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Interference Management in OFDMA FemtocellNetworks: Issues and Approaches

Nazmus Saquib, Ekram Hossain, Long Bao Le, and Dong In Kim

Abstract—One of the effective techniques of improving thecoverage and enhancing the capacity and data rate in cellularwireless networks is to reduce the cell size (i.e., cell splitting)and transmission distances. Therefore, the concept of deployingfemtocells over macrocell has recently attracted growing interestsin academia, industry, and standardization forums. Varioustechnical challenges towards mass deployment of femtocellshave been addressed in recent literature. Interference mitigationbetween neighboring femtocells and between the femtocell andmacrocell is considered to be one of the major challenges infemtocell networks because femtocells share the same licensedfrequency spectrum with macrocell. Further, the conventionalradio resource management techniques for hierarchical cellularsystem is not suitable for femtocell networks since the positionof the femtocells is random depending on the users’ servicerequirement. In this article, we provide a survey on the dif-ferent state-of-the-art approaches for interference and resourcemanagement in orthogonal frequency-division multiple access(OFDMA)-based femtocell networks. A qualitative comparisonamong the different approaches is provided. To this end, openchallenges in designing interference management schemes forOFDMA femtocell networks are discussed.

Index Terms—Femtocell, macrocell, OFDMA, interferencemanagement, LTE-Advanced systems, WiMAX.

I. INTRODUCTION

One of the major challenges for next generation wirelesscommunication systems is to improve the indoor coverage andprovide high-data-rate services to the users in a cost-effectivemanner and at the same time, to enhance network capacity.One of the traditional approaches of solving this problem isto make the transmitters and receivers closer to each other.However, this approach may not be economically feasiblesince it involves deploying more base stations (BSs) within thenetwork. In this regard, home base stations, commonly knownas femtocells, are considered as a promising option for the mo-bile operators to improve the network coverage, especially inthe interiors of houses and buildings and to provide ubiquitoushigh speed connectivity to the end users or User Equipments(UEs). Femtocells or Femto Access Points (FAPs) are small,short-ranged (10∼30 m) low powered (10∼100 mW) accesspoints developed to provide cost-effective and high-bandwidthservices in next generation wireless communication systems.Femtocells operate in licensed spectrum owned by the mo-bile operator and enable Fixed Mobile Convergence (FMC)service by connecting to the cellular network via broadbandcommunications links (e.g., DSL) [1].

One of the main advantages of femtocell deployment is theimprovement of indoor coverage where macrocell base station(referred to as MeNB) signal is weak. Femtocells providehigh data rate and improved quality-of-service (QoS) to the

subscribers. It also lengthens the battery life of the mobilephones since the mobile phones do not need to communicatewith a distant macrocell base station. Femtocells can easilybe deployed by the end users in indoor environments ona “plug-and-play” basis. It saves the backhaul cost for themobile operators since femtocell traffic is carried over wiredresidential broadband connections and reduces the traffic inten-sity at the macrocell network. Finally, femtocells can also beconsidered as an option towards the convergence of landlineand mobile services. A recent study conducted by a marketresearch company Informa Telecoms & Media estimates thatby 2014, 114 million mobile users will be accessing mobilenetworks through femtocells [2]. This signifies that in theupcoming years femtocells could be an integral part of thenext generation wireless communication systems.

In recent years, different types of femtocells have beendesigned and developed based on various air interface tech-nologies, services, standards, and access control strategies. Forexample, 3G femtocells use Wideband Code-Division MultipleAccess (WCDMA)-based air interface of Universal MobileTelecommunication system (UMTS), which is also known asUMTS Terrestrial Radio Access (UTRA). The 3rd GenerationPartnership Project (3GPP) refers to these 3G femtocellsas Home Node Bs (HNBs). On the other hand, WiMAX(Worldwide Interoperability for Microwave Access) and LongTerm Evolution (LTE) femtocells use Orthogonal Frequency-Division Multiple Access (OFDMA). The LTE femtocells arereferred to as Home evolved Node Bs (HeNBs).

In general, femtocells are designed to operate in one ofthree different access modes, i.e., closed access mode, openaccess mode, and hybrid access mode [3]. In closed accessmode, a set of registered UEs belonging to Closed SubscriberGroup (CGS) are allowed to access a femtocell. This typeof femtocell access control strategy is usually applicable inresidential deployment scenarios. However, in public placessuch as airports and shopping malls, open access mode offemtocells can also be used where any UE can access thefemtocell and benefit from its services. This access mode isusually used to improve indoor coverage. In hybrid accessmode, any UE may access the femtocell but preference wouldbe given to those UEs which subscribe to the femtocell.In small business or enterprise deployment scenarios hybridaccess mode of femtocells may be used [3].

II. TECHNICAL CHALLENGES IN FEMTOCELLDEPLOYMENT

The mass deployment of femtocells gives rise to severaltechnical challenges. One of the major challenges is in-

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terference management between neighboring femtocells andbetween femtocell and macrocell. In general, two types ofinterferences that occur in a two-tier femtocell network ar-chitecture (i.e., a central macrocell is underlaid/overlaid with3G/OFDMA femtocells, respectively) are as follows:

• Co-tier interference: This type of interference occursamong network elements that belong to the same tierin the network. In case of a femtocell network, co-tierinterference occurs between neighboring femtocells. Forexample, a femtocell UE (aggressor) causes uplink co-tier interference to the neighboring femtocell base stations(victims). On the other hand, a femtocell base station actsas a source of downlink co-tier interference to the neigh-boring femtocell UEs. However, in OFDMA systems,the co-tier uplink or downlink interference occurs onlywhen the aggressor (or the source of interference) andthe victim use the same sub-channels. Therefore, efficientallocation of sub-channels is required in OFDMA-basedfemtocell networks to mitigate co-tier interference.

• Cross-tier interference: This type of interference occursamong network elements that belong to the different tiersof the network, i.e., interference between femtocells andmacrocells. For example, femtocell UEs and macrocellUEs (also referred to as MUEs) act as a source ofuplink cross-tier interference to the serving macrocellbase station and the nearby femtocells, respectively. Onthe other hand, the serving macrocell base station andfemtocells cause downlink cross-tier interference to thefemtocell UEs and nearby macrocell UEs, respectively.Again, in OFDMA-based femtocell networks, cross-tieruplink or downlink interference occurs only when thesame sub-channels are used by the aggressor and thevictim.

Femtocells are deployed over the existing macrocell net-work and share the same frequency spectrum with macrocells.Due to spectral scarcity, the femtocells and macrocells haveto reuse the total allocated frequency band partially or totallywhich leads to cross-tier or co-channel interference. At thesame time, in order to guarantee the required QoS to themacrocell users, femtocells should occupy as little bandwidthas possible that leads to co-tier interference. As a result, thethroughput of the network would decrease substantially dueto such co-tier and cross-tier interference. In addition, severeinterference may lead to “Deadzones”, i.e., areas where theQoS degrades significantly. Deadzones are created due toasymmetric level of transmission power within the networkand the distance between macrocell UE and macrocell basestation. For example, a macrocell UE located at a cell edgeand transmitting at a high power will create a deadzone to thenearby femtocell uplink transmission due to co-channel inter-ference. On the other hand, in the downlink transmission, dueto high path-loss and shadowing effect, a cell edge macrocellUE may experience severe co-channel interference from thenearby femtocells. Thus, it is essential to adopt an effective androbust interference management scheme that would mitigatethe co-tier interference and reduce the cross-tier interferenceconsiderably in order to enhance the throughput of the overall

network.In OFDMA-based femtocell networks, due to the flexibility

in spectrum allocation, orthogonal sub-carriers can be assignedto femtocells and macrocells. This gives OFDMA-based fem-tocells an edge over CDMA systems in terms of utilizing thefrequency spectrum resources efficiently. Fig. 1 illustrates allpossible interference scenarios in an OFDMA-based femtocellnetwork. If an effective interference management scheme canbe adopted, then the co-tier interference can be mitigatedand the cross-tier interference can be reduced which wouldenhance the throughput of the overall network.

Index Aggressor Victim Interference  Type

Transmission  Mode

Symbol

1 Macrocell UE Femtocell BS Cross-­‐tier Uplink

2 Macrocell BS Femtocell UE Cross-­‐tier Downlink

3 Femtocell UE Macrocell BS Cross-­‐tier Uplink

4 Femtocell BS Macrocell UE Cross-­‐tier Downlink

5 Femtocell UE Femtocell BS Co-­‐tier Uplink

6 Femtocell BS Femtocell UE Co-­‐tier Downlink

Femtocell BS

Internet

Mobile Core Network

Broadband Router

Macrocell BS

Femtocell UE

Macrocell UE

Macrocell UE

Femtocell BS

Femtocell BS

Index 1

Index 5

Index 6

Femtocell UE

Femtocell UE

Index 2 Index 4

Index 3

Fig. 1. Interference scenarios in OFDMA-based femtocell networks.

Other challenges in femtocell deployments include: hand-off and mobility management, timing and synchronization,auto-configuration, and security. An effective and efficientmobility management and handover scheme (macrocell-to-femtocell, femtocell-to-macrocell and femtocell-to-femtocell)is necessary for mass deployment of femtocells in UMTSand LTE networks. The scheme should have low complexityand signaling cost, deal with different access modes andperform proper resource management beforehand for effi-cient handover. Timing and synchronization is one of themajor challenges for femtocells since synchronization overIP backhaul is difficult, and inconsistent delays may occurdue to varying traffic congestion. Since the femtocells arerequired to operate on a “plug-and-play” basis, it is importantthat femtocells can organize and configure autonomously andaccess the radio network intelligently so that they only causeminimal impact on the existing macrocell network. Sincefemtocells could be vulnerable to malicious attacks (e.g.,masquerading, eavesdropping, man-in-the-middle attack etc.),

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enhanced authentication and key agreement mechanisms arerequired to secure femtocell networks.

In this article, we give an overview of the different inter-ference management techniques for OFDMA-based femtocellnetworks presented in the recent literature. To this end, wewill provide a qualitative comparison among these techniquesbased on some important criteria. We will conclude the articleoutlining some open challenges related to interference man-agement in OFDMA-based femtocell networks.

III. INTERFERENCE MANAGEMENT APPROACHES

Different techniques such as cooperation among macrocellBSs (i.e., MeNBs) and femtocell BSs (i.e., HeNBs), formationof groups of HeNBs and exchange of information (such as pathloss, geographical location, etc.) among neighboring HeNBs,accessing the spectrum intelligently, etc. can be considered toreduce co-tier and cross-tier interferences. In the following, weprovide an overview of the different approaches for interfer-ence mitigation in two-tier OFDMA femtocell networks. Theseapproaches consider uplink and/or downlink transmissions aswell as co-tier and/or cross-tier interference.

A. Femto-aware spectrum arrangement scheme

In [4], Yi Wu et al. propose a femto-aware spectrumarrangement scheme to avoid uplink cross-tier interferencebetween a macrocell and femtocells. In this scheme, theallocated frequency spectrum for any macrocell coverage areais divided into two parts: the macrocell dedicated spectrum partand macrocell-femtocell shared spectrum part. It is assumedthat shared spectrum allocated to femtocells (i.e., HeNBs) isconfigured by the mobile operator. Thus, the macrocell basestation (i.e., MeNB) has adequate knowledge of the sharedfrequency spectrum. Based on this knowledge, the MeNBdevelops an interference pool which includes the macrocellUEs that pose a threat to the nearby HeNBs. These macrocellUEs are thus assigned a portion of the spectrum dedicated formacrocell usage which reduces/mitigates the uplink cross-tierinterference and solves the uplink deadzone problem.

Fig. 2 illustrates the femto-aware spectrum arrangementscheme, where macrocell UE4, macrocell UE5, and macrocellUE6 pose potential threat of cross-tier interference on theirprospective nearby HeNBs. Therefore, these macrocell UEsare put into the femtocell-interference pool by the MeNBand are assigned a dedicated portion of the total frequencyspectrum in order to mitigate co-channel interference. On theother hand, since other macrocell UEs (i.e., macrocell UE1,macrocell UE2, and macrocell UE3) are not close to anyHeNB, they share the rest of the frequency spectrum alongwith the femtocell UEs (i.e., femtocell UE1, femtocell UE2,and femtocell UE3). However, this scheme does not considerinter-HeNB interference and may be inefficient if the numberof macrocell UEs near the HeNB increases.

B. Clustering of femtocells

In [5], a framework is presented to reduce downlink inter-ference (both cross-tier and co-tier) and enhance the spectral

Femtocell BS

Macrocell BS

Femtocell UE 1

Macrocell UE 1

Femtocell UE 2

Macrocell UE 6

Macrocell UE 5

Macrocell UE 2

Macrocell UE 4

Femtocell UE 3

Macrocell UE 3

Femtocell-interference Pool Macrocell UE 4 Macrocell UE 5 Macrocell UE 6

Macrocell UE 1Macrocell UE 2Macrocell UE 2Femtocell UE 1Femtocell UE 2Femtocell UE 3

Macrocell dedicated spectrum

Macrocell-Femtocell shared spectrum

Fig. 2. Femto-aware spectrum arrangement scheme.

efficiency for an OFDMA-based closed access femtocell net-work. In this framework, a Femtocell System Controller (FSC)per macrocell obtains all the necessary knowledge of HeNBsystem configuration (i.e., position information of HeNBs andmacrocell UEs) and performs the necessary computations. Tomitigate interference, the scheme encompasses a combinationof dynamic frequency band allocation among HeNBs andMeNB, and clustering of HeNBs based on their geographicallocations. In this scheme, a portion of the entire frequencyband is dedicated to the MeNB users and the rest is reusedby the MeNB and HeNBs. The advantage of allocating aportion of the frequency band strictly for MeNB users isthat it can solve the MeNB UE downlink deadzone problemand guarantee users’ QoS requirement. However, the portionof the frequency band, which is shared, is determined bythe total number of HeNB clusters obtained through a clus-tering algorithm. The clustering algorithm allocates HeNBsinto different frequency reuse clusters and UEs of differentHeNBs in the same cluster use the same sub-channels allocatedfrom the shared frequency band. Based on the geographicallocations of the HeNBs, the threshold distance for clusteringinterference is calculated. If the Euclidean distance betweenany two HeNBs is less than the threshold distance, then theyare assigned to different clusters to avoid co-tier and cross-tier interferences. Simulation results show that high spectrumefficiency is achieved as the probability of cross-tier spectrumreuse becomes higher than 97.4%. This signifies that theproblem regarding macrocell UE downlink deadzone aroundHeNBs is effectively solved (e.g., the probability of onemacrocell UE lying in the deadzone is below 2.4%). For theproposed scheme, simulation results also show a significantimprovement of the femtocell user capacity (at most 200HeNBs per macrocell coverage area).

In [6], an energy-efficient interference mitigation schemeis presented for closed access HeNBs grouped in a neigh-borhood area based on their geographical locations. In thisscheme, inter-femtocell or co-tier interference among neigh-boring HeNBs is minimized by reducing the unnecessaryAvailable Intervals (AI) in Low Duty Operation (LDO) modefor HeNBs. According to the IEEE802.16m standard, a HeNBin the operation state may enter the LDO mode if no UE

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exists in its coverage zone, or if all UEs in the coverageare in sleep/idle mode. In the LDO mode, a HeNB switchesalternately between available interval (AI) and unavailableinterval (UAI) modes. During UAI, a HeNB becomes inactiveon the air interface. During AI, the HeNB may becomeactive on the air interface by transmitting preambles to thenew incoming UE for synchronization purposes. However,the HeNB in the LDO mode still has AIs even though thereis no UE that will access the HeNB in near future. Theseunnecessary AIs cause co-tier interference for CGS HeNBs.

In the proposed scheme, unnecessary AIs are decreasedwhich results in reduction of co-tier interference amongneighboring HeNBs. The main idea behind reducing suchinterference is to cluster/group the neighboring HeNBs basedon their geographical locations. In each cluster, one HeNB isdesignated as the leader and the adjacent HeNBs are referredto as members. According to the IEEE 802.16m standard, anewly installed HeNB scans the surrounding area to search forneighbor HeNBs in its initialization state. Since it is assumedthat the network has global knowledge about the topologyof the network, the scanning report may include the groupconfiguration in the network (i.e., the leader and the membersof the group based on the HeNB ID). If a newly installedHeNB receives the preamble signal from the leader above adefined threshold then it becomes a member of the group,otherwise, it will form a new group and assign itself as theleader of the group. The leader requires having AIs in its LDCpattern so that the arrival of a UE at the group can discoverthe existence of the group by detecting the leader, even thoughthe members in the group stay in UAI. As soon as the leadersenses the arrival of the UE, it sends a message to the targetHeNB to activate its AI in the LDC pattern so that the UEcan detect the target HeNB and connect to it. In this pattern,the unnecessary AI in the LDO mode of HeNB is reducedresulting in power conservation of HeNB and at the sametime the co-tier interference is minimized. Through analysisand simulation it is shown that for the proposed scheme, thegain in terms of co-tier interference reduction time and energysaving is up to 90% in comparison with conventional LDOscheme in the IEEE802.16m standard.

C. Beam subset selection strategy

The authors in [7] propose an orthogonal randombeamforming-based cross-tier interference reduction schemein closed-access two-tier femtocell networks. The macrocellbeam subset selection strategy is based on the number ofmacrocell UEs and the intensity of HeNBs in the network. TheMeNB selects the beam subset and the users for each channelbased on the signal-to-interference-plus-noise (SINR) informa-tion for all the channels which is fedback by the macrocellUEs. The main objective is to enhance the throughput ofthe network by optimizing the trade-off between multiplexinggain and multiuser interference (cross-tier) based on adaptiveselection of optimal number of beams using max-throughputscheduler at the MeNB. The adaptive selection of the num-ber of beams decreases cross-tier interference, and providesspatial opportunity to HeNBs to access the spectrum in an

opportunistic manner. In addition, distributed power controlmechanism for HeNBs integrated with the proposed schemereduces cross-tier interference significantly.

D. Collaborative frequency scheduling

Co-channel uplink and downlink cross-tier interference canbe mitigated if a HeNB can avoid using the macrocell resourceblocks that belong to its nearby macrocell UEs throughefficient spectrum sensing. However, the spectrum sensingresults for HeNB may be impaired due to misdetection, falsealarm, and improper timing synchronization. To deal with thisproblem, a framework for OFDMA-based HeNBs is providedin [8] where the scheduling information for macrocell UEs’(both uplink and downlink) is obtained from the MeNBthrough backhaul or air interface. This information is used toimprove the spectrum sensing results for HeNB and to utilizethe resource blocks associated with far-away macrocell UE inthe uplink and downlink transmission. The key features of theproposed framework are as follows:

• HeNB receives the macrocell UEs scheduling informationfor uplink and downlink from the MeNB.

• HeNB performs spectrum sensing for finding the oc-cupied parts of the spectrum. The occupied parts ofthe uplink spectrum can be determined through energydetection.

• HeNB compares the spectrum sensing results with theobtained scheduling information to decide about thespectrum opportunities.

Since the HeNB accesses the spectrum in an opportunisticmanner, the authors analyze the impact of Inter-carrier Interfer-ence (ICI) from macrocell UEs to femtocell which is severein the uplink transmission. The ICI is basically due to theasynchronous arrival of macrocell UE signals at the femtocell.Through simulation (using Okumura-Hata model of radiopropagation) it is shown that the variation of the ICI powerdepends on center frequency, height of the femtocell, and thesize of the Cyclic Prefix (CP). A lower center frequency anda higher femtocell height increase the received ICI power atHeNB. In addition, if the macrocell UEs’ signal arrival time atHeNB exceeds the CP duration then the orthogonality betweenthe sub-carriers is disrupted leading to ICI. Also, different sub-carrier assignment schemes result in different ICI.

E. Power control approach

Power control methods for cross-tier interference mitigationgenerally focus on reducing transmission power of HeNBs.These methods are advantageous in that the MeNB and HeNBscan use the entire bandwidth with interference coordination.Dynamic or adjustable power setting, which is preferredover fixed HeNB power setting, can be performed either inproactive or in reactive manner each of which again can beperformed either in open loop power setting (OLPS) or closed-loop power setting (CLPS) mode. In the OLPS mode, theHeNB adjusts its transmission power based on its measure-ment results or predetermined system parameters (i.e., in aproactive manner). In the CLPS mode, the HeNB adjusts its

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transmission power based on the coordination with MeNB(i.e., in a reactive manner). Also, a hybrid mode can be usedwhere the HeNB switches between the two modes according tothe operation scenarios [9]. Another related concept is powercontrol for HeNBs on a cluster basis in which the initial powersetting for the HeNBs is done opportunistically based on thenumber of active femtocells in a cluster (Fig. 3) [10]. For this,centralized sensing can be used by which an MeNB can esti-mate the number of active femto cells per cluster and broadcastthe interference allowance information to femtocells for theirinitial power setting. Alternatively, distributed sensing can beused where each cell senses if the others are active in the samecluster and adjusts its initial power setting accordingly.

!(a) Centralized sensing (b) Distributed sensing

!!Fig. 3. Sensing-based opportunistic power control (from [10]).

Game theoretic models can be used to design and analyzedistributed power control methods in a heterogeneous cellularwireless network with macrocells and femtocells. Two broadcategories of game theoretic models are noncooperative andcooperative game models. In [15], a distributed power controlallocation problem is formulated for downlink transmission ofOFDMA-based femtocells overlaid upon a macrocell network.The problem is modeled as a noncooperative game, namely, aStackelberg game, where the throughput of each station in thenetwork is maximized under power constraints. In this game,the macrocell UEs are referred as the leaders and the femtocellUEs are considered to be the followers. The game is dividedinto two sub-games: the sub-game comprised of the set ofleaders, referred to as the upper sub-game, and the sub-gamecomprised of the set of followers, referred to as the lowersub-game. The players in each sub-game compete with eachother in a non-cooperative manner to reach a sub-game Nashequilibrium, which is the solution of the power control game.

F. Fractional Frequency Reuse (FFR) and resource partition-ing

The basic mechanism of this method divides the entirefrequency spectrum into several sub-bands. Afterwards, eachsub-band is differently assigned to each macrocell or sub-areaof the macrocell. Since the resource for MeNB and HeNB isnot overlapped, interference between MeNB and HeNB canbe mitigated. In [11], the authors propose a frequency shar-ing mechanism that uses frequency reuse coupled with pilotsensing to reduce cross-tier/co-channel interference between

macrocell and femtocells. In this scheme, FFR of 3 or aboveis applied to the macrocell. When a HeNB is turned on, itsenses the pilot signals from the MeNB and discards the sub-band with the largest received signal power, and thus usesthe rest of the frequency sub-bands resulting in an increasedSINR for macrocell UEs. The overall network throughput isenhanced by adopting high-order modulation schemes.

In [12], another interference management scheme for LTEfemtocells is presented based on FFR. The scheme avoidsdownlink cross-tier interference by assigning sub-bands fromthe entire allocated frequency band to the HeNBs that are notbeing used in the macrocell sub-area. In the proposed scheme,the macrocell is divided into center zone (corresponding to63% of the total macrocell coverage area) and edge regionincluding three sectors per each region. The reuse factor ofone is applied in the center zone, while the edge region adoptsthe reuse factor of three. The entire frequency band is dividedinto two parts and one of them is assigned to the center zone.The rest of the band is equally divided into parts and assignedin the three edge regions.

Fig. 4(b) illustrates the allocation of frequency sub-bandswithin the macrocell sub-areas. The sub-band A is used inthe center zone (Cl, C2, and C3), and sub-bands B, C, andD are used in regions Xl, X2, and X3, respectively. Now,when a HeNB is turned on, it senses the neighboring MeNBsignals, compares the Received Signal Strength Indication(RSSI) values for the sub-bands, and chooses the sub-bandswhich are not used in the macrocell sub-area. In addtion, ifthe HeNB is located in the center zone then it excludes thesub-band that is used in the center zone as well as the onethat is used by the mactocell in the edge region of the currentsector. For example, if a HeNB is located in edge region X1,then it would exclude sub-band B which is used by macrocellUEs, and select sub-band A, C, or D. However, if a HeNB islocated in center zone C1, then it avoids sub-band A and atthe same time sub-band B since the RSSI for this sub-band iscomparatively higher for that HeNB. In this way, this schememitigates co-tier and cross-tier interference. Simulation resultsshow that, the scheme offers throughput gains of 27% and 47%on average, when compared with the FFR-3 scheme (with nocenter zone) and a scheme with no FFR, respectively.

(a) (b)

A

B

C

D

X1Macro:  BFemto:  A,C,D

X3Macro:  DFemto:  A,B,C

       X2Macro:  CFemto:  A,B,                            D

Macro:  A

C1Femto:  C,D

C3Femto:  B,C

C2Femto:  B,D

1

2

3

4

5

7

6

Fig. 4. Interference management scheme using FFR.

The two schemes described above use a fixed partitioning,which would cause a loss in throughput performance due

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to inefficient use of the bandwidth resources. A dynamicpartitioning scheme (in both time and frequency domain) canbe used for bandwidth sharing which minimizes cross-tierinterference. In [13], an adaptive FFR scheme is presentedto minimize downlink interference caused by the HeNBs inthe vicinities of a macrocell. The proposed scheme adoptsFFR radio resource hopping or orthogonal FFR radio resourceallocation based on the density (e.g., high or low) and locationinformation (e.g., inner region or outer region) of the HeNBs.The location information of the HeNBs may be obtainedand maintained within the network through using registeredphysical address associated with the broadband IP (InternetProtocol) address that a HeNB uses. The proposed schemeonly deals with the cross-tier interference posed by the HeNBslocated (inner region) near the MeNB. If the HeNB is situatedin a high dense inner region, then orthogonal sub-channels areadopted by the HeNBs. Otherwise, the HeNB selects a sub-channel arbitrarily, utilizes it for a certain period of time, andthen hops to other sub-channels. The proposed scheme reducesdownlink cross-tier interference.

Note that resource partitioning method can be used alongwith power control (thus resulting in a hybrid approach) toreduce co-tier and cross-tier interferences.

G. Cognitive approachCognitive radio approach based on distributed spectrum

sensing can be used for interference mitigation in femtocellnetworks. In [14], an efficient downlink co-tier interferencemanagement scheme for an OFDMA-based LTE system isproposed where the path-loss information is shared amongHeNB neighbors. In addition, adjacent HeNBs share theinformation related to the usage of LTE Component Carriers(CC), achieved based on carrier aggregation technique leadingto a sub-channel, in a distributed manner. The exchangeof information between HeNBs may be done via femtocellgateway (HeNB GW) or over-the-air (OTA) method. TheHeNB GW is considered to be an intermediate node betweenHeNBs and mobile core network that manages the inter-HeNBcoordination messages via S1 connection. On the other hand,the OTA method includes a direct link between HeNB andMeNB.

In the proposed scheme, when a HeNB is turned on, itidentifies the adjacent neighbors and obtains the knowledgeof the CCs used by the neighbors. The main idea of thescheme is that, each HeNB estimates the co-tier interferencebased on the path-loss information, capitalizes the knowledgeof the usage of CCs by the neighbors, and accesses thespectrum intelligently to minimize interference. The selectionof CC is done in such a way that, each HeNB selects theCC which is not used by the neighbor or the CC that isoccupied by the furthest neighbor or the CC that is occupiedby the least number of neighbors (in a chronological order asmentioned). Simulation results show a significant reduction inco-tier interference and signaling overhead within the networkwhen compared with another cognitive based HeNB co-tierinterference management technique.

Fig. 5 illustrates a scenario of co-tier interference manage-ment (downlink) of HeNBs through cognitive approach. In this

scenario, let us consider that the available CCs for HeNBsare CC1, CC2, CC3, and CC4. In Fig. 5(a), since HeNB1and HeNB3 are adjacent to each other, they select differentCCs. On the other hand, since HeNB2 is a neighbor of neitherHeNB1 nor HeNB3, it selects any one pair of available CCs(e.g., CC1 and CC2). Now, under such femtocell deployment,when HeNB4 is turned on, it discovers its adjacent neighbors,i.e., HeNB1 and HeNB2. Through inter-HeNBs coordinationmechanism, HeNB4 obtains the information related to theusage of CCs of its adjacent neighbors. Thus, in order toavoid co-tier interference, when HeNB4 selects CCs for thedownlink transmission, it selects the CCs (i.e., CC3 and CC4)which are different from those used by from HeNB1 andHeNB2. Furthermore, in Fig. 5(b), when HeNB5 is turned on,it identifies the adjacent neighbors (i.e., HeNB1, HeNB3, andHeNB4), and obtains the knowledge of the CCs used by theneighbors. Under these circumstances, HeNB5 selects the CCsthat are occupied by the furthest neighbor, i.e., HeNB1. To thisend, HeNB5 selects CC1 and CC2 for downlink transmissionto reduce co-tier interference.

CC1 CC2

CC1 CC2 CC1 CC2

CC1 CC2

CC3 CC4

CC3 CC4

Component Carriers (CC): CC3 CC4CC1 CC2

CC3 CC4

CC3 CC4

CC1 CC2

(a) (b)

HeNB1

HeNB3

HeNB2

HeNB4HeNB4

HeNB3

HeNB1

HeNB2

HeNB5

Femtocell : HeNB

Fig. 5. Interference management through cognitive approach.

IV. QUALITATIVE COMPARISON AMONG INTERFERENCEMANAGEMENT APPROACHES

Table I provides a qualitative comparison among differ-ent interference management schemes. The “efficiency” of ascheme depends on whether it (i) mitigates/significantly re-duces both co-tier and cross-tier interferences; (ii) is applicablefor both uplink and downlink transmissions; (iii) considerscoordination among HeNBs and MeNB, or capitalizes onminimal amount of information, i.e., path-loss, geographicallocation, or usage of the spectrum or sub-band among nearbyHeNBs and/or among HeNBs and MeNB; (iv) handles ICI(e.g., by using frequency scheduling or any other method); (v)adopts an adaptive power control mechanism; (vi) correspondsto opportunistic access of the spectrum by the HeNBs based onRSSI value from MeNB signals; (vii) reduces the unnecessaryAIs of LDO mode for HeNBs; (viii) is scalable and robust,i.e., implementable for mass deployment of HeNBs; and (ix)is applicable for all 3 types of access modes (i.e., closed,open, and hybrid). If any scheme attains majority (more than5) of these attributes, then we consider the efficiency of the

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scheme to be high. We consider the efficiency of a scheme tobe moderate if it attains 3-5 of the aforementioned attributes.

For example, the efficiency of cognitive approach is con-sidered to be moderate since it is capable of handling bothcross-tier and co-tier interferences with minimal amount ofinformation (i.e., information about usage of sub-bands) ex-change among neighboring HeNBs, applicable for all types ofaccess modes of HeNBs, and more importantly, it accesses thespectrum in an opportunistic manner causing minimal harmto the nearby macrocell UEs. The collaborative frequencyscheduling scheme is considered to be highly efficient sinceit significantly reduces cross-tier and co-tier interferences formass deployment of HeNBs in both uplink and downlinktransmission, handles ICI problem, and allows the HeNBsto opportunistically access the spectrum based on only thescheduling information of macrocell UEs that is exchangedamong HeNBs and MeNB.

The “complexity” of each scheme increases with (i)the amount of information exchanged between neighboringHeNBs, (ii) the amount of information exchanged betweenHeNBs and MeNB, (iii) formation of clusters among HeNBs,(iv) algorithm executed in the HeNBs and/or in the MeNB toallow the HeNBs to access the spectrum opportunistically etc.The more information exchanged among HeNBs or betweenHeNBs and MeNB, the more signaling overhead is introduced,and more processing is done in both HeNBs and MeNB,increasing the complexity of the scheme. For example, thecomplexity of the beam subset selection strategy scheme isconsidered to be high since it requires the channel stateinformation from all macrocell UEs to determine the optimalnumber of beams every time along with extensive coordinationbetween HeNBs and MeNB regarding the spectrum access(thus increasing the signaling overhead). Also, the HeNBshave to run iterative power control algorithm to minimizeinterference.

Selection of an interference management scheme dependson the desired trade-off between complexity and efficiency.We recommend to adopt FFR as an interference managementscheme for two-tier femtocell networks since it requires min-imal/no coordination among HeNBs and MeNB (and hencereduces the signaling overhead, and thus the complexity ofthe system), opportunistically accesses the spectrum based ononly RSSI value from MeNB signals, and it effectively solvesthe problem of cross-tier and co-tier interferences in uplinkand downlink transmission for difference access modes ofHeNBs. Consequently, it can increase the throughput of thenetwork by a large margin, and can be used when the averagenumber of HeNBs per macrocell is very high (about 180-200)while maintaining the QoS requirements of macrocell UEs.Currently, FFR is being considered as an effective interferencemanagement scheme for OFDMA-based two-tier femtocellnetworks [2].

V. OPEN CHALLENGES

To enable mass deployment of femtocells, it is essential todevelop distributed interference management schemes whichprimarily satisfy the QoS requirements of macrocell and

femtocell UEs and at the same time enhances the capacity andcoverage of the network. Such schemes should incur low over-head for coordination among macrocell BSs (i.e., MeNBs),and also should be able to integrate mobility management withdifferent access modes and synchronization issues while keep-ing the complexity as minimal as possible. The interferencemanagement solution would strongly depend on the employedradio access technology (e.g., CDMA or OFDMA) and accessmode (i.e., closed, open, or hybrid). In particular, adaptive ad-mission control, power control, and advanced communicationstrategies such as interference cancellation and beamformingfor multiple-antenna transceivers are important techniques tomitigate co-tier and cross-tier interferences. For example, byusing beamforming techniques femtocells can form antennabeams toward their UEs while nulling interference caused tomacrocell UEs. In addition, macrocells would have higherpriority in accessing the spectrum; therefore, suitable admis-sion control mechanisms should be activated when femtocellscreate intolerable interference for macrocell UEs.

For OFDMA-based femtocell networks, if different sets ofsubchannels are assigned to macrocells and femtocells, cross-tier interference can be completely eliminated. However, toimprove the spectrum utilization, a more efficient spectrumassignment method can be adopted.

Also, hybrid interference management schemes which com-bine power control with resource partitioning are promising.Power control schemes are advantageous in that MeNB andHeNB can use the entire bandwidth with interference coordi-nation for both control and data channels. However, for this,the HeNB measurement scheme for power setting would needto be standardized. Also, such a scheme may not be fully effec-tive when a macro UE is located very close to a HeNB. Withresource partitioning schemes, interference between MeNBand HeNB can be eliminated. However, multiple frequencybands are required. The merits of both the approaches can beexploited in a hybrid scheme, the design of which is not trivial.

VI. CONCLUSION

The femtocell technology can provide many advantagesto the mobile subscribers and the service providers. Thus,femtocells could be viewed as a promising option for next gen-eration wireless communication networks such as OFDMA-based LTE-Advanced and WiMAX networks. We have pro-vided a survey of different techniques to cope with the co-tierand cross-tier interference problem in OFDMA-based two-tierfemtocell networks. With efficient interference managementschemes, the network capacity and coverage can be increasedthat benefit both the subscribers and the operators.

ACKNOWLEDGMENT

This research was also supported in part by NSERC,Canada, and in part by the MKE (Ministry of KnowledgeEconomy), South Korea, under the ITRC (Information Tech-nology Research Center) support program supervised by theNIPA (National IT Industry Promotion Agency (NIPA-2011-(C1090-1111-0005)).

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TABLE IQUALITATIVE COMPARISON AMONG DIFFERENT INTERFERENCE MANAGEMENT SCHEMES

Scheme Transmission Cooperation among Access Complexity Efficiency Type ofmode HeNBs and MeNB mode interference

Femto-aware Uplink Required Closed Moderate Low Cross-tierspectrum management

Clustering of Downlink Required Closed Moderate Moderate Co-tier andfemtocells cross-tier

Beam subset Downlink Not required Closed High Moderate Cross-tierselection strategy

Collaborative Uplink and Not required Closed Moderate High Cross-tier andfrequency scheduling downlink inter-carrier interferences

Power control Downlink Not required Closed and open Moderate High Cross-tierCognitive Downlink Required Closed and Moderate Moderate Cross-tier

openFractional Downlink Not required Closed, open, Low High Co-tier and

frequency reuse and hybrid cross-tier

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Control Strategy in UMTS and LTE,” IEEE Communications Magazine,vol. 47, no. 9, pp. 117–123, Sept. 2009.

[4] W. Yi, Z. Dongmei, J. Hai, and W. Ye, “A Novel Spectrum ArrangementScheme for Femtocell Deployment in LTE Macrocells,” Proc. IEEE 20thSymposium on Personal, Indoor and Mobile Radio Communications, pp.6–11, 13-16 Sept. 2009.

[5] H. Li, X. Xu, D. Hu, X. Qu, X. Tao, and P. Zhang,“Graph Method BasedClustering Strategy for Femtocell Interference Management and SpectrumEfficiency Improvement,” Proc. IEEE 6th International Conference onWireless Communications Networking and Mobile Computing (WiCOM),pp. 1–5, 23-25 Sept. 2010.

[6] H. Widiarti, S. Pyun, and D. Cho, “Interference Mitigation Based onFemtocells Grouping in Low Duty Operation,” Proc. IEEE 72nd VehicularTechnology Conference Fall (VTC’10-Fall), pp. 1–5, 6-9 Sept. 2010.

[7] S. Park, W. Seo, Y. Kim, S. Lim, and D. Hong,“Beam Subset SelectionStrategy for Interference Reduction in Two-tier Femtocell Networks,”IEEE Transactions on Wireless Communications, vol. 9, no. 11, pp. 3440–3449, Nov. 2010.

[8] M. E. Sahin, I. Guvenc, Moo-Ryong Jeong, H. Arslan, “Handling CCIand ICI in OFDMA Femtocell Networks through Frequency Scheduling,”IEEE Transactions on Consumer Electronics, vol. 55, no. 4, pp. 1936–1944, Nov. 2009.

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[14] L. Zhang, L. Yang, and T. Yang, “Cognitive Interference Managementfor LTE-A Femtocells with Distributed Carrier Selection,” Proc. IEEE72nd Vehicular Technology Conference Fall (VTC 2010-Fall), pp. 1–5,6-9 Sept. 2010.

[15] S. Guruacharya, D. Niyato, E. Hossain, and D. I. Kim, “HierarchicalCompetition in Femtocell-Based Cellular Networks,” Proc. IEEE Globe-com’10, Miami, FL, USA, 6-10 Dec. 2010.

PLACEPHOTOHERE

Nazmus Saquib received his B.Sc. degree inElectronics and Communication Engineering fromBRAC University, Bangladesh, in 2008. He is cur-rently working towards his M.Sc. degree in Elec-trical and Computer Engineering at the Universityof Manitoba, Winnipeg, MB, Canada. For academicexcellence in undergraduate studies, Saquib won theVice Chancellor’s Gold Medal from BRAC Univer-sity. Also, he has been awarded the University ofManitoba Graduate Fellowship. His research inter-ests include interference management and resource

allocation in OFDMA based heterogeneous networks.

PLACEPHOTOHERE

Ekram Hossain (S’98-M’01-SM’06) is a Profes-sor in the Department of Electrical and ComputerEngineering at University of Manitoba, Winnipeg,Canada. He received his Ph.D. in Electrical En-gineering from University of Victoria, Canada, in2001. Dr. Hossain’s current research interests in-clude radio resource management in wireless/mobilecommunications networks and cognitive radiosystems (http://www.ee.umanitoba.ca/∼ekram). Heserves as the Area Editor for the IEEE Transactionson Wireless Communications in the area of “Re-

source Management and Multiple Access”, an Editor for the IEEE Transac-tions on Mobile Computing, the IEEE Communications Surveys and Tutorials,and IEEE Wireless Communications. Dr. Hossain has several research awardsto his credit which include the University of Manitoba Merit Award in 2010(for Research and Scholarly Activities) and the 2011 IEEE CommunicationsSociety Fred Ellersick Prize Paper Award. He is a registered ProfessionalEngineer in the province of Manitoba, Canada.

PLACEPHOTOHERE

Long Bao Le (S’04-M’07) received the Ph.D. de-gree from University of Manitoba, Canada, in 2007.He is currently an Assistant Professor at INRS-EMT,University of Quebec, Montreal, Quebec, Canada.His current research interests include cognitive radio,cooperative diversity and relay networks, stochasticcontrol and cross-layer design for communicationnetworks.

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PLACEPHOTOHERE

Dong In Kim (S’89-M’91-SM’02) is a Pro-fessor and SKKU Fellow in the School ofInformation and Communication Engineering atSungkyunkwan University (SKKU), Suwon, Korea(http://wireless.skku.edu). He received his Ph.D. de-gree in Electrical Engineering from University ofSouthern California, Los Angeles, in 1990. From2002 to 2007, he was a tenured Full Professor in theSchool of Engineering Science, Simon Fraser Uni-versity, Burnaby, BC, Canada. His current research

interests include cooperative communications, in-terference management for heterogeneous networks, cross-layer design andwireless security. He serves as an Editor for Spread Spectrum Transmissionand Access for the IEEE Transactions on Communications and an AreaEditor for Cross-layer Design and Optimization for the IEEE Transactionson Wireless Communications. He also serves as co-Editor-in-Chief for theJournal of Communications and Networks.