Wireless Pers Commun (2014) 78:85–106DOI 10.1007/s11277-014-1737-8

A Review on Femtocell and its Diverse InterferenceMitigation Techniques in Heterogeneous Network

Afaz Uddin Ahmed · Mohammad Tariqul Islam ·Mahamod Ismail

Published online: 28 March 2014© Springer Science+Business Media New York 2014

Abstract In third generation wireless network, the demand of wireless multimedia service isresponsible for the enormous growth of data traffic. Cellular operators throughout the worldare facing challenges in increasing system capacity, coverage and residential connectivity insub-urban and urban environments due to the huge investment that follows. Femtocell offersan economically appealing way to improve quality, coverage and service in the existingnetwork. Currently the use of femtocell is being supported primarily by the argument ofimproved indoor coverage for consumers and substantial cost savings for operators due tocapacity offload. The Quality of Service is mainly effected by the limited bandwidth inwireless links. Femtocell can be an effective alternative to divert and carry out a big portionof the traffic from the Macro Base Station. The only issue in its vast implementation is lack ofeffective schemes to mitigate interference that creates challenge in maintaining customers’satisfaction. This paper presents the role of femtocell in heterogeneous network and discussesdiverse interference mitigation techniques.

Keywords Femtocell review · Femtocell interference · Interference mitigation ·Heterogeneous network · Radio-frequency interference

1 Introduction

Cellular operators these days are aiming towards the growing market of data and voice call.The companies are trying to provide more multimedia contents to attract the new generationcustomers for further increment of their revenues. The voice service is mostly characterizedby the subscribers’ number while the data services are congested by the use of vast number ofapplications and protocols. As the data traffic is increasing exponentially, the need for properresource management has increased drastically in indoor regions. Radio signals are weak in

A. U. Ahmed (B) · M. T. Islam · M. IsmailDepartment of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia,43600 Bangi, Selangor, Malaysiae-mail: afazbd@gmail.com

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indoor environment due to the path loss, lognormal shadowing and fast fading effects. Alsothe bandwidth of the backhauls which are used for Macro Base Station (MBS) is not sufficientas they can provide speed around 3–8 M-bits/s [1]. Due to the attenuation-loss and penetrationloss, providing high-quality indoor service is a big challenge for the macro networks. Rapidincrease of concrete and steel structures also creates problem for the high- spectrum wirelessservice providers. To overcome this, both MBS and cell phone have to transmit more powerthat is complicated in uplink as cell phone batteries are small and contain low capacity.Another hurdle arises when the performance of data transfer degrades due to the low qualityof signals. As a result, more MBS or access points are required to provide adequate dataservice. This problem is even greater in dense metropolitan and compact urban areas. Theoutlays of operators’ businesses are linearly related to the deployment density of MBS. Butthe Average Revenue per User (ARPU) doesn’t rise proportionally as such, providing qualitydata service appears to be a great financial challenge to mobile operators [2].

In the metric measurement system, “Femto” means one quadrillionth. Although, in real,Femtocell covers areas (e.g. residence, office, and multi-floored building) up to 40 m inradius. It is a mobile base station that has been shrunk down to the size of a paperback book.It is a wireless voice and broadband service providing device which has a low power accesspoint with limited range. Femtocell looks similar to a wireless internet router and is easyto install at office and residence. It is a mini base station for the indoor purpose. It can becategorized as a modern version of In-Building Solution (IBS) used in 2–2.5 G network,where small repeaters receive poor outdoor signals and deliver them inside the building witha high antenna gain. IBS works as a macrocell’s segment and connects via transmissionlink to the macrocell. Picocell was the previous solution to the coverage problems withinconfined structures. Femtocell is a modernized version of the Picocell for home networkpurpose. Instead of being linked with a Base station controller, it uses the backhaul internetconnection to get connected to the operator’s network. It uses an open 3 GPP based standardsthrough the end’s household broadband internet connection. It utilizes internet protocol (IP)and flat base-station architectures. Outdoor users usually connect to the macrocell but whenthey enter into their home, they connect to the femtocell network. This provides a smoothconnection for the femtocell users and maximal coverage is ensured inside the house [3].

Femtocell Access Point (FAP) which are known as Home Node Base Station (HNB) is ashort-ranged, low-powered and low-cost base station which is connected with a third partybackhaul internet connection; like cable modem or digital subscriber line (DSL) or opticalfiber as available. Figure 1 shows us the connection diagram of the femtocell to the existingnetwork. Both femtocell and macrocell communicate with the end-users in the same wayusing the same frequencies with the same signaling protocols. The mobile phone signalis converted and sent via broadband internet connection through the femto gateway. Voiceservices are provided by the same “Mobile Switching Center” and data services by the same“GPRS Support Node” that are used in the existing outdoor network [4]. The main advantagesof femtocell users are:

• It can provide high quality coverage to the subscribers when there is no coverage ofmacrocell. It assists the network to conceal the coverage problem in deep indoor situationsand underground structures where the macrocell cannot reach. The presence of a femtocellinside an enclosed structure can ensure a “5 bar” coverage which results in less call drops,better voice quality, higher data rate and much quicker and easier roll-out compared tooutdoor cell sites.

• Mobile phone networks are congested with high level of data traffic. Femtocell increasesthe capacity of the network by diverting the data flow through the wired connection.

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Fig. 1 Femtocell connection to an operator’s network

• For enterprise users, having femtocell instead of Digital enhanced cordless telecommu-nication (DECT) phones allow them to use a single phone which provides the optionto use single address book, single billing account for cell phone, land-line phone andbroadband.

• Different services (data mainly) can be provided by the operators as the femtocell iden-tifies the precise location and presence of the user’s, which allows to present a differentmenu at home.

• Depending on the company pricing policy, calls placed under the same femtocell networkcan offer special tariffs.

When mobile operator provides coverage and service, the first thing they have to do is to buildup these cellular towers which covers two to five miles (varies on urban, sub - urban, andrural) each. Network is then shared by phones that are within that coverage radius. Massivecosts are associated with the deployment of cellular towers (installation, maintenance, leasinglands etc.). However, with the use of femtocell, a huge portion of the cost can be avoided.Femtocells possess the potential to save operators’ millions of dollars in tower building forthe coverage purposes. Moreover, it shifts the burden of providing quality coverage on to thecustomer’s hand. Mobile operators these days encourage their users to buy and use femtocellwherever they want: as long as they possess a backhaul internet connection.

2 Deployment of Femtocell Technology

The demand of voice and data traffic has grown rapidly over the last few years mostlybecause of the technology advancement and computational market strategy of the cellular

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operators that features lower rates of the services. After the introduction of the 4G-LTEtechnology, the existing system has to fulfil the higher demands of the subscribers to maintainthe ARPU. In fact, the operator has to offer higher bit rate with lower cost than previous. Thismade the operator and service providers to upgrade the network by increasing the numberof base stations and spectrum efficiency. In LTE network, the frequency reuse factor 1 isintroduced to coup up with the vast data demand. Still the spectrum is a limited resourceand they are assigned in a fixed number to each operator. Moreover, the main challengecomes when the coverage radius of the base station reduces along with the number of activesubscribers. This leaves with no alternatives to increase the number of base stations [5].Then again, building up new base stations is a very costly solution when more than threeforth of the traffic is generated from indoor region. Ensuring better coverage from the outdoormacrocell to the indoor subscriber especially in dense urban areas will be quite hard. Femtocellbrings a techno-economical solution to this problem. Femtocell possesses the potential toimprove Area Spectral Efficiency (ASE) several folds [6]. Not only it increases the capacityand coverage of network in indoor environment, it also ensures high Quality of Service(QoS) allowing the operator to get rid of expending extra cash for the expansion of thenetwork. Since discovering such a role of femtocell in modern day’s telecommunicationsectors, researchers are trying to make it more flexible, less expensive with more user-friendlyfeatures so that it attracts the highest possible attention of the subscribers. As an outcomeof the researches, an energy effective femtocell deployment strategy for higher bandwidthenterprise and residence customers is mention in article [7]. The deployment strategy issupported by both wireless and optical backbone which links with the main network. Anotherdeployment and isolation density is studied is in article [8]. The study shows that, when thenumber of unplanned deployment of femtocell crosses a certain limit, the average throughputof the users degrades.

Deployment of femtocell opens up wide range of possibilities. Residence user gets fullcoverage inside their house while enterprise user enjoys high bandwidth. Under the samefemtocell the operators can promote new tariffs and additional facilities which can make itmore convenient for the users [9]. It reduces the traffic load of the macrocell in the peak hoursthrough the wired backhaul connection to the core network. In rural places where DSL orcable connection is not available, it can provide broadband access with backhaul supportedfrom the satellite or HAP (Higher altitude platform). A “Techno-economic model” in [10] isused to evaluate the LTE-network related cost and the turnover in business environment alongwith the market parameters, market assumptions, network architecture and cost assessment.In another research authors find out that an appropriate femtocell deployment with properplanning to increase the network coverage almost double while increasing the data rate 28 %on an average [11]. Since femtocell is considered as a mini base station, National RegulatoryAuthorities (NRA) treated its protocol with the same regulation regarding spectrum licensingor emergency calls processing. However, OFCOM—Independent regulator and competitionauthority for the UK communications industries suggested in a public consultancy to reservea portion of the 2.6-GHz band for exclusive femtocell deployment [12]. Though later it wasnot followed up as to ensure higher priority to spectrum efficiency over interference issues.The deployment of femtocell has reached from residence, enterprise use to public places likeshopping mall, stations and airports. Now operators are looking forward to deploy femtocellin public transports like bus, train with backhaul supported from macrocell, satellite, or HAP.May be in near future femtocell can be found deployed in street furniture and structure servingoutdoor subscribers along with macrocell.

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3 Interference in Femtocell

Interference is one of the major issues associated with femtocell development. There areconcerns coming along with the interference issues and all of these have to be investigatedthoroughly. Only after solutions to these problems are found, the successful deployment offemtocells can be ensured. Femtocell has to provide both high-quality voice service and high-speed mobile data service at a fraction of the cost of the macro node. Its output power must beflexible so that it can avoid generating interference in dense femtocell network while usingthe same spectrums. The maximum output power that femtocell uses is less than 25 dBm.It has receiver (Rx) sensitivity up to −107 dbm and adjacent channel selectivity (ACS) of−101 dBm [13]. However, like an unplanned network, the femtocell network is subjectedto massive interferences since it utilizes the frequency that is assigned for MBS. Due to thelow throughput in areas that are densely covered by femtocells, the users at the edge of thecoverage area suffer from low QoS [14]. These problems are identified as the key issuesof investigation. Considerable efforts have been given to minimize the interference so thatit does not hinder the mass deployment of femtocell. Research groups have been studyingabout several aspects of femtocells and have come out with a number of schemes that havebeen developed to ensure minimum interference. Users now can install femtocell without theneed to worry about any mechanical and technical issues.

Two types of interference scenarios are present in two-tier network. Co-tier interference isone of the kind that occurs between the same elements of the network, e.g. interference amongfemtocells. Cross tier interference occurs between different elements of the network e.g.interference between macrocell and femtocell. The situations that cause major interferenceare mentioned below.

• Femtocells interfering with Macrocell on the same spectrum.• Macrocell interfering with femtocell on the same spectrum• Neighboring femtocells that are close, interfering with each other on the same spectrum.• User hand-equipment transmitting with high power may reach the macrocell that influ-

ences the level of noises received by the macrocell.

The designers claim that femtocell can provide the best coverage within a range of 40 m andcan support connection to users who are travelling at speed up to 120 Km/h. So the optimumdistance between two femtocells should be around 80 m [15]. Though based on the femtocellclass (1–3), the coverage range varies from 10–40 m. The classes of the femtocell variesamong these three based on its operation in residential and enterprise application. In an idealheterogeneous network, there is very little chance of co-tier interference if femtocells aredeployed with proper planning. However, the “plug and play” feature that femtocell offerscannot be included in any network planning. So mitigating interference in heterogeneous net-work is a big problem for network planners. In the heterogeneous network, the target is to useminimum frequency reuse factor. In Fig. 2, a realistic femtocell network is considered wherethe network planners do not have any role on the severity of interference due to unplannedand dense deployment of the cells. In an ideal femtocell network, the coverage areas of fem-tocell do not overlap with each other. But in practice, the appearance of overlapping eventsare frequent. There also exists dead zones where the users hardly have the quality coverageto set up a call, but gets affected by a number of serving cells. As shown in the Fig. 2, fourHome Node Base stations (HNB)—HNB1, HNB2, HNB3, HNB4 and a sector of macrocellare deployed. All of the cells are serving in a particular area. Each HNB has a 40 m radiuscoverage region where they can deliver quality coverage. U1, U2 and U3 are three users underthe cells’ coverage looking for a better signal quality. Due to unplanned femtocell deploy-

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Fig. 2 Unplanned femtocell configuration

ment, the distance between the HNB1 and HNB3 are not appropriate. Any user between thesetwo HNB is subjected to strong interferences. The macrocell also transmits strongly at samechannels in the same area, which results the interference to increase further. In case of U2,situated in the inner coverage area of HNB1, it is a fractional distance away from the HNB2’scoverage area but still leaks into the coverage area of HNB1 and gets affected. But it is quiteless than that of U1. U3 is in the middle of the four femtocells and in the null-coverage regionwhere none of the femtocell has quality coverage. It is within the region of 80 m of the allthe femtocells. Even though the macrocell can deliver a good coverage to U3, all the otherfemtocells interfere in the downlink if they use the same spectrum.

To determine the SINR at the users end, we need to determine path loss of each userfrom each femtocell and macrocell. The path loss models of the users from femtocell andmacrocell are formatted as below.

Pathlossm(db) = 15.3 + 37.6 log10 D (1)

Pathloss f (db) = 38.46 + 20 log10 D + 0.7d2D,indoor + 18.3n

(n−2n+1 −0.46

)+ wLiw (2)

where D, w, n, 0.7d2D,indoor , Pathlossm and Pathloss f are the distance, number of walls,number of floors, penetration loss inside the house, path-loss of macrocell and the path-lossof femtocell, respectively.

For x number of femto users and y number of sub-carriers, the expression of SINR isgiven as:

SINRx,y = PF,y Gx,F,y

N0� f + PM,y Gx,M,y + ∑F PF ′,y Gx,F ′,y

(3)

where N0,� f , PF ′,y and PM,y are the white noise power spectrum density, sub-carrier spac-ing, gain (depends on path-loss), transmitting power of neighboring femtocell and the trans-mitting power of neighboring macrocell, respectively [16] (Fig. 3).

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Fig. 3 Path-loss and SINR of the users

A simulation of path-loss and SINR for the users is done considering the transmissionpower of macrocell and femtocell 46 and 25 dbm, respectively, white noise power density−174 dBm/Hz and sub-carrier spacing 15 KHz.

4 Interference Mitigation Techniques

Many techniques have been tested and deployed to mitigate the interferences in femtocellnetwork. They can be classified based on different criteria and according to their ability toimprove the link reliability as well as the capacity.

The user centric approach imitates the satisfaction of the users on the service of thenetwork. The users gets the main focus and the resources are allocated to guarantee usersatisfaction. As a result, it maximizes the available resources of the system that can beformatted as:

maxp(n)q(n)

K

k∑k=1

N∑n=1

1/

kU(

r (n)k

)q(n)

k

N∑n=1

p(n) ≤ Pmax,

K∑k=1

q(n)k ≤ 1, ∀n (4)

where q(n)k and U (r (n)

k ) are the variable sub-carrier assignment variable and the unity function,respectively. When sub-carrier is assigned to users, the value of is 1 otherwise its 0.

On the other hand, system centric approach concentrates on the QoS of the system. Insteadof monitoring individual’s satisfaction, it studies the overall performance of the network.System centric arrangement can be classified into two sub-systems: Radio Adaptive systemand Margin adaptive system. In radio adaptive system, the QoS of the users are maximizedwhich can be formatted as:

maxp(n)

N∑n=1

F/N log2

(1 + p(n)(G(n))2

σ 2

)

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N∑(n=1)

p(n) ≤ Pmax, p(n) ≥ 0, ∀n (5)

where F , p(n), G(n)and N are the total bandwidth, transmitting power, channel gain and thenumber of subscribers, respectively.

Conversely, Margin Adaptive system emphasizes on minimizing the transmission powerof FBS that can be expressed as

minp(n)

N∑n=1

p(n),

N∑n=1

F/N log2

(1 + p(n)(G(n))2

σ 2

)≥ R (6)

where R is the required data rate of the user [17].Multi-antenna techniques are used for forming and smart clustering deployment in arti-

cle [18,19]. Increasing BTS capacity and tuning parameters of the resources [20] are alsoincluded in the hardware approach in terms of dealing interferences. Based on resourcepartitioning and power management, self-organized approaches can be categorized as cen-tralized method and distributed method [21]. Below some of the algorithms which based oninterference mitigation are discussed.

4.1 Resources Allocation

The basic mechanism of resource allocation is blocking and denying access to the down-link resources which are subject to greater co-tier and cross-tier interferences [22]. It dealswith both type of interferences which makes it more efficient as well as complex. In [23],authors recommended a RADION system which is a framework for distribution of time fre-quency resources based on OFDMA. A core building block enables femtocells to find theavailable resources in a completely distributed resource management framework that effec-tively manages the interference. FERMI—FEmtocell Resource Management for Mitigationof Interference by Arslan et al. [24], is one of the first resource allocation concept in OFDMAbased femtocell network which introduces power pooling access cells, resource allocation,resource assignment, isolation of frequency domain, client classification and schedulingacross the entire network using centralized data collection. Even though, FERMI analysesthe collected data, it is not recommended for residential setting where femtocell is self-configured without any radio planning. Comparing with it, RADION is instructed keepingmainly the residential femtocell in mind. Based on their interference level, it divides thesubscriber in two classes, one that can reuse the spectrums and another who needs resourceisolation to mitigate the interference. It categories the existing zones under each cell into threesub-zones consisting of reuse zone, isolation zone and transition zone. Allocation of differenttransmit power to different sub-channel according to the users demand and channel condi-tion, the system can lead to a self-organizing network behavior. By minimizing the transmitpower up to users requirement, higher spectral efficiency can be ensured with less interfer-ence occurrences [25]. In another scheme [26], femtocell senses the energy of sub-channelsand detects the channel occupation in order to achieve the dynamic allocation of spectrum.The femtocell can determine the available unoccupied frequencies using the sensors. Groupwise arrangement is developed to occupy the existing environment and to gather data fromthe network. It has the advantage of achieving noticeable spectral efficiency gain over somepreceding schemes. Authors in [27] proposed a distracted selection of sub-channels for theOFDMA-based femtocell systems. An energy efficient resource optimization is proposedin [28] where authors have investigated the distributed radio resource allocation scheme to

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maximize the performance of the femtocell network. Here distributed sub channel and powerallocation algorithms were discussed which solves most of the existing resource allocationproblems. A block diagonalization is done by Jang et al. [29] where each femtocell generatesa pre-coding matrix by forming a cluster and it considers the interference effect on the nearbymacro user. As interference increases with the number of femtocell in a confined area, anantenna selection algorithm comes into act to mitigate the interference. Getting access toprivate residence for onsite maintenance is not easy. By the Home e-Node base-station man-agement system (HeMS) which is based on TR-069 CPE WAN management protocol, it ispossible to control and configured the HNB. It allows remote access to HNB to configure andcontrol the parameters that are specified by HeMS. It also provides the option to calculate themaximum allowable interference power constraint at the MBS. Considering the challenges,a framework has been designed using Stackelberg game concept assuming MBS as the lead-ers and FAPs as the follower in the article [30]. The macrocell base stations have enoughinformation to predict the response of the femtocells for given macrocell power profile. Onthe contrary, femtocell maximizes their individual capacity under power constraints iteratinga waterfilling strategy among themselves to reach a sub-game Nash equilibrium.

Orthogonal Frequency division multiplexing has been introduced in 3GPP Long TermEvolution focusing on features like: reuse of spectrums and high spectrum efficiency [31].However, due to frequency reuse, neighboring cells transmitting in the same sub- channelleads to performance degradation. As a matter of fact, allocation of spectrums in macro-femtocell network brings up a burning topic for researchers to work on interference mitigationin heterogeneous network. The entire frequency spectrum is divided into sub-bands and areassigned intelligently to each macro-femto cell so that the frequency does not overlap. Theseco-ordinates are measured through DL-HII (Downlink High Interference Indicator) [32].Though frequency reuse, the division of spectrum into sub-groups are an old practice to dealwith the interference problem. Still it is a better solution to coup up with the required usercapacity and overall network performance. Authors in [33] introduced frequency splittingtechniques to solve the interference problem in femtocells network where multiple accessinterference (MAI) and Inter-Carrier Interference (ICI) influence the system’s performance.Clustering Algorithms are proposed in [34,35] where the whole bandwidth is allotted intotwo bands. One is dedicatedly assigned to femto-users and another is for macro-users. Theratio of the bands is fixed by the given “Clustering” technique. Meanwhile to decrypts theoptimal clustering problems an Adaptive Clustering Heuristic Algorithm (ACHA) algorithmis also recommended.

To control the cross-tier co-channel interferences between femtocell and microcell, pilotsensing frequency reuse coupled-mechanism is mentioned in [36]. According to the proposedscheme, pilot sensing signal increases the SINR by discarding the sub bands with the highestreceived signal power thus using the rest of the frequency bands for radio transmission. Anautonomous component carrier selection is demonstrated in [37], where a distributed and scal-able solution is achieved based on minimal information exchange and negotiation betweenbase stations. The cell configures the most suitable frequency by themselves; improving thequality of coverage without compromising the capacity. In [38,39], a dynamic resource parti-tioning is proposed where downlink interference co-ordination between macro and femtocellis measured and conveyed via an X2 connection to neighboring FBS and MBS so that HNBdenies to get access to the downlink resources that are assigned for macro user in their vicin-ity. The DL-HII generates the necessary signal and share through their wired backbone. Asubcategory of orthogonal sub-carriers is assigned to all FBSs in that region. The strongestsignal from the FBS is synchronized with the desired macro-signal (Fig. 4).

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Fig. 4 Channel allocation in femtocell network

Fractional frequency reuse (FFR) is the combination of different frequency reuse schemes.FFR in different regions of the network can be determined by dividing the coverage areainto sub-zones [40]. Most of the FFR feature divides the cell coverage into two regions:cell-edge region and cell-center region. In most of the spectrum partitioning scheme, cell-center uses a set of fixed spectrums while the cell edge region adopts different algorithmsin assigning them. A twin layer FFR scheme has been discussed in [41] where a spectrumswapping strategy is introduced in macrocell to overcome the system level interferences. In[42], authors proposed a FFR based time resource allocation scheme. For the cell-edge and thecell-center region, feasible frequency sub-band and time resource is allocated. Co-channelsare used in low commodity channel index (CCI) and orthogonal channels are used in highCCI as a useful policy to handle the interference in the overlay cellular network. Accordingto the proposed spectrum partitioning scheme, femto-cells are allowed to transmit their datathrough the unused sub-bands to specific macro users without effecting the nearby macro-users. In [43], authors investigated the advantage of using FFR by allocating orthogonalbandwidth. Introduction of FFR embedded the spectrum efficiency high as it reduces thethroughput degradation of the macro-users. Similar strategy is taken into account in [44]where the macrocell coverage is spited into two zones and the whole frequency is split intotwo portions. The two zones; center zone and edge zone includes three sub-zones aligningwith the three poles of macrocell. Considering the cell coverage and hand-off, event anefficient hybrid-frequency assignment technique for mitigating the interference is proposedin [45].

To reduce the CCI among the femto-macro cells, three factors are considered: frequency,time and space [46]. The cell center region uses one subset of frequency and the cell-edgeregion uses three subset of frequencies. The neighboring macro-users use the orthogonalsub-channels in order to avoid interference. Therefore, the users in the cell center region ofmacro-cell can be served simultaneously during their service time as CCI from neighboringMBS is limited. In [47], a centralized method, namely dynamic frequency planning (DFP),which focuses on superior spectrum allocation to femtocell network through allocating thespectrums to femtocells network is described. However, authors did not consider the cross-tired interference during the measurement. Since allocation of spectrums and power resourcesis an effective approach in mitigating interference in macro-femtocell network, it also might

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Fig. 5 Different access control mechanism in femtocell network

limit the spectral efficiency of the network. The performance of the methods in a co-channeldeployment scenario might not be satisfactory.

4.2 Access Control

Access control limits the users to get access to personal femtocells. The performance offemtocell greatly depends upon mechanism that decides either the user can connect to thecell or not. The selection of access control has a vast effect on network performance. Openaccess, closed access, and hybrid access are the three types of access control mechanism usedin femto network. As the name shows, open access allows all the subscribers of that operatorto access the network whenever they get within the range of the femtocell. In the case of closeaccess, only particular users can get access into the femtocell thus avoiding unwanted traffic.Third generation partnership project (3GPP) referred this group as closed subscriber group(CSG) [48]. In the hybrid access, a limited amount of resources are available to all the usersof the operator who are within the coverage range, but only the CSG possesses the privilegeto use full resources. In open access, the number of handover and congestion of signalingincreases as the femtocell allows all the users to get connected through it. It also has a sharingand security concern. The close access does not have that much congestion of signaling as thefemtocell denies the access of subscribers who are not in CSG even if it provides best signalquality. This induces a new set of interfering signals that makes the interference problemeven more. Hybrid access deals with both problems and come out with an optimum solutionby tuning the resources according to the number of femtocell owners and subscribers [49].In Fig. 5 the basic mechanism of three access control in femtocell network is illustrated.

A closed user group access control feature is proposed in [50] that allows a particular fem-tocells’ user group like CGS to get access to the service. In the scheme, two-tier interference

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Fig. 6 Load balancing in femtocell network

is minimized and unwanted interference is lessened allowing it to be more energy efficient. In[51] the authors studied access control mechanism under the timing misalignment of uplinkand showed that the average performance of closed access OFDMA femtocell depends on itsdistance to the connecting FBS and on the number of users. If the timing misalignment amongthe users is greater than the cyclic prefix duration, inter-symbol interference and inter-carrierinterference occurs in the revised signaling at the femtocell end. A “3-ON-3 Femtocell Clus-tering Architecture” concept is build-up in [52] where 3 Femto base stations (FBS) formsa cluster which transmits 3 levels of power to serve three kinds of users according to theirpriority. Based on Adaptive Hard Reuse Scheme (AHRS), the entire bandwidth is utilized byeach femtocell. Access control mechanisms are still not deployed by the operators in the fieldlevel. Right now the femtocell LTE/4G network is in initial stage. However, it is expectedto be a potential candidate for interference mitigation in future radio planning and networkstrategy.

4.3 Load Balancing

Load balancing scheme distributes the workload across multiple cells, links and units of thenetwork. It ensures maximum user satisfaction and optimum QoS by proper management ofthe users assigned to each cell according to their capability [53]. Distribution and sharingof users according to the available resources makes it more effective to deal with the co-tierinterference with less complexity. To increase the number of femtocell users and adjust thetraffic load of macrocell according to the resources, a relay method is introduced in [54].This method balances the traffic load of the mobile network by redirecting from a macrocellto overlaid femtocells. Femtocell offers a relay service for neighboring users connected tothe MBS who has no permission to get access to the nearby femtocell or who is outside offemtocells “quality coverage area”. The FBS users get a reward for assisting in balancingthe traffic load. Nearly similar method is proposed in [55] where a femtocell relay methodaccelerates the number of users connected with femtocell and reduces the macrocell traffic.This process redirects the active users from a macrocell to draped femtocells so that thefemtocell owners gets the privilege of the relay service. In the core side, proxy mobile IPv6protocol is used to activate the relay features. A reward mechanism is also applied based onthe data amount served from that femtocell so that femtocell owner get inspiration by therelay service.

The split of resources can be coordinated between macro and femto base stations andadapted to load dynamics in the network. However, the femto network architecture allows toimproved capacity, whereby resources allocated to the macro BS can be reused by some ofthe femto BS. Like in Fig. 6, one femtocell is serving 4 clients while the second femtocell

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is serving 2 clients. Since the two clients are under both femtocells coverage, but connectedto one, the other femtocell brings down the coverage creating a weak interference. Sharingone client each, both femtocell can reuse the channels for other two clients. In [56] a heuris-tic algorithm is introduced to reduce the complexity in interference mitigation scheme. Thealgorithm monitors the interference condition of every sub-carrier using the resource allo-cation method. Analyzing the performance, it blocks the sub-carriers that suffer from highinterference. The transmission power of the sub-carrier is also measured and tuned to avoidthe effects of high transmission effect to the nearby FBSs. Monte-Carlo simulation is done tomonitor the performance compare to the existing solution. Considering the fairness of eachuser equipment capacity, authors in [57] proposed a load balancing scheme where the systemis spitted into two major steps: interference recognition and load balancing. Analyzing thefemtocell cooperation message and user equipment measurement-UMR (collects path-lossrelationship between Femto users and FBS), interference detection mechanism executes theload balancing stage by relocating the users to other FBSs according to their better mutualrelationship. Load balancing is a centralized technique that requires neighboring relationamong the femtocell and macrocell. The radio network controller (RNC) assists both cells tocommunicate through it to share information. Such a network helps to distribute the trafficload among the neighboring cells. In some cases, excess interaction among the cells throughthe network node might result in a delay response and signaling congestion during the peakhour of the network.

4.4 Cognitive Approach

Cognitive radio is considered as a promising technique in wireless network these days. Itintroduces a set of concept that offers cognitive and operational logic to get aware of theexisting radio environment. It also enables the equipment to have the “autonomy” of theavailable resources [58]. The major effort is given in physical layer enhancement; degreesof spectrum reuse, IP based backhaul modification and wider spectrum adoption. Cognitiveradio network assists a radio base station to gain an optimum performance by adoptingand analyzing the radio environment [59]. It guarantees an enhanced QoS by utilizing theresources that are not occupied by other base stations. Sensing and collecting informationfrom the surrounding environment, it compares it with the previous data and improves theconsequent behavior of the system [60]. Figure 7 illustrates the operation of a cognitivefemtocell operation. Since the femtocell are supposed to have cognitive sense, they are used tocategorize the user class or in some cases, select the service area based on the interference levelin the network. There are mainly two modes for spectrum sharing in cognitive radio network:Underlay mode and Overlay mode. In underlay mode, the FBSs can use the spectrums as longas the interference is under a predefined threshold. In the overlay mode, the FBS can use thespectrums only when they are not occupied by the nearby network [61]. Using evolutionarygame concept and reinforcement learning algorithm, authors in [62] proposed a cell selectiontechnique in access control femtocell network. Here the users gradually improves their utilityfunction in selecting the best cell to reach the evolutionary state with the help of learningalgorithm. Authors in [63] presented a closed access mode (femtocell) cognitive interferencemitigation scheme using a LTE-A system level simulator. The scheme is differentiated withfrequency reuse techniques. Cognitive feature handles both the co-layer and cross-layerinterference using cognitive resource allocation scheme in the downlink. In cellular system,degrees of spectrum reuse are measured by frequency reuse factor, which might be 1, 3, 4,7 and so on. It is determined based on how much co-channel interference the network cantolerate. In [64], authors studied a flexible spectrum reuse scheme for femtocell network. The

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Fig. 7 Cognitive femtocell operation

concept is, it uses adaptive frequency reuse factor according to the position of the femtocellin the macrocell network.

The concept of Cognitive Radio Resource Management (CRRM) in [65] demonstrateshow the cognitive system allows the femto network to sense the periodic channel and toestimate the radio usage of the macro-network. This CRRM categorizes the QoS capabilityof the system by allocating and optimizing the available resources. In [66] a cognitive radioarchitecture is described which enables opportunistic multi-tier access in advanced cellu-lar wireless systems. The proposed architecture is a combination of conventional femtocellideas with infrastructure-based overlay cognitive radio. A decentralized cognitive approachis shown in article [41] where dynamic resource allocation technique is presented to enhancethe spectrum efficiency. Using cognitive concept to assign femtocell network with more chan-nel allocation, authors in [67] focused on sharing spectrum band not only from macrocellbut also from other licensed systems like TV system. Employing mixed primal and dualdecomposition methods in downlink spectrum sharing, they looked for capacity incrementand of course reducing interference. Observation of SINR of both macrocell and femtocellare read to know about the system and to take decision of the effected frequencies whichis one of the common phase of cognitive radio. After getting the feedback from the basestation, the system learns about the probability distribution of the transmission configura-tion such that a minimum time-average SINR can be guaranteed in the macrocell at theequilibrium. In [68], authors introduced an equilibrium concept combining the game theoryand stochastic approximation. After collecting the surrounding data, the system optimizesthe network by tuning the frequency bands and power levels. Similar concept is formulatedin [69] where an energy saving feature of spectrum sharing and power allocation is stud-

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ied in three steps. In the first step, the primary network offers the spectrum-selling price tothe cognitive base station. In the second step, cognitive base station decides to acquire thespectrums from the primary network and assigns them to femtocells or macrocell secondaryusers. In the third step, the femtocell base station performs power allocation for the fem-tocell secondary users. Cognitive approach has a great prospect in future wireless network.It’s an effective method in decentralized technique where femtocell can configure and oper-ate in an ad-hoc manner without proper radio planning and pre-knowledge about its nearbyenvironment.

4.5 Q-Learning

Q-learning is a corroboration learning framework that build-up a concept about the existingenvironment by using an action-value function. It gives the expected utility of taking agiven action in a given state and following a fixed policy thereafter [70]. The main featureof Q-learning is: it compares the predictable efficacy of the obtainable actions without anyreference model of the surrounding environment. Q-learning can be mentioned under the sub-categories of the cognitive radio. However, it has a better efficiency than the typical cognitiveradio approaches. A distributed reinforcement-learning framework is analyzed in [71] wherea macrocell network is supported by femtocells sharing the same spectrum. Each FBS interactwith the surrounding environment and gradually learns about the system through trails anderror process. After understanding the system very well, it adapts intelligent channel selectionprocess to minimize the interference problem due to sharing of the spectrum. This processalso mitigates the interference towards the macrocell network. Authors in [72] discussedabout a decentralized Q-learning algorithm by estimating the behavior and the intelligencelevel of the femtocell based on intelligent and self-organizing network concept. Here newlyactivated femtocells share the updated expertness of the femtocell that they measure and storeafter a certain time. As in Q-learning based approach, it may take more time and iterations torealize the environment. Therefore, this dynamic learning method speeds up the process byknowledge exchange. In normal femtocell network, the cell is being selected without knowingmuch about its behaviors. In [70], authors demonstrated modern trade-off model where cellselection is done predicting their impending future state based on Q-learning algorithm. Auser capacity based handover executing model was discussed in [73]. User had equipmentdetermine the level of service it can deal at a time. At first, the authors studied the case withonly one user performing a handover in single-agent multiple-state RL with adaptive actionselection (SAMSRL-AAS). After that, they studied the multiple user performing handovernamed as MAMSRL-SS (Multiple-Agent Multiple-state RL with Soft-max Selection). In[74], authors tried to solve the interference problem with power management technique basedon decentralized Q-learning: real-time multi-agent reinforcement learning which directlyinteracts with the environment by unitizing the past experiences. The model engages eachfemtocell as an agent in the multi-agent network (femtocell network) which is in-charge ofmanaging the radio resources to their users. Another model [75] shows the same concept butit extended to share some of the experience during the next learning session in order to speed-up their performance. In general, Q-learning represents the cognitive approach but it focusesmainly on the machine learning process. The femtocell learns from the previous performancesby analysis the network parameters. This approach is more effective than a general cognitiveapproach since it utilizes the previous experiences of the femtocell. However, it comes alongwith a complicated network architect in a dense femtocell network.

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A Review on Femtocell and its Diverse Interference Mitigation Techniques 101

4.6 Comparison

As discussed earlier, plenty of strategies are formatted and some of them are already beenimplemented in heterogeneous network. There are also other schemes which focus on otheraspect like CDMA hoping adaptive frequency use [76], self-optimizing network [77], han-dover [78], optimized admission control scheme [79] etc. Like this a multi antenna method isdemonstrated in [80] that controls the relationship between the user and the adjacent closedfemtocell for the mitigation of co-channel interference using the suboptimal methods thatuses beam forming method in downlink. Through the analysis, it is shown that practicaltransmission beam forming methods can be used to suppress the interference in adjacentFBSs. To improve the capacity and energy efficiency of the FBS, cell biasing techniques areproposed in article [81] and [82]. Cell biasing is applied to offload a desired portion of thefemtocell so that the load can evenly managed and total network capacity gets improved.A price based resource allocation scheme for two-tier networks is introduced in [83] wheretwo practical femtocell network simulations were scrutinized: “densely deployed situation”which is for urban areas and “sparsely deployed situation” which is for rural areas. Themacrocell interference-mitigation strategy is based on the pricing of interference levels. Formultiuser wireless communication, an interference tolerance OFDMA scheme with specificapplication is proposed in [84]. Each user gets an interleaved set of dedicated sub-carrierand a proper cyclic extension provides the opportunity to mitigate both kind of interfer-ences. Finally a “distributed interference price bargaining” algorithm is formulated in [85]using Stackelberg game. In Table 1, a brief comparison of different interference mitigationtechniques is presented.

5 Conclusions

Femtocell is a promising technology for the next generation wireless network. It ensuresproper utilization of the radio resources of a network by increasing spectral efficiency andproviding better QoS, especially in indoor situations. Femtocell fills the coverage gaps andenhance the capacity of heterogeneous network even though the true potential of femtocellis yet to be fully discovered. Currently the implementation of femtocell technology facescertain issues such as latency problems, due to the backhaul connection via internet, which isquite important for delay sensitive multimedia services, handover signaling, and multimediastream routing to support mobility. In this paper reviews some of the interference mitigationtechniques in two-tier network. Successful mitigation of interference results in a strongconnection among neighboring femtocells and macrocell that results in a stronger coverageand smoother service. Due to its key features, like low cost, high speed, plug and play etc.,femtocell influences the mobile operators to look for vast deployment if the interferenceissues can be handled properly. However the contributions are not still enough and needmore ideas need to be developed. The use of femtocell in designing smart home system andcontrolling electronic devices will diversify the use of mobile under femtocell coverage andwill make more revenue for the operators.

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Afaz Uddin Ahmed doing M.Sc. on wireless communication and alsoworking as a research assistant in Universiti Kebangsaan Malaysia(UKM). He received his BSc in Electrical and Electronic Engineeringfrom Chittagong University of Engineering and Technology (CUET),Bangladesh. He also worked in Ericsson Bangladesh, Banglalink GSM(ORASCOM Telecom Bangladesh Ltd.) and GMG Airlines Ltd. Hisfield of interest is LTE, heterogeneous network, Cognitive radio andnext generation wireless communication system. He is a student mem-ber of IEEE and associate member of IEB (Institute of Engineers,Bangladesh).

Mohammad Tariqul Islam is a Professor at the Center for Space Sci-ence of the Universiti Kebangsaan Malaysia (UKM). He is also thegroup leader of Radio astronomy Informatics group at UKM. Prior tojoining UKM, he was a lecturer in Multimedia University, Malaysia.He is a senior member of the IEEE, regular member of Applied Com-putational Electromagnetic Society (ACES) and serving as the Editor-in-Chief of the International Journal of Electronics and Informatics(IJEI). Prof. Tariqul has been very promising as a researcher, with theachievement of several International Gold Medal awards, a Best Inven-tion in Telecommunication award and a Special Award from Vietnamfor his research and innovation. Over the years, he has carried outresearch in the areas of communication antenna design, radio astron-omy antennas, satellite antennas, and electromagnetic radiation analy-sis. His publication include over 180 research journal papers, nearly150 conference papers, and few book chapters on various topics relatedto antennas, microwaves and electromagnetic radiation analysis with 7

inventory patents filed. Thus far, his publications have been cited 1112 times, and the H-index is 20 (Source:Scopus). For his contributions, he has been awarded “Best Researcher Award” in 2010 and 2011 at UKM. He

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is now handling many research projects from the Ministry of Science, Technology, and Innovation (MOSTI),Ministry of Higher Education Malaysia (MOHE) and some International research grants from Japan.

Mahamod Ismail joined the Department of Electrical, Electronics andSystem Engineering, Faculty of Engineering and Built Environment,Universiti Kebangsaan Malaysia (UKM) in 1985. He is currently theHead of the Department and a Professor in Communication Engineer-ing. He received the B.Sc. degree in Electrical and Electronics fromUniversity of Strathclyde, U.K. in 1985, the M.Sc. degree in Com-munication Engineering and Digital Electronics from University ofManchester Institute of Science and Technology (UMIST), ManchesterU.K. in 1987, and the Ph.D. from University of Bradford, U.K. in 1996.He was with the first Malaysia Microsatellite TiungSat Team Engineersin Surrey Satellite Technology Ltd. U.K. from June 1997 until March1998. In the summer semester 2003, he served as a Guest Professorin Computer Engineering in the University of Duisburg-Essen, Duis-burg Germany funded by the German Academic Exchange Services(DAAD). His research interests include mobile and satellite communi-cation, and wireless networking particularly on the radio resource man-

agement for the next generation wireless communication network. He served as Chair of the Institute of Elec-trical and Electronics Engineers (IEEE), USA Malaysia Section from 2011 to 2012.

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