Reducing Unnecessary Handovers: Call Admission ControlMechanism for Handover betweenWiMAX Macrocells and Femtocells

Rekha Singoria and Talmai OliveiraTechnical Report No. 002.12.2010

School of Computing Sciences and InformaticsCenter for Distributed and Mobile Computing

University of CincinnatiCincinnati, OH 45221-0030, USA

singorra@mail.uc.edu, oliveitb@mail.uc.edu

Abstract—Femtocells are capable of providing services inshadowed areas of the WiMAX macrocell (cell coverage en-hancement) and can relieve traffic from the macrocell net-works, reduce infrastructure costs for the network operators,allow for network capacity increase, and provide neededservice quality in indoor environments. However, conventionalhandover methods cannot support a good enough performanceunder different mobility patterns and dynamic network con-ditions. Unnecessary handovers pose serious problems since itmay cause reduction in the user’s QoS level and the systemcapacity. This work introduces an appropriate call admissioncontrol and proposes a resource management scheme thatsignificantly reduce unnecessary handovers. Simulation resultsare shown to validate our performance predictions.Keywords-WiMAX Femtocell, Handover

I. INTRODUCTIONIt is reasonable to assume that the demand for high data

rates wireless networks will only increase, as users expectproviders to support IP telephony, ultra broadband Internetaccess, gaming services, streamed multimedia and manyother activities. Existing wireless communication systems,however - even as they evolve to a true 4G network -face an increased number of challenges [1]. Although notthe only solution, WiMAX (Worldwide Interoperability forMicrowave Access) system is one of the most attractivetechnologies for this future scenario [2] as both the coveragearea as well as the capacity of existing cellular networks sys-tems are expanded. Unfortunately, offering QoS is difficult inWiMAX when indoors [3, 4] due to high frequency OFDMAused with MIMO, as it has been shown that higher thetransmission frequency is, larger will be penetration lossesthrough walls. To makes matters worse, more than half ofvoice calls and data usage are performed indoors [5, 6].Wireless service providers could deploy additional

WiMAX macrocells, but this would increase their cost ofoperation. The use of relay stations (RS) in the border areahave been advocated to enhance the quality of wirelesssignals. Such a solution is effective in the WiMAX celledges, but is rather cumbersome to deploy indoors or in

internal areas of the cell. Here, we propose to employ fem-tocells which are low-power access points that are capable ofduplicating the role of the macrocells and operate in licensedspectrum, whilst providing wireless voice and broadbandservices to customers and reducing infrastructure costs. Keybenefits of femtocells include load sharing, infrastructurecost reduction, and signal quality enhancement [7].In this paper, a simple handover mechanism is described

between macrocell and femtocells for WiMAX networks thatsignificantly reduces the amount of unnecessary handoversin networks with hybrid access WiMAX Femto Access Point(WFAP).

II. ADVANTAGES OF THE FEMTOCELL APPROACHThe need for higher data rates has traditionally been dealt

with by increasing the bandwidth of the radio frequencycarriers, better modulation and channelization techniquesand by spectrum reuse through division of the coveragearea into smaller cells. For continuous indoor coverage,however, there are two alternative technologies: WiFi andRelay stations.According to [8], seamless roaming between a WiFi

Access Point (WiFi AP) and the cellular network can occurwhen the MS is a dual-mode handset and once withintransmission range, voice/data traffic is carried over theIP network. However, from the network operator point ofview, this can be less favorable since they cannot charge forservices carried over WiFi APs. For subscribers, dual-modeMSs have the undesired property of a much higher cost andstringent power constraints. Finally, QoS and security overunlicensed spectrum can be of a great concern.Relay stations [9] can be used to extend point-to-

multipoint links between the macrocell BS and the MS, byintelligently relaying data between the macrocell BSs andthe MSs located in the border area of the cell. Relay stationsconduct all transmission through their wireless interfaces -not requiring wires nor IP backhaul - and provide a cost-effective way to extend coverage and capacity. This strategy

has been proposed in the IEEE 802.16j task group draft [ 10,11]. However, since it uses wireless backhaul, the amount ofspectrum available for access further is reduced. Moreover,relay support is completely transparent to the WiMAX MS,and therefore the MS is never aware that it is receivingcontrol messages from the macrocell BS and data trafficfrom the RS. The MS is then unable to control interferencenor to distinguish between the two entities when measuringsignal strength of interfering stations. Another limitationis that the underlying topology is a tree, while a moregeneral mesh or distributed control functionality would allowthe network to operate in a more efficient manner as cellsize is decreased when the number of RSs is increased.Furthermore, it is difficult to utilize RSs in the interior ofthe cell, making it especially impractical to use RSs indoor.On the other hand, Femtocells are installed indoors, in

the customer’s residence, and data packets received bythe WFAP are relayed back and forth, utilizing cellulartechnology with IP backhaul through the customer’s fixedbroadband Internet connection to the provider’s network. Aconsiderable amount of traffic handled by the macrocells canthen be transfered to the femtocell system. But, in order forthis scenario to take place, unique technical challenges mustbe tackled, including handoff, synchronization, QoS andinterferences in the implementation of the WFAP. Networkintegration of WiMAX and femtocells is not predeterminedby the network operator, but through a random and un-known rationales determined by the users, greatly affectingthe interference measurements intrasystem and intersystemamong WFAP’s and macrocells. Furthermore, if the numberof neighboring WFAP’s are large within a macrocell area,unnecessary handoffs need to be eliminated.Equipped with self-organization capabilities, femtocells

make the network easy to operate and manage, and a moredistributed handover decision process can make the overallnetwork more efficient. Not to mention the fact that WFAPscan support only low-mobility users with high data rates,while existing macrocell and RS deployments can serveusers with high mobility. Therefore, competing technologiessuch as WiFi and RS may coexist in the network [8].

III. WIMAX ARCHITECTUREThe WiMAX network architecture is based on IEEE

802.16e standard [12], a WiMAX Forum Network WorkingGroup specification. It specifies everything from the PHYand MAC of the radio link, as well as the end-to-endarchitecture. It also differentiates the functional and businessdomains, providing modularity and flexibility in deployment.Instead of describing the whole standard, a basic descriptionof the key elements will now be covered from [7, 12, 13].A Mobile Station (MS) consists of the user equip-

ment, providing wireless connectivity between a user and aWiMAX network. Like any other wireless network, WiMAXrequires a fixed transceiver that has radio coverage over

a wide area which enables MS to communicate over arange of 4 to 5 miles. The Network Access Provider (NAP)is a business entity that provides WiMAX radio accessinfrastructure through the Macrocell Base Stations (BS)and the Access Service Network gateways (ASN-GW) thatserves as a point of entry for the MS to the WiMAX network.A NAP is deployed as one or more ASNs. The ASN-GW plays an important role as it aggregates subscriber andcontrol traffic from the BSs, as this entity permits networkoptimization and enables provides to deploy highly scalablebroadband networks. The Network Service Provider (NSP) isthe business entity that manages user subscriptions, providesIP connectivity according to a negociated service levelagreement, and manages the WiMAX services. NSPs includea home agent, authentication, authorization and accounting(AAA) as well as other corresponding servers and databases.NSPs shares NAPs, and a single NSP may control multipleNAP’s.

Figure 1. WiMAX femtocell high-level architecture

In a femtocell populated WiMAX network, for scalabilityreasons, similar entities must be deployed, and so theFemto-NAP, Femto-NSP, Femto-Security-GW and Femto-ASN-GW are introduced. Logically separated from theirrespective conventional WiMAX counterparts, these enti-ties are reponsible for MSs’ subscriptions, AAA opera-tions, femtocell management and self-organization subsys-tems, femto-specific functionalities such as closed subscribergroup, subscriber admission control as well as interferencemanagement. The Femto-Security-GW provides IP securityfor the WFAPs with IPSec tunnels and is responsible forauthentication and authorization to other funcional entities inthe WiMAX network. Furthermore, the inherent potential ofmany WFAPs being deployed in overlay coverage of macro-cell BSs or of neighboring WFAPs, the operating parameterssuch as network performance, coverage and capacity must be

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well monitored, and handover to/from macrocell and to/fromfemtocells need to be efficient. Figure 1 shows the describedentities.One aspect still not mentioned is that of the access

preference to the WFAPs. Since these will use the residentialbackhaul - paid by the primary femtocell subscriber - somesort of access restriction is expected, or at least a preferentialaccess should be given to the users associated with theprimary femtocell subscriber. Depending on the business anddeployment models, WFAPs may be configured to operate ineither open access mode with no access limits on admission,closed subscriber group mode where only a selected list ofMSs have access, or in a hybrid access mode where certainMSs are admitted with a higher level of QoS, but non-members are admitted (at a lower level of QoS) as longas there exists any surplus bandwidth.

IV. HANDOVER CHALLENGESSeamless handover between a WFAP and a macrocell,

or to other WFAPs is one of the key technical challenges inthe development of femtocell products, as well as one of thereasons that limits their widespread deployment. As mobilityis inherent to MS, providing continuous service withoutinterruption can only be achieved by WFAPs and macrocellsthat support handoff. However, MSs with varied speeds maymove through an area with deployed WFAPs and performmultiple handovers, most of which are unnecessary, speciallyfor high speed users [14]. Considering dense urban neigh-borhoods, WFAPs may be installed in a large number ofhomes, therefore the Femto-ASN-GW may need to managethousands of femtocells [15]. The conventional advertisingtechnique of broadcasting the WFAP’s status leads to verylarge neighborhood advertising message sizes, high delaysduring the scanning period, and wasted resources [4, 13].Even the access mode of the cell may affect handover 1. In

an open access mode, frequent and unnecessary handoverscould occur under a dense scenario. In the closed accessmode, while exclusive access is limited to a list of MSs,un-registered MSs in range of the WFAP may cause highlevels of interference, thus lowering the service quality [16].Any handover procedure needs to be able to optimize

and improve the performance of both the femtocell andthe conventional WiMAX network. For these reasons, somemodification of the handover procedure is required in orderto improve the performance of femtocell/macrocell net-works.

A. Existing Handover SchemesHandover procedures can normally be divided into 3

phases: a preparation phase where measurements are madeand information about surrounding BSs and WFAPs are

1In fact, only hybrid access mode is capable of reducing unnecessaryhandovers [6]. For this reason, we assume that WFAPs use hybrid accessmode in this paper.

accumulated. Authentications are also acquired for securitypurposes [6, 15]. A processing phase where the best candi-date is selected and a handover decision is made, and finally,an execution phase, where termination with the servingBS/WFAP is executed, and network entry is carried outthrough the new BS/WFAP. As defined in the IEEE standard,3 basic handover modes are supported to enable continuousdata transmission and services when a MS moves across thecell boundaries of macrocells, and they are: Hard Handover(HHO), Macro Diversity Handover (MDHO) and Fast BaseStation Switching (FBSS) [3].In HHO, a break-before-make approach is taken, which

is less complex but high latency may be exhibited, possiblyinterrupting delay sensitive application. HHO uses differentfrequencies between neighboring BSs, and only allows a MSto be connected to one BS at a time. Before the handoverrequest is made, a suitable target BS is selected. In MDHO,all the BSs use the same frequency. A “diversity set” thatincludes several BSs that may be involved in the handoverprocess is kept for the MS, and the MS may communicatewill all BSs in the set simultaneously. The diversity set isupdated based on the long-term statistical signal strengthof BSs, and whenever a BS is above (or below) a pre-determined threshold, it will be added (or removed) fromthe set. Among these, one is selected as an Anchor BSfor the transmission of management messages. In FBSS,a similar set is maintained, but the MS may only connectto the Anchor BS for both data and management messagetransmission. The Anchor BS may be changed accordingto some MS requirement, or whenever the MS is handoff.Out of these 3, however, only the HHO is mandatory, butalthough very similar to that used in cellular technologieslike EV-DO [17] and HSDPA [18, 19], the HHO schemein WiMAX is highly bandwidth efficient, fast, smooth andnearly glitch-free [20].Comparatively, HHO is simple to implement, minimizes

handover overheads and, most of the time tends to ensuresufficient QoS without significant interruptions and degrada-tions of QoS. However, depending on the network load, thismay not be true, and long delays may be experienced. HHOis not very good for handling voice-based requirements withhigh-speed MSs. MDHO and FBSS, on the other hand, aredesigned to allow full seamless mobility at a much higherMS speed. Both the macro-diversity handover schemes usedby these are designed to provide better performance withrespect to multi-access interference, flexibility and coverage.However, according to [20], there is still a long way to gobefore adequate support measures for these two techniquescan be developed and deployed in WiMAX networks. Abrief summary of WiMAX handover techniques can be seenin Table I.For these reasons, several fast handover schemes have

been proposed. In [21], a target BS selection algorithm isproposed to reduce the redundant scanning. The basic idea

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Table ICOMPARISON OF WIMAX HANDOVER TECHNIQUES.

Parameters HHO FBSS MDHOLatency High Medium LowComplexity Low Medium HighReliability Low Medium HighPacket Loss High Low LowCost Low Medium HighSpeed Low Medium HighLink Quality Low Medium High

is to select only one target BS to be scanned or associatedbased on the estimated mean carrier to interference-plus-noise ratio (CINR) among neighboring BSs. Although thisreduces time spent on multiple rounds of scanning, it doesnot provide a feasible way to estimate the CINR from theneighboring BSs. In [22], data transmission is immediatelyallowed after handover synchronization. Normally, the MScan receive data only in the normal operation mode afterthe whole handover process is completed, which may causeloss of data and impair real-time services. On the other hand,Ma [3] has argued that it could only be applicable to real-time downlink services such as video streaming. The workof [23] allows for both uplink and downlink transmissionsbefore the handover process is completed, through priorreservation of resources. But, due to limited number ofresources that can be reserved, this scheme is recommendedexclusively for real-time applications.

V. PROPOSED HANDOVER SCHEMEOur proposed handover scheme optimizes the selection

functionalities in the femtocell/macrocell handover. Bothmacrocell to femtocell handover, as well as femtocell tomacrocell handover are complex in many ways. The formermay have several possible target femtocells for handover.The appropriate WFAP needs to be selected by consideringthe mobility factors, interference levels, as well as authoriza-tion (access model). The latter is significantly easier, sincewhenever a user moves away from a femtocell network, thereis no other option other than the macrocell networks. But,one very important issue in that of the handover time, whichshould be very small.

A. Macrocell to Femtocell HandoverIn the WFAP selection phase, to determine a suitable

one for performing handover, a MS may scan or associatewith neighboring WFAPs. This step is carried out before thehandover request is made, following the steps of HHO. Themacrocell serving BS periodically broadcasts the topologyinformation and the channel information of neighbor WFAPswith Mobile Neighbor Advertisement (MOB-NBR-ADV)message. A BS may also obtain that information over thebackbone. The MS can acquire this information and use it forcell selection before starting any particular scanning process.

Later, a Mobile Scanning Request (MOB-SCN-REQ) maybe initiated by the MS to request allocation of scanningintervals. Then, these scanning intervals are allocated andacknowledged by serving BS via Mobile Scanning Response(MOB-SCN-RSP). During the scanning process, the MSmeasures the channel quality or signal strength of eachneighboring WFAP. A list of neighboring WFAPs may beselected as a candidate for the actual handover. The cellselection is performed prior to actual handover, and theconnection to the serving BS is still maintained.Both the MS and the serving BS can request a handover

activity, and thus a handover process is initiated by MSHandover Request message (MOB-MSHO-REQ) or MobileBS Handover Request message (MOB-BSHO-REQ) whenthe conditions to perform handover are satisfied. If thehandover is requested by the MS, the MS may send aMOB-MSHO-REQ and indicate the possible target WFAPsbased on the analysis of the metrics measures from thescanning procedure. The serving BS may negotiate withthe recommended target WFAPs via backbone network,and it sends acknowledgement to the MS with Mobile BSHandover Response message (MOB-BSHO-RSP). On theother hand, if the handover is requested by the servingBS, it sends a MOB-BSHO-REQ to the MS, in whichneighboring WFAPs are suggested. The MS can conducthandover to one of the recommended WFAPs, or reject thislist, and attempt to perform handover to some other WFAP.During the message exchange, dedicated ranging opportunitymay be allocated to speed up the ranging process for laternetwork re-entry.After this exchange of information, authoriza-

tions/registration of the MS onto the network is re-started.In this phase, the MS needs to synchronize with downlinktransmission and obtain downlink and uplink transmissionparameters with target WFAP through the Femto-ASN-GW.A Ranging Request message (RNG-REQ) is to start aranging process. A dedicated ranging opportunity may beavailable if it is allocated in the previous step, so as toavoid contention-based ranging. Later, Ranging Responsemessage (RNG-RSP) is transmitted, and in which there-entry management messages that can be omitted areindicated. After the channel parameters are adjusted, the MScan communicate with target WFAP (once again through theFemto-ASN-GW) to negotiate channel capability, performauthorization and conduct registration. This happens throughthe transmission of a Basic Capability Request (SBC-REQ)message and authentication with Privacy key managementmessages (PKM-REQ/PKM-RSP). To speed up the process,some information about the MS may be transmitted to thetarget WFAP via the internet backhaul. Finally, MobileHandover Indication message (MOB-HO-IND) is issued toterminate the connection with the serving BS. The handoverprocedure is completed thereafter, and the data transmissionbetween the MS and the new serving WFAP can be started.

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Figure 2. MAC layer macrocell to femtocell handover procedure

The handover procedure at the MAC layer is illustratedin Figure 2.

B. Femtocell to Macrocell HandoverAs previously stated, the handover from a WFAP to a

BS can be a bit less complex due to the fact that as aMS moves away from a WFAP, there is likely to be onlyone option of macrocell BSs. Time is of essence, and thehandover should be executed as quickly as possible. Sincethere is no complex interference analysis and multi-levelauthorization check, this process is somewhat simpler. Dueto space limitations, a full description will not be included,but the reader may notice the similarities in the messagessent during the handover procedure at the MAC Layerbetween macrocell to femtocell. Figure 3 demonstrates theprocedure.

C. Reducing Unnecessary HandoverIn Section IV we listed some of the most important

challenges that need to be tackled when a handover occurs ina femtocell enabled WiMAX network. Of these, two aspects

Figure 3. MAC layer femtocell to macrocell handover procedure

deserve to be mentioned. First, since BSs and WFAPs broad-cast a neighbor list used by the MS to learn where to searchfor potential handover targets. If the number of adjoiningcells are large, the MAX overhead becomes significant dueto the increased message size of the neighboring cells list.Second, since a femtocell coverage area is possibly small,a high speed MS can enter many such areas in a shorttime, causing two or more unnecessary handover due tomovement. This leads to reduced end-to-end QoS level aswell as decrease in the overall capacity of the system. So,minimization is highly desired, if not absolutely necessary.For this reason, we also propose a new Call AdmissionControl (CAC) mechanism. Whenever a WFAP receives ahandover request from the Femto-ASN-GW or from the MS,the WFAP makes a decision to allow the handover accordingto the proposed CAC seen in Figure 4. The reader is onceagain reminded that femtocell are preferred to operate in ahybrid access mode, which is more flexible and eliminates anumber of unnecessary handover [24], but may also operatein open or closed mode.

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Important parameters considered during the modeling ofour CAC are the detected signal level, the expected timeof residence in a cell coverage area given the MS speed,the duration of time the MS is capable of maintaining theminimum required signal level and signal-to-interference(SIR) level, and the capacity (bandwidth) that one femtocellcan accept.

Figure 4. Proposed Call Admission Control (CAC)

Since users with high velocity may cross multiple WFAPboundaries, the MSs’ QoS may not be maintained. There-fore, the work of [14] persuades that, unless the MSs’ trafficrequires a strict real-time service, the handover can waitas delay and packet loss can be tolerated to some extent.Therefore, a mobile state is initially defined as Low, Mediumor High.

• Low mobile state: from 0 to 15km/h, slow walk,stationary.

• Medium mobile state: from 15 to 30km/h, speedequivalent to that of riding a bicycle.

• High mobile state: anything above 30km/h.Depending on the sensitivity of the MSs’ requirements,

the CAC carefully handles the handover. The threshold isthe minimum level required for the handover to be executedfrom the serving BS or serving WFAP. If this threshold ishigher, then the available bandwidth is taken into account.Otherwise, we check if the serving BS or serving WFAP haslowered their signal quality. If this occurs, then handovermay still be the best option. The access mode of the WFAPaffects the handover, and therefore, the MS may need toauthenticate (when closed or hybrid), and to check if thesignal level is greater then a threshold time specified by thenetwork operator. This is needed since, sometimes the MSdetects a signal larger than the threshold, but for a very shorttime-span. Finally, the SIR levels are tested and the MS willbe accepted to handover to that WFAP or not.

VI. PERFORMANCE EVALUATION

In order to reinforce the benefits of our proposed CAC,simulations are performed. Table II shows the simulationparameters used. We calculate the angle of movement ofa MS, update the MS position according to the movementspeed, and scan for available WFAPs. We assume 100WFAPs within a macrocell coverage area. 10, 000 runs areexecuted and the average results are used for the plots.

Table IISIMULATION PARAMETERS

Shape of WFAP coverage CircularRadius of WFAP coverage area 10 mMS Speed Low: (0-15km/h)MS Speed Med (15-30km/h)MS Speed High (30-60km/h)Number of WFAP within Macrocell 100Number of MS 1 to 33Number of Runs 10,000

Figure 5 demonstrates that the number of handovers arereduced when using our CAC. The differentiated treatmentof the signal threshold and the expected time of residencein a cell coverage area given the MS speed, the durationof time the MS is capable of maintaining the minimumrequired signal level and signal-to-interference (SIR) level,and the capacity (bandwidth) that one femtocell can support.This can significantly reduce the amount of unnecessaryhandovers and is expected to provide better service andQoS. It also shows how the handovers are affected by theMS speed, when not relying on our CAC. The slower theMS moves, the more effectively WFAPs can withstand anyhandover. Obviously, at very high speeds, it will encounterfewer WFAPs at any given time, thereby reducing thenumber of handovers.

Figure 6 shows the average number of users that eachWFAP serves. Note that, without CAC, on an average, every

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Figure 5. Simulation results of the number of handovers for the users thatmove from Macrocell to WFAP coverage

WFAP serves a MS when more than 30 are present. How-ever, quickly choosing another target WFAP, significantlyreduces the QoS. On the other hand, using CAC reduces theoccurrence of this event.

Figure 6. Simulation results of the average number of users that are servedby WFAPs

VII. CONCLUSION

From a technical standpoint, femtocell deployment ofWiMAX services have a very difficult uphill battle. Seamlesshandover between a WFAP and a macrocell, or to otherWFAPs is one of the key technical challenges in enhancingacceptance of femtocell products, and that is what is con-sidered in this work. Unnecessary handovers significantlyaffect QoS - the result of which usually degrades thecommunication of the overall broadband wireless system -

and the improvement of handover performance depends onhow resources are managed.Our call admission control procedure, not only considers

mobility (a inherent characteristic of MS), but also signal-to-interference level and available bandwidth in such a waythat allows for WFAPs to provide continuous service withoutinterruption while MSs execute handover procedures. Simu-lations results reinforces our intuition that these parameterscan be used to improve conventional schemes. As a futurework, we plan on analyzing how a network-assisted WFAPmanagement scheme can be used to further reduce thenumber of scanning operations and the size of the neighboradvertisement message, such as proposed in [4].

REFERENCES

[1] D. P. Agrawal and Q. A. Zeng, Introduction to Wireless andMobile Systems. Cengage Learning, 3rd ed., 2011.

[2] J. G. Andrews, A. Ghosh, and R. Muhamed, Fundamentalsof WiMAX: Understanding Broadband Wireless Networking(Prentice Hall Communications Engineering and EmergingTechnologies Series). Upper Saddle River, NJ, USA: PrenticeHall PTR, 2007.

[3] M. Ma, Current Technology Developments of WiMax Systems.Springer Netherlands, 2009.

[4] J. K. Nam, W. K. Seo, D. W. Kum, J. I. Choi, and Y. Z. Cho,“A network-assisted femto base station management schemein ieee 802.16e system,” in CCNC’10: Proceedings of the7th Annual IEEE Consumer Communications & NetworkingConference, IEEE, 2010.

[5] G. Mansfield, “Femto cells in the us market - businessdrivers and femtocells in the us market - business drivers andconsumer propositions,” in FemtoCells Europe, AT&T, 2008.

[6] J.-S. Kim and T.-J. Lee, “Handover in umts networks withhybrid access femtocells,” in ICACT’10: Proceedings of the12th International Conference on Advanced CommunicationTechnology, 2010.

[7] R. Y. Kim, J. S. Kwak, and K. Etemad, “Wimax femto-cell: Requirements, challenges, and solutions,” Comm. Mag.,vol. 47, no. 9, pp. 84–91, 2009.

[8] S.-P. Yeh, S. Talwar, S.-C. Lee, and H. Kim, “Wimax fem-tocells: a perspective on network architecture, capacity, andcoverage,” IEEE Communications Magazine, vol. 46, no. 10,pp. 58–65, 2008.

[9] J. Sydir and R. Taori, “An evolved cellular system architectureincorporating relay stations - [wimax update],” IEEE Commu-nications Magazine, vol. 47, no. 6, pp. 115–121, 2009.

[10] I. I, “Mobile multihop relay working group: 802.16 mobilemultihop relay tutorial,” 2006.

[11] U. M., “Usage model ad-hoc working group: Harmonizedcontribution on 802.16j (mobile multihop relay) usage mod-els,” 2006.

7

[12] W. Forum, “Requirements for wimax femtocell systems,version 1.0.0,” April 2009.

[13] M. Ergen, “The access service network in wimax: The roleof asn-gw,” 2006.

[14] H. Zhang, X. Wen, B. Wang, W. Zheng, and Y. Sun, “A novelhandover mechanism between femtocell and macrocell for ltebased networks,” vol. 0, (Los Alamitos, CA, USA), pp. 228–231, IEEE Computer Society, 2010.

[15] M. Z. Chowdhury, W. Ryu, E. Rhee, and Y. M. Jang,“Handover between macrocell and femtocell for umts basednetworks,” in ICACT’09: Proceedings of the 11th Interna-tional Conference on Advanced Communication Technology,(Piscataway, NJ, USA), pp. 237–241, IEEE Press, 2009.

[16] V. Chandrasekhar, J. G. Andrews, and A. Gatherer, “Femto-cell networks: A survey,” CoRR, vol. abs/0803.0952, 2008.

[17] 3GPP2, “Cdma 2000 high rate packet data air interfacespecification. 3gpp2 c.s0024-a, standard, ver. 2.0,” October2000.

[18] 3GPP, “Physical layer aspects of utra high speed downlinkpacket access. 3gpp tr 25.848, 3gpp specification series,”2000.

[19] 3GPP, “Technical specification group radio access network:Physical layer-general description. 3gpp tr 25.201, 3gpp spec-ification,” 2000.

[20] S. Ray, K. Pawlikowski, and H. Sirisena, “Handover in mobilewimax networks: The state of art and research issues,” IEEECommunications Surveys & Tutorials, vol. 12, no. 3, 2010.

[21] D. H. Lee, K. Kyamakya, and J. P. Umondi, “Fast handoveralgorithm for ieee 802.16e broadband wireless access sys-tem,” in Proceedings of the 1st International Symposium onWireless Pervasive Computing, 2006.

[22] S. Choi, G.-H. Hwang, T. Kwon, A.-R. Lim, and D. Cho,“Fast handover scheme for real-time downlink services inieee 802.16e bwa systems,” in Proceedings of the IEEE 61stVehicular Technology Conference, 2005.

[23] W. Jiao, P. Jiang, and Y. Ma, “Fast handover scheme for real-time applications in mobile wimax,” in Proceedings of theIEEE International Conference on Communications, 2007.

[24] L. Ho and H. Claussen, “Effects of user-deployed, co-channelfemtocells on the call drop probability in a residential sce-nario,” in Proceedings of the 18th IEEE International Sympo-sium on Personal, Indoor and Mobile Radio Communications(PIMRC 2007), pp. 1–5, 2007.

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