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Handover Mechanism for Device-to-Device Communication Ho-Yuan Chen, Mei-Ju Shih, Hung-Yu Wei∗ Graduate Institute of Communication Engineering, National Taiwan University, Taiwan ∗
[email protected]
Abstract—Device-to-Device (D2D) communication is a proximity-based technique used by Long Term EvolutionAdvanced (LTE-A) systems. When a ProSe-enabled UE in D2D communication moves across a cell boundary, seamless handover is expected. However, LTE-A does not specify D2D handover procedures, and the current LTE-A handover procedures cannot support the ProSe service continuity requirements [1]. Therefore, an efficient D2D handover mechanism is necessary to meet the requirements. In this paper, a D2D handover mechanism, which considers the signal quality of D2D pairs, is proposed. The proposed mechanism includes a Joint Handover procedure and a Half Handover procedure based on a D2D handover decision method. Simulation of Urban Mobility (SUMO) [2] is adopted for the mobility model and performance evaluation simulations. The properties of this mechanism satisfy the ProSe service continuity requirements while decreasing the D2D HO failure rate as well as reducing the amount of information exchanged between the source eNB and the target eNB. Index Terms—device-to-device, ProSe, handover, service continuity, mobility, SUMO.
I. I NTRODUCTION D2D communication in Long Term Evolution-Advanced (LTE-A) systems has been a hot issue nowadays, for the great demand increases in the wireless communication system. D2D communication is selected as a promising technology to realize Public Safety (PS) and for commercial usage. However, implementation of D2D communication faces a number of challenges. With regard to PS scenarios, distributed D2D communication should be made possible. A distributed synchronization method is proposed to achieve D2D discovery and synchronization at the same time [3]. To reach collision free in a short time, a distributed feedbackless resource allocation scheme for D2D broadcast communication is proposed [4]. In this work, we handle another important issue: the mobility management problem caused by D2D communication handover. Nevertheless, little research is available on handover of D2D communication. A D2D-aware handover (D-A HO) solution is proposed to permit a UE to postpone the handover to a target eNB until signal quality of the source eNB becomes lower than a threshold [5]. When the signal quality of the target eNB is able to meet the D2D HO conditions for both UEs, both UEs will handover to the target eNB simultaneously. The prior work does not account for the signal quality of the D2D pairs nor does it discuss mobility in different directions. The third generation partnership project (3GPP) agrees that D2D discovery and communication will become one of the new features to be studied during 3GPP Rel-12 and Rel13 under the LTE ProSe study item [6]. According to the scope of Rel-13 [7], the ProSe continuity is considered as an important feature for D2D communication enhancement. "Copyright 2015 held by Ho-Yuan Chen, Mei-Ju Shih, and Hung-Yu Wei. Publication Rights Licensed to IEEE"
Fig. 1. After UE2 does LTE handover, ProSe service continuity is interrupted
Device mobility, which affects performance of ProSe service continuity, is one of the factors deserved to be considered. When the ProSe-enabled UE in D2D communication moves across the cell boundary, seamless handover is expectedso as to provide PS service continuity. How to provide the ProSe service continuity is challenging. II. P ROBLEM S TATEMENT D2D handover problem occurs because legacy LTE system cannot support D2D handover. When D2D pairs perform LTE HO, several drawbacks might emerge, such as latency, extra resource wasting, extra signaling exchange, and interrupted D2D link. For example, as shown in Fig. 1, the ProSe-enabled UEs perform ongoing D2D communication in the same cell and move forward the same direction at first. Then the UE2 might be handed over to its neighboring cell. The D2D communication link may be interrupted because the UE2 performs LTE handover. After LTE HO, cross-cell D2D communication is re-established. When the UE1 is handed over to the same neighboring cell, the cross-cell D2D communication link may be interrupted again. Afterwards, D2D communication is reestablished in the same neighboring cell. As a result, how to provide more reliable D2D communication and maintain the ProSe service continuity support is an important direction. III. P ROPOSED SCHEME A. Joint Handover Procedure Considering 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 the ProSe-enabled UEs are often in close proximity to each other, it might be useful for the network to consider the joint handover to another cell so as to provide the ProSe service continuity. In Fig. 2, we further discuss the details of the Joint Handover procedure from four perspectives: Measurement scheme (step 1 - step 2), X2-interface scheme (step 3 - step 6), joint handover scheme (step 7 - step 11), and path switch scheme (step 12 - step 18).
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ID to measure the RSRP of the target eNBs. Regarding D2D communication handover, the ProSe UE measures not only signal to the target eNB, but also signal among its ongoing D2D communication ProSe UEs. That is, D2D pair or D2D group should be assigned a new ID to measure the D2D communication signal among each other. Measurement Object: Measurement Object, specified by Measurement ID, provides information about E-UTRA networks to be measured by a UE, such as frequency channel number, Physical Cell ID (PCI) of the cells to be measured. In LTE handover, a UE will not measure the signal strength to a cell belonging to different Public Land Mobile Network (PLMN). That is, the UE always measures the signal strength to eNBs within the same PLMN.
2) X2-interface scheme: As illustrated in Fig 2 (Step 3 - Step 6), the source eNB requests a handover by sending a Handover Request message to the target eNB. Through this message, the source eNB delivers the stored context information of ProSe UEs. •
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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) signaling, Measurement Control, through an RRC (Radio Resource Control) Connection Reconfiguration message when RRC connection is established. Measurement Control indicates the required information of measurement, including Measurement ID, Measurement Object, Reporting Configuration, Measurement Gap, etc. The source eNB sends Measurement Control to the ProSe UEs by Multicast Control Channel (MCCH). The reason is that the ProSe UEs use the same context and message. Then the UE reports the Measurement Reports to the eNB periodically or by events through Dedicated Control Channel (DCCH). • D2D Measurement ID: Measurement ID is used to identify Measurement Objects, to which the UE should measure its signal strength. The UE uses Measurement
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Handover Request message: This message is delivered by the source eNB to the target eNB. The information included in the message is as follows [8]: – UE Context information: the ProSe UE context stored at the source eNB. ∗ D2D bearer: the information of ongoing D2D communication resource [9] [10]. ∗ E-RAB to be setup: E-RAB (EUTRAN Radio Access Bearer) of the ProSe UE information stored at the source eNB. ∗ UE Security Capability: security algorithms supported by the ProSe UE (encryption and integrity algorithm). – D2D handover type: the source eNB decides the D2D handover type, based on the D2D Handover Decision method. Admission Control: Upon receiving the Handover Request message, the target eNB begins handover preparation to ensure seamless service provision for the ProSe UE. Using AS (Access Stratum) security keys, the target eNB can communicate securely with the ProSe UEs over the radio link when the UE accesses. Next, the target eNB, based on the information of E-RAB to be setup and D2D bearer, checks if the same QoS (Quality-ofService) provided by the source eNB is available at the target eNB as well. If available, it establishes an uplink (UL) S1 bearer connecting to service gateway (S-GW) by using the information of UL S1 bearer and D2D resource reservation stored at the source eNB. The target eNB notifies the ProSe Function that D2D is performing handover and the ProSe Function authenticates identification of the ProSe UEs. On the basis of the information of the E-RAB, D2D bearer and QoS, the target eNB reserves RRC resources for the UE to use over the radio link and allocates Cell Radio Network Temporary Identifier (CRNTI). Handover Request Acknowledge message: This message is delivered by the target eNB to the source eNB
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if the target eNB has successfully completed resource allocation. • Handover Preparation Failure message: This message is delivered by the target eNB to the source eNB if resource allocation at the target eNB fails. 3) Joint handover scheme: As illustrated in Fig. 2 (Step 7 - Step 11), once the source eNB completes the handover preparation with the target eNB, it commands the ProSe UE to perform a handover by sending a Joint Handover Command message. The UE detaches from the source eNB and accesses to the target eNB. The target eNB becomes capable of sending and receiving packets once the UE has successfully accessed to it. After receiving the Joint Handover Command message, the ProSe UEs obtain C-RNTIs, dedicated random access channel (RACH) Preambles and Target Data Radio Bearer (DRB) ID for the target cell. The ProSe UEs detect the synchronization signal from the target eNB. Once synchronized with the target eNB, the ProSe UEs initiate non-contention based random access. The target eNB sends the ProSe UEs the information of timing alignment and D2D Grant [11]. The ProSe UEs send the target eNB a Handover Confirm message. Now, the UE can send/receive packets to/from the target eNB and use D2D resources 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 to the target eNB successfully, the bearer path of the ProSe UE is connected to the target eNB. The target eNB informs Evolved Packet Core (EPC) and sends a Path Switch Request message to Mobility Management Entity (MME), so that the Evolved Packet System (EPS) bearer path can be modified accordingly. MME requests S-GW for S1 bearer modification. Upon request, the S-GW establishes a downlink (DL) S1 bearer that connects to the target eNB. Then it stops sending DL packets to the source eNB, and begins to send them to the target eNB through the newly established DL bearer. The MME informs the target eNB that the DL S1 bearer path has been modified. The target eNB sends the source eNB a UE Context Release message, allowing the source eNB to release the D2D resource. B. Half Handover Procedure In some cases, Joint Handover procedure cannot perform because the ProSe UEs in ongoing D2D communication are not in close proximity to each other, or because one of the ProSe UEs may be handed over to its neighboring cell. In view of this, we proposed a Half Handover procedure to maintain service continuity. In Half Handover procedure, one of the ProSe UEs can be handed over to the target eNB, and the other still in the source eNB. Measurement scheme (step 1 - step 2) and X2-interface scheme (step 3 - step 6) are the same as the Joint Handover procedure. In Fig. 3, we discuss the details of the Half Handover procedure from two perspectives, half handover scheme (step 7 - step 15) and path switch scheme (step 16 - step 22). 1) Half handover scheme: As illustrated in Fig. 3 (Step 7 - Step 15), once the source eNB completes the handover preparation with the target eNB, it orders the UE to perform a handover by sending a Half Handover Command
Fig. 3. Half Handover procedure
message. A Half Handover Command message contains CRNTI, dedicated RACH Preamble and DRB ID to be used at the target cell. The ProSe UE will perform Half Handover when it receives a Half Handover Command specified for it (e.g., UE1 ). If the ProSe UE receives a Half Handover Command not specified to it (e.g., UE2 ), it will store the Half Handover Command message and wait for next trigger. The UE1 detects the synchronization signal from the target eNB. Once synchronized with the target eNB, the UE1 initiates non-contention based random access. The target eNB sends the UE1 the information of timing alignment and D2D Grant. In the meanwhile, D2D communication still continues using D2D resources of the source eNB. The UE2 waits to trigger a handover to the target eNB. However, the target eNB that the UE2 measures should be the same as the eNB to which the UE1 hands over. When the UE2 satisfies the handover condition, it uses the stored Half Handover Command message to conduct the same procedure as the UE1 . Now, the ProSe UEs can send/receive packets to/from the target eNB and uses the D2D resources of the target eNB to perform D2D communication. The D2D seamless handover has completed. HoTimer is a timer which starts counting when the UE2 receives a Half Handover Command. When HoTimer times out, it will cause Half Handover failure. Half Handover failure has 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 is below the D2D threshold and this phenomenon continues longer than the duration of HoTimer, which means the ProSe UEs may move away from each other. The ProSe UEs perform mode switch (from D2D/cellular mode to cellular/D2D mode) [12]. The reason is the signal strength of D2D pair will degrade as time passes by. 2) Path switch: If the source eNB performs Half Handover successfully, the path switch scheme is the same as Joint Handover procedure. Nevertheless, if not, the ProSe UEs perform mode switch and D2D communication is interrupted. The UE1 will performs path switch to the target eNB alone. C. D2D Handover Decision Method The handover decision method, as illustrated in Fig. 4, is a basic but effective handover algorithm consisting of several variables, Handover Margin (HOM), Time to Trigger (TTT) timer, LTE threshold (LTET h ), D2D threshold (D2DT h ) and Time to Trigger of D2D (TTTD ). These variables assist in making 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 the difference between the received signal strength to the source eNB and the received signal strength to the target eNBs. The received signal strength is called reference signal receiving power (RSRP) in an LTE system. HOM ensures the target eNB as the most appropriate eNB for the ProSe UE to camp on. A TTT value is the time interval that is required to satisfy HOM condition [13]. A handover action can only be performed after the TTT condition has been satisfied. The ProSe UEs can use different values of TTT. Both HOM and TTT are used for reducing unnecessary handovers, called “Ping-Pong effect”. When the ProSe UE is experiencing the Ping-Pong effect, it will perform a handover from the source eNB to the target eNB and back again during a short period of time. In such a case, the required signaling exchanges and resources increase, which in turn decreases the system throughput and increases the data traffic delay caused by buffering the incoming traffic at the target eNB during each handover. Therefore, it is essential to
prevent unnecessary handovers. A handover is triggered when the triggering conditions, Eq. (1) and Eq. (2), are both satisfied. RSRPT > RSRPS + HOM
(1)
HO T rigger > T T T
(2)
,where RSRPT and RSRPS are the RSRP received from the target eNB and the source eNB, respectively and HOTrigger is the handover trigger timer which starts counting when Eq. (1) gets satisfied [13]. LTET h is a constant variable that represents whether the source eNB can provide basic services to the ProSe UEs. If the RSRP from the source eNB is greater than LTET h , the source eNB can provide the services to the ProSe UEs. In contrast, if the RSRP from the source eNB is less than the LTET h , the source eNB cannot provide the services to the ProSe UEs. D2D signal is the radio signal strength between ProSe UEs. We propose D2DT h to determine the radio signal strength of D2D quality. D2D Trigger is a D2D signal quality timer which starts counting when Eq. (3) gets satisfied. A Time to Trigger of D2D (TTTD ) value is the time interval that is required to satisfy Eq. (3). The eNB makes the D2D handover decision based on the condition in Eq. (1), (2), (3) and (4) illustrated in Fig. 4. D2D signal > D2DT h
(3)
D2D T rigger > T T TD
(4)
Based on our D2D handover decision method, we list all combinations of signal quality, as shown in Table I. Signal quality to the source eNB is in the “S” column. A “+” indicates that the UE can receive basic services from the source eNB. In contrast, a “-” indicates that the source eNB cannot provide services to the ProSe UE. Signal quality to the target eNB is in the “T” column. A “+” indicates that the triggering conditions in Eq. (1) and (2) are both satisfied. In the D2D column, a “+” indicates the condition that signal strength in the D2D pair is greater than D2DT h continues longer than TTTD , which means that the ProSe UEs may be in close proximity. On the other hand, a “-” in the D2D column indicates the condition that the signal strength in the D2D pair is higher than D2DT h cannot continue a period of TTTD ; this means that the ProSe UEs may be moving away from each other. The HO types are indicated in the type column, i.e., Joint, Half, and no HO type. TABLE I D2D D ECISION TRUTH TABLE UEA S T + + + + + + + + + + -
UEB S T + + + + + + + + + + +
D2D
type
none none + none none none none none none
No No Joint No No No Joint No Half
UEA S T + + + + + + + + + -
UEB S T + + + + + + + + -
D2D
type
+ none none none none none none
Joint Half Half No Half Half Half No No
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Topology Traffic light Road 1000
quality in the D2D pair or the RSRP to source eNB is so poor. Secondly, D2D mode ratio, indicated in Eq. (5), represents the duration in D2D communication in the simulation time. Thirdly, the amount of LTE HO, D2D HO and mode switch is the total number of event trigger in the simulation time.
y (m)
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T he duration in D2Dmode Simulation time
(5)
B. Simulation Results −500
−1000 −1000
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Fig. 5. Network layout
TABLE II PARAMETER SETTINGS Parameter Network layout Inter-Site distance Number of D2D pairs Carrier frequency Macro BS downlink power D2D transmission power Minimum association RSRP for D2D communication Cellular path loss and fading D2D path loss and fading HOM & TTT TTTD Simulation time Mobility model UE velocity
Setting Hexagonal grid, 19 sites 500 m 120 2 GHz 46 dBm 23 dBm -112 dBm Macro Urban [6] Winner + B1 [6] 3dB & 100 ms 50 ms 180 sec SUMO [2] 3, 15, 30 km/h
IV. S IMULATION A. Simulation methodology We use SUMO [2] to simulate the network layout in an urban setting. As is shown in Fig. 5, UEs are randomly and uniformly placed on roads, which are indicated by red dotted lines. We randomly select the UEs, which are above the minimum association RSRP, to form D2D pairs. Based on 3GPP discussion [14], we assume that ProSe UEs which form D2D pairs must be connected to the same eNB and independently traced in SUMO. As such, moving as a pair rarely occurs. The D2D pair conduct mode switch based on Eq. (3) and (4). If the ProSe UEs stay in a cellular mode, it performs LTE handover. We also set the traffic lights on each intersection, as they play an important role on UE mobility and handover times. The main simulation settings follow the simulation guidelines recommended by 3GPP [6] and are shown in Table II. Three performance metrics under consideration are D2D HO failure rate, D2D mode ratio, and amount of LTE HO, D2D HO and mode switch. First, D2D HO failure rate is ratio that the number of D2D HO failures compared with the total number of D2D HO. D2D HO failure happens when the signal
The proposed scheme maintains ProSe service continuity while reducing the D2D HO failure rate and the number of LTE HO, D2D HO, and mode switch. Compare to the LTE A3 HO scheme, Yilmaz’s D2D-aware handover (D-A) [5] scheme, only the Joint HO procedure (our Joint scheme), and only the Half HO procedure (our Half scheme), the proposed scheme uses both Joint HO and Half HO procedures based on the D2D handover decision method. As shown in Fig. 6 and Fig. 7, the D2D HO failure rate of the proposed scheme is lower than that of other schemes. In addition, the D2D mode ratio is also greater than that of other schemes. The proposed scheme enable the ProSe UEs to stay in the D2D mode for a longer time so that it maintains better service continuity. As the speed of the ProSe UEs increases, D2D mode ratio decreases and D2D HO failure rate increases. That is because ProSe UEs move not only straight at the same direction but also away from each other in SUMO mobility model. In LTE HO scheme and our Joint HO scheme, the D2D HO failure occurs and D2D communication is interrupted when both UEs in a D2D pair do not hand over to the same eNB simultaneously. In SUMO mobility model, each ProSe UE is independently traced. The condition that UEs simultaneously move as a pair to the same cell rarely happens. Thus, the D2D pairs experience a higher D2D HO failure rate and a lower D2D mode ratio in LTE HO scheme and our Joint HO scheme. In D-A HO scheme, a UE, having a better signal strength to the target eNB, waits for the paired UE, until the signal quality of the target eNB is able to meet the HO threshold for both UEs. However, if the signal quality of the source eNB becomes lower than a predefined HO threshold, a HO failure will occur and the D2D link will be severed. In our Half HO scheme, the ProSe UE, which is specified to hand over to the target eNB via the HO command, still continues to use the D2D resources of the source eNB for D2D communication. As such, the UE which has already handed over to the target eNB will have a good signal quality, as opposed to UEs which use the D-A HO scheme. Although the time for the D2D pair to both hand over to the target eNB and use new D2D resources may take longer in our Half HO scheme than that in D-A HO scheme, at least the our Half HO scheme guarantees the D2D communication free from interruption. In Fig. 8, the amount of LTE HO, D2D HO and mode switch of our proposed scheme is lower than that of other schemes. This attributes to the fact that our proposed scheme has a low D2D HO failure rate and a high D2D mode ratio. When the speed of the ProSe UE increases, the D2D HO failure rate increases and D2D mode ratio decreases. That is, the time duration for a ProSe UE to stay in D2D mode is short, so the number of D2D handover trigger decreases. It
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LTE A3 HO D−A HO Our proposed Our Joint HO Our Half HO
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Fig. 8. The number of LTE HO, D2D HO and mode switches. Fig. 6. The duration in D2D mode
LTE A3 HO
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ACKNOWLEDGEMENT This work was in part supported by Industrial Technology Research Institute, and Ministry of Science and Technology under Grants MOST 103-2221-E-002-086-MY3 and 1022221-E-002-077-MY2.
0.8 0.7 D2D HO failure rate
account the signal quality between UEs in a D2D pair to decide the proper timing to handover.
0.6 0.5 0.4
R EFERENCES
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Fig. 7. D2D handover failure rate
shows that our proposed scheme effectively reduces the extra signalling associated with mode switch, D2D communication re-establishment, the number of LTE HO and D2D service interruption. V. C ONCLUSION In this paper a D2D handover mechanism, which fulfils the ProSe service continuity requirements for D2D communication, is proposed. On the basis of our D2D handover decision method, the proposed scheme uses a hybrid Joint and Half handover scheme. When the Joint Handover procedure is triggered, all the ProSe UEs hand over to the target eNB together. In the Half Handover procedure, the first ProSeenabled UE will hand over to the target eNB, while the second UE still remains connected to the source eNB. The Half Handover scheme allows the second ProSe-enabled UE to hand over seamlessly in a short period of time. The simulation results prove that our scheme not only reduces the number of LTE HO, D2D HO, and mode switch, but also successfully minimizes the D2D HO failure rate. Different from the previous work [5], our proposed scheme takes into
[1] 3GPP TR 36.300 V12.5.0, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN); Overall description; Stage 2,” Mar. 2015. [2] “http://sumo-sim.org/.” [3] S.-L. Chao, H.-Y. Lee, C.-C. Chou, and H.-Y. Wei, “Bio-Inspired Proximity Discovery and Synchronization for D2D Communications,” IEEE Commun. Lett., vol. 17, no. 12, December 2013. [4] M.-J. Shih, G.-Y. Lin, and H.-Y. Wei, “A Distributed Multi-Channel Feedbackless MAC Protocol for D2D Broadcast Communications,” IEEE Wireless Commun. Lett., vol. 4, no. 1, pp. 102–105, Feb 2015. [5] O. Yilmaz, Z. Li, K. Valkealahti, M. Uusitalo, M. Moisio, P. Lunden, and C. Wijting, “Smart Mobility Management for D2D Communications in 5G Networks,” in IEEE WCNCW, April 2014, pp. 219–223. [6] 3GPP TR 36.814 V9.0.0, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Further advancements for E-UTRA physical layer aspects,” Mar. 2010. [7] 3GPP RP-150441, “Enhanced LTE Device to Device Proximity Services,” Mar. 2014. [8] 3GPP TR 36.423 V12.5.0, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP),” Mar. 2015. [9] K. Doppler, M. Rinne, C. Wijting, C. B. Ribeiro, and K. Hugl, “Deviceto-Device Communication as an Underlay to LTE-Advanced Networks,” IEEE Commun. Mag., vol. 47, no. 12, pp. 42–49, 2009. [10] M. J. Yang, S. Y. Lim, H. J. Park, and N. H. Park, “Solving the Data Overload: Device-to-Device Bearer Control Architecture for Cellular Data Offloading,” IEEE Veh. Technol. Mag., vol. 8, no. 1, pp. 31–39, March 2013. [11] D. Tsolkas, E. Liotou, N. Passas, and L. Merakos, “LTE-A Access, Core, and Protocol Architecture for D2D Communication,” in Smart Device to Smart Device Communication. Springer, 2014, pp. 23–40. [12] K. Doppler, C.-H. Yu, C. B. Ribeiro, and P. Janis, “Mode Selection for Device-to-Device Communication Underlaying an LTE-Advanced Network,” in IEEE WCNC, 2010, pp. 1–6. [13] 3GPP TR 36.331 V12.5.0, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio Resource Control (RRC);Protocol specification,” Mar. 2015. [14] 3GPP TR 36.843 V12.0.1, “Study on LTE Device to Device Proximity Services; Radio Aspects,” Mar. 2014.
