research article resource efficient handover strategy for...

Research ArticleResource Efficient Handover Strategy for LTE Femtocells

Sungkwan Youm,1 Jai-Jin Jung,2 Youngwoong Ko,3 and Eui-Jik Kim4

1Department of Information and Communication, Cheju Halla University, 38 Halladaehak-ro,Jeju-si, Jeju-do 690-708, Republic of Korea2Department of Multimedia Engineering, Dankook University, 152 Jukjeon-ro, Suji-gu, Yongin-si,Gyeonggi-do 448-701, Republic of Korea3Department of Computer Engineering, Hallym University, 1 Hallymdaehak-gil, Chuncheon-si,Gangwon-do 200-702, Republic of Korea4Department of Convergence Software, Hallym University, 1 Hallymdaehak-gil, Chuncheon-si,Gangwon-do 200-702, Republic of Korea

Correspondence should be addressed to Eui-Jik Kim; [email protected]

Received 17 October 2014; Accepted 8 December 2014

Academic Editor: Damien Sauveron

Copyright © 2015 Sungkwan Youm et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Long Term Evolution (LTE) networks that are composed of macrocells and femtocells can provide an efficient solution to notonly extend coverage of macrocells but also deal with the growth of traffic within macrocells. LTE is now being considered to bea vital connectivity solution for the success of the Internet of Things (IoT) because it can provide broadband connectivity to thegrowing number of sensing and monitoring devices and even the wireless sensor networks (WSNs). However, it is still challengingto properly allocate radio frequency resources in the handover procedure between a macrocell and a femtocell. In this paper, wepropose a new handover algorithm that increases the efficient utilization of a radio frequency resource and thereby maximizes thecapacity of the overall LTE network, including the femtocells within network. The handover decision criteria take into account thestrength of the received signal, the radio resource reuse, and the overall capacity of the network throughput. The performance ofthe proposed algorithm is verified through a simulation, and the simulation results indicate that the proposed handover algorithmimproves the reusability of the cell bandwidth and increases the overall capacity of the network.

1. Introduction

Personal wireless terminals, such as smart phones, can accessan expanding supply of video and audio content. A largeamount of multimedia services, such as mobile IPTV, isnowadays available for users, and this has exponentiallyincreased the data traffic load on a mobile communicationnetworks [1–3]. Moreover, the latest Long Term Evolution(LTE) technology has vast potential in that it is fast becominguniversally available and can be used to connect millionsof devices to the Internet that have never been connectedbefore. LTE is now being considered to be a vital connectivitysolution for the success of the Internet ofThings (IoT) [4]. IoTaims to connect the growing number of sensing andmonitor-ing devices with the goal of exchanging data or controlling

these devices. Thus, LTE can be an attractive technology toprovide broadband connectivity to these potentially remotedevices and even the wireless sensor networks (WSNs) [5]. Inthis regard, the greatest challenge is to efficiently manage thelimited frequency resources of an LTEnetwork and to provideguaranteed services to the user. Recently, the use of femtocellshas been proposed in order to expand the bandwidth avail-able in LTE networks by reusing frequency resources.

Femtocells are equipped in a very small base station, andtheir low output power allows for a short transmission range.Frequency resources can be efficiently managed by reas-signing the same frequency to the neighboring femtocells,and furthermore femtocells can be easily installed by endusers by attaching the equipment to broadband connections,such as ADSL, Cable, and FTTH, that can then connect to

Hindawi Publishing CorporationInternational Journal of Distributed Sensor NetworksVolume 2015, Article ID 962837, 8 pageshttp://dx.doi.org/10.1155/2015/962837

2 International Journal of Distributed Sensor Networks

the core mobile communications network. Mobile networkoperators (MNOs) can also reduce the cost of the installationof base stations and of network management as a result ofthe presence of femtocells in their mobile communicationsnetwork. Consequently, LTE femtocells can be a catalyst forfuture IoT applications.

An efficient frequency allocation scheme is required inan LTE network that includes macro- and femtocells becausethe femtocells exist within the coverage area of a macrocell.Frequency allocation is divided into two schemes: separatecarriers allocation and shared carriers allocation. In theseparate carriers allocation scheme, different frequencies areassigned to macro- and femtocells, and, in the shared carriersallocation scheme, both the macro- and the femtocells areallocated in the same frequency. However, the shared carriersscheme requires additional mechanisms that can avoid inter-ference on the same frequency [6, 7].

The shared carriers allocation can implement eitherorthogonal or cochannel allocation methods. For example,WiMAX or 3GPP LTE with OFDMA uses a number oforthogonal subchannels that are divided within a singlefrequency resource.The orthogonal allocationmethod exclu-sively allocates each subchannel to a macrocell and a fem-tocell. Thus, it can minimize the interference between themacro- and the femtocells. The cochannel allocation methodis considered to be the best solution to share access to anysubchannel to a macrocell and a femtocell [8, 9]. However, itshould be supplemented with an additional mechanism thatcan avoid allocating a subchannel tomacro- and femtocells atthe same time, in order to minimize interference.

The orthogonal allocation method can be divided intoa dynamic subchannel allocation method and a fixed sub-channel allocation method. When there is a small amount ofuser equipment (UE) connected to femtocells assigned withunchangeable subchannels, the fixed subchannel allocationmethod has a low efficiency because unchangeable subchan-nels are unused. A dynamic subchannel allocation methodwith limited frequency resources can allocate a subchannelto macrocells and to femtocells dynamically according to theload of the traffic in the cell, increasing the overall systemcapacity.

In this paper, we propose a new handover algorithm thatimplements the orthogonal allocation method with dynamicsubchannel assignment. In the proposed handover algorithm,when a handover request is triggered as a result of theUE’s movement towards a femtocell, new decision criteriaare applied considering the number of required subchannelsfor handover completion. The handover request is acceptedor rejected according to the calculation of the number ofsubchannels required for the handover so that the overallresources of LTE network can be efficiently managed.

The remainder of this paper is organized as follows.In Section 2, we further describe LTE femtocells. Section 3describes the proposed handover method. The performanceof the proposed handover method is evaluated through sim-ulations in Section 4. Finally, Section 5 concludes this paper.

FemtoPico Micro Macro

<30m ∼500m∼100m >1km

Figure 1: Comparison of the cell sizes.

2. LTE Femtocells

2.1. Femtocells in LTE Network. The macrocell transmis-sions sometimes cannot reach users in indoor environ-ments because the thick walls of the buildings can attenuatethe transmission power. Therefore, femtocells have beendeployed in the indoor environments to overcome this issue.Compared to a macrocell, a femtocell is more easily deployedsince it does not require modification of the network equip-ment. A femtocell transmits a radio signal at 20 dBm. Theservice area of a femtocell is limited to short range of up totens of meters [10]. Therefore, a femtocell is characterized asa low-cost, low-power equipment. Cells for different coverageareas are usually referred to as macro-, micro-, pico-, andfemtocells, which are listed in order of decreasing base stationpower, as shown in Figure 1 [11].

2.2. Frequency Management Usage in LTE Femtocells. Fem-tocells are located within a service area of a macrocell andare operated using overlay characteristics. Due to limitedfrequency resources, an efficient frequency resource man-agement method is necessary in order to enable seamlessLTE communications services. In general, carrier allocationschemes are simply categorized into either the separatecarriers method or the shared carriers method, dependingon whether the network operators are using the same carrierfrequency. Note that carrier frequencies are officially assignedto the network operator through a license, and each frequencyincludes several subchannels that can be assigned to amacro-cell or femtocell. Figure 2(a) shows an example of the separatecarriers method where each different carrier frequency isseparately assigned to macro- or femtocells. In this method,the interference between the femtocell and themacrocell doesnot occur. Figure 2(b) shows an example of the shared carriersallocation where the macrocell and the femtocell are bothassigned to the same common carrier frequency. This sharedcarrier method ensures high frequency utilization whencompared to the separate method, but an additional solutionis required to avoid interference between the femtocell andthe macrocell. Most MNOs prefer to use the shared carriersmethod because it is usually difficult to obtain sufficientfrequency resources.

As seen in Figure 3, the shared carriers method can alsobe divided into an orthogonal allocation method and acochannel allocation method, depending on the approachused for the subchannel allocation. Figure 3 shows examplesof these two types of shared allocation methods. One fre-quency resource is divided intomultiple orthogonal subchan-nels which are then used in 3GPP LTE or WiMAX-basednetworks using OFDMA. An orthogonal allocation method

International Journal of Distributed Sensor Networks 3

Femtocell

MacrocellFrequency

Frequency

Carrier 1: Carrier 2:used by femtocells only used by macrocells only

(a)

Carrier 1:

Femtocell

Macrocell

Frequency

Frequency

shared between macroand femtocells

(b)

Figure 2: Frequency allocation schemes: (a) separate carriersscheme, (b) shared carriers scheme.

Macrocell subchannel Femtocell subchannel

(a) Orthogonal allocation methods

Macrocell subchannel

Femtocell subchannel

(b) Cochannel allocation methods

Figure 3: Shared allocation methods.

aims to provide exclusive access to each subchannel withinthe macrocell and the femtocell. As shown in Figure 3(a), afrequency resource is divided into multiple subchannels, andthese are separately assigned to macrocells and femtocells.Thus, the orthogonal allocation method could be the best interms of minimizing the interference between the macrocelland the femtocell. Figure 3(b) shows an example of thecochannel allocation method where every subchannel canbe assigned to both macro- and femtocells. Thus additionalmeasures are required to avoid interference.

In the orthogonal allocation method, on the other hand,a subchannel could be assigned to a macro- or a femtocell ata given time. The subchannel could be fixed or dynamically

assigned to the cell. When a small number of users areconnected to a femtocell assigned with a fixed subchannel,the assigned frequency usage in the femtocell is low becausethe subchannel could otherwise be consumed by another userin the macrocell. In such a case, the dynamic subchannelassignment of the orthogonal allocation method should beconsidered.

The orthogonal assignment method is the most effectivebecause it allows for subchannel reuse in the femtocell. Also,there is no need to provide a separate interference avoidancemethod. In general, the subchannel is assigned to a new callor through a handover. The subchannel of the new call mustbe assigned without exception while the subchannel assignedto the handover call could be determined by the status ofthe UE and network. An efficient assignment method shouldbe addressed for the handover on the subchannel allocationassignment in the orthogonal allocation method.

2.3. Network Architecture of LTE Femtocells. Figure 4 showsthe network architecture of the LTE femtocells as defined inthe 3GPPRelease 10 Specification [11].The femtocells within ahome or a small office are provided with broadband network(ADSL, Cable, and FTTH) access to the core network. Thecosts of the mobile communication network can be reducedfor both the user and the network operator by allowing forfewer installations of new base stations [12].

Figure 4 shows how the LTE femtocell network consistsof enhanced NodeB (eNB) and Home enhanced NodeB(HeNB). The HeNB gateway (GW) may be placed betweenthe mobility management entity/serving gateway (MME/S-GW) and the HeNB in order to manage a large numberof HeNBs. One HeNB will only be connected to either theHeNB GW or the MME, where a link is made through theS1 interface to the MME/S-GW or directly to the HeNB GW.The advantages of using an HeNB are that it can be deployedanytime and at any place depending on the occasion. TheHeNB is connected to a different HeNB GW depending onthe location of theHeNB.TheHeNBGWtransfers the controlmessages of the HeNB and the traffic data between the HeNBand the MME/S-GW to the evolved packet core (EPC), andit relays the data traffic from the EPC to the HeNB. In otherwords, the HeNB GW plays the role of eNB and MME fromthe perspective of the MME and HeNB, respectively. The X2interface between the HeNBs is defined in 3GPP Release 10,and, therefore, a handover procedure could be made betweentheHeNBs throughX2 interfacewithout anymediation of theMME [13].

3. Proposed Handover Algorithm forLTE Femtocells

In this section, we describe the LTE femtocell transmissionnetwork structure and the proposed handover algorithm,considering the overall resource management. In this paper,we assume that the LTE network includes a macrocell andfemtocells. Therefore, the proposed handover algorithm canbe applied in micro- or picocells as well. The handoveralgorithm operates on a UE moving from the macrocell


MME/S-GW MME/S-GW MME/S-GW

eNB

eNB

eNB

HeNB

HeNB

HeNB GW

E-UTRAN

HeNB

S1

X2

X2

X2

X2

X2

S1S1

S1S1 S1 S1

S1 S1

S1S1

S5

Figure 4: LTE femtocell network architecture.

eNB64QAM16QAM

QPSK

RMS(resource management

system)

HeNB HeNB HeNB

Figure 5: Network architecture of LTE femtocell.

to a femtocell, and the handover request could either beaccepted or be rejected by using the proposed algorithm.Theoverall resources could increase by preventing a handoverthat requires additional resources.

3.1. Femtocell Network Architecture. Figure 5 shows the net-work architecture of the LTE femtocell network that is con-sidered in our work. When the UE moves to the coverageof a femtocell, the femtocell should provide a schedulingblock (SB) to the UE. Note that the SB is the basic unitused to manage resources. If no resource is available atthe femtocell, the femtocell sends a request message to theresource management system (RMS) in order to increase theSBs provisioned for the femtocell. The granted SB is thenoffered to the UE incoming to the cell. The RMSmanages theavailability of the SBs by responding to request messages sentfrom the femtocells. The RMS exists by providing a form ofthe functionality of the HeNB GW of the core network andthe eNB.

3.2. Proposed Handover Procedure. Algorithm 1 shows theproposed handover algorithm.The decision for the executionof the handover is made according to the type of UE serviceand the requested resource. The resource requested for

handover to the femtocell is differentiated by the type of UEservice, which could require a guaranteed bit rate (GBR) ornon-GBR. The handover admission is decided according tothese requested resources. In order to provide continuousservice for the UE, GBR traffic that is triggered to a femtocelldemands QPSK modulation while non-GBR traffic demands64QAMmodulation, corresponding to the best signal quality.The detailed description of the procedure is as follows.

(1) UE that is connected to a macrocell moves to the areaof the HeNB femtocell and detects the signal of theHeNB.

(2) UE compares the signal strength against the thresholdvalue configured in the macrocell. The handoverprocedure is initiated in the case where the signalstrength is larger than the threshold value.

(3) eNB requests the data rate to the femtocell HeNBcorresponding to the current service of the UE.

(4) The HeNB decides whether the request can beaccepted or not. If the SB that is to be assigned to UEis available in order to continue the service of UE, therequest is accepted. However, if the SB availability isnot adequate to continue the current service of theUE, HeNB asks the RMS to assign more SBs.


Monitor signal strength of HeNB in macrocellIFHeNB signal Strength >Handover thresholdTrigger handover procedureIF UE’s GBRSend QPSK handover required

ELSESend 64QAM handover required

ENDIFIF (Available SB in HeNB)Accept handover request

ELSEIF (More SBs than the present SB)Reject handover request

ELSE (The same or less SB of UE is not available in HeNB)Request SB expansion to RMS

ENDIFSWITCH (Capacity after expanding SB for femtocell)case Good or Normal:Expand SB in femtocell and admit handover

case Bad:Reject handover required

ENDSWITCHENDIF

Algorithm 1: The proposed handover procedure of LTE femtocell network.

(5) The RMS compares the increase in the SB used infemtocell against the SB used in the macrocell. Thatis, a comparison is performed between the presentresources and the subsequent resources following thehandover for the RMS to accept the handover requestfrom the femtocell. The request is accepted in thecasewhere the upcoming resources increase, and thenthe resources are granted to femtocell. However, ifthe upcoming resources are less than the currentresources, the request is denied.

We hereby describe the method used to calculate the SBin step (5) of the above handover procedure. In LTE systemsthat use modulation techniques, such as the OFDM, thefrequency resources are divided into a number of orthogonalsubcarriers. In addition, these subcarriers include a bundleof subchannels. The SB consists of a set of 12 subcarriers thatcorrespond to a 180KHz bandwidth. The SB also has a singlesubframe in the LTE frame. The subframe accounts for 1msin the time domain. Equation (1) denotes the data rate of theSB used for the transmission:

𝑟(𝑗)=

𝑅(𝐶)

𝑗log2 (𝑀𝑗)𝑇𝑆𝑁𝑆

𝑁𝑆

∑

𝑆=1

𝑁(𝑑)

SC (𝑆) , (1)

where 𝑇𝑆 is the total number of SBs in one subframe. 𝑅𝑗 isthe minimum data rate requested at the 𝑗 user. And 𝑁𝑆 and𝑁SC represent the consecutive OFDM symbols for one SB inthe time domain and the number of consecutive subcarriersin the frequency domain, respectively. 𝑗 is the index numberwhere 𝑗 is 𝑗 ∈ {1, 2, . . . , 𝐽}.𝑀𝑗 and 𝑇𝑆, respectively, representthe size of the MSC (e.g., 64QAM, 16QAM, and QPSK) and

the OFDM symbol duration. In order to denote whether theSB is used or not,

𝜌𝑘,𝑛 = 1 (2)

represents the 𝑛th SB used for user 𝑘. The zero of this valueindicates no usage of the SB. Equation (3) is the achievabledata rate user 𝑘, following (2):

𝑟𝑘 =

𝑁tot

∑

𝑛=1

𝜌𝑘,𝑛𝑟(𝑗). (3)

The number of SBs for the 𝑘th user is acquired as in (4),where the minimum data rate required by user 𝑘 is set to 𝑅𝑘Mbit/s and the 𝑟(𝑗) date rate is configured at the SB of theMCS𝑗 index against the total number of SBs𝑁tot:

𝑅𝑘

𝑟(𝑗)= 𝑁𝑘. (4)

That is, 𝑁𝑘 is changed depending on the MCS index 𝑗 atthe same data rate, 𝑅𝑘. The number of SBs remaining in thesingle subframe in the following equation is derived from (4):

𝑁remain = 𝑁tot −𝐾

∑

𝑘=1

𝑁tot

∑

𝑛=1

𝜌𝑘,𝑛 − 𝐹SB. (5)

The 𝐹SB in (5) represents the number of SBs used in thefemtocells. If the number of femtocells is small after the SBin the femtocell increases due to handover request, the UEcamping on the macrocell cannot move to the femtocell dueto lack of resources in the femtocell. Therefore, when theavailability of SBs in femtocell is evaluated, the service cov-erage of the femto- and macrocell is considered. Generally,


Table 1: Parameters for femtocell service coverage.

Macrocell intersite distance 500mMacrocell radius 350mMacrocell coverage 3502 𝜋m2

Femtocell radius 18mFemtocell coverage 182 𝜋m2

the ratio of the macrocell radius to the femtocell radius isassumed to be 20 : 1, based on the power transmission rangeof cell. The total number of SBs could be assigned based onthe total coverage ratio of the macrocell and femtocell, andthe urban model of the 3GPP model is referred to in orderto obtain the ratio value of the cell [10]. Table 1 shows thereference value that is applied to the ratio. The coverage ratiofor macro- and femtocells noted in Table 1 is derived from

182𝜋

2502𝜋

=

1

380

. (6)

The SB availability in the femtocells distributed in the net-work could be derived from

𝐹available = 𝐹Add ×1

380

× 𝑁femto. (7)

𝐹Add in (7) represents the available SBs in the femtocellthat is newly installed.𝑁femto is the number of femtocells, andthe available SBs in the total network are obtained by adding(5) and (7). Thus,

𝑁remain + 𝐹available. (8)

The system’s overall throughput is represented in thefollowing equation by using (8) and substituting𝑁remain and𝐹available with (5) and (7), respectively:

𝑇tot =𝐾

∑

𝑘=1

𝑟𝐾 +

𝑁femto

∑

𝑛=1

𝐶

∑

𝑐=1

𝑟𝑐,𝑛. (9)

𝑇tot is the overall throughput of the system where 𝐶 and𝑟𝑐,𝑛 represent the amount of femtocell UE and the data ratesat the 𝑐th UE and 𝑛th femtocell, respectively.

4. Simulation Results and Analysis

In this paper, the proposed method is applied to the LTEfemtocell network configuration, as shown in Table 2.

The parameters configured in the LTE femtocell networkare listed in Table 2. We assume that the UE moving tothe coverage of the femtocell executes the handover proce-dure for the neighboring femtocell. The UE camps on thefemtocell after completing the handover procedure. Figure 6shows the bandwidth variation within the femtocell. In thefixed method, although the UE moves to the femtocell, thehandover procedure is not executed because no additionalresources would be allocated to the femtocell.

In the dynamic method of the legacy allocation method,the handover procedure is executed for the femtocell without

Table 2: Simulation parameters.

The amount of UE in femtocell 10 per cell (total 30)The number of femtocells 10 per cell (total 30)The amount of UE camping ona femtocell 2 per cell (total 60)

Minimum data rate per UE 1.0MbpsMacrobandwidth versusfemtobandwidth ratio in fixedallocation method

9 : 1

System total bandwidth 20MHzMacrocell path loss (128.1 + ( 37.6 × log 10 (𝑟/1000)))Femtocell path loss (127 + (30.0 × log 10 (𝑟/1000)))

Coverage 350m (macrocell)/18m(femtocell)

Power strength 46 dBm (macrocell)/20 dBm(femtocell)

The distance between femtocells 10m

1 2 3 4 5 6 7 8 91

1.5

2

2.5

3

3.5

The amount of UE

Avai

labl

e ban

dwid

th (M

Hz)

DynamicProposed

Fixed

Figure 6: Available system bandwidth.

considering the resources of the femtocell. The UE movingto the femtocell either suffers from a degradation in servicesor moves back to the macrocell, and the resources used atUE moving to the femtocell could not be allocated to otherusers in the macrocell anymore. On the other hand, in theproposed method, this handover procedure is prohibitedbecause the resources for the macrocell could be reduced.This prohibition results in an increase in the resource of themacrocell or in accommodating more users in the femtocell.The available capacity of the system increases in the proposedmethodwhen compared to the dynamic legacymethodwhenthe UE tries to move to the femtocell.

In the proposed method, the number of handoversand the available resource are observed as the UE movesthrough the femtocell.We assume that theUEmoves throughthe femtocell at 1m/s. The handover is triggered when


DynamicProposed

Fixed

0 50 100 150 200 250 300 3500

20

40

60

80

100

120

140

160

180

Macrocell radius (m)

Han

dove

r cou

nt

Figure 7: Number of handovers.

DynamicProposed

Fixed

0 50 100 150 200 250 300 3501.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Macrocell radius (m)

Avai

labl

e ban

dwid

th (M

Hz)

Figure 8: Variation in available bandwidth according to the han-dover to the femtocell.

the strength measured by the UE is more than the thresholdconfigured in the macrocell. Figure 7 shows that the numberof handovers increases according to the movement of theUE.The handover is then triggered without consideration forthe system resources. This frequent handover degrades theperformance of the system and of the UE. The variation inthe resources available for the system is shown in Figure 8 as anumber of handovers increase.The resources of themacrocellare not taken into consideration in the legacy method. Onthe other hand, in the proposed method, the handover isnot triggered in the case where there is a possibility ofreducing the system resources.The figure indicates howmoreresources are required in the existing methods (i.e., fixed

and dynamic methods) than in the proposed method as theUE moves to the boundary of macrocell. In the proposedalgorithm, unnecessary handovers are prevented, resulting ina decrease in the probability of a dropped session as well.

5. Conclusions

The handovers in a femtocell that covers a small area havebeen addressed in order to accommodate the exponentialincrease in traffic volume of LTE networks. In this paper,we present a novel handover algorithm that considers theresource allocation of femto- and macrocells as well as theoverall resources of the LTEnetwork.This algorithmprovidesUE with service continuity and offers an efficient allocationof overall resources. The admission of the handover requestis decided by considering the availability of resources at thefemtocell. Although the available resources may allow for ahandover, resource efficiency may be improved by rejectingthe handover request in the case where more resources areused by UE after the handover. The purpose of the proposedalgorithm is to increase the overall resources available, andits performance was verified through a simulation. The pro-posed method efficiently manages resources for situations inwhich there is an increase in the availability of SBs of an LTEnetwork, and it also provides guaranteed service to the UE.

Conflict of Interests

The authors declare that they have no conflict of interests.

Acknowledgments

This work was supported by the ICT R&D Program ofMSIP/IITP (B0101-14-0059, Human Resource DevelopmentProgram for Future Internet). This research was also sup-ported by Basic Science Research Program through theNational Research Foundation of Korea (NRF) funded by theMinistry of Education (NRF-2014R1A1A2057641).

References

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[2] D. Calin, H. Claussen, and H. Uzunalioglu, “On femto deploy-ment architectures and macrocell offloading benefits in jointmacro-femto deployments,” IEEE Communications Magazine,vol. 48, no. 1, pp. 26–32, 2010.

[3] J. G. Andrews, H. Claussen, M. Dohler, S. Rangan, and M. C.Reed, “Femtocells: past, present, and future,” IEEE Journal onSelected Areas in Communications, vol. 30, no. 3, pp. 497–508,2012.

[4] http://www.sequans.com/lte-internet-of-things/.[5] M. A. Mehaseb, Y. Gadallah, and H. El-Hennawy, “WSN appli-

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[6] D. Lopez-Perez, A. Valcarce, A. Ladanyi, G. de La Roche, andJ. Zhang, “Intracell handover for interference and handovermitigation in OFDMA two-tier macrocell-femtocell networks,”Eurasip Journal on Wireless Communications and Networking,vol. 2010, Article ID 142629, 2010.

[7] Z. Bharucha, A. Saul, G. Auer, and H. Haas, “Dynamic resourcepartitioning for downlink femto-to-macro-cell interferenceavoidance,” Eurasip Journal on Wireless Communications andNetworking, vol. 2010, Article ID 143413, 2010.

[8] D. Lopez-Perez, A. Valcarce, G. de la Roche, and J. Zhang,“OFDMA femtocells: a roadmap on interference avoidance,”IEEE Communications Magazine, vol. 47, no. 9, pp. 41–48, 2009.

[9] J.-H. Yun andK. G. Shin, “Adaptive interferencemanagement ofOFDMA femtocells for co-channel deployment,” IEEE Journalon Selected Areas in Communications, vol. 29, no. 6, pp. 1225–1241, 2011.

[10] J. Zhang and G. de la Roche, Femto Cells: Technologies andDeployment, John Wiley & Sons, Chichester, UK, 2010.

[11] 3GPP TR 36.814, “Further advancements for E-UTRA physicallayer aspects, version 9.0.0,” March 2010.

[12] M. Peng, D. Liang, Y. Wei, J. Li, and H.-H. Chen, “Self-configuration and self-optimization in LTE-advanced heteroge-neous networks,” IEEECommunicationsMagazine, vol. 51, no. 5,pp. 36–45, 2013.

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