Algorithm for an Improved MacroFemto Handover ...

Algorithm for an Improved MacroFemto Handover Decision in Mobile WiMAX Saeed Mahfooz, Abeera Ilyas Department of Computer Science, University of Peshawar Peshawar, KPK, Pakistan [email protected], [email protected] Abstract— WiMAX network has always been a suitable candidate for providing high data rate broadband access. With the increasing demand of further improved services, femtocells (FC) are introduced in WiMAX networks. Femtocells not only increase the cell coverage and system capacity, they also provide high throughput indoors. However, some ambiguity regarding macro /femto handover and quality of service (QoS) still exists. This paper suggests a procedure for handover between macro and femtocells. The method proposed provides better services to the mobile station (MS). Furthermore, it lessens the number of unnecessary handover decisions to the FC(s). Keywords— WiMAX, Femtocells, handover delay, cell selection algorithm

I. INTRODUCTION Higher data rates in wireless networks have always been a demand of modern day users. Voice over IP (VOIP), ultra broadband access, real- time and streamed multimedia, gaming services and many other such activities require increased bandwidth for flawless communication. Many new technologies are being introduced with higher data rates but it would take a lot of time to deal with the challenges they may face [1] [2]. Mobile WiMAX and Long term evolution (LTE) are among the future wireless technologies which may form the basis of 4G wireless networks. 4G wireless network is not only backward compatible to old technologies but will also make widespread use of femto, pico and micro cells for increased capacity of network, load balancing with macro cells and higher throughput [3]. WiMAX network is the most attractive wireless technology which fulfills the demand of both larger coverage area and support for increased number of users. However, it has some issues regarding quality of service (QoS) for indoor users due to higher frequency orthogonal frequency division multiple access (OFDMA) being used with multiple input and multiple output (MIMO). According to research, more than 70% of the voice calls and data transmission is performed indoors, so a solution has to be suggested [4]. One approach can be the installation of additional WiMAX BS but it will lead to increased operational cost. Another approach might be the use of relay stations for enhanced QoS. Unfortunately, relay stations can perform better at cell borders only but are not effective for indoor services [1]. In order to provide better coverage indoor, femtocells are the most suitable

ISBN: 978-1-902560-27-4 © 2014 PGNet

option. Femtocells (FC(s)) are low powered base stations (BS) which are capable of providing a coverage area of about 10-30 meters. FC(s) is generally deployed by consumers and connected to service provider at backhaul similar to Wi-Fi access points. Distinguishing features of FC(s) include low cost, load sharing with macrocells, less infrastructure complexities and enhanced signal quality for indoor services. Due to the various distinguishable features and ease of installation, deployment of FC(s) may reach upto 50 million by 2014 [10]. Mobile WiMAX is specified in the new amendment of IEEE standard 802.16-2009 which is a revision of IEEE standards 802.16-2004[5], IEEE 802.16e-2005[6] and IEEE 802.16j-2009[7], Network Working Group (NWG) and WiMAX forum specifications. The concept of FC(s) was introduced in the release 1.5 of the WiMAX Forum [8][9]. It is basically a small coverage area of about a house or small office, home office (SOHO). According to current cellular industry definition, FC(s) may refer to the coverage area or device itself, however to be specific, a Femto Access Point (FAP) is a small plug-and-play BS which can provide services to the users under its coverage area. The consumers of FAPs can receive enhanced indoor data speeds with service quality and improved battery life since MS need not to connect with macro BS [11].However some issues of this technology are still to be addressed such as mobility management, access methods, security and efficient handovers between femtocell and macrocells. Considering the future demand of femtocells in WiMAX networks, section 1.2 discusses previous work done in the field and its background. An easy and simple handover approach is suggested between WiMAX macrocells and FC(s) in section 1.3. The proposed handover procedure will reduce the number of handover decisions. Also, the load balancing approach suggested for a hybrid access WiMAX FAP is expected to provide better services to the MS connected with FAP. Section 1.4 concludes the paper with a discussion on future work and conclusions drawn from the proposed model.

II. LITERATURE REVIEW AND BACKGROUND A. Literature Review Initial researches were focused majorly on the defining the architecture and evaluating the network performance of the femtocells [12-14]. A few researches are also done on handover issues. Authors in [15] have proposed a handover algorithm for hierarchical macro/femto cell networks. They have suggested an algorithm considering Received Signal Strength (RSS) but no QoS profile or load balancing is considered. In another research [16], an access and handover method is proposed for closed and hybrid FC(s). The approach uses a femto-initiated handover and depends on the backhaul for communication with macro base station (BS) which may lead to delay in handover decision for mobile stations (MS). In [17] an enhancement to the existing handover method is done by considering FAP’s backbone characteristic and time spent by a MS under a FAP’s coverage area. The proposed algorithm considers only load balancing parameter as a decision for selecting a suitable cell. Dependence on a single parameter leads to an average cell selection, thus not choosing the best option for MS to perform handover. Previously, no QoS measures were taken into consideration for MS. However in [3] QoS classes are introduced to differentiate the services required by various incoming MS. The method proposed reduces the scanning time interval but the QoS mechanism is complex and delay prone for handover procedure. Additionally, the proposed scheme does not take interference of FAP and speed of MS into account. Researches [18][19] suggests a handover decision for load balancing. The proposed approach in [18] uses a queuing theory and a cell selection algorithm at time when failure occurs in a FC. The approach makes sure a handover is not blocked when FC is not operational due to some fault. The method is only suitable for indoor FC(s) handover and no parameter other than cell load is considered in cell selection. Scheme suggested in [19] also suggests load balancing among neighboring FC(s) using transmission power control but the approach is not suitable for macro/femto handover decision. Authors in [1] have proposed a mechanism for reduced handover decisions. The cell selection algorithm considers parameters such as RSS, interference and speed of MS but no cell load and QoS measures for MS are taken into account. B. Background 1) WiMAX Architecture: WiMAX architecture is based on IEEE standard 802.16e [6]. An overview of the key architecture features is covered from [1]. A MS is equipped with the WiMAX wireless connectivity. Similar to any wireless network, WiMAX requires a fixed BS which is capable of providing radio coverage for a wide area about 4 to 5 miles. A MS can communicate with the respective BS if it comes under its coverage area. The Network Access Provider (NAP) is an entity (business) which provides WiMAX infrastructure between BS and WiMAX Network Access Gateway (WASN-

GW). WASN-GW serves as an entry point for WiMAX MS(s). A NAP can contain one or more Access Service Networks (ASNs). WASN-GW is important for handling all traffic coming from the BS in an ASN or vice versa. Subscribers are aggregated by gateway. Network optimization and deploying a scalable network is also the responsibility of a WASN-GW.

Figure 1: Relationship of NSP with NAP and ASN

A Network Service Provider (NSP) is an entity which contains Home Agent (HA), Authentication, Authorization and Accounting (AAA) services as well as databases containing user information. NSP is shared among various NAPs which mean NSP is connected to one or more NAPs. 2) Femto/WiMAX Network Architecture: The hierarchy of FAPs lies on top of the existing WiMAX network. A FAP not only provides indoor coverage but is also capable of providing services to low mobility users where as high speed users will remain associated to macro BS [11]. In a FC(s) supported WiMAX network, similar entities as WiMAX network are required for a functional femto-WiMAX network. So femto-NAP (FNAP), femto-NSP (FNSP), femto-access network gateway (FASN-GW), femto-security-gateway (FSec-GW) entities were introduced. Although these entities are logically separate from the conventional WiMAX network but they are responsible for MS’s registration, authentication, FC(s) organization and management, admission control and interference issues. Femto-security-gateway (FSec-GW) offers security to WiMAX FAP(s) for IPSec tunnels in IP networks. The gateway is responsible for providing authorization and authentication when MS is registered with WiMAX networks or registration of new stations with the WiMAX FAP. Along with all basic functions, the operational parameters such as network performance, coverage area, capacity and efficient hand over issues to/ from macrocells from/to FC(s) also is handled by the FSecGW.  Femto Access Control:

Classification of FC(s) greatly affects any handover approach. FC(s) has so far three access control mechanisms i-e Closed Subscriber Group (CSG), Open Subscriber Group (OSG) and Hybrid Subscriber Group (HSG). For CSG FAP(s), the owner and the subscribers’ list of FAP(s) have all reservations and access to the resources of the FC(s). In OSG, any MS is free to connect with the FC(s) if it comes into its coverage area. A balanced approach of HSG provide services to both the subscribers (with high quality) and to nonsubscribers (with less quality) [10]. Generally, an OSG and HSG is a much preferred FAP(s) access control in order to avoid interference with macro/micro BS and neighboring FAP(s) since both uses the same carrier frequency of 10MHz [13].

process begins with MOB_MSHO-REQ which can be provoked by MS, current BS or the network. The BS collects the information related to candidate BS(s) and informs MS with MOB_MSHO-RSP. MS confirms the selection of target BS and handover is initiated by message MOB_HO-IND sent to current BS. MS synchronizes itself with the target BS and basic parameters for connection are exchanged (uplink and downlink parameters). After a handshake on parameters is done, an association with target BS is created by ranging. Ranging process indicates the transmission parameters and power levels to MS. On receiving an affirmative response, MS performs network re-entry and the connection with current BS is released.

Figure 2: Mobile WiMAX/femto Network Architecture [1]

III. EXISTING MOBILE WIMAX HANDOVER SCHEME The process of handover [7] has six stages starting with cell selection, handover decision and then initiation, synchronization with the target BS, ranging, which is followed by the termination of service with current BS and handover completion or network re-entry. The cell selection phase is the stage where a suitable target cell is chosen for MS to perform handover. BS sends a MOB_NBR-ADV message periodically to identify network and advertise the list of neighboring BS(s) to MS. BS has a list of MAC addresses and mapping indexes of the neighboring BS(s). The list is advertised in network by MOB_NBR-ADV message. A scanning message MOB_SCNREQ is sent by MS to the current BS for requesting a scan interval. The current BS responds with a MOB_SCN-RSP to MS. During this interval a MS identifies a suitable candidate for a handover. Meanwhile, all the traffic sent to MS is buffered, enabling MS to scan candidates. The scanning session can be a threat to the QoS of the MS. Therefore, the time span of scanning a target BS should be as short as possible. Before connecting with the target BS, MS has to synchronize the uplink and downlink channels. The handover

Figure 3: Existing mobile WiMax handover[1]

IV. PROPOSED HANDOVER SCHEME A handover is performed when MS receives a degraded signal from serving BS. In a macro/femto environment, the MS needs to search for a new BS or a FC whichever is suitable for connection. Since the MS now has a large number of target BS(s) and FAP(s) to select from, therefore a handover decision is complex in many ways. The proposed scheme in this paper optimizes the functionalities of cell selection in macrocell/ FC. The suitable FAP can only be selected by considering parameters as cell load, mobility factors, signal strength, interference and access control. All the parameters are equally important for a quality handover decision. The handover procedure can be segmented into three phases. A. Network Discovery After the introduction of FAP(s) in WiMAX environment, an efficient handover method is needed to select a suitable cell

in minimum possible time. As mentioned in the standard architecture of mobile WiMAX [7] MS receives the MOB_NBR-ADV message from network which contains a list of candidate target FC(s) and BS(s). Considering the coverage area of FC(s) which is 10-30m, a macrocell may contain a large number of FC(s) in it. So to reduce the scanning interval time, FC(s) are organized in groups. Each group has one representative FAP which is assigned an ID (FAP_ID) by the BS. A BS selects the representative FAP on the basis of not only received signal strength (RSS) but also cell load (CL). As minimum the CL of a FAP, the better service it is capable of providing to its users. The parametric information of a FAP can be taken from the backhaul or by signaling the FAP. The MOB_NBR-ADV message will send a list of FAP_ID(s) and BS index list of candidate FC(s) and neighboring BS(s) respectively. B. Scanning Phase At time when a handover decision has to be made, MS requests for a scanning interval by sending a message MOB_SCN-REQ to current BS. The reason for a scanning interval is to have a list of available FAP(s) or target BS(s). MS analyzes the list and selects the most suitable candidates for a handover. During the scanning interval, MS is unable to send or receive any traffic; therefore the span of interval should be minimized as much as possible. Due to the filtered list of FAP(s) in network discovery phase, scanning interval is reduced in the suggested method.

Figure 4: WiMAX FAP cell re-selection

For this reason, a list of FAP(s) is formed excluding FAP(s) which are overloaded, have dropped received signal strengh (RSS), and CSG access mode. All these parameters can be fulfilled through exchanges between Femto-Sec-GW and Femto-ASN_GW. However, an alternate approach is to acquire the information through the backhaul by BS.

The Femto-ASN_GW maintains a profile of all subscribers and BS connected with it. Profile contains security context and MS specifications. This profile is exchanged between BS and target FAP while handover is performed for authorization of a MS. A periodic advertisement message is responsible for maintaining a list of FAP(s) and their neighboring BS(s) along with the access rights, CL, interference level and RSS parameters between Femto-Sec_GW and Femto-ASN_GW. This information is required during the handover decision to make an optimal choice for the procedure. The current BS sends the MAC address and mobility rate of MS to the Femto-Sec_GW. Femto-Sec_GW has an index table for all the neighboring FAP(s) to which MS can perform a handover. CAP_REQ message is to verify that whether FAP can satisfy the requirements of the MS without being overloaded. Femto-Sec_GW then sends the ordered list of FAP(s) according to the requirements to the current BS which then allocates a scanning interval for MS. The scanning interval compares the lists achieved by MOB-NBR_ADV and MOBSCN_RSP. A common FAP in both the lists is chosen for the handover. If there isnt any match in the lists then a MOBSCN_UPD message is sent to the MS for selection of another suitable target cell from the compared list. In this way our approach guarantees a shorter span of scanning interval by only comparing a minimized number of FAP(s) for a handover. C. Cell Selection and Algorithm The proposed algorithm presents a cell selection method for the handover procedure. The handover process can either be initiated by the MS or serving BS. Firstly the proposed algorithm reduces the number of FAP(s) to be scanned. Secondly, by selecting cell load as its primary cell selection parameter, a certain level of QoS is provided to MS without implementing any complex techniques. Finally, it reduces the number of unnecessary handovers for MS(s) with high mobility rate. The mechanism is shown in figure 4. Important parameters considered for the proposed approach are CL for improved quality to MS without any complex mechanisms since lesser the number of MS associated with a FAP, the better is its service for them and RSS. Another parameter is the mobility rate of MS to avoid unnecessary hand over, access mode and interference level of FAP.

After all the preceding actions are performed a handover indication message (MOB-HND-IND) is sent to terminate connection with macro BS. V. CONCLUSION AND FURTURE WORK Seamless handover between FAP and macrocell or FAP is one of the significant technical challenges faced in the deployment of FC(s) in a WiMAX environment. In this paper, a concept for the improvement in femto/macro handover decision is presented which may not only minimize the delay due to scanning interval phase but also might reduce unnecessary handovers. QoS requirements are desired by any MS that connects with a FAP. This paper also suggests an efficient cell selection algorithm which can provide a certain level of quality services to MS without implementing any complex mechanisms. In future work, an implementation of the proposed concept is intended. Furthermore, a comparison of the suggested scheme for evaluation with the existing procedures for macro/femto handover is also planned. REFERENCES [1]

[2] [3]

[4]

[5]

[6]

[7] Figure 5: Cell selection algorithm

MS with high mobility rates may not achieve better services if they perform handover frequently. A research [18] indicates that a delay and packet loss in services of nonrealtime traffic during handover is sustainable. Therefore mobility of MS can be divided into categories as Low(0-15 km/hr), Medium (15-30 km/hr) and High (above 30 km/hr). The proposed cell selection algorithm carefully handles all the necessary steps needed to perform a handover. The access mode (OSG/HSG) of FAP also effects the handover decision, and, therefore the MS is first authenticated with FAP and then the signal level is compared with the thresh to avoid signal measurement in vain as the signaling between MS and FAP before performing handover adds to network traffic and MS battey consumption. Finally, SIR level is checked after bandwidth availability for MS.

[8]

[9] [10]

[11]

[12]

[13]

Rekha Singoria, Talmai Oliveira, Dharma Agarwal, “Reducing Unnecessary Handovers: Call Admission Control Mechanism Between WiMAX and Femtocells”, IEEE Global Telecommunication Conference (Globecom 2011) 2011 Proceedings. Tom Preibe, “Current Development and Innovations in the area of Femtocells” http://www.itu.int/ITU-D/ict/statistics/, 2012. Rami ELLOUZE, Mourad GEUROUI, Adel,M ALIMI, “MacroFemto Cell Handover with enhanced QoS in Mobile WiMAX”, in wireless Telecommunication Symposium (WTS), pp 1-6, Apr 2011. G. Mansfield, “Femto cells in the US market - business drivers and femto cells in the US market” - business drivers and consumer propositions,” in Femtocells Europe. AT&T, 2008. IEEE standard for local and metropolitan area networks–Part 16: air interface for fixed broadband wireless access systems, IEEE Standard 802.16, 2004. IEEE 802.16e-2005, ‘‘Amendment for Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands”, February 2006. IEEE Std 802.16-2009, ‘‘IEEE standard for Local and Metropolitan Area Networks – Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, May 2009. WiMAX Forum Network Architecture Release 1.5 Version 1 - Stage 2:Architecture Tenets, Reference Model and Reference Points, November 2009. WiMAX Forum Network Architecture Release 1.5 Version 1 Draft O Stage 3: Detailed Protocols and Procedure, November 2009. Jeffery G.Andrews, Holger Claussan, Mischa Dohler, Sundeep Rangan, Mark C. Reed, “Femtocells: Past, Present and Future”, IEEE Journal on Selected Areas in Communications, Vol 30, No.3, April 2012. Shu-Ping Yeh, Shilpa Talwar, Seong-Choon, Heechang Kim, “ WiMax Femtocells: A Perspective on Network Architecture, Capacity and Coverage, IEEE Communications Magazine, Oct 2008. Zeng H., Cho C .and Chen W.,”System Performance of Self Organizing Network Algorithm in WiMAX Femtocells”, in proceedings of 4th ACM Iinternational Conference Proceeding Series, ICST, Brussels, Belgium, pp.1-9, 2008. David Lopez Perez, A. Valcarce, G. Da LaRoche, E. Liu and J. Zhang, “Access Methods to WiMAX Femtocells: A downlink system-level Case Study”, in IEEE Conference on Communication Systems, pp 16571662, Nov 2008.

[14] H.Claussen, “Performance of Macro and Co-channel Femtocells in a Hierarchical Cell Structure”, in IEEE 18th International symposium on PIMRC 2007, Athens, Greece, pp 1-5, Sept 2007. [15] Moon J and Cho D, “ Efficient Handoff algorithm for inbound mobility in hierarchical macro/femto cell networks”, IEEE Communication Letters, pp 755-757, China, Nov 2008 [16] Zhong Fan, Yong Sun, “Access and Hanover Management for Femtocell Systems”, 71st International IEEE Vehicular Technology Conference(VTC), pp 1-5, may 2010. [17] Zdenek Becvar, Pavel Mach, “ On Enhancement of Handover Decision in Femtocells”, IEEE Conference on Wireless Days (WD), IFIP, Oct 2011.

[18] Kyuwhan Lee, Sunghun Kim, Seunghyun LEE, Joongsoo Ma, “ Load Balancing with transmission power control in Femtocell Networks”, ICACT, pp 13-16, Feb 2011. [19] Yong Jin Kwon, Dong-Ho Cho, “ Load Based Cell Selection Algorithm for Faulted Handover in Indoor Femtocell Network”, 73rd IEEE Conference on Vehicular Technology Conference (VTC Spring), pp 1-5, May 2011. [20] Nair. Suresh, Feder, Peretz, “Methods for Managing Co-located Macro and femto Base Stations deployments and methods for initiating Mobile Station Handoff”, Patent Publication No: WO/2011/025709, US 2011/0047029 A1, Feb 2011.