Handover Scheme in LTE-based Networks with Hybrid Access ...€¦ · Handover Scheme in LTE-based...

Handover Scheme in LTE-based Networks with Hybrid Access Mode Femtocells Shih-Jung Wu, Steven K.C. Lo

Journal of Convergence Information Technology, Volume6, Number 7, July 2011 Handover Scheme in LTE-based Networks with Hybrid Access Mode

Femtocells

*1Shih-Jung Wu, 2Steven K.C. Lo *1, Department of Innovative Information and Technology, Tamkang University

, E-mail: [email protected] 2, Department of Information Management, Jinwen University of Science and Technology,

E-mail: [email protected] doi : 10.4156/jcit.vol6.issue7.9

Abstract

Femtocell networks that use Home evolved NodeB (HeNB) and existing networks for backhaul connectivity can fulfill the upcoming demand for high data rates in wireless communication systems as well as extend the coverage area. We consider some parameters for handovers, including interference, velocity, RSS and quality of service (QoS) level. We propose a new handover strategy between the femtocell and the macrocell for LTE (Long Term Evolution) -based networks in hybrid access mode. This strategy can avoid unnecessary handovers and can reduce handover failure. In this paper, we analyze three scenarios for handover decision strategy procedures: hand-in for Closed Subscriber Groups, hand-in for non-Closed Subscriber Groups, and hand-out.

Keywords: Femtocells, LTE, Handover, Hybrid Access Mode

1. Introduction

As the use of multimedia grows, the demand for high data rates in wireless communications increases. In the next generation, wireless communication systems will develop gradually towards ultra-wide band technology. Today, wireless communication systems must face the challenges of supporting broadband data access. However, surveys show that, in personal communications systems, approximately two-thirds of the calls and over ninety percent of the data service occurs indoors. To improve the coverage of personal communication systems and to increase system capacity, system operators cope with large demands from mobile devices. Femtocells are a popular method to extend mobile network coverage and to enhance the system capacity and are expected to become a mainstream solution. There are many special characteristics for femtocells; for example, femtocells have a small communication range, low power, and low cost. Femtocells can also be used in home based stations and can be set up by both system operators and consumers. The use of femtocells allows for better-quality communication services and data transmission [1, 2, 12]. As a result, using femtocell technology will be good for both users and system operators. For users, good signals enhance transmission reliability and capacity, and offer energy saving features such as reduced electrical interference and reduced power wasting. For system operators, femtocells solve shortages in radio resources and reduce the load of macrocells; their advantages also save the construction cost of base stations. It is estimated that by 2012 there will be 70 million households in the world that have femtocells, servicing about 1.5 million users. Although femtocells are very popular, there are also many challenges, including the proposed integration architecture and access control management, security, mobility management, and interference management. First, system operators must integrate the architecture that already exists, such as GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunications System), LTE (Long Term Evolution), WiMAX and other backbone internet systems. The system operators will perform integration between the original network and the femtocells. Smooth interoperability is necessary with networks, network security, network monitoring software, integration application services systems, standardization and equipment upgrades, and new and old equipment compatibility issues. Furthermore, handover procedures for existing networks are needed to support macrocell/femtocell integrated networks. In a large number of femtocells, there are too many pre-handover and unnecessary handover processes that occur. To have seamless mobility

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011 between femtocells and macrocells, it is necessary to design appropriate handover strategies [7-11]. In access control management, there are currently three femtocell access methods. The first method is a closed access mode; if you are not a CSG (Closed Subscriber Group) member, then you cannot access the CSG's femtocell. As an alternative, open access mode is open to let everyone use. Hybrid access mode is similar to closed access mode, but there are some restrictions on users of non-CSGs. How to choose the appropriate access mode and the proper management are also very important. As femtocells emphasize the convenience of building, the exact coverage area planned may not be available (cell planning). The interference between femtocells and macrocells may be very serious. We must have an adaptable self-configuration network, interference cancellation, or a way to control power. Avoiding and/or reducing the interference between two femtocells or between a femtocell and a macrocell is also an important challenge. To avoid unnecessary handoff and to reduce excessive interference, we present a handover strategy between Femtocells and Macrocells that is based on the QoS (Quality of Service) level under an LTE-based network. In the hybrid access mode, the QoS level and the other handover parameters are considered in our method. 2. Related works 2.1. Basic concepts of femtocells

Femtocells (femto) can provide indoor and small office coverage, substantial capacity, high quality and high transmission, for wireless communications services to balance the loading of a macrocell. Backhaul uses broadband internet, and the installation and configuration of backhaul uses plug and play components. A femtocell is a low-power and small-capacity base station. The power range of a femtocell is between 13~20dBm on the same floor; the maximum coverage is approximately 15 to 50 meters (the location and the actual environment would affect the coverage). Because floor to floor signals usually cross through thick reinforced concrete, the signals would have high attenuation. In this case, femtocells only cover immediate upper and lower floors. Figure 1 shows the basic structure for femtocell access [1,3].

Figure 1. The basic structure for femtocell access.

The technical standards related to the UMTS (Universal Terrestrial Radio Access Network) is

mainly formed by 3GPP (3rd Generation Partnership Project) and by Broadband Forum (BBF). In addition, two other agencies, Femto Forum and NGMN (Next Generation Mobile Network), also make contributions. Femto Forum was established in July 2007, and is mainly composed of mobile services, and hardware and software manufacturers. In May 2008, Femto Forum's discussions created 3GPP in the Iuh interface and BBF in the TR-069 family, which represent significant progress. Table 1 shows related terminology for femtocells.

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011

Table 1. Terminology for femtocells. 3GPP terminologies Popular names HNB (home NodeB)

Called HNB in UMTS Called HeNB in LTE

Femtocell (FAP)

HNB-GW (HNB Gateway in UMTS) HeNB-GW (HeNB Gateway in LTE)

FAP-GW (FAP Gateway)

HMS (HNB management system) ACS (Auto-Configuration Server)

Figure 2 shows the logical view of the HNB access network architecture and reference model. The

HNB connects to the mobile network through the HNB-GW. The HNB-GW is like a concentrator to aggregate a large number of HNBs. This interface, between the HNB and the HNB-GW, is called Iuh. In addition, this connection goes through a security gateway (SeGW), which provides a security mechanism. The HNB Management System (HMS) is based on the TR-069 family of standards. This system facilitates HNB-GW discovery and provides configuration data and authentication to the HNB. [1, 7]

Figure 2. Iuh reference model

2.2. Access control of a femtocell

There are three types of access control models for femtocells: open, closed, and hybrid. The closed access mode service is only for CSG users. However, the system operator will set a different service level for the needs of CSG users. In this case, management is more complex because femtocells must also be able to know a user's payment level. Under open access mode, any MSs (mobile stations) can use femtocell services. However, in mixed mode, non-CSG services can get limited service. Of course, the access control mode depends on the management of the system operators. Figure 3 shows the access control management. Using CSG requires having a CSG_id, which gives a user the CSG femtocell service [5]. In an office building, the HeNB is an open cell that can serve any UE (including UE2). The CSG_id of UE2 is “none”; as a result, in this case, service can be only open cell or hybrid cell. CSG_id 2 HeNB in a coffee shop is a hybrid cell that can serve the UE3 and provide restricted service to UE2. The closed cell at home offers normal service to the set of UEs belonging to the CSG_id=1.

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X

Figure 3. The access control scenario.

2.3. LTE-based network architecture of a femtocell

For Femtocell and Macrocell network integration, there are some feasible choices. The E-UTRAN (Evolved Universal Terrestrial Radio Access Network) HeNB architecture discussed in LTE femtocell standards has not been finalized. The architecture that we want to have would follow the all-IP principles and would integrate the evolved packet core (EPC) smoothly. The HeNB management system is similar to HMS in UMTS. The reference of the LTE femtocell architecture is shown in Figure. 4 [7,13].

Figure 4. E-UTRAN HeNB logical architecture.

3GPP has specified two standard interfaces, which are X2 and S1 for the Evolved Packet System.

The X2 interface provides functionality to support mobility and the exchange of information between eNodeBs (macrocells). The S1 interface supports relations between MME (Mobility Management Entity)/SAE GW (System Architecture Evolution Gateway) and eNodeB. Furthermore, S1 is used for communication between HeNB and MME/SAE GW. If HeNB worked at the control plane, it could communicate through S1-MME. Otherwise, the connection between HeNB and MME/SAE GW could use the S1-U interface for the user plane. For a better integration, we could design HeNB-GW as an

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011 eNodeB for the MME/SAE GW view. The entire E-UTRAN architecture with HeNB is shown in Figure 5.

X2

Figure 5. The complete E-UTRAN architecture with HeNB.

3. Handover scheme in hybrid access mode femtocell networks

The handover process in LTE femtocell networks should be modified. In this paper, we consider the received signal strength, the velocity and the QoS level, and we design a new handover scheme for hybrid access modes in femtocell networks. Although there are many related papers, few consider the hybrid access mode. Here, we utilize the interference level as the primary factor with regard to non-CSG users. Our idea can avoid unnecessary handovers and can reduce handover failures. 3.1. Handover scenario

The handover between the LTE macrocell and the HeNB should operate seamlessly and smoothly. This goal is a big challenge for the LTE, including the femtocell network. In Figure 6, we describe several cases for handover scenarios in femtocell networks utilizing the hybrid access mode.

Figure 6. Handover scenario for femtocell networks

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The first case is hand-in. The hand-in type of handover is from a macrocell service to an HeNB. Accordingly, for the CSG or non-CSG user, we can divide two subcases in the hand-in case, which are hand-in (CSG) and hand-in (non-CSG). The second case is hand-out. The hand-out case represents a handover from the HeNB service to a macrocell. The third case is an inter-HeNB-HO (handover). This case describes the handover between two HeNBs. However, our handover decision strategy does not consider the inter HeNB-HO. 3.2. Interference scenarios

In the femtocell network, interference is divided into the following: Cross-layer - femtocell interference; macrocell users interfering with each other; Co-layer - femtocell interference; and femtocell users interfering with each other. To cope with interference, recommendations in [15] are given for the use of power control in CDMA systems and intelligent subchannel allocation in OFDM systems. Co-layer interference always occurs in two close femtocells without obstruction. The Cross-layer interference often occurs with macrocell users in a femtocell service area, and both the uplink and the download has occurrences of interference. Assume that our LTE is based on an OFDM, as shown in Figure 7 and Figure 8; these figures describe the interference scenarios for downlink (Figure 7) and uplink (Figure 8). The M-user was serviced by a macrocell (eNodeB) and the F-user was serviced by a femtocell (HeNB). In the case of downlink, if the M-user and the F-user use the same subchannels, then this sharing of subchannels will cause interference, as shown in Figure 7. If the signal is strong enough to satisfy the user, then the signal can cause interference easily. This scenario is similar to the case of uplink (Figure 8) [1,4,8].

Figure 7. Interference scenario for downlink transmission.

Figure 8. Interference scenario for uplink transmission

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011 3.3. A new handover scheme

We propose a simple handover strategy in Figure 9, which operates under an LTE-based network between a femtocell and a macrocell under the Hybrid access mode. The handover strategy considers the received signal strength (RSS), the velocity (V), the available bandwidth, the QoS, and the interference level. Our algorithm can decrease the interference and reduce unnecessary handover. This algorithm is divided into two parts: Handover from HeNBs to macrocells (hand-out) and Handover from macrocells to HeNBs (hand-in). We did not discuss the handover between one HeNB and another HeNB (inter HeNB-HO). In this case, the hand-in has two sub-components: one sub-component is for CSG users and the other sub-component is for non-CSG users. We design two velocity levels, Vt1 and Vt2, for which Vt1>Vt2. (For example Vt1=30 kmph and Vt2=15kmph.) The system operator can set up this configuration by himself, using professional judgment. In the case of hand-out, the considerations are simple but the management is more complex. As a result, both CSG and non-CSG users need to perform handover into macrocell service when they experience bad communications. Therefore, we first consider V of MS (mobile station). We check whether V is over Vt1 kmph, and we check the availability of the bandwidth, then we perform handover into a macrocell. If the MS is lower than Vt1 kmph, then we use RSS to decide whether to perform handover or not. The HeNB physical layer cannot be a high-speed service. In the case of a hand-in for a hybrid access mode, first we check whether the user is CSG or not. If the MS is CSG, then we see whether the RSS is lower than a threshold and we check whether the velocity is over Vt1 kmph or not. If V>Vt1 kmph, then there is no handover. If Vt1 kmph>V>Vt2 kmph, then we see whether there is real-time service or not. If the answer is yes, then the handover will be executed in the bandwidth that is available. If V<Vt2 kmph, then the handover will also be executed in the bandwidth that is available. If the MS does not belong in the CSG, then in a normal situation the HeNB does not need to provide service. However, if the MS causes too much interference (as in Figure 7 and 8), then we hope that the MS can perform handover to the HeNB, to reduce interference. When V>Vt2 kmph, then handover is not allowed because we believe that this interference will pass soon. While V<Vt2 kmph and achieves an interference level, handover will be requested. This handover is different from a normal situation, where handover is initiated by the HeNB.

Figure 9. The new handover strategy

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011 4. Handover procedures 4.1. Hand-in procedures

The handover procedures between traditional LTE macrocell are presented in [13] and [16], and we modify the handover mechanisms include femtocells. The handover from a macrocell to a femtocell has two kinds of procedures. The first procedure is for the CSG. Normally, the MS should choose the most appropriate target for the HeNB. This scenario is shown in Figure 10. For the non-CSG, the handover procedure has some differences from the normal situation. The initialization of the handover is triggered by the HeNB, to avoid interference, as can be seen in Figure 11.

Figure 10. The hand-in procedures for CSG users

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011 4.2. Hand-out procedures

The hand-out procedures are not complicated. The procedures for hand-out are the same [14] shown

in Figure 12. The UE has no option to select the target cell because there is only the macrocell eNodeB. If the UE has a high velocity or the RSS is decreasing continuously, then either condition will trigger the hand-out procedure.

5. Conclusions and future work

In this paper, we propose a simple and effective handover algorithm for LTE-based femtocell networks. We offer a new idea in which, according to an interference level trigger, a handover procedure is performed for a non-CSG user. This algorithm could reduce unnecessary handover initialization and could eliminate cross-layer interference. However, we do not consider co-layer interference and inter-HeNB HO. In addition, we do not discuss the inter-HeNB-HO that involves handover between HeNB and HeNB. In the future, we hope to add co-layer interference consideration and to perform simulations to compare other handover strategies.

Figure 11. Hand-in procedures for non-CSG users

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Figure 12. Hand-out procedures

6. References [1] Jie Zhang, Guillaume de la Roche, "Femtocell:Technologies and Deployment", John Wiley &

Sons Ltd, UK, 2010. [2] Vikram Chandrasekhar, Jeffrey G. Andrews, Alan Gatherer, "Femtocell networks: a survey", IEEE

Commun. Mag., pp. 59-67, Sept. 2008. [3] Douglas N. Knisely, Frank Favichia, "Standardization of femtocells in 3GPP2", IEEE Commun.

Mag., pp. 76-82, Sept. 2009. [4] David Lopez-Perez, Alvaro Valcarce, Guillaume de la Roche, Jie Zhang, "OFDMA femtocells: a

roadmap on interference avoidance", IEEE Commun. Mag., pp. 41-48, Sept. 2009.

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Journal of Convergence Information Technology, Volume6, Number 7, July 2011 [5] Assen Gulaup, Mona Mustapha, Leo Boonchin, "Femtocell access control strategy in UMTS and

LTE", IEEE Commun. Mag., pp. 117-123, Sept. 2009. [6] M. Chowdhury, W. Ryu, E. Rhee, Y. Jang, "Handover between Macrocell and Femtocell for

UMTS based Networks", in Proceeding of the IEEE ICACT, pp. 237-241, FEB. 2009. [7] Haijun Zhang, Xiangming Wen, Bo Wang, Wei Zheng, Yong Sun, "A Novel Handover

Mechanism between Femtocell and Macrocell for LTE based Networks", in Proceeding of the 2nd International Conference on Communicaiton Software and Networks, pp.228-231, 2010.

[8] Zhong Fan, Yong Sun, "Access and handover management for femtocell systems", in Proceeding of the Vehicular Technology Conference, pp.1-5, 2010.

[9] Tarek Bchini, Nabil Tabbane, Emmanuel Chaput, Sami Tabbane, Andre-Luc Beylot, "Interconnection & Handover between IEEE 802.16e & UMTS", IJACT : International Journal of Advancements in Computing Technology, Vol. 1, No. 2, pp. 99-09, 2009.

[10] Hasina Attaullah , Muhammad Younus Javed , "QoS based Vertical handover between UMTS, WiFi and WiMAX Networks", JCIT: Journal of Convergence Information Technology, Vol. 4, No. 3, pp. 59-64, 2009.

[11] Pyung-Soo Kim, Yong Jin Kim, "Hierarchical Mobile IPv6 Based Fast Vertical Handover using IEEE 802.21 Media Independent Handover Function", JCIT: Journal of Convergence Information Technology, Vol. 2, No. 4, pp.41-45, 2007.

[12] Femto Forum, http://www.femtoforum.org [13] 3GPP-TS36.300 v8.5.0, “E-UTRAN Overall Description”. 2008. [14] Ardian Ulvan, Robert Bestak, Melvi Ulvan, "The study of handover procedure in LTE-based

femtocell network", in Proceeding of the Wireless and Mobile Networking Conference (WMNC), pp.1-6, 2010 .

[15] 3GPP, "3G Home NodeB Study Item Technical Report", 3rd Generation Partnership Project – Technical Specification Group Radio Access Networks, Valbonne (France), Tech. Rep. 8.2.0, Sep. 2008.

[16] 3GPP TS 36.331, “Radio Resource Control (RRC).”

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