FemtoCells

1A Comparative Study of Different DeploymentModes for Femtocell Networks

Hisham A. Mahmoud and Ismail GuvencDOCOMO Communications Laboratories USA, Inc., 3240 Hillview Avenue, Palo Alto, CA, 94304

Email: {hmahmoud, iguvenc}@docomolabs-usa.com

Abstract Femtocell networks have the potential of signifi-cantly improving the capacity of next-generation cellular systems.However, interference to/from the macrocell network is a criticalproblem affecting the capacity. Co-channel operation with closed-subscriber group (CSG) access offers a large bandwidth to beshared by the users of the femtocell network; however, it alsointroduces significant interference to/from the macrocell network.On the other hand, femtocell networks using dedicated spectrumassignment and open access mode have insignificant interferenceconcerns, with the trade-off of available spectrum limitations.In this study, we investigate interference vs. bandwidth trade-offs in different types of femtocell network deployments throughthe help of channel capacity formulations. Related downlinksimulation results are provided to compare the capacities ofdifferent deployment modes and the impact of key systemparameters.

Index Terms Adjacent channel, capacity CDF, co-channel,CSG, dedicated channel, femto-cell, femtocell, HeNB, HomeNodeB, HNB, LTE, open access, power control.

I. INTRODUCTIONFemtocell networks have recently received considerable

attention from industry and academia due to their tremendouspotential for capacity improvements [1], [2]. Some otherbenefits of femtocells include improvements in cellular net-work coverage (especially indoors), capability of working withexisting handsets, longer battery lives for cell-phones dueto shorter communication ranges, and enhanced emergencyservices. However, interference between the femtocell andmacrocells, and between neighboring femtocells, remain to bea key problem that needs to be addressed [3].

How interference affects the system performance is directlyrelated to the deployment configurations of femtocells. In [4],three different deployment criteria have been specified forfemtocell networks: 1) Dedicated channel vs. co-channel,2) Open access vs. closed subscriber group (CSG), and 3)Fixed downlink (DL) transmit power vs. adaptive DL transmitpower. The prior-art works related to femtocells typicallyinvestigate the throughput/capacity performance of femtocellsand/or macrocells under one of these three deployment criteria.For example, comparison of public (open) access and private(CSG) access femtocells has been presented in [1], [5], [6]through computer simulations. Co-channel deployments offemtocells have been preferred compared to dedicated channeldeployments in [5]-[8]. Spectrum allocation for femtocellshave been discussed in [9], [10], while different power controltechniques have been proposed in [6], [11], [12]. A compre-hensive collection of simulation results for several importantinterference scenarios of interest area documented in [3].

Despite several studies, to our best knowledge, a compar-ative study that captures several trade-offs among differentfemtocell deployment criteria is not available in the prior-art.The goal of this paper is to provide a comparative study ofthe three different femtocell deployment modes through thehelp of channel capacity formulations, and discuss various

Fig. 1. Dedicated channel operation vs. co-channel operation of femtocelland macrocell networks.

related trade-offs. Moreover, comprehensive set of simulationresults in realistic macrocell/femtocell settings and channelmodels will be presented to discuss the impact of key designparameters.

II. CAPACITY OF MACROCELL USERS WITHOUT ANYFEMTOCELLS

Consider a macrocell network, where there are M macrocellmobile stations (mMSs) communicating with a macrocell basestation (mBS). Let Bmac denote the bandwidth of the spectrumavailable for the macrocell network. Also, consider a simplescheduler that assigns equal bandwidth Bm,i = Bmac/M toeach of the mMSs. Then, in the absence of any femtocells,the capacity of an mMS can be written as

C(nf)m,i =

Bmac

Mlog

(1 +

MPm,iBmacN0

), (1)

where (nf) refers to a no-femtocell scenario, Pm,i is thereceived power for the ith mMS, and N0 denotes the noiselevel.

As apparent from (1), the capacity of mMSs will improvewith smaller M , larger Bmac, and larger Pm,i. Therefore, thecapacity of the indoor users will be relatively lower comparedto outdoor users; they suffer from wall penetration loss, whichresults in lower Pm,i values. This problem may be handledthrough deploying femtocells within indoor locations, whichwill result in larger received powers (and in typical settings,larger bandwidths) for indoor users, yielding higher channelcapacities. In the next section, we will review three differentfemtocell deployment modes and investigate how they affectthe channel capacity for macrocell and femtocell users.

978-1-4244-5213-4/09/ $26.00 2009 IEEE

2III. FEMTOCELL DEPLOYMENT SCENARIOSA. Dedicated Channel vs. Co-channel

1) Dedicated Channel Deployment: For dedicated channelassignments, femtocells are assigned a separate spectrum (ofbandwidth Bfem) than that of the macrocell, as illustratedin Fig. 1(a). Even though this mostly eliminates potentialinterference from the macrocell1, frequency resources are notefficiently utilized. The capacity of an mMS with dedicatedchannel assignment can be written as

C(dc)m,i =

Bmac BfemM

log(1 +

MPm,i

(Bmac Bfem)N0

), (2)

where (dc) refers to a dedicated channel deployment scenario,i is the index for the mMS, Bfem denotes the bandwidthdedicated to the femtocell networks, and M < M is thenumber of mobile stations (MSs) associated with the macro-cell2. Comparing (2) with (1), it is observed that with theintroduction of femtocells there is less available spectrum forthe macrocell network. However, also, M M of the usersare shifted to the femtocell networks, and they no longer usethe macrocells frequency resources. Hence, in general, thecapacity of macrocell users may improve for smaller valuesof Bfem and M .

On the other hand, capacity of a femtocell mobile station(fMS) with dedicated channel assignment can be written as

C(dc)f,i =

BfemN

log(1 +

NPf,i

BfemN0

), (3)

where N is the number of users per femtocell and Pf,i isthe received signal power from the femtocell base station(fBS). Comparing (3) with (1), even though the bandwidthper user

(Bf,i = BfemN

)may be similar to the bandwidth of

indoor macrocell users without any femtocell deployment, thereceived powers Pf,i would typically improve significantlywith femtocell deployments, hence improving the capacity ofindoor users.

2) Co-channel Deployment: Co-channel deployment offemtocells enables more efficient utilization of the availablespectrum. As illustrated in Fig. 1(b), both the macrocell andfemtocell will have larger bandwidth available per user withco-channel deployments (Bm,i > Bm,i and Bf,i > Bf,i,respectively). Moreover, cell-search process for an mMS be-comes easier since it will not have to search cells in differentfrequency bands (e.g., for handover purposes). However, fem-tocells and the macrocell will observe co-channel interferencefrom each other.

The channel capacity of an mMS with co-channel femtocelldeployment can be written as

C(cc)m,i =

BmacM

log(1 +

Pm,iIfem +BmacN0/M

), (4)

where (cc) refers to a co-channel deployment scenario andIfem is the total interference observed from all the nearbyfemtocell networks. Comparing (4) with (2), we observe thatthe bandwidth available per user improves with co-channel

1There may still be co-channel interference between neighboring fem-tocells, and adjacent channel interference between the macrocell and thefemtocells. Even though we neglect both in capacity formulations withinthis section, impact of inter-femtocell interference will be taken into accountduring simulation results.

2After deploying femtocells in the macrocells, some of the MSs that wereoriginally associated with the macrocell are assumed to make hand-off tofemtocells due to better signal quality.

deployment (i.e., Bm,i > Bm,i). However, the mMSs also ob-serve interference Ifem from nearby femtocell networks, whichmay degrade the capacity if it is significant. Hence, whetherthe capacity improves or not with respect to a dedicatedchannel scenario depends jointly on Bfem and Ifem. Similarly,comparing (4) with (1), whether the capacity improves or notwith respect to a no-femtocell scenario depends jointly on Mand Ifem.

On the other hand, the capacity of an fMS with co-channeldeployment can be written as

C(cc)f,i =

BfemN

log(1 +

Pf,iImac +BfemN0/N

), (5)

where Bfem = Bmac Bfem, which implies significantincrease in available bandwidth per femtocell user. This comesat the expense of interference Imac observed from macrocellusers and the mBS. Since bandwidth affects the channelcapacity linearly, and interference affects the channel capacitythrough the logarithm function, as also discussed by severalother work in the literature ([2], [5], [6], [8]) co-channeldeployments of femtocells typically result in better overallcapacities compared to dedicated channel deployments. Thiswill be verified through computer simulations in Section IV.

B. Open Access vs. Closed Subscriber Group (CSG)For open access femtocell networks, any mMS is allowed

to join a particular femtocell network. For CSG type offemtocells, on the other hand, the mMSs that may join to aparticular femtocell network are restricted to a certain group.Therefore, for the CSG mode, a particular femtocell mayreceive significant interference from (and cause significantinterference to) a close-by co-channel mMS since it will notbe granted admission to the femtocell [1], [5], [6].

How the channel capacity changes for CSG and openaccess modes can be interpreted using equations (4) and (5).For the open access mode, a femtocell will be serving to alarger number of fMSs since some close-by mMSs will bemaking hand-off to the femtocell. Therefore, based on (5),the bandwidth available per fMS user

(BfemN

)will be smaller.

However, those mMSs that join the femtocell would typicallybe the ones that were causing the strongest interference to thefemtocell. Hence, interference term Imac will also decreasesignificantly for the open-access mode and only the far-awaymMSs will be still causing interference. This may typicallycompensate for the bandwidth reduction per fMS and improvethe femtocell capacity. From macrocell networks perspective,based on (4), it is easy to see that open access mode willincrease the available bandwidth per macrocell user

(BmacM

),

improving the capacity of remaining users associated with themacrocell.

Example simulation results in [1], [5], [6] show that openaccess mode yields better overall system throughput andcoverage, while [1] also shows that CSG results in largerareal capacity gains in general (defined as the ratio of systemcapacity with femtocells to the system capacity without anyfemtocells). Note that compared to CSG mode, open accessoperation may also have several concerns such as privacyissues, extra burden on the backhaul of a femtocells owner,etc. In Section IV, we will investigate when CSG may bepreferable over open access mode through a simple example.

C. Fixed DL Transmit Power vs. Adaptive DL Transmit PowerFor fixed transmission power, the fBS fixes its maximum

transmission power to a pre-determined value, which is typi-

3TABLE ISIMULATION PARAMETERS.

Parameter ValueCentral frequency 2.1 GHzBandwidth 5 MHzCoverage (radius) (mBS, fBS) 0.5 km, 8 mMaximum transmit power (mBS, fBS) 41.75 dBm, 20 dBmThermal noise density 174 dBm/HzWall penetration loss (WL) 10 dB, 20 dBAntenna gain (mBS, fBS) 17 dBi, 2 dBiFeeder/cable loss (mBS, fBS) 3 dB, 1 dBAntenna heights (mBS, fBS, MS) 15 m, 1.5 m, 1.5 mHouse size 15 m 15 mStreet width 10 mDistance between grid points 1.7 mNumber of users per macrocell 150Indoor area vs. outdoor area 18% vs. 82%Scheduling strategy Equal bandwidth per userIndoor to indoor path loss model ITU P.1238Indoor to outdoor path loss model ITU P.1411 + Wall LossOutdoor to outdoor path loss model ITU P.1411Outdoor to indoor path loss model ITU P.1411 + Wall Loss

cally set as 13 dBm or 20 dBm. With adaptive transmissionpower, on the other hand, the fBS may adjust its transmitpower based on the interference caused/received to/from themacrocell and other neighboring femtocells. For example,femtocells closer to the mBS may transmit at the maximumpower level (due to significant interference from the mBS),while the femtocells closer to the cell edge may decrease theirtransmit powers since the mBS interference will be weaker.Based on (4), reduction of the fBS transmit power would alsoreduce the interference Ifem to the macrocell network. Thisresults in larger capacity for macrocell users at the expense ofsome decrease in the capacity of certain femtocell networkswith lower transmit powers. A typical approach for powercontrol is to set the fBS transmit power so that signal tointerference ratio (SIR) is equal to 0 dB at the borders ofthe femtocell [6], [13], which is also adopted in this paper.

IV. SIMULATION RESULTSComputer simulations are made to evaluate several trade-

offs discussed in Section III. Simulation parameters are se-lected based mostly on [3] and key set of parameters aresummarized in Table I. The downlink of a macrocell/femtocellscenario as in Fig. 2 is considered with an mBS located inthe center of the cell (shown with a triangle). The buildings(squares) and 150 mobile users (circles3) are uniformly scat-tered over a 600600 grid, within the (hexagonal) borders ofthe macrocell, where 18% of the area is inside the buildingsand 82% of the area is outdoors. Cell selection is based onthe SIR metric, where MSs join the femtocell/macrocell thathas the best SIR. In all simulations, idle femtocells (with nousers) are detected and disabled (i.e., their transmit power areset to zero) to minimize interference.

Based on the fixed building/macrocell layout in Fig. 2and simulation parameters in Table II, cumulative distributionfunctions (CDFs) of the capacities of indoor users, outdoorusers, and all users are plotted for different scenarios. First,Fig. 3 includes the capacity plots in the absence of anyfemtocells and for two different wall penetration losses 10 dBand 20 dB, where the results are averaged over 50 differentrealizations of user locations. Results show that compared tooutdoor users, the capacity of indoor users suffer significantly

3Brown circles for indoor users and green circles for outdoor users.

Fig. 2. Macrocell/femtocell simulation scenario in consideration. Receivedpower from the mBS (in dBm/Hz) is also illustrated.

0.8 1 1.2 1.4 1.6 1.8

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Capacity (Mbit/sec)

Capa

city

CDF

Indoor users (WL = 20 dB)Indoor users (WL = 10 dB)All users (WL = 20 dB)All users (WL = 10 dB)Outdoor users (WL = 20 dB)Outdoor users (WL = 10 dB)

IndoorUsers

OutdoorUsers

AllUsers

Fig. 3. Comparison of indoor and outdoor capacity CDFs with no femtocells.

from the wall penetration loss, which can be addressed throughdeploying femtocells. In the upcoming subsections, capacityimprovements due to introducing femtocells with differentdeployment modes will be demonstrated through simulations.

A. Dedicated Channel vs. Co-ChannelFig. 4 and Fig. 5 compare the capacity CDFs of the

outdoor and indoor users for dedicated channel and co-channeldeployments, respectively, using an open access mode ofoperation and without any power control at femtocells. InFig. 4, a dedicated channel femtocell deployment scenariois considered, where it is assumed that all the buildingshave femtocells (100% femtocell penetration), there is nointerference between femtocells and the macrocell, while thereoccurs interference among neighboring femtocells. Macrocellusers use 90% of the available bandwidth Bmac, while 10% ofthe bandwidth is assigned to femtocell users. The bandwidth isequally distributed among the users of the femtocells and themacrocell based on the number of users associated with each

45 10 15 20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Capacity (Mbit/sec)

Capa

city

CDF

Outdoor users (WL = 10 dB)Outdoor users (WL = 20 dB)All users (WL = 10 dB)All users (WL = 20 dB)Indoor users (WL = 10 dB)Indoor users (WL = 20 dB)

Outdoorusers

Allusers

Indoorusers

Fig. 4. Comparison of indoor and outdoor capacity CDFs for dedicatedchannel femtocell deployments.

cell. Results in Fig. 4 show significant capacity improvementof indoor users compared to Fig. 3 (no femtocells), especiallywhen the wall penetration loss is large. This is becausefor larger WL, interference from neighboring femtocells islower, yielding larger signal-to-interference (SIR) ratios atfemtocells4.

In Fig. 5, on the other hand, a co-channel deploymentscenario is considered for femtocells. In addition to inter-femtocell interference, we also consider interference betweenthe femtocells and the macrocell for co-channel deployment.However, available bandwidth per femtocell user is signifi-cantly improved, since femtocells have access to the wholespectrum that is used by the macrocell (see e.g., (5)). Resultsin Fig. 5 show significant gains compared to dedicated channeldeployment results in Fig. 4. This verifies our discussion inSection III-A that in general, the large bandwidth gain com-pensates for the increase in interference, making co-channeldeployments preferable over dedicated channel deployments.

B. CSG vs. Open AccessA simple single-femtocell simulation scenario as in Fig. 6(a)

is considered for the comparison of channel capacities withopen access and CSG modes of co-channel femtocells. Theuplink (UL) transmission of an fMS at a distance 7 meters tothe fBS is considered, with fBS being located at the center ofa 15 m 15 m apartment. The UL signal received at the fBSis interfered by Nm mMS signals, located at a distance d2 tothe fBS. If the femtocell operates in open access mode, thesemMSs are accepted to and served by the femtocell, sharing itsbandwidth. Otherwise, with CSG mode of operation, they areserved by the macrocell, causing interference to the femtocellusers.

Mean capacities for the users associated with the femtocellare plotted in Fig. 6(b) for open access and CSG modes,for Nm = 1, 2, 3, 4 and d2 [3, 6, ..., 39] meters. When themMS(s) are located within the borders of the apartment, thereis significant UL interference in the CSG mode, which resultsin poor channel capacities for the femtocell users. On the otherhand, in open access mode, the channel capacity with indoor

4Note that the staircase type of behavior of the CDF is due to gridresolution, which amounts to a 9 9 gird within the buildings.

20 40 60 80 100 120

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Capacity (Mbit/sec)

Capa

city

CDF


OutdoorUsers

AllUsers

IndoorUsers

Fig. 5. Comparison of indoor and outdoor capacity CDFs for co-channelfemtocell deployments.

mMSs is acceptable even with large number of mMSs. Forthe mMSs located outdoors, CSG mode of operation becomespreferable for longer distances between the fBS and mMSs,and for larger number of interfering mMSs.

C. Impact of Power ControlTo test the performance of femtocells under adaptive power

control, the following setup was employed. Femtocell transmitpower is adjusted to maintain an SIR level of 0 dB (withrespect to macrocell signal) at edge of the coverage area. Theminimum femtocell range is set to 8 m with the assumptionthat the coverage area is indoors. Thus, the SIR just outsidethe building is 20 dB or 40 dB for WL of 10 dB or 20 dB,respectively. This setup reduces interference to macrocellusers, especially users in proximity of femtocells located nearthe edge of the macrocell area.

The impact of power control on the capacity CDFs ofmacrocell and femtocell users is illustrated in Fig. 7. Whencompared with the results in Fig. 5, we note that adaptivepower control can, as expected, improve the macrocell/outdooruser capacity due to reduction in interference levels fromfemtocells. However, the degradation to femtocell users, repre-sented by indoor users in this case, is clearly more significant.This is explained by the fact that macrocell users only usea small fraction of the total bandwidth when compared tofemtocell users who typically utilize 50% 100% of totalbandwidth (1 2 users per femtocell). As such, the reductionin capacity in femtocells are more sensitive to the SIR levelwhen compared to macrocell users.

The median capacities for the three simulation scenariosin Figs. 3-5 and in Fig. 7 are summarized in Table II forindoor users, outdoor users, and all users. Results showthat co-channel femtocell deployments result in tremendouscapacity gains for indoor users, which improve with largerwall penetration loss values. This comes at the expense ofinsignificant capacity degradation for outdoor users due tointerference from femtocells.

V. CONCLUSIONIn this paper, different deployment modes for femtocell net-

works are compared through the help of capacity formulations

5TABLE IICOMPARISON OF MEDIAN CAPACITIES WITH AND WITHOUT FEMTOCELLS.

No Femtocells With Femto (dc) With Femto (cc, no pc) With Femto (cc, with pc)All users (WL = 20 dB) 1.15 Mbps 4.13 Mbps 14.2 Mbps 5.11 MbpsIndoor users (WL = 20 dB) 0.80 Mbps 16.2 Mbps 72.2 Mbps 21.7 MbpsOutdoor users (WL = 20 dB) 1.23 Mbps 1.35 Mbps 1.23 Mbps 1.41 MbpsAll users (WL = 10 dB) 1.19 Mbps 1.23 Mbps 8.60 Mbps 4.74 MbpsIndoor users (WL = 10 dB) 1.03 Mbps 1.70 Mbps 42.9 Mbps 20.9 MbpsOutdoor users (WL = 10 dB) 1.23 Mbps 1.12 Mbps 0.98 Mbps 1.15 Mbps

(a) Simulation scenario for the comparison of open access andCSG types of femtocell access modes.

5 10 15 20 25 30 35

1

2

3

4

5

6

x 107

d2 (meters)

Mea

n Ca

pacit

y of

fMS

User

(bps

)

CSG mode (Nm

=1)Open access mode (N

m=1)

CSG mode (Nm


m=2)

CSG mode (Nm


m=3)

CSG mode (Nm


m=4)

Apartment Wall

(b) Comparison of mean capacities for CSG and open access modes.

Fig. 6. Simulation results comparing the mean femtocell UL capacities forCSG and open access femtocell deployments. Different number of mMSslocated at distance d2 to the fBS are considered.

and computer simulations. Bandwidth and interference areshown to be two critical parameters inversely affecting the ca-pacity. However, due to low transmission powers of femtocells,it is typically desirable to have co-channel operation whichfavors bandwidth gains over interference-free communications.In cases where interference is intolerable (e.g., for mMSsinside a femtocell), open access mode is preferable over CSGtype of operation. Finally, femtocell power control can beutilized for limiting the amount of interference caused tothe macrocell users, which comes at the expense of somedegradation in the femtocell capacity.

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[2] V. Chandrasekhar, J. G. Andrews, and A. Gatherer, Femtocell networks:a survey, IEEE Commun. Mag., vol. 46, no. 9, pp. 5967, Sep. 2008.

10 20 30 40 50 60 70 80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Capacity (Mbit/sec)

Capa

city

CDF


OutdoorUsers

AllUsers

IndoorUsers

Fig. 7. Comparison of indoor and outdoor capacity CDFs for co-channelfemtocell deployments with power control (pc).

[3] FemtoForum, Interference management in UMTS fem-tocells, White Paper, Dec. 2008. [Online]. Available:http://www.femtoforum.org/femto/Files/File/Interference Managementin UMTS Femtocells.pdf

[4] 3rd Generation Partnership Project; Technical Specification GroupRadio Access Networks; 3G Home NodeB Study Item Technical Report(Release 8), 3GPP, 3GPP TR 25.820, March 2008.

[5] D. L. Perez, A. Valcarce, G. D. L. Roche, E. Liu, and J. Zhang;, Accessmethods to WiMAX femtocells: A downlink system-level case study, inProc. IEEE Int. Conf. Commun. Syst. (ICCS), Guangzhou, China, Nov.2008, pp. 16571662.

[6] H. Claussen, Performance of macro- and co-channel femtocells in ahierarchical cell structure, in Proc. IEEE Int. Symp. Personal, Indoor,Mobile Radio Commun. (PIMRC), Athens, Greece, Sep. 2007, pp. 15.

[7] L. T. W. Ho and H. Claussen, Effects of user-deployed, co-channelfemtocells on the call drop probability in a residential scenario, in Proc.IEEE Int. Symp. Personal, Indoor, Mobile Radio Commun. (PIMRC),Athens, Greece, Sep. 2007, pp. 15.

[8] V. Chandrasekhar and J. G. Andrews, Uplink capacity and interfer-ence avoidance for two-tier cellular networks, in Proc. IEEE GlobalTelecommun. Conf. (GLOBECOM), Washington, DC, Nov. 2007, pp.33223326.

[9] D. L. Perez, G. D. L. Roche, A. Valcarce, A. Juttner, and J. Zhang;,Interference avoidance and dynamic frequency planning for WiMAXfemtocells networks, in Proc. IEEE Int. Conf. Commun. Syst. (ICCS),Guangzhou, China, Nov. 2008, pp. 15791584.

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[11] Nokia Siemen Networks, LTE Home Node B downlink simulationresults with flexible Home Node B power, Shangai, China, Oct. 2007,3GPP TSG-RAN Working Group 4 (Radio) Meeting. [Online]. Avail-able: ftp://ftp.3gpp.org/tsg ran/WG4 Radio/TSGR4 44bis/Docs/R4-071540.zip

[12] V. Chandrasekhar, T. Muharemovic, Z. Shen, J. G. Andrews, andA. Gatherer, Power control in two-tier femtocell networks, submittedto IEEE Trans. Wireless Commun., 2009.

[13] I. Guvenc, M. R. Jeong, F. Watanabe, and H. Inamura, A hybridfrequency assignment for femtocells and coverage area analysis for co-channel operation, IEEE Commun. Lett., vol. 12, no. 12, pp. 880882,Dec. 2008.

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