2D path loss and fading Winner + B1 [6]HOM & TTT 3dB & 100 msTTTD 50 msSimulation time 180 secMobility model SUMO [2]UE velocity 3, 15, 30 km/h

IV. SIMULATION

A. Simulation methodology

We use SUMO [2] to simulate the network layout in anurban setting. As is shown in Fig. 5, UEs are randomlyand uniformly placed on roads, which are indicated by reddotted lines. We randomly select the UEs, which are abovethe minimum association RSRP, to form D2D pairs. Basedon 3GPP discussion [14], we assume that ProSe UEs whichform D2D pairs must be connected to the same eNB andindependently traced in SUMO. As such, moving as a pairrarely occurs. The D2D pair conduct mode switch based onEq. (3) and (4). If the ProSe UEs stay in a cellular mode, itperforms LTE handover. We also set the traffic lights on eachintersection, as they play an important role on UE mobilityand handover times. The main simulation settings follow thesimulation guidelines recommended by 3GPP [6] and areshown in Table II.

Three performance metrics under consideration are D2DHO failure rate, D2D mode ratio, and amount of LTE HO,D2D HO and mode switch. First, D2D HO failure rate is ratiothat the number of D2D HO failures compared with the totalnumber of D2D HO. D2D HO failure happens when the signal

quality in the D2D pair or the RSRP to source eNB is so poor.Secondly, D2D mode ratio, indicated in Eq. (5), representsthe duration in D2D communication in the simulation time.Thirdly, the amount of LTE HO, D2D HO and mode switchis the total number of event trigger in the simulation time.

D2D mode ratio =The duration in D2Dmode

Simulation time(5)

B. Simulation Results

The proposed scheme maintains ProSe service continuitywhile reducing the D2D HO failure rate and the number ofLTE HO, D2D HO, and mode switch. Compare to the LTE A3HO scheme, Yilmaz’s D2D-aware handover (D-A) [5] scheme,only the Joint HO procedure (our Joint scheme), and only theHalf HO procedure (our Half scheme), the proposed schemeuses both Joint HO and Half HO procedures based on the D2Dhandover decision method.

As shown in Fig. 6 and Fig. 7, the D2D HO failure rateof the proposed scheme is lower than that of other schemes.In addition, the D2D mode ratio is also greater than thatof other schemes. The proposed scheme enable the ProSeUEs to stay in the D2D mode for a longer time so thatit maintains better service continuity. As the speed of theProSe UEs increases, D2D mode ratio decreases and D2DHO failure rate increases. That is because ProSe UEs movenot only straight at the same direction but also away fromeach other in SUMO mobility model. In LTE HO scheme andour Joint HO scheme, the D2D HO failure occurs and D2Dcommunication is interrupted when both UEs in a D2D pairdo not hand over to the same eNB simultaneously. In SUMOmobility model, each ProSe UE is independently traced. Thecondition that UEs simultaneously move as a pair to the samecell rarely happens. Thus, the D2D pairs experience a higherD2D HO failure rate and a lower D2D mode ratio in LTEHO scheme and our Joint HO scheme. In D-A HO scheme,a UE, having a better signal strength to the target eNB, waitsfor the paired UE, until the signal quality of the target eNBis able to meet the HO threshold for both UEs. However, ifthe signal quality of the source eNB becomes lower than apredefined HO threshold, a HO failure will occur and theD2D link will be severed. In our Half HO scheme, the ProSeUE, which is specified to hand over to the target eNB viathe HO command, still continues to use the D2D resourcesof the source eNB for D2D communication. As such, the UEwhich has already handed over to the target eNB will have agood signal quality, as opposed to UEs which use the D-A HOscheme. Although the time for the D2D pair to both hand overto the target eNB and use new D2D resources may take longerin our Half HO scheme than that in D-A HO scheme, at leastthe our Half HO scheme guarantees the D2D communicationfree from interruption.

In Fig. 8, the amount of LTE HO, D2D HO and modeswitch of our proposed scheme is lower than that of otherschemes. This attributes to the fact that our proposed schemehas a low D2D HO failure rate and a high D2D mode ratio.When the speed of the ProSe UE increases, the D2D HOfailure rate increases and D2D mode ratio decreases. That is,the time duration for a ProSe UE to stay in D2D mode isshort, so the number of D2D handover trigger decreases. It

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Fig. 6. The duration in D2D mode

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Fig. 7. D2D handover failure rate

shows that our proposed scheme effectively reduces the extrasignalling associated with mode switch, D2D communicationre-establishment, the number of LTE HO and D2D serviceinterruption.

V. CONCLUSION

In this paper a D2D handover mechanism, which fulfilsthe ProSe service continuity requirements for D2D commu-nication, is proposed. On the basis of our D2D handoverdecision method, the proposed scheme uses a hybrid Joint andHalf handover scheme. When the Joint Handover procedureis triggered, all the ProSe UEs hand over to the target eNBtogether. In the Half Handover procedure, the first ProSe-enabled UE will hand over to the target eNB, while thesecond UE still remains connected to the source eNB. TheHalf Handover scheme allows the second ProSe-enabled UEto hand over seamlessly in a short period of time. Thesimulation results prove that our scheme not only reduces thenumber of LTE HO, D2D HO, and mode switch, but alsosuccessfully minimizes the D2D HO failure rate. Differentfrom the previous work [5], our proposed scheme takes into

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Fig. 8. The number of LTE HO, D2D HO and mode switches.

account the signal quality between UEs in a D2D pair to decidethe proper timing to handover.

ACKNOWLEDGEMENT

This work was in part supported by Industrial TechnologyResearch Institute, and Ministry of Science and Technol-ogy under Grants MOST 103-2221-E-002-086-MY3 and 102-2221-E-002-077-MY2.

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