handover signaling for mobile user in relay lte-advanced environment

Abstract—In order to enhance work in LTE-Advanced with

the existing of a RN, the discussion about the available architecture and the available handover procedure are discussed. A new architecture and signaling for X2 handover from a RN to a target eNB in different cell for Architecture A alt. 2 are introduced in this paper. It shows how the handover signaling on the RRC protocol works. The alt. 2 implements the DeNB + “X2 home eNB GW” thus the difference of it is discussed in this paper. The signaling highlight the important of the measurement report to the handover performance. Without measurement report packet will be loss during the handover execution and this will degrade the performance of the network.

Index Terms—LTE-Advanced, handover, alt. 2, measurement report.

I. INTRODUCTION Mobile user needs a handover process to at least maintain

the ongoing service. The handover is a process of changing the current radio channel with a new radio channel to avoid the termination of a current service. There are three types of handover in LTE-Advanced (Long Term Evolution-Advanced) which involves three types of measurements. In LTE-Advanced, the UE (User Equipment) should perform at least three measurement types for mobility. The measurements are classified as Intra-frequency E-UTRAN (Evolved Universal Terrestrial Radio Access Network) measurements, Inter-frequency E-UTRAN measurements, Inter-RAT (Inter-Radio Access Technology) measurements for UTRAN and GERAN (GSM EDGE) and Inter-RAT measurements of CDMA2000 HRPD or 1XRTT frequencies [1]. These measurements are used by E-UTRAN to order the UE to start measurements, modify measurements or stop measurements.

In LTE-Advanced, network controlled UE-assisted handovers are usually performed. This means that the network makes the handover decision. The UE makes the measurement of the neighboring cell and reports to the network for the handover decision. There are three reporting criteria used; event triggering reporting, periodic reporting and event triggering periodic reporting [1]. The event triggering reporting is based on measurement threshold. The periodic reporting is based on time and the event triggering periodic reporting is a combination of measurements threshold and time.

LTE-Advanced implements a hard handover [2]. The hard handover is also known as a break beforemade handover

because the previously associated network is terminated before the new network connection is established. Therefore there is an interruption time in ongoing service during the handover procedure. The use of X2 interface among the eNBs (evolved NodeB) in LTE-Advanced offers low latency. Qualcomm Inc. claims that LTE-Advanced offers fast and seamless handover. The seamless handover is intended to provide QoS (Quality of Service) even during the handover procedure.

The LTE-Advanced heterogeneous network has a macrocell as the largest coverage and in the macrocell there are picocells, femtocells and relays. BS (Base Station) in LTE and LTE-Advanced is known as an eNB so from this point onward the eNB will be used instead of the BS. The macrocell and picocells eNB uses dedicated backhaul and open to the public access. The difference is the picocells eNB is a smaller eNB with lower power for a smaller coverage compare with the macrocell. The femtocells cover a small area and it is normally inside a building where the outside signals cannot be reached well. The eNB of the femtocell is also dedicated backhaul but not open to the public access. A RN (Relay Node) can be same as the macrocell eNB depends on which layer it operates. The RN connects to an eNB, which is referred to as a DeNB (Donor eNB) for that particular RN [3]. The RN is wirelessly connected to the DeNB.

The relaying technique provides some beneficial aspects such as improving the coverage area of the DeNB, improving the cell edge coverage and extending coverage of heavily shadowed areas in the cell such as urban areas. To do so the communication among the UE, the RN and the DeNB should be well investigated to make sure the service is continuously although the user is moving. While moving, the handover is an important criterion to look into to make sure the service is not in a disruption state.

This paper investigates the handover signaling for the architecture of the alt. 2 (alternative 2) when the UE is moving from one RN to another eNB. This work proposed a handover decision made by the RN to avoid redundant signaling between RN and the DeNB + “home eNB GW” because RN Layer 3 has the ability to perform a handover by itself.

The structure of this paper is organized as follows. Section II depicts the architecture of LTE-Advanced with RN. Section III discusses on the handover stages. Section IV shows the handover signaling. Finally, Section V concludes this paper.

II. ARCHITECTURE OF LAYER 3 RELAY In [4], there are two architectures for an environment with

RN which are Architecture A and B. Three alternatives of

Handover Signaling for Mobile User in Relay LTE-Advanced Environment

Nurzal Effiyana Ghazali and Sharifah Hafizah Syed Ariffin Faculty of Electrical Engineering

Universiti Teknologi Malaysia {effiyana,sharifah}@fke.utm.my

2011 IEEE International Conference on Control System, Computing and Engineering

978-1-4577-1642-3/11/$26.00 ©2011 IEEE 216

architecture are discussed in Architecture A whilst only one alternative in Architecture B. The difference between the alternatives in the Architecture A is that in the grouping, there are different functional network elements within or out of the DeNB. Fig. 1 shows the diagram of the alternatives of Architecture A refer as alt. 1, alt. 2 and alt. 3.

Fig. 1: Architecture A [4]

Fig. 1 shows that in alt. 1, the DeNB function is a single entity connects to the Relay-UE’s MME (Mobility Management Entity) and Relay-UE’s SGW/PGW (Serving Gateway/Packet Data Network Gateway). Thus it is also called as full L3-relay, transparent for DeNB because all the signaling between RN and SGW/PGW are invisible to the DeNB.

In contras alt. 3 combines DeNB function and Relay-UE’s SGW/PGW becoming one entity makes it known as RN bearers terminate in DeNB. This enhancement makes the routing path optimized because packets from User-UE’s SGW/PGE do not have to traverse via a second relay (Relay-UE’s SGW/PGW).

In alt. 2, DeNB function, Relay-UE’s SGW/PGW and Relay GW are combined to be one entity. Relay GW (Gateway) is an optional entity and it is transparent to the relay, the core network of the UE and other eNBs. Alt. 2 also known as Proxy S1/X2 because S1-MME and X2 interfaces terminate in a proxy sense in the DeNB whilst the other alternatives terminate in the RN. Alt. 2 is a candidate for the Release 10 and it is considered for future work.

Next, the user plane and the control plane of alt. 2 are shown in Fig. 2 and Fig. 3, respectively.

Fig. 2: User plane protocol stack

Fig. 3: Control plane protocol stack

From the Fig. 2, note that there is a GTP (GPRS

Tunneling Protocol) tunnel per UE bearer, mapped from the SGW/PGW of the UE to the DeNB. Then in the DeNB it is switched to another GTP tunnel going to the RN which is one-to-one mapping.

The control plane shown in Fig. 3 illustrating the function of NAS (Non-Access Stratum) which runs between the MME on the network side and the UE on the terminal side which is EPS (Evolved Packet System) bearer of management, authentication and security control [5] [6]. The NAS packets are carried by the SCTP (Stream Control Transmission Protocol) which operated above IP layer. The RRC (Radio Resource Control) performs the system information broadcast, paging, radio bearer control, RRC connection management, mobility functions and UE measurement reporting and control. Therefore this layer is important in handover decision.

III. HANDOVER STAGES IN LTE-ADVANCED In [2], two stages of handover procedures are discussed

which are backward and RLF (Radio Link Failure) handover. However, no RN is considered. Both of these handover procedures require the source eNB to prepare a target cell for handover concurrently with the handover decision, otherwise, the UE transitions to idle-state where it attempts to complete the handover procedure by transitioning back to connected-state via a procedure called NAS recovery.

Fig. 4, 5 and 6 show the handover procedures implement in LTE system.

Fig. 4: Backward handover

In backward handover related information is exchanged

between the UE and the source eNB via the old radio path. The radio link conditions have to be good enough for both the source eNB and the UE to receive the Measurement Report and Handover Command, respectively. There is a short interruption in service between the time that the UE


217

receives the Handover Command from the source eNB and the time that the target eNB receives the Handover Confirm from the UE. Data forwarding and in-order delivery guarantees that none of the data buffered in the source eNB will be lost.

Fig. 5: RLF handover

RLF handover function as a recovery mechanism when the backward handover fails. When the UE detects radio link problems, it starts the RLF timer, a typical setting is 500ms or 1000ms [2]. The timer is tuned by the service provider based on extensive drive tests within the network. The UE searches for a suitable target cell or better cell than current cell and attempts to re-establish its connection with the target cell while it still in connected-state. This procedure incurs additional delay compare with the backward handover but it guarantees that none of the data buffered in the source eNB will be lost.

Fig. 6: NAS recovery procedure

The NAS recovery procedure is the last resort that the UE

has, when the RLF handover does not work and it exceed the RLF timer. Thus it will affect the data session.

The source eNB does not prepare the target cell for handover. With NAS recovery, the UE does not remain in connected-state. Instead, on re-establishment failure, the UE transitions from connected-state to idle-state and attempts to establish a new connection. This process cause additional delay. All the data buffered will not send to the target eNB thus they will be lost.

IV. HANDOVER SIGNALING Handover signaling illustrates relay mobility between

DeNBs for alt. 1 already discussed in [4]. Fig. 7 depicts it.

Fig. 7: RN mobility-alt. 1

This section focuses on the handover of moving UE in alt.

2. Consider a scenario in Fig. 8 where it shows the UE is connected to the RN, is moving towards the target eNB. The RN is wirelessly connected to its DeNB. The DeNB + “X2 home eNB GW” is connected to the Target eNB via X2 interface therefore the handover process is called X2 handover process. It is assumed that the DeNB + “X2 home eNB GW” and the Target eNB under the same MME and SGW/PGW serving the UE and during the handover the MME unchanged without relocation of the SGW/PGW.

As we have discussed in Section II, in alt. 2 the DeNB, relay-UE’s SGW/PGW and relay GW is considered as one entity which is called DeNB + “X2 home eNB GW”. This is the alt. 2 eNB’s criteria. Hence there will be no more Relay SGW/PGW because its function is taken by the new eNB in this alternative. MME serving UE is needed because in this case the UE is moving. The DeNB + “X2 home eNB GW” and the target eNB are connected to the MME serving UE by the S1-MME (UE) interface whereas X2 interface is interconnecting between DeNB + “X2 home eNB GW” and the target eNB. S11 (UE) interface is used to connect between the MME (UE) and the SGW/PGW (UE) whilst S1-U (UE) interface for connecting the SGW/PGW (UE) with the DeNB + “X2 home eNB GW” and the target eNB. The timing diagram signaling of this architecture is shown in Fig. 9.


218

Fig. 8: Architecture of X2 handover from a RN to a target eNB-alt. 2

Fig. 9: X2 handover from a RN to a target eNB-Alt 2

The difference between Fig. 7 and Fig. 9 is Fig. 9 is a

handover signalling when the UE mobility between a RN and target eNB for alt. 2 whilst Fig. 7 illustrating relay mobility between DeNBs for alt. 1. Hence, new entity and new way of signaling are introduced in Fig. 9. In Fig. 9, the “NEW” labels introduce the new elements in the alt. 2 and the new handover procedures compared to the Fig. 7 and works done in [7].

In [10], they proposed HO framework based on centralized relaying and decentralized relaying. The centralized relaying is proposed for the L1 and L2 RN whilst decentralized relaying for L2 and L3 RN. In this work the signaling can be considered as decentralized relaying technique because the architecture involved is for L3 RN. In [10], the HO decision is done by the DeNB but in this work it is done by the RN itself. The HO Req. message is send to the Target eNB via DeNB. The message will be transparent in alt. 1 and alt. 3 but not in alt. 2 where the DeNB reads the target cell ID from the message then sends to the Target eNB.

The handover is triggered by the UE that sends a Measurement Report message to the RN. The Measurement Report message is controlled by the RN with the message of

Measurement Control. The RN makes the handover decision based on the Measurement Report message. The handover preparation phase starts by sending the HO Req message from the RN to the target eNB via DeNB + “X2 home eNB GW”. This message contains all the relevant handover information (UE-RAN, PDCP (Packet Data Control Protocol) context etc.) [8]. When the DeNB + “X2 home eNB GW” received the HO Req message it reads the target cell ID and forwards the message to the Target eNB. In this case the Target eNB understands that the UE is coming from a cell under DeNB + “X2 home eNB GW”. The target eNB saves the context, makes a preparation for the handover process and responds to the RN with a HO Req Ack message which provides information for the establishment of the new link. The information includes the new C-RNTI (Cell Radio Network Temporary Identity). Then, the RN transfers all the information to the UE in the HO Command message and the handover preparation phase end.

Now, the UE performs the radio link establishment to the target eNB and it involves detaching from the current RN and synchronizing to the target eNB. This time is called Detach Time where no radio connectivity means the UE is not connected to the RN or to the target eNB. During this Detach Time, packet forwarding is done to avoid packet loss. The UE informs the target eNB about the success of the new establishment by sending HO Complete message. After receiving this message, the target eNB starts transmitting the buffered data which it receives during the packet forwarding to the UE.

The target eNB sends a Path Switch Req message to the MME serving the UE to inform that the UE has changed cell. Next, the MME serving the UE updates the information of the UE to the SGW/PGW serving UE by sending User Plane Update Req message. The SGW/PGW serving the UE performs the path switching and after this packets are directly sent to the target eNB. In order to assist the reordering function in the target eNB, the SGW/PGW serving the UE sends the End Marker packet on the old path immediately after the switching path [9]. The SGW/PGW serving the UE confirms the path switching by sending User Plane Update Response message to the MME serving the UE. The MME serving the UE updates with target eNB with Path Switch Req Ack message. By sending the UE Context Release from the target eNB to the RN via DeNB + “X2 home eNB GW”, the target eNB informs success of the handover and this message triggers the RN to release of resources.

In this case, it is proved that the handover depends on the measurement report. Without measurement report there will be no handover decision and this will cause packet loss. It is expected that the measurement report is influenced by the mobility of the UE and also system load. In [7] a scenario of 1km cell radius, 250km/h UE speed and highly loaded system, 97% of the handover failures are due to the transmission of the measurement reports. Further results are available in [7].

V. DISCUSSION AND CONCLUSION LTE-Advanced with RN implementation is still in its

infancy. A few architectures are introduced and the advantages and disadvantages of the architectures are still in research. However in [4], it concludes that alt. 2 is a possible candidate for Release 10.


219

Backward handover and RLF handover are implemented in LTE-Advanced. Both handover shows that the measurement report is important to make sure that there is no packet loss during the handover. The evidence of it has been discussed in Section V.

In this paper, the timing diagram signaling of X2 handover from a RN to a target eNB for alt. 2 and its architecture are presented. From the signaling, it is understood that the LTE-Advanced implements network-controlled UE assisted handover. In conclusion, the measurement report is significant in the handover performance.

ACKNOWLEDGEMENT This work was supported by Minister of Higher Education (MOHE) under Research University UTM grant QJ130000.7123.01H39 and Research Management Center UTM.

REFERENCES [1] 3GPP TS 36.300, “Evolved Universal Terrestrial Radio Access (E-

UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description; Stage 2 (Release 8)”, version8.12.0, March 2010.

[2] Qualcomm Incorporated, “LTE Mobility Enhancements”, February 2010. [Online]. Available: http://www.qualcomm.com/documents/files/lte-mobility-enhancements.pdf

[3] Peter Szilagyi and Henning Sanneck, “LTE Relay Node Self-Configuration”, 12th IFIP/IEEE International Symposium, May 2011.

[4] 3GPP TR36.806 V9.0.0: “Evolved Universal Terrestrial Radio Access (E-UTRA); Relay Architectures for E-UTRA (LTE-Advanced),” March 2010.

[5] Anthony Lo and Ignas Niemegeers, “Multi-hop Relay Architecture for 3GPP LTE-Advanced,” Proceedings of the 2009 IEEE 9th Malaysia International Conference on Communications, December 2009, pp. 123-127.

[6] Farooq Khan, “LTE for 4G Mobile Broadband Air Interface Technologies and Performance,” Cambridge University Press, 2009.

[7] Konstantinos Dimou, Min Wang, Yu Yang, Muhammad Kazmi, Anna Larmo, Jonas Petterson, Walter Muller and Ylva Timner, “Handover within 3GPP LTE: Design Principles and Performance”, Vehicular Technology Conference Fall (VTC2009-Fall), IEEE 70th, 2009.

[8] Lajos Bajzik, Peter Horvath, Laszlo Korossy and Csaba Vulkan, “Impact of Intra-LTE Handover with Forwarding on the User Connections”, Mobile & Wireless Communications Summit, 16th IST, 2007.

[9] 3GPP TS 23.401, “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Access (Release 9)”, version9.4.0, March 2010.

[10] Oumer Teyeb, Vinh Van Phan, Bernhard Raaf and Simone Redana, “Handover Framework for Relay Enhanced LTE Networks,” IEEE Workshop on Communications, 2009.


220

handover signaling for mobile user in relay lte-advanced environment

Documents