Novel Signaling Scheme for l x CDMA-basedFemtocell Networks

Kuen-Chool Y00, Tae-Jin Lee

School ofInformation and Communication Engineering,

Sungkyunkwan University, Suwon, 440-764, South Korea

{kcyoo2000, tjlee }@ece.skku.ac.kr

Abstract-Femtocell networks can support iosolated coveragearea and provide high data rates for mobile users by usingwired Internet lines such as xDSL and cable TV networks forbackhaul connectivity. Femtocell networks can be classified into3GPP UMTS-based networks and Ix CDMA-based networks.Although 3GPP UMTS-based femtocells are standardized in theform of Home Node-B, Ix CDMA-based femtocells are notcurrently standardized. So, operators and communicationsystem vendors are developing and launching such networkswith proprietary specifications. From the viewpoint of anoperator seeking to reduce its setup-costs, the adoption of theFemto Access GateWay (FAGW) is preferred. If femtocellnetworks based on the FAGW are adopted, an efficientsignaling between the femtocells and the FAGW is required.The SIP-based signaling technique may suffer from an overloadto support large number of femtocells. In this paper, wepropose a novel signaling mechanism for FAGW and evaluateits performance in comparison with the SIP-based signalingmethod.

Keywords- Ix CDMA-based femtocell, Femto Access Gateway,Signaling mechanism.

I. INTRODUCTION

Femtocells can be deployed in small areas such as homeor office with low-power and low-cost cellular base stations.Mobile Stations (MSs) can be connected to a femtocell basestation which provides high speed backhaul for Internetaccess [1]. The 3GPP UMTS-based femtocell is standardizedwith lu-h interface between a Home Node-B and FemtoAccess Gateway (FAGW) [2]. The lx CDMA-basedfemtocells are being launched by some equipmentmanufacturers and communications system operators.Various integration models between Ix CDMA-basedfemtocells and a Ix CDMA core network have beenproposed in the femto forum [3]. By default, because theInternet is used for backhaul connectivity, a security gatewayfor IPSec tunneling between the femtocells and a corenetwork is configured as a separate node.

Three main configurations have been proposed:femtocells are connected 1) directly to the Mobile SwitchingCenter (MSC), 2) indirectly to the MSC through a gatewaysuch as a Base Station Controller (BSC)-based gateway, or

3) directly to an IP Multimedia Subsystem (IMS). Among them,the best solution in terms of the cost-effectiveness is to deploya gateway. The gateway should have various functionalities,Le., BSC functions, interconnections for lots of femtocells,No.7-based Interoperability Specification (lOS) signaling withMSC, and EI/TI-based Time Division Multiplexing (TDM)functions for voice traffic. The gateway configuration methodrequires standardizing the signaling interface between thefemtocells and the FAGW, e.g., 3GPP lu-h interface.

The interface uses a secure tunnel such as IPSec. Theinteroperability issue between two network elements connectedby the Internet may occur. In this context, some appropriatesignaling interfaces have been suggested by variousmanufacturers. Among the proposals, one of the representativemechanisms uses a Session Initiation Protocol (SIP) as thesignaling interface and Real Time Protocol (RTP) as the datatraffic interface. This approach may be appropriate in terms ofthe IMS network migration and the Internet signaling protocol.However, it incurs some compatibility issue with the existingcore networks. Therefore, we propose a new signaling protocolthat refers to the 3GPP2 lOS specifications. By comparing theperformance of the proposed new signaling mechanism withthat of the basic SIP-based signaling, it is shown that theproposed signaling is appropriate.

When an SIP message is delivered, the Al messages shouldbe transferred as the contents. So, we have to defme a new typeof contents to discriminate. And, there is mismatch between theair interface flow connecting femtocell and the Al flowconnecting the MSC. Therefore, the basic SIP flow must bemodified and the additional flows are required. Since themodifications of the SIP signaling flow between the femtocellsand FAGW may limit the performance and interoperability, asignaling flow has to be carefully designed. In this paper, wefirst consider a basic SIP-based signaling flow and evaluate itsperformance. And, we propose a novel signaling flow takinginteroperability into consideration. The new signaling approachbetween the femtocells and FAGW is based on the existing Alinterface flow and the Signaling Connection Control Part(SCCP).

The organization of this paper is as follows. Section 2describes the basic SIP-based and the proposed signaling-basednetwork architectures. Section 3 describes the basic SIP-basedand the proposed signaling flow for the Ix CDMA femtocells.

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II. FEMTOCELL SYSTEM ARCHITECTURE

Section 4 presents the simulation environment and comparesthe performance of the SIP-based and the proposed signalingflows. Finally, Section 5 provides conclusion and presents adesign guideline.

III. PROPOSED FEMTOCELL NETWORK'S SIGNALING

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Fig. 4 shows the basic SIP-based signaling flow. The SIP­based signaling protocol should translate the SIP messages andtheir contents into AI messages and parameters. Then, theFAGW can transmit the Al message to the MSC. And, thepaging message needs to be transmitted only to the femtocell atwhich the subscriber is registered. So the FAGW shouldsupport a database containing the subscriber's and femtocell'sidentities. Thus the FAGW has to perform operations thatconvert the SIP message and contents into No.7-based Almessages and parameters.

In order to meet the requirements, a new signalingmechanism is proposed. The proposed AI signaling flow issimilar to the 3GPP2 lOS flow. However, the parameters forthe bearer setup are different (see Fig. 5). The paging messagefor incoming call setup is only delivered to the femtocell atwhich the subscriber is registered, not to all of the femtocells.

The FAGW receives the SIP messages and then parses theAl parameters. So, the FAGW should provide a decisionfunction to determine what kinds of AI parameters exist. TheSIP layer delivers the messages of the Al interface related toCall Control (cq, Radio Resource management (RR), andMobility Management (MM). In fact, an FAGW should supporta lot of femtocells through SIP signaling. And as a transportlayer protocol, User Datagram Protocol (UDP) is used.

Fig. 2 shows the proposed femtocell network architectureand its protocol stack. This network refers to the IP transportlayer of 3GPP2 lOS and interoperates with the existing MSC.Its interoperability with the MSC is similar to that of the basicSIP-based architecture. Since the A2-bearer's interface deliversthe network data traffic through the RTP, it is basically thesame as that of the SIP-based signaling. The lx CDMAfemtocell network architecture proposed in this paperminimizes the modification of the legacy CDMA core network,and the FAGW is recognized as one of the BSCs. Therefore,the femtocell network can be installed without anymodification to the existing communication network. Fig. 3shows the proposed Al femtocell network and the legacyCDMA network. This femtocell network configuration enablesrapid deployment of femtocells.

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A femtocell is generally installed indoors and is connectedto a core network through access network, e.g., an xDSL orcable TV networks [5]. Since it goes through an insecurenetwork, the signaling and traffic information need to beforwarded through an IPSec tunnel. In this environment, aninterface such as a gateway can be configured for the sake ofits interoperability with the existing macrocell network.

We consider connecting an FAGW with femtocells. Fig. Ishows the general femtocell network architecture based onthe SIP signaling and the overall protocol stack. From thefemtocells to the Femto Security Gateway (FSG), IPSec isenabled by default. In general, the signaling for the FAGWuses SIP. The received SIP messages should be changed tothe Al Interface messages (see Fig. I). Because the MSCrecognizes the FAGW as one of the BSCs, to achieve the Alsignaling with the MSC, the functions of the FAGW need tobe modified. Since the femtocell includes most of thefeatures of the BSC or Base station Transceiver System(BTS), the SIP message body, which contains the AIparameters, should be forwarded to the FAGW.

Fig. 2. Femtocell architecture based on the proposed signaling. Fig. 3. The proposed At femtocell network and the lxCDMA network.

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Fig. 6. The simulaton environment for the basicSIP-based Al signalingflow.

Fig. 7. The simulation environment for the proposed SCCP-based Alsignaling flow.

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after certain duration, each session should be released. TheMSC simulator interoperates with the FAGW and issues pagingmessages for the purpose of terminating call-setups . Typically,it supports No.7-based protocols and issues request messages.The simulators do not support issuing voice payloads, but doessupport delivering information for a radio traffic channel andcircuit channel. The FAGW is composed of variousapplications, including the SIP stack, No.7 stack and the UNIXserver. To implement most of the functions of the FAGW, weconsider three parts, interoperability with multiple femtocells,interoperability with the MSC using No.7 protocols, andsession management and information handling for the femtocellidentity and the mobile station information.

Fig. 6 shows the virtual simulation environment used toevaluate the performance of the basic SIP-based signaling flow.The SIP function exists between the femtocell simulator andthe FAGW. The simulation environment for the proposedSCCP-based signaling flow is shown in Fig. 7. The simulationstarts at the femtocell simulator and the MSC simulator sendsresponse messages and paging messages. In order to measurethe performance of the modules supporting the major functionswithin the FAGW, the top utility is used, which measures theCPU load. The mean holding time of the call sessions is set to50 seconds and the registration for all MSs completes beforecall-setup.

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IV. SIMULATIONS

Fig. 4. The basic SIP-based Al signaling flow.

Fig. 5. The proposed SCCP-based signaling flow.

To evaluate the performance of the basic SIP-basedmechanism and that of the proposed, we build a simulationenvironment and simulators to simulate the MSC andfemtocell functions. The femtocell simulator should be ableto encapsulate the A I signaling information into the SIPmessage, to load the message into the UDP layer and to run alot of femtocell applications simultaneously. In addition,

To perform these functions, a database that can store thesubscriber information and the femtocell's identity ismaintained. The information should be updated when theLocation Update (LU) is performed. Also, a function thatconverts the proposed signaling to No.7-based Al signalingis provided in the FAGW. The functionality that converts theIP-based traffic into the Pulse Code Modulation (PCM)­based data traffic is performed in the FAGW through a trans­coding function. Fig. 5 shows the proposed signaling flow.

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Table 1. Simulation Paramters.

Parameter Value

Total number offemtocells 100,000Total number ofMSs 500,000Average call duration 50 secondNumber offemtocells 10,000 - 100,000Server SUN N440 Ultra SPARC lIIi

1.6GHzCalls /second 20

90 - - 1- - - 1- - -, - - 1 - - T - - r - - r - -1- - -I - - -, - - I - - l

I I I I I I I I I I I I80 - -+- Bas ic SIP Interface - - r - - r - - 1- - - 1- - I - - - 775 l

I I I I I I I70 - --- Propos ed SeeP-based - - I" - - 1- - - 1- - - , - 1 - - 1 - - i

Interface I I I , 65.7 I 63.1 I60 - - 1- - - 1- - - I - - 1 - - T - - r - - r - - - -I - - I - I - - l

50 - - :- - - : - - - : - - ~ - - ~ - - ~ - 51.7_ :_ - I 48~4 - : - - ~ - - ~I I I I I I I I I I I

40 --1---1---1-- 1- 405 1 - - 1- - 1---1---1--1--1

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I 5 I I I I I I20 I I" - - T - - I" - - 1- - -1- - -I - - - I - - I - - i

10 - -~~2- : - - -:- - ~ - -+- - ~ - - :- - -:- - -:- - -:- - ~ - - ~I I I I I

10000 20000 40000 60000 80000 100000

need to configure other kinds of expensive hardware platforms.However, by adopting the proposed signaling mechanism, wecan flexibly add new femtocells with improved performance.

V. CONCLUSION

One of the easiest ways to install lx CDMA-based femtocellnetworks is to employ an FAGW. For this networkconfiguration, the signaling interface between the FAGW andthe femtocells may be SIP-based. In this paper we haveproposed a new SCCP-based Al signaling interface . Throughthe performance evaluation of the two signaling methods, it hasbeen shown that the proposed SCCP-based AI signaling ismore efficient and provides improved performance. When acore network is migrated to an IMS core network, our proposedSCCP-based A I signaling is expected to improve theperformance of the FAGW and to provide fast and easydeployment offemtocells.

Femtocell Number

Fig. 8. CPU load for various numbers of femtocells.

Table I shows the parameters for simulation. Fig. 8 showsthe simulation result. The proposed SCCP-based A Isignaling flow occupies less CPU time for various numbersof femtocells . The CPU occupation time pertaining to thefunction modules of interoperating with the MSC is similarfor both kinds of signaling mechanisms. In the case that thenumber of femtocells is small, the difference between thetwo signaling methods is small. As the number of femtocellsincreases, the performance gap becomes larger. To installmore femtocells at one FAGW in the basic system, we may

[I]

[2]

[3][4]

[5]

REFERENCES

V. Chandrasekhar and 1. G. Andrews, "Femtocell Networks: A Survey,"IEEE Communications Mag., vol. 46, no.9, pp. 59-67, Sept. 2008.3GPP TS 25.467 V8.1.0, "UTRAN Architecture for 3G Home NodeB ,"April 2009.Femto Forum, http://www.femtoforum.org.3GPP2 A.SOOOI2-e Version 2.0, "Interoperability Specification (lOS)for CDMA2000 Access Network Interface, " June , 2001.M. Z. Chowdhury, W. Ryu, E. Rhee, and Y. M. lang, "Handoverbetween Macrocell and Femtocell for UMTS based Networks," in Proc.of 2009 ICACT Conference, pp. 1-3, Feb., 2009.

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