Inbound Mobility Management on LTE-Advanced Femtocell Topology Using X2 Interface Sangchul Oh, Hongsoog Kim, Byunghan Ryu and Namhoon Park Femtocell System Research Team Mobile Convergence Research Department ETRI Daejeon, KOREA 305–700 Email: [email protected], [email protected], [email protected], [email protected]

Abstract—The femtocell access point of LTE-Advanced system, which can be deployed under macrocell coverage, was applied to the paper. The paper consists of two analyses which are the interference impact by the femtocell and the macrocell at a user equipment and the load impact to the control plane in evolved packet core network by X2 and S1 handover signaling messages. On the results of investigations by performance simulations with regard to the receiving power and signal-to-noise ratio at a user equipment side, the unavailable zone to the inbound handover from macrocell to femtocell occurred due to a severe intercell interference depending on a femtocell topology. Therefore, we found that an intercell interference coordination between macrocell and femtocell in order to make an inbound handover is needed and that X2 interface is the better solution rather than S1 interface for an inbound handover by investigating the X2 and S1 handover procedure of 3GPP. Ultimately, it is shown that the X2 interface between macrocell and femtocell can play an important role in an high reliable inbound handover and a simple interference control implementation on femtocell environment.

I. I NTRODUCTION Recently, many companies are developing a self-organizing network (SON) and femtocell technologies in terms of a small base station, which can reduce the capital expenditure (CAPEX) and operational expense (OPEX) of a service provider by an active encounter about user requirements under a radius of a cell with a small area environment such as a home and office, which can cut down the time and maintenance cost on a additional deployment of a cell, and which can serve a wireless environment reflecting a upgrade of service quality of users and launching a new market of manufacturers. A SON technology of a femtocell can be organized by initial auto-installation, self-configuration and self-optimization technologies. The initial auto-installation technology performs an initial auto-installation of a base station by creating installation parameters according to an internal configuration of the base station for itself when deploying an additional base station. The self-configuration technology makes an establishment and registration of an inter-relationship and identification among neighbor base stations, and connections with its core network. The self-optimization technology carries out a power control of a base station by making full use of information about signal and traffic patterns among neighbor base station, and an optimization of handover parameters. Moreover, it is necessary to apply a mobility technology, a base station

selection technology using closed subscriber group (CSG), and an interference mitigation and avoidance technologies. A femtocell base station is a nomadic or mobile base station with small size supporting a cheap wired and wireless convergence service by connecting a mobile phone with an internet in an indoor environment such as home and office. Although it is similar to a WiFi access point (AP) in a functional aspects point of view, it is different that a primary role of a femtocell base station is to relay a mobile phone call unlike a WiFi AP. It is an evolved technology in a view point of serving an internet as well as a voice unlike a legacy wired and wireless convergence service such as Onephone and Homezone services. In a long term, a femtocell base station based on LTE-Advanced will become a practical application of integration of a voice and data service in a home and office. It is also able to recognize an exact location and an indication of coming or going of their families through an individual femtocell access in terms of a location-based service (LBS), and to provide new supplementary services by combining various home local area network (LAN) technologies. II. E-UTRAN A RCHITECTURE Fig.1 shows a logical architecture for the HeNB that has a set of S1 interfaces to connect the HeNB to the EPC [1]. The evolved universal terrestrial radio access network (E-UTRAN) consists of the macro eNode B (eNB), the home eNode B (HeNB) and the evolved packet core (EPC) corresponding to the mobility management entity (MME) and the serving gateway (SGW). Recently, they are connected to one another only with the S1 interface. The current version of the specification does not support X2 connectivity of HeNBs [1]. The E-UTRAN architecture may deploy a Home eNB Gateway (HeNB GW) to allow the S1 interface between the HeNB and the EPC to scale to support a large number of HeNBs. The HeNB GW serves as a concentrator for the control-plane (C-Plane), specifically the S1-MME interface. The S1-U interface from the HeNB may be terminated at the HeNB GW, or a direct logical user-plane (U-Plane) connection between HeNB and SGW may be used. The HeNB GW appears to the MME as an eNB. The HeNB GW appears to the HeNB as an MME. The S1 interface between the HeNB and the EPC is the same whether the

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TABLE I E NB SYSTEM ASSUMPTIONS MME / S-GW

MME / S-GW S1

S1

S 1

S1

S 1

eNB

S 1

S 1

1 S

X2 X2

eNB

1 S

E-UTRAN

S 1

eNB HeNB

Fig. 1.

HeNB

Assumption

Cell Radius

500 m

Carrier Frequency

2000 MHz

Carrier bandwidth

HeNB GW

X2

Parameter

HeNB

III. SNR P ERFORMANCE A NALYSIS A. Simulation Environment Fig.2 describes the simulation environment of the paper. The femtocell access point, HeNB, which can be deployed under macrocell coverage, was applied to the paper. the simulation environment applied a typical London suburban road. The femtocell cluster consisted of five HeNBs and was horizontally 200 meters away from the macro eNB. Every house was 10 m * 10 m square in shape and every HeNB was placed on a center of each house. HeNBs were every 10 m and a user could be anywhere on the dotted line which was 1 m in front of every house. the user equipment (UE) needs to read the master information block (MIB) and system information block (SIB)1 information which is broadcasted in the broadcasting channel (BCH) of HeNB in order to make an inbound handover to HeNB. Ultimately, source eNB makes a decision of inbound handover by the information sent by the UE. One of the key performance index for inbound handover is the capability of reading the MIB/SIB1 information of the UE. Therefore, the performance analysis with respect to the receiving (RX) power and signal-to-noise ratio (SNR) at the UE side are addressed by the simulation. Several deployment configurations with regard to a radio frequency channel usage for HeNB such as a co-channel, a partial co-channel, and a dedicated channel have been considered in [2]. Frequency distribution methods can be classified as a fractional frequency reuse (FFR), a soft frequency reuse

path

P L(dB) = 15.3 + 37.6log10 R

eNB antenna gain after cable loss

14 dB

noise power

-95 dBm

eNB Tx power

46 dBm

Overall E-UTRAN Architecture with deployed HeNB GW [1]

HeNB is connected to the EPC via a HeNB GW or not. The HeNB GW shall connect to the EPC in a way that inbound and outbound mobility to cells served by the HeNB GW shall not necessarily require inter-MME handovers. Here, the inbound mobility means the handover from the macro eNB to a HeNB, and the outbound mobility means the handover from the HeNB to a macro eNB. One HeNB serves only one cell. The functions supported by the HeNB shall be basically the same as those supported by an eNB and the procedures run between a HeNB and the EPC shall be the same as those between an eNB and the EPC.

10 MHz

Distance-dependent loss

TABLE II H E NB SYSTEM ASSUMPTIONS Parameter HeNB Channel

Assumption Frequency

same frequency and same bandwidth as macro layer

Distance-dependent path loss

P L(dB) = max(15.3 37.6log10 R, 38.46 + 20log10 R)

+

wall penetration loss

0.7∗R+Low , where Low is the penetration loss of an outdoor wall, which is 10 dB.

HeNB antenna gain after cable loss

5 dBi

noise power

-95 dBm

eNB Tx power

20 dBm

(SFR), and a partial frequency reuse (PFR). A frequency reuse means that same frequency can be used in different cells for improving the system capacity greatly. To date there has been a few investigations related to a FFR method in [3]. In this paper, we assume that the HeNB commonly used the same frequency and same bandwidth with the macro eNB like a co-channel configuration. This would likely be the worst case. Simulation parameters in [4] were applied to the paper. Table I and table II respectively show system parameters of the macro eNB and the HeNB. The relationship among these parameters was defined in equations below. GT X (dB) = Gant − (Lpath + Lpenetration )

(1)

Equation (1) indicates GT X , the gain of transmission power passed from the eNB or the HeNB to the UE. • • •

Gant : the antenna gain of eNB and HeNB. Lpath : the path loss passed from the eNB or the HeNB to the UE. Lpenetration : the penetration loss caused by a wall. this is only applied to the HeNB, not eNB. PRX = PT X ∗ GT X

(2)

Equation (2) indicates PRX , the received power at the UE. PT X : the transmission power of eNB and HeNB.

•

Femtocell Horizontal Cluster (HeNBs)

10m HeNB 1

HeNB 3

HeNB 2

HeNB 4

HeNB 5

10m eNB

1m UE

200m

Fig. 2.

Simulation Environment

−39

Signal-to-noise ratio (SNR), which is defined as the power ratio between a signal and the interference, can be expressed as follows:

i=1 (PT X(HeN B(i))

= 5

i=1

∗ GT X(HeN B(i)) ) + Pnoise (3)

PRX(eN B) PRX(HeN B(i)) + Pnoise

(4)

Equation (3) indicates the SNR with regard to the eNB signal received at the UE. This equation means that all signals of HeNBs except for the signal of the eNB serve as an interference. where Pnoise is the noise power at the UE. Equation (3) can be reformulated by Equation (2) to Equation (4). SN RHeN B(i) =

Rx Power of eNB/HeNB signal at the UE (dBm)

PT X(eN B) ∗ GT X(eN B)

SN ReN B = 5

−43

−44

−45

−46

−47

200

220

Fig. 3.

240 260 eNB−UE distance R (m)

280

300

RX power of eNB and HeNB at UE

30 eNB HeNB1 HeNB2 HeNB3 HeNB4 HeNB5

25

20

15 SNR at the UE (dB)

In this section, we will present the results of the simulation. Fig.3 presents the received power with respect to eNB and HeNBs signal at the UE, PRX . According to the results, it is shown that the received signal power of macro eNB is greater than that of the HeNB within the point 225 m away from macro eNB. this information indicates that we cannot make an inbound handover to the HeNB within this area because the UE can not read the MIB/SIB1 system information of the HeNB. Hence, we found that HeNBs should be installed at least 225 m away from the macro eNB to be able to perform an inbound handover successfully. For a more detailed analysis, the analysis of SNR will be introduced in Fig.4. Fig.4 shows the SNR with respect to eNB and HeNBs signal at the UE, SN ReN B of equation (4) and SN RHeN B(i) of equation (5). Based on these results, it is shown that the SNR of HeNBs is partially greater than that of the eNB in

−42

−49

(k=i)

B. Simulation Results

−41

−48

PRX(HeN B(i)) 5 PRX(eN B) + k=1 PRX(HeN B(k)) + Pnoise

(5) Equation (5) indicates the SNR with regard to each HeNB, i-th, signal received at the UE. This equation means that all signals of eNB and HeNBs except for the signal of the HeNB, i-th, itself serve as an interference.

eNB HeNB1 HeNB2 HeNB3 HeNB4 HeNB5

−40

10

5

0

−5

−10

−15

−20

200

220

Fig. 4.

240 260 eNB−UE distance R (m)

280

300

SNR of eNB and HeNB at UE

limited areas even after the point 225 m away from macro eNB. It means that the UE can receive the MIB/SIB1 system information of HeNBs in that limited areas and make an inbound handover in that limited areas only. Limited areas are the 225 m ˜ 228 m to the HeNB(2), the 243 m ˜ 249 m

30

25

25

NumberofsignalingmessagesforS1/X2HO

NumberofsignalingmessagesforS1/X2HO

20

20

S1HO X2HO

15

10

15 S1HO X2HO

10

5

5

0 intraͲMMEandintraͲSͲGWS1/X2HO

0

intraͲMMEandinterͲSͲGWS1/X2HO

intraͲMMEandintraͲSͲGWS1/X2HO(EPC)

Fig. 5.

Number of signaling messages for all S1/X2 handover procedures

to the HeNB(3), 262 m ˜ 271 m HeNB(4), 281 m ˜ 292 m HeNB(5). There is no possible area for an inbound handover to the HeNB(1) as shown in Fig.4. IV. A NALYSIS OF S IGNALING M ESSAGES FOR S1/X2 I NBOUND H ANDOVER Here, we performed the analysis based on handover signaling procedures and messages in [1] and [5] in order to investigate the load impact to the control plane of the EPC by X2 and S1 handover signaling messages. The analysis included the radio resource control (RRC) and the general packet radio service tunnelling protocol-control (GTP-C) signaling message in relation to the inbound handover. The inter-mobility management entity (MME) handover was excluded from the analysis since there was no X2 handover scenario. Fig.5 presents the comparison of the number of signaling messages between S1 and X2 handover scenario in case of the intra-MME and intra-serving gateway (SGW), or the intraMME and inter-SGW. In case of the intra-MME and intraSGW, a total of twenty signaling messages for S1 handover and a total of fifteen signaling messages for X2 handover had been used respectively. Moreover in case of the intra-MME and inter-SGW, a total of thirty signaling messages for S1 handover and a total of seventeen signaling messages for X2 handover had been used respectively. Therefore, we found that the 25 % of signaling messages needed more in S1 handover when compared to X2 handover in case of the intra-MME and intra-SGW. We also found that the 43 % of signaling messages needed more in S1 handover when compared to X2 handover in case of the intra-MME and inter-SGW. Fig.6 describes the comparison of the number of signaling messages with respect to the EPC only between S1 and X2 handover scenario in case of the intra-MME and intra-SGW, or the intra-MME and inter-SGW. In case of the intra-MME and intra-SGW, a total of fifteen signaling messages for S1 handover and a total of six signaling messages for X2 handover had been used respectively. Moreover in case of the intra-

Fig. 6. only

intraͲMMEandinterͲSͲGWS1/X2HO(EPC)

Number of signaling messages for S1/X2 handover relative to EPC

MME and inter-SGW, a total of twenty five signaling messages for S1 handover and a total of eight signaling messages for X2 handover had been used respectively. Therefore, we found that the 60 % of signaling messages needed more in S1 handover when compared to X2 handover in case of the intra-MME and intra-SGW. We also found that the 68 % of signaling messages needed more in S1 handover when compared to X2 handover in case of the intra-MME and inter-SGW. Ultimately it is shown that the S1 handover causes more severe load impacts to the EPC compared to the X2 handover. Considering the LTE-Advanced environment which has relatively small cell size compared to 2G/3G network and a significant amount of HeNB, we can expect that the majority of handovers will be triggered more. The frequent S1 handover will be increased dramatically and place a tremendous burden to the EPC. Here, the addition of X2 interface and the permission of X2 handover to the HeNB can be a sort of solution to decrease the load impact to the EPC and to increase the reliable inbound handover. V. C ONCLUSION The HeNB, which can be deployed under macrocell coverage, was applied to the paper. We focused on the intercell interference effect at the UE on our performance simulation for an inbound handover. So, we looked at that there were unavailable zones to the inbound handover from macrocell to femtocell because of that severe intercell interference and the HeNB should be installed at least 225 m away from the macro eNB to be able to perform an inbound handover on our simulation environment. But this is also partial even after the point 225 m away from macro eNB. The results of our investigations show that a femtocell topology and location is important factor on a HeNB deployment from a intercell interference point of view.

We propose the intercell interference coordination method by the X2 interface between macrocell and femtocell. We confirmed that X2 interface is the better solution rather than S1 interface by investigating 3GPP specifications. Ultimately, we found that the X2 interface between macrocell and femtocell can play an important role in an high reliable inbound handover and a simple interference control implementation on femtocell environment. We believe that the easiest way to add the X2 interface between macrocell and femtocell is to use the HeNB GW, which can manage X2 connections of a significant amount of HeNB, served as a concentrator. We leave for further research the study of the intercell interference impact to an inbound handover on a diverse femtocell topologies such as a grid model. ACKNOWLEDGMENT This work was supported by the IT R&D program of MKE/KEIT. [KI002129 , Development of SON and Femtocell Technologies for LTE-Advanced System] R EFERENCES [1] 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2,” 3rd Generation Partnership Project (3GPP), TS 36.300, Sep. 2010. [Online]. Available: http://www.3gpp.org/ftp/Specs/html-info/36300.htm [2] 3GPP, “Home Node B (HNB) Radio Frequency (RF) requirements (FDD),” 3rd Generation Partnership Project (3GPP), TR 25.967, 2009. [Online]. Available: http://www.3gpp.org/ftp/Specs/html-info/25967.htm [3] D. Kim, J. Y. Ahn, and H. Kim, “Downlink transmit power allocation in soft fractional frequency reuse systems,” ETRI Journal, vol. 33, no. 1, pp. 1–5, Feb. 2011. [4] 3GPP, “Simulation assumptions and parameters for FDD HeNB RF requirements,” 3rd Generation Partnership Project (3GPP), Contribution R4-092042, May 2009. [5] 3GPP, “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access,” 3rd Generation Partnership Project (3GPP), TS 23.401, Sep. 2010. [Online]. Available: http://www.3gpp.org/ftp/Specs/html-info/23401.htm