X2 Based Local Mobility Management for Networked Femtocells

Ahmed Salim Chekkouri, Abdellatif Ezzouhairi, Samuel Pierre, Member, IEEE Mobile Computing and Networking Research Laboratory

Department of Computer Engineering École Polytechnique de Montréal Montréal, Canada

{ahmed-salim.chekkouri, abdellatif.ezzouhairi, samuel.pierre}@polymtl.ca

Abstract- One of the most attractive features considered by the recent LTE-A standard is the introduction of femtocells in its main architecture. Rapidly, femtocell technology has been recognized as a potential solution to offload cellular networks since its deployment is expected to grow up considerably in the near future. However, the small coverage of femtocells may lead to frequent handoffs that need permanent support of the core network. Hence, to ensure seamless handovers through such networks, local mobility management remains an important task to be addressed. In this paper, we propose an efficient X2-based local mobility scheme that allows mobile users to perform handovers with minimum signaling load compared to the mobility procedures proposed in the 3GPP LTE-A standard. Index Terms—Mobility management; Long-Term Evolution-Advanced (LTE-A); Femtocell; Local handoffs.

I. INTRODUCTION

Demand for mobile broadband data services is increasing at huge rate as a result of an increasing level of penetration of Mobile smart devices (Smart Phones, Tablets, and other devices) and data-intensive applications. Mobile networks operators are facing increasing challenges to deliver the ever growing capacity demand and to provide a high-quality consumer experience. In order to meet those challenges, mobile networks are rapidly evolving to new technologies that increase capacity and coverage (i.e. 3.5G, 4G). Even though the 4G-LTE Long Term Evolution is delivering important capacity improvement over the 3G networks, but still insufficient on its own to address future expected capacity demands and coverage issues.

On the other hand, the most part of mobile data traffic demand is increasingly localized indoors, typically within buildings (e.g. offices, homes, shopping centers, campuses), and due to the radio signals attenuation, it is very challenging for mobile networks operators to extend wireless coverage and deliver High data rates inside the buildings using only Macro-cell base stations. To help meet these challenges, the new LTE-Advanced (LTE-A) standard proposes the integration of a new generation of small-cell mobile stations (femtocells) in its main architecture [4]. Femtocells, also called the Home evolved Node B (HeNB), offer a highly cost-effective method to extend the wireless coverage of the macro-cellular network while at the same time improving the end user data experience

because of the reduced distance between the mobile and femtocell radio. Mobile data offloading is one of the most important advantages of femtocells, indeed offloading traffic frees capacity and therefore improves the data experience of users on the macro network. Nevertheless, the small coverage, massive deployment and long Internet backhaul of femtocells present new challenges for mobility management. More specifically, in some Multi-femtocell deployment scenarios such as campuses, shopping centers, conference events, it is luckier to assist to frequent interfemto handovers. Accordingly, the frequent data path switch operations will cause significant signaling load to the core network entities and may degrade drastically the user's Quality of Experience (QoE). That is why this paper proposes a smart X2-based local mobility scheme that aims to allow interfemto roaming with minimum signaling load. The rest of this paper is organized as follows. The related work is presented in Section II. Section III describes the system architecture. In Section IV, we introduce the proposed mobility scheme. The analytical model is developed in Section V. In Section VI, numerical results are presented. Section VII concludes the paper.

II. RELATED WORK

In [9], an intermediate node called HeNB gateway (HeNB GW) is proposed to solve the scalability and security of femtocell deployment. Two mobility management schemes are proposed to be applied in the HeNB GW. In the first scheme, HeNB GW acts as a mobility anchor to control handovers among femtocells. While in the second scheme, HeNB GW acts as a relay between HeNB and EPC. To investigate the handover signalling cost, both methods are compared using an analytical model [10]. The first method is shown to reduce the signalling traffic in EPC and is more suitable for enterprise and campus use. The second method is similar to S1-based handover in LTE, it has small impact on E-UTRAN standards but it is shown to have 30% more signalling cost than the first method. However, given the fact that the HeNB GW is still located on the mobile operator’s premise, unnecessary signalling overhead still reaches the mobile core network. Recent work [11], propose two schemes for mobility management of LTE-A femtocells to reduce the path switch cost during a handover. In fact, in the current 3GPP femtocells architecture, even though the two femto users reside in the same area (e.g. campus), the call signalling and data traffic are

2013 IEEE 9th International Conference on Wireless and Mobile Computing, Networking and Communications (WiMob)

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routed through the core network, causing unnecessary overhead. The proposed schemes are based on the pointer forwarding which was first introduced in [12] to reduce network signalling and database loads for location management in cellular networks. The same idea is used in tunnelling mechanisms in Mobile IP and WiMax. For instance, to avoid the extra delay of performing relocation during a HO between two WiMax base stations that belong to different Access Service Networks (ASNs), the old ASN become the traffic anchor node, which receives and then tunnels the traffic to the new ASN gateway, the ASN relocation can later be performed. The schemes proposed in [11] use local mobility management based on X2 traffic forwarding. Instead of switching the data path after each HO, an X2 based traffic forwarding chain is established between the current serving femtocell and the local anchor femtocell. A trade-off between the path switch cost and the traffic forwarding is balanced based on defined threshold of the forwarding chain. The first scheme cascades the target femtocell to the previous source femtocell through the local X2 path after each HO. The second scheme improves the previews one by implementing a local path switch if the target femtocell has shorter path from the original anchor point than the cascading path. Simulation studies of the analytical model show that the proposed schemes can significantly reduce the signalling overhead of the mobile core network compared to the standard 3GPP scheme.

III. LTE-A FEMTOCELLS ARCHITECTURE

In this section, we present an overview of the networked femtocells architecture as well as the 3GPP LTE-A HO procedure used for HeNBs.

A. Femtocells network architecture

Based on the recent 3GPP LTE-A enhancements [1], a networked femtocells can be deployed as shown in Fig. 1. The HeNB, includes the E-UTRAN eNodeB function with smaller transmission power. Each HeNB is connected to the evolved packet core (EPC) through a security gateway and an optional HeNB gateway. So that, a HeNB may be configured with restricted access, i.e. only the members of closed subscriber group (CSG) are allowed to access it.

Fig. 1. 3GPP LTE-A networked femto architecture

Practically, the HeNB exchanges between the Mobility Management Entity (MME) and the serving gateway (S-GW) in the EPC are completed respectively through S1-MME and S1-U interfaces. The security gateway ensures controlled access of the user equipments (UE) to the operator's network through the deployed HeNB, while the optional HeNB gateway aggregates a number of HeNBs towards the EPC. Hence, the core network entities (MME/S-GW) see multitude of HeNBs as a single home base station whereas these units appear to a HeNB as an MME.

One thing to be noted is that the X2 interface, which directly interfaces two base stations, is not allowed between a HeNB and a macro eNB [2]. The CSG provisioning network elements includes the CSG List Server and the CSG Administration Server. They are responsible for CSG membership maintenance for femto cells and provide the CSG membership information to a UE and network. Additionally, the UE maintains a list of allowed CSG identities, i.e. the CSG white list in the USIM or the non-volatile memory. The network can configure the UE via the OMA Device Management (DM) protocol or over-the-air (OTA) procedures to add or remove one or more CSG IDs in the CSG white list. The HeNB broadcasts a CSG Indicator and a CSGID so that a UE can tell if it can access the HeNB or not [3].

B. 3GPP LTE-A X2 based HO procedure

The 3G and LTE network architectures were designed with a focus on macrocells, femtocells being a relatively new addition to the existing components [1]. Fig. 1 illustrates the relevant components of the LTE network architecture from the handover perspective. The Serving Gateway (S-GW) supports user data and provides routing and forwarding functionality between eNBs (or HeNBs) and the Packet Data Network (PDN). It also acts as the mobility anchor during handovers between LTE and other 3GPP systems. All base stations (eNBs and HeNBs) connect to the MCN and the Packet Data Network (PDN) through the S-GW for control signaling. The Mobility Management Entity (MME) is the key control node for LTE access network. It provides the control plane function for mobility between LTE and other access networks, and is responsible for choosing the right S-GW for a UE and for authenticating them. A HeNB Gateway (HeNB-GW) is used to provide interface scalability and support to a large number of HeNBs. It works as a concentrator for the control plane [4].

A UE periodically scans all available channels to measure signal strengths and reports the measurements to its associated HeNB. If the measurement report indicates that there is a candidate HeNB with good signal strengths, a handover process is triggered after a HO decision. Fig. 2 shows how the legacy handover procedure used in LTE-A networked femtocells. The source HeNB is the femtocell the UE is currently associated with and the target HeNB is the femtocell that the UE is to be handed-over to. The 3GPP LTE-A Handoff procedure is described as follows:

• Once Handoff Decision is completed by the Source HeNB, it sends a HO Request message to the Target HeNB.

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• The Target HeNB then performs Admission Control for the UE, and responds with a HO request Ack message.

• The Source HeNB issues the Handover Command to the UE, which then detaches from the associated femtocell and tries to handoff to the new one.

• In the meantime, the Source HeNB starts to buffer the application layer data it receives from the S-GW. It also sends out a SN Status Transfer message to the Target HeNB via the MME and then begins forwarding the data to it.

Fig. 2. 3GPP HeNB-based Handover Procedure

• Then, the Target HeNB, begins to buffer the data being forwarded by the Source HeNB and accepts the Handover Confirm message from the UE, thus allowing it to associate with itself.

• The Target HeNB then begins transmitting the buffered data to the UE. The data at this time, goes from the S-GW to the Target HeNB via the Source HeNB, traversing the public internet twice.

• Finally, the Target HeNB issues a Path Switch Request to the S-GW, which then switches the data path and responds with a Path Switch Response message.

• The S-GW then sends an End Marker data packet to the Source HeNB and then switches the data path so that it now streams the data directly to the Target HeNB. The Source HeNB forwards the End Marker packet when it is done forwarding all the data it has been receiving and buffering from the S-GW.

• The Target HeNB, after receiving the End Marker packet, begins to transmit data from the S-GW directly to the UE. This marks the end of the legacy handover process.

IV. PROPOSED X2-BASED LOCAL MOBILITY SCHEME

In this section we first present the main motivations based on the functional scenario depicted in Fig. 3. Then, we introduce the proposed mobility scheme that deals with local mobility in networked femtocells.

A. Motivations

Let us consider a UE that moves from HeNB1 to HeNB8 as illustrated in Fig. 3. In this roaming scenario, the UE has to perform five consecutive handovers. Particularly, the UE has to complete path switching with the EPC whenever it performs a handoff between two adjacent HeNB. It is clear that without a local mobility management, the LTE-A's handover procedure, presented in Fig. 2, generates additional signaling load between the target HeNB and the MME/S-GW entities. Moreover, the handoff decision is based only on signal measurement report, which cannot provide appropriate choice concerning the HeNB destination. Indeed, with the presence of multiple HeNBs, for instance, in a shopping mall or in dense residential area, it may happen that the selected HeNB do not guaranty a good sojourn time. Hence, it is luckier that a UE will undergo frequent handoffs during its movement in the aforementioned areas.

Fig. 3. Functional scenario

B. Local Anchor HeNB election mechanism

As stated earlier, the recent 3GPP HO procedure [1] designed to deal with mobility in networked femtocells suffer from high signaling load. Indeed, the UE needs the support of the core EPC network whenever it crosses a new HeNB's femto. Accordingly, when the number of crossed HeNB is relatively high, one may expect that the generated signaling load will increase considerably. To reduce the access to the core network during the handoff process, we propose a new local mobility management scheme that considers a number of HeNBs as anchor gateways to facilitate the X2 traffic forwarding. The determination of these local anchor HeNBs

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can be done during the deployment phase. This means that the operator may select in advance a list of HeNBs that are candidates to become anchor entities. Note that this task can be also dynamically completed by a self-organization approach. In this paper, we assume that local anchor HeNB is determined during the deployment phase.

C. X2-based local mobility management (X2-LMM)

In order to take benefit from the deployed femtocells, we assume that an operator or an independent authority may motivate personal users to be a member of networked femtocells. Particularly, a number of HeNBs can be designed, in an intelligent manner, to operate as local anchor HeNBs as described in the previous section. In these circumstances, a UE needs a HO procedure to move inside a local HeNB domain as well as another one to perform handovers between different anchor HeNB domains. These two HO procedures will be described in what follows.

1) Intra anchor HeNB domain HO procedure The proposed HO scheme inside an anchor HeNB domain

is described in Fig. 4. Notice that in this case, the target HeNB does not need to perform path switching with the EPC whenever a HO occurs. Moreover, the HO decision is completed by the local anchor point so that the decision concerning the target HeNB will be made based on additional information such as UE location and pattern mobility. Note that details concerning HO decision are not considered here since this task is out of the scope of this paper. Recall that the local anchor HeNB is endowed with a general view of the network topology since all the HeNB are connected through X2 interfaces.

Fig. 4. Intra anchor HeNB domain HO procedure

The HO procedure is described as follows: - Periodically, the UE sends a measurement report to the source HeNB which checks whether the predefined thresholds

are satisfied. If it is the case, a HO Request is sent to the local anchor HeNB. - Based on its HO decision module, the local anchor HeNB selects the appropriate target HeNB by sending a HO Request notification. - The Target HeNB then performs Admission Control for the UE, and responds with a HO Request Ack message. - Then, the local anchor HeNB forwards the HO Request Ack to the source HeNB. - When the Source HeNB receives the Handover Response, it issues the Handover Command to the UE, which then detaches from the associated femtocell and tries to handoff to the new femtocell. - After completing synchronization, the target HeNB starts sending data to the UE as well as a Local Path Switch Request to its serving local anchor HeNB. - Finally, the target HeNB sends a UE Context Release message to source HeNB to trigger radio and CP resource release.

2) Inter-domain local anchor HeNB HO procedure

Fig. 5. Inter anchor HeNB domain HO procedure

Once the handoff triggering occurs based on the measurement report, a HO Request is sent to the local anchor HeNB which performs a HO decision. If the candidate target HeNB is not served by the current local anchor HeNB, a HO Request is immediately sent to the target local anchor HeNB. The HO request is then forwarded to the target HeNB that performs admission control for the UE. Then, A HO Request Ack response is sent to source HeNB through the new and the old local anchor points as illustrated in Fig. 5. Similar to the previous HO procedure, the source HeNB issues a Handover Command to the UE, which then detaches from the associated femtocell and tries to handoff to the new femtocell. Once the UE is successfully synchronized with its new source HeNB, a

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path switching with the EPC is requested by the new local anchor HeNB as described in Fig. 5. Finally, the target HeNB sends a UE Context Release message to the old source HeNB to trigger radio and CP resource release. Notice that the proposed HO procedures in both cases can be implemented over existing HeNB units without extensive costs.

V. ANALYTICAL MODEL

To study the effectiveness of the proposed mobility scheme, we consider the recent 3GPP-A mobility procedures [1] as a benchmark comparison model. More specifically, we develop an analytical model to evaluate the proposed X2-based local mobility management (X2-LMM) compared to the 3GPP-A X2-based mobility. Note that in this section we focus on signaling cost analysis since the handoff latency remains unchangeable for both approaches.

A. Preliminary and notations

Fig. 6 illustrates a typical UE's roaming scenario derived from the functional scenario considered in Fig. 3. During its movement, a UE can perform either a handoff of type (a) or (b) as illustrated bellow. Where: (a) : refers to handoffs between two HeNBs belonging to the

same anchor mobility domain, (b) : refers to handoffs between HeNBs belonging to different anchor mobility domains.

Fig. 6. UE movement topology

Let rμ be the border crossing rate of a UE through any

HeNB,

Let dμ be the border crossing rate of an UE through anchor

HeNB domains,

Let Iμ be the border crossing rate through HeNBs when the

UE remains inside the same anchor HeNB domain, Iμ is

defined as: Iμ = rμ - dμ .

According to [5], if we assume that an anchor HeNB coverage domain is composed of M circular HeNB subnets, the border crossing rates can be expressed as:

−⋅=

=

M

M

M

rI

rd

1μμ

μμ

(1)

Under the fluid-flow mobility [6], rμ can be defined as:

π

νρ sR⋅⋅, where: ρ is the user density, v the UE average

velocity and Rs the perimeter of a subnet. Let ε be the probability of handoffs performed inside an anchor HeNB domain, i.e., percentage of handoffs completed in case (a) as shown in Fig. 6. Hence ε is defined as :

anchor HeNB

HeNB

(N -1)= 1-

N (2)

Where:

anchorHeNBN : number of anchor HeNBs, 2anchorHeNBN ≥ ,

HeNBN : number of HeNBs, 2HeNBN ≥ .

In order to obtain accurate performance analysis, we have to model the probability distribution of the number of boundary crossing during a UE session. Several distribution models have been proposed in the literature like hyper-Erlang, Gamma and Pareto as reported in [7]. However, it is demonstrated in [8] that the exponential model remains an appropriate choice. Thus, for simplicity we assume that the UE residence time in an HeNB subnet and in an anchor HeNB domain follow exponential distribution with parameters

rμ and bμ

respectively, while session arrival process follows a Poisson distribution with rate

sλ . Consequently, if we denote:

)( rNE as the average number of HeNB subnet crossing,

)( dNE as the average number of anchor HeNB domain

crossing and )( lNE as the average number of HeNB subnet

crossing performed inside the same anchor HeNB domain we can define the above averages as:

s

rrNE

λ

μ=)( (3)

s

ddNE

λ

μ=)( (4)

)()( rl NENE ⋅= ε (5)

In what follows, we use the above equations to analyze both signaling and packet delivery costs of the studied mobility schemes.

B. Total cost analysis

We define the total cost ( totalC ) as:

deliverysignaltotal CCC += (6)

The signaling cost refers to the amount of signaling traffic while the packet delivery cost refers to the network overhead.

The signalC and deliveryC are modeled during an inter-session

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arrival time that refers to the interval time between the arrival of the first packet of a data session and the arrival of the first packet of the next data session (i,e., one session lifetime).

1) X2-based local mobility management (X2-LMM) total cost

The X2-LMM total cost is defined as: X2-LMM X2-LMM X2-LMMtotal signal deliveryC = C +C (7)

• X2-LMM / 3GPP LTE-A signaling cost

The X2-LMM signaling cost is incurred when an UE performs either (a) or (b) handoffs is given by:

X2-LMM HeNB anchor HeNBsignal I DC = E(N )×C + E(N )×C (8)

Where :

HeNBC : refers to the signaling cost when an UE performs a handoff of type (a)

HeNBanchorC : refers to the signaling cost when an UE performs a handoff of type (b)

If we assume that a handoff preparation is always followed by

a handoff execution, the expressions relevant to HeNBC and

HeNBanchorC are given in Table 1.

Where YXT , refers to the transmission cost between node X

and node Y, ZP is the processing cost at node Z. Note that, for

simplicity we have considered UE's location before and after

am handoff as the same. The YXT , cost can be expressed as:

X,Y cX,Y hop hopT = (N -1+ )×T (9)

Where, YX

hopN , : number of hops between node X and Y,

δ : a proportionality constant to illustrate that the transmission cost for wireless hops are superior to those of wired hops,

chopT : transmission cost per hop.

To illustrate the impact of the UE's mobility as well as the average session arrival on the signaling cost, we introduce a session-to-mobility factor (SMR) which represents the relative ratio of session arrival rate to the mobility rate.

The SMR factor is expressed by : s

r

SMR =

(10).

Hence, if we consider equations (1), (3), (4), (5) and (10), the equation (8) becomes:

⋅+⋅=− HeNBanchorHeNBLMMXsignal C

MC

SMRC

112 ε (11)

• X2-LMM packet delivery cost

Let pA be the average packet received and sent by a UE

during one session lifetime. Based on Fig. 6, the UE can perform either handoffs of type (a) or (b). Hence the X2-LMM packet delivery cost is given by :

( )

( )

,

2

arg arg

( )

( )

UE anchor HeNB

X LMMdelivery p I source anchor HeNB HeNB source anchor HeNB HeNB

d t et anchor HeNB t et anchor HeNB

T

C A E N l l

E N l

η η

η

− = ⋅ + ⋅ + + +

+ ⋅ +

2) 3GPP LTE-A X2-based total cost

The 3GPP LTE-A X2-based total cost is defined as:

GPPdelivery

GPPsignal

GPPtotal CCC 333 += (13)

• 3GPP LTE-A X2-based signaling cost

Based on the 3GPP LTE-A handoff procedures depicted in Fig. 2 and the proposed procedures depicted in Fig. 4 and Fig. 5, the signaling cost is given by:

3GPP LTE- A3GPPsignal I dC = (E(N )+ E(N ))×C (14)

TABLE 1. Expression of signaling costs

source HeNB, target HeNB UE,source HeNB UE,target HeNB

3GPP LTE-Atarget HeNB,MME MME,S-GW S-GW,source HeNB

source HeNB target HeNB MME S-GW

5 T +T +T

C +2 T +2 T +T

+4 P +3 P +2 P +P

⋅

= ⋅ ⋅

⋅ ⋅ ⋅

3

3

source HeNB,source anchor HeNB

source anchor HeNB,target anchor HeNB

source HeNB,target HeNB UE,target HeNB

anchor HeNBtarget anchor HeNB,MME MME S-GW

MME,S-GW target anchor HeNB,S-GW

source

T

T

+3 T +T

C +T +2 P +P

+2 T +T

+4 P

⋅

+ ⋅

⋅

= ⋅

⋅

⋅ HeNB source anchor HeNB

target anchor HeNB target HeNB

+P

+5 P +5 P⋅ ⋅

target HeNB,anchor HeNBsource HeNB,anchor HeNB

HeNBUE,source HeNB anchor HeNB UE,target HeNB

source HeNB,target HeNB source HeNB target HeNB

3 T +4 T

C +T +3 P +T

+3 T +4 P +4 P

⋅ ⋅

= ⋅

⋅ ⋅ ⋅

To express equation (14) as a function of the SMR factor, we use equations (1), (3), (4), (5) and (10).

ALTEGPPGPPsignal C

MSMRC

−⋅+⋅=

33 )1

(1

ε (15)

• 3GPP LTE-A X2-based packet delivery cost

The packet delivery, in this case, is given by:

(12)

627

,3

( ( ) ( )) ( )UE HeNB GWGPP

delivery p

I d HeNB GW HeNB HeNB GW HeNB

TC A

E N E N l l η η

−

− −

= ⋅+ + ⋅ + + +

(16)

VI. PERFORMANCE ANALYSIS

Based on the analytical model presented in the previous section we present, in what follows, the main results. The parameters used in this simulation are given in Table 2.

TABLE 2. Parameters used for performance analysis

Parameters Symbols Values Subnet perimeter (m) Rs 200 Border crossing rate of HeNB

rμ 0.6

Border crossing rate of HeNB domains dμ 0,1

Number of circular HeNB M 50 Session arrival rate

sλ 1

Average packets received/sent by UE pA 50

Lookup cost at the HeNB HeNBl 1

Packet tunneling cost at the HeNB HeNBη 1

Fig. 7 UE's mobility impact on signaling traffic

Fig. 7 shows the impact of UE's speed on the total signaling cost. We notice that even in low mobility, the proposed X2-LMM shows lower signaling load compared to the 3GPP standard.

Fig. 8 Impact of user density on signaling load

In Fig. 8, we illustrate the impact of user's density on the proposed mobility scheme. We particularly notice that in the presence of a low UE's density, both the X2-LMM and 3GPP

proposals generate aprroximately the same load. Nevertheless, in dense areas, for instance, we notice that X2-LMM reduces considerably the generated load.

To study the scalability of the proposed solution, we increase the number of the HeNB of the networked femto. We notice that the estimated signaling load for the proposed X2-LMM is relatively low compared to the 3GPP standard. This situation can be explained by the introduction of local anchor HeNB in the proposed X2-LMM which avoids path switching with the EPC core whenever an UE changes its serving HeNB.

Fig. 9 Impact of network scalability

Finally, Fig. 10 shows the impact of ongoing traffic on the generated signaling cost. It is clear that when the average packet arrival increases, the X2-LMM’s associated load is relatively lower that the one generated by the standard 3GPP LTE-A. This result is quite normal since the processing load is managed locally in the X2-LMM approach rather than the EPC core as it is the case with the 3GPP LTE-A proposal. This means that the processing cost of the centralised EPC units like MME or S-GW is relatively high compared to the local units such as HeNB and anchor HeNB.

Fig. 10 Average packet arrival vs processing load

VII. CONCLUSION

Femtocells offer huge opportunities for both mobile network operators and end-users. Still they provide new challenges for mobility management. Specifically, in multi-femtocell deployment scenarios, it is luckier to assist to frequent interfemto handovers. Therefore, the frequent data path switch operations will cause significant signalling load to

628

the core network entities and may degrade drastically the user's Quality of Experience. In this paper, a smart X2-based local mobility scheme is proposed, the aim of this scheme is to allow interfemto roaming with minimum signalling load. Based on analytical model, numerical results show the effectiveness of the proposed X2-based local mobility scheme in reducing the signalling load of interfemto handover.

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