Throughput enhancement through femto-cell deployment

EUROPEAN TRANSACTIONS ON TELECOMMUNICATIONSEur. Trans. Telecomms. 2010; 21:469–477Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/ett.1428

Throughput enhancement through femto-cell deployment†

Zubin Bharucha1*, Harald Haas1, Andreas Saul2 and Gunther Auer2

1Institute for Digital Communications, Joint Research Institute for Signal and Image Processing, School of Engineering and Electronics,The University of Edinburgh, Edinburgh EH9 3JL, UK

2DOCOMO Euro-Labs, Landsberger Strasse 312, 80687 Munich, Germany

SUMMARY

This paper studies the impact of a femto-cell underlay deployment that shares radio frequency resourceswith urban macro-cells. Due to their random and uncoordinated deployment, femto-cells potentially causedestructive interference to macro-cells and vice versa. On the other hand, femto-cells promise to substantiallyenhance the spectral efficiency due to an increased reuse of radio resources. The performance of networkswith indoor Home Evolved NodeB (HeNB) deployment is compared to a system where all users, includingindoor users, are served by the outdoor macro Evolved NodeB (eNB). In addition, the impact of closed-access and open-access femto-cell operation is examined. It is demonstrated that significant capacity gainscan be achieved through such HeNB deployment, regardless of whether closed-access or open-access isconsidered. Results clearly indicate that the capacity gains through femto-cell deployment outweigh theadditional interference they introduce. Copyright © 2010 John Wiley & Sons, Ltd.

1. INTRODUCTION

The rapidly increasing demand for increased user and sys-tem capacity in wireless networks is served by higher band-width allocation, but since bandwidth is scarce and expen-sive, a key to substantial capacity enhancement is to improvethe spatial reuse of radio frequency resources. One of themost powerful methods of boosting wireless capacity is byshrinking the cell size. The reason for this is that smallercell sizes effectively increase spatial reuse of spectrum [1].The shorter transmission distances enhance link capacitydue to higher channel gains [2]. While decreasing the cellsize boosts system capacity, the cost involved is becomingincreasingly prohibitive due to the required installation ofnew network infrastructure.

Studies indicate that a large proportion of data trafficoriginates indoors [3]. Poor signal reception caused byhigh penetration losses through walls severely compromisesthe operation of indoor broadband data services. Thus,

* Correspondence to: Zubin Bharucha, DOCOMO Euro-Labs, Landsberger Strasse 312, 80687 Munich, Germany. E-mail: [email protected] Bharucha was formerly with the Institute for Digital Communications, Joint Research Institute for Signal and Image Processing, School ofEngineering and Electronics, The University of Edinburgh, Edinburgh EH9 3JL, UK.†Parts of this paper were presented in the 7th International Workshop on Multi-Carrier System and Solutions (MC-SS 2009), Herrsching, Germany.

the concept of femto-cells has recently attracted consid-erable interest. Third generation (3G) femto-cells are nowbeing commercialised, while the specifications of beyond3G (B3G) femto-cells are currently discussed in standard-isation bodies, such as 3rd generation partnership project(3GPP) Long Term Evolution (LTE). Femto-cells consistof user-deployed HeNBs, which are cheap, low-power andplug-and-play base stations. HeNB deployment extends andimproves macro-cell coverage indoors. As HeNBs are con-nected to the backbone network via cable, they offload in-door users from the macro-cell, thus potentially enhancingthe capacity both indoors as well as outdoors.

A number of proposals have been made in recent years toincrease indoor capacity. In Reference [4], it has been pro-posed that buildings be equipped with distributed antennasin order to improve coverage. However, the implementa-tion of this system requires careful planning. The proposalin Reference [5] calls for the separation of user equipment(UEs) based on their mobility classes and serving them

Received 21 March 2010Copyright © 2010 John Wiley & Sons, Ltd. Accepted 31 March 2010

470 Z. BHARUCHA ET AL.

within micro and macro-cell accordingly. This proposal re-quires dual-mode UEs since different access methods areused, based on the mobility class within which the UE falls.In References [6–9], the authors propose the TDD under-lay concept. Owing to the asymmetric nature of traffic, theunderloaded frequency division duplex (FDD) band can besplit in time such that the HeNB transmits and receivesinformation from its associated UEs in a time division du-plex (TDD) fashion. In this paper, both macro and femto-cell operate in the same radio frequency spectrum in FDDmode, compliant with 3GPP specifications [10]. The HeNBbackhauls data on a wired interface to the cellular operatornetwork.

Through femto-cell deployment, the operator is ableto concentrate on providing better service to the outdoormacro-cell UEs. Another benefit for operators is that broad-band coverage is extended to the indoor environment.

However, HeNBs are deployed without network plan-ning. Since HeNBs operate on the same bands as macro-cell eNBs, their deployment introduces additional interfer-ence [11]. Furthermore, questions regarding security andaccess to the femto-cell are raised with the deployment ofHeNBs. The discussion is therefore taken in the directionof open-access versus closed-access systems. Open-accessallows any UE to connect to the HeNB, whereas closed-access only allows a specified set of UEs to connect to theHeNB. Closed-access systems are therefore more likely toimpose high interference to indoor UEs that lie in closevicinity of a femto-cell, but are not connected to a HeNB.

One of the first studies on femto-cells [12] investigates thegeographic distribution of throughput for a deployment ofopen-access femto-cells in a cellular network. The impact ofHeNB deployment on macro-cell performance is analysedin Reference [13]. The focus of the work presented in thispaper is to conduct a rigorous assessment of the impact offemto-cell deployment on a traditional cellular network. Wealso analyse the differences between open and closed-accesssystems through extensive system-level simulations. A sys-tem without HeNB deployment, where all users (indoors oroutdoors) are exclusively served by macro-cell eNBs (as in atraditional cellular system) serves as the benchmark systemfor comparison. Systems with HeNB deployment are distin-guished between open and closed-access. We demonstratethat HeNB deployment, regardless of the access method,has a clear advantage over the benchmark system in termsof user and overall system capacity.

The remainder of this paper is organised as follows. Thefollowing section describes the system model and simulatorsetup. Thereafter simulation results are presented and finalsection highlights the key findings.

2. SYSTEM MODEL AND SIMULATION SETUP

The system bandwidth B is divided into N resource blocks(RBs), B = NBRB, where the RB represents the basic or-thogonal frequency division multiple access (OFDMA)time-frequency unit and BRB is the bandwidth of one RB.The set of available RBs,N , with cardinality |N |=N, is dis-tributed uniformly by eNBs among their associated macroUEs. Within the femto-cells, for closed-access systems, theHeNB allocates all RBs to the femto UE. In the case ofopen-access systems, if additional UEs from the macro-cellare assimilated into the femto-cell, the RBs are uniformlydistributed among all the UEs served by that HeNB. The re-ceived signal power observed by receiver r from transmittert (in either uplink or downlink) at RB n is given by

Yrn = Gr, t

n Ptn + Ir

n + η (1)

where Gr, tn is the channel gain between r and t, and Ir

n ac-counts for the interference observed at RB n by the receiverr. The thermal noise per RB is represented by η. The trans-mit power per RB, Pt

n, varies depending on the transmitter,i.e. a (H)eNB or a macro UE or a femto UE.

2.1. Interference and spectral efficiency calculation

Universal frequency reuse is assumed such that every macroand femto-cell utilises the entire system bandwidth, B.As macro-cell and femto-cell share the available time-frequency resources, the aggregate interference, Ir

n, is com-posed of macro-cell and femto-cell interference

Irn =

∑i∈Mint

Gr, in Pm

n +∑

j∈Fint

Gr, jn P f

n (2)

The sets of macro and femto-cell interfering eNBs andHeNBs are denoted by Mint and Fint, respectively. In thedownlink, a UE suffers from interference from neighbour-ing HeNBs and eNBs. Likewise, in the uplink, a (H)eNBexperiences interference from macro and femto UEs in ad-jacent cells. In the uplink, Pm

n denotes macro UE transmitpower and in the downlink, it denotes eNB transmit power.Similarly, P f

n denotes femto UE and HeNB transmit powerin the uplink and downlink, respectively. In particular, forclosed-access systems, macro UEs lying within the cover-age of a femto-cell are exposed to severe femto interferenceby the HeNB in the downlink, while HeNBs lying withinthe coverage of macro UEs are exposed to severe macro UEinterference in the uplink. Figure 1 highlights how the inter-ference situations change for different access modes. The

Copyright © 2010 John Wiley & Sons, Ltd. Eur. Trans. Telecomms. 2010; 21:469–477DOI: 10.1002/ett

THROUGHPUT ENHANCEMENT THROUGH FEMTO-CELL DEPLOYMENT 471

Figure 1. The different access methods and the benchmark sys-tem. Interference scenarios are highlighted with dashed lines.

signal-to-interference-plus-noise ratio (SINR) observed byreceiver r amounts to

γrn = Gr, t

n Ptn

Irn + η

(3)

where Ptn denotes the transmit power of transmitter t on RB

n. Given the SINR and the set of RBs used by r, Nr, thecapacity, Cr, of receiver r is calculated using the Shannonbound

Cr =∑n∈Nr

BRB log2

(1 + γr

n

)(4)

Since the eNB must distribute the set of RBs, N , among itsassociated UEs, each macro UE gets allocated a subset of thetotal number of RBs. However, since every closed-accessHeNB serves exactly one femto UE, this UE is allocatedall RBs, N . In the case of an open-access femto-cell, theHeNB needs to share the available RBs between the femtoUE and, in addition, the assimilated macro UE(s). There-fore, in the uplink, for closed-access systems, the macroUE interferes with a subset of the RBs as seen by theHeNB. In the downlink, since the HeNB utilises all RBs, themacro UE experiences interference on all its allocated RBs,Nr.

2.2. Uplink power control

Power control is applied in the uplink in both the femtoand macro layer. According to Reference [14], the transmitpower per RB for a UE, Pt

n, is adjusted as a function of thecombined uplink path loss and shadowing, L = L(d) + Xσ ,as

Ptn = min

{Pmax, max

[Pmin, Pmax

(L

α

)ξ]}

(5)

where L(d) is the distance-dependent path loss, Xσ is thelog-normal shadowing value (in dB) with standard deviationσ as described in Reference [15], Pmax is the maximumtransmit power per RB,Pmin is the minimum transmit powerper RB and α is the k-percentile path loss, a constant whosevalue determines the critical path loss value above whicha UE transmits with full power. The balancing factor, ξ,determines how steeply the transmit power increases withincreasing path loss. In the femto layer, the maximum femtoUE transmit power is set such that it cannot be higher thanthe HeNB transmit power.

2.3. Channel model

The channel gain, Gr, tn , between a transmitter t and a re-

ceiver r on RB n as defined in Equation (1), is composedof distance dependent path loss, log-normal shadowing andthe channel variations due to frequency-selective fading andis calculated as

Gr, tn = ∣∣Hr, t

n

∣∣2 10−L(d)+Xσ

10 (6)

where Hr, tn accounts for the channel transfer function on

RB n between transmitter t and receiver r. While the chan-nel response, Hr, t

n , generally exhibits time and frequencydispersions, channel fluctuations within a RB are neglectedbecause the RB dimensions are significantly smaller thanthe coherence time and coherence bandwidth of the chan-nel [16]. The delay profiles associated with the applicablepropagation conditions that generate frequency-selectivefading of Hr, t

n are provided in References [15, 17].

2.4. Path loss models

Three path loss models (along with their respective de-lay profiles) as described in References [15, 17] are used:the urban micro (UMi) model for the macro-cell chan-nel, the indoor hotspot (InH) model for the femto-cellchannel and the indoor-to-outdoor model to simulate the



channel between entities lying indoors and outdoors. Foreach link in the system, the probability of line of sight(LoS) is calculated as specified in the appropriate path lossmodel. Based on the LoS condition the associated pathloss model is applied to the respective link. Furthermore,a wall penetration loss is included for outdoor-to-indoorlinks.

2.4.1. UMi model. This model is designed specifically forsmall cells with high user densities in dense urban ar-eas. The path loss for the LoS condition is calculated asshown in Equation (7). Here, the distance between trans-mitter and receiver is d, the effective breakpoint distance iscalculated as d′

BP = 4h′eNBh′

UEfc/c, where fc is the cen-tre frequency in Hz, c is the speed of light in m/s andh′

eNB and h′UE are the effective antenna heights for the

eNB and UE, respectively, with values h′eNB = 24 m and

h′UE = 0.5 m. Log-normal shadowing values are spatially

correlated according to the correlation model in Refer-ence [15], where the values of their standard deviations canalso be found. The non-LoS model is computed as shown inEquation (8).

LUMi,LoS ={

22 log10 (d) + 42 + 20 log10 (fc[GHz]/5) ; 10 m < d < d′BP,

40 log10 (d) + 9.2 − 18 log10(h′eNB) − 18 log10(h′

UE) + 2 log10(fc[GHz]/5); d′BP < d < 5 km

(7)

LUMi,NLoS = 36.7 log10 (d) + 40.9 + 26 log10 (fc[GHz]/5) ;

10 m < d< 2 km (8)

The LoS probability is a function of distance and is cal-culated as

Pr(LoS) = min(18/d, 1)(1 − e−d/36) + e−d/36.

2.4.2. InH model. This model is used to model the channelfor indoor links. The LoS path loss is calculated as shownin Equation (9) and the non-LoS model is calculated asshown in Equation (10). Finally, the probability of LoS fora link is computed as

LInH,LoS = 16.9 log10(d) + 46.8 + 20 log10(fc[GHz]/5);

3 m < d < 100 m

(9)

LInH,NLoS = 43.3 log10(d) + 25.5 + 20 log10(fc[GHz]/5);

10 m < d< 150 m (10)

Pr(LoS) =

⎧⎪⎨⎪⎩

1; d < 4 m,

e− d−43 ; 4 < d < 60 m,

0; d > 60 m

2.4.3. Outdoor-to-indoor (and vice versa) model. TheUMi path loss model is used to model only the indoor-to-outdoor path loss and vice versa as

Loi = Lb + Ltw + Lin; 50 m < d < 5 km

Here, Lb is the basic path loss calculated using the UMimodel as Lb = LUMi(dout + din). The parameter dout refersto the distance between the femto-cell wall and the outdooreNB/UE and din refers to the indoor distance between theUE/HeNB and the femto-cell wall. The parameter Ltw, isthe wall penetration loss and Lin, dependent on the indoor

distance alone, is calculated as Lin = din/2. In this case, theprobability of LoS is zero.

2.5. User distribution

The simulation area is a two-tier, tessellated hexagonal celldistribution. In order to eliminate edge effects, two addi-tional tiers are simulated; however, statistics are taken onlyfrom the first two tiers. Femto-cells have a circular area,each with the same radius, and they are uniformly dis-tributed with a given density. One femto UE and one HeNBare randomly distributed in each femto-cell.

In the macro-cell, UEs are randomly distributed over theentire area. These UEs are served by the macro-cell eNB.Depending on their density and the density of femto-cellsthere is a certain probability (approximately 6%) that someof these UEs lie within a femto-cell. In the case of open-access systems, such UEs are assimilated into the femto-celland served by the HeNB. For comparison, we also considera benchmark system where all UEs are served by the macro-cell (whether located indoors or outdoors).



Once the users are distributed, log-normal shadowingmaps containing correlated shadowing values in space aregenerated. Using these and the scenario specific path lossmodels described in section ‘Path loss models’, macro UEsare associated with the eNB to which they have the leastpath loss. Depending on whether open or closed-access isconsidered, macro UEs that lie within femto-cells are eitherassimilated or left to communicate with the eNBs.

3. RESULTS

Simulations are run for a full-buffer traffic model. Perfectsynchronisation in time and frequency is assumed so thatinterference between neighbouring RBs is avoided. The pa-rameters used in the simulation are taken from the 3GPPspecifications [10, 18–21] and are shown in Table 1.

The average sum system capacity in the uplink and down-link per macro-cell is depicted in Figure 2. The benefit offemto-cell deployment is clearly demonstrated here. Thenormalised sum system capacity is generated as the sumof the individual user capacities, divided by the number ofmacro-cells considered. For systems with HeNB deploy-ment, the aggregate femto and macro user capacity is com-pared against the benchmark system where all users areserved by the eNB. These results highlight the achievedgains through HeNB deployment. It is seen that the sumcapacity for systems with HeNB deployment is more thantwo orders of magnitude higher than that for the benchmarksystem in both the uplink and downlink. Certainly, without

Table 1. Simulation parameters.

Parameter Value

Avg. femto-cells per macro-cell 15Avg. macro UEs per macro-cell 10Femto UEs per femto-cell 1Inter-site distance 200 mFemto-cell radius 10 mBandwidth 10 MHzUplink FDD band 2.505 GHzDownlink FDD band 2.625 GHzTot. number of available RBs 50RB bandwidth 180 kHzThermal noise −174 dBm/HzTot. eNB Tx pow. 38 dBmTot. HeNB Tx pow. −30, −10 and 20 dBmMax. macro UE Tx pow. 24 dBmMax. femto UE Tx pow. −30, −10 and 20 dBmMin. macro/femto UE Tx pow. −30 dBmWall penetration loss, Ltw 20 dBk-percentile path loss, α 124 dBPath loss balancing factor, ξ 1

−30 −10 20

100

1000

HeNB Tx Power [dBm]

Su

m S

yste

m T

hro

ug

hp

ut

[Mb

ps]

Benchmark ULBenchmark DLClosed ULClosed DLOpen ULOpen DL

Figure 2. Normalised sum system capacity for all three systems.

femto-cells, the benchmark system experiences the leastinterference in most cases, as is observed in the coming fig-ures. However, these gains are offset by the fact that in thebenchmark, the indoor users are not offloaded from the eNB.Since the same set of resources are available to the eNBsin all systems, each user, on average, gets allocated fewerresources in the benchmark system. These results illustratehow resources in a macro-cell can be effectively reused byfemto-cells to significantly boost the overall system capac-ity. A higher transmit power in the downlink results in animproved SINR at the femto UE. Despite the higher interfer-ence this causes to macro UEs, the sum capacity increaseswith increased HeNB transmit power. Due to the severe wallpenetration losses the increased transmit power results in alower increase in interfering power than useful power. Inthe uplink, due to power control, the transmit power valuesare typically below the maximal power. Thus, the uplinkperformance is independent of the maximum HeNB power.In the benchmark system, since HeNBs do not exist, thecapacity remains constant regardless of the HeNB transmitpower.

Before proceeding with the remaining results, we define:

Set A: All femto UEs in the system.Set B: All macro UEs in the system, regardless of whether

they lie indoors or outdoors.Set C: Those (affected) macro UEs lying physically inside

femto-cells.

Furthermore, macro UEs may lie indoors as well as out-doors. Femto UEs strictly lie indoors.

3.1. Downlink

The downlink capacity for Set A for varying HeNB trans-mit powers is shown in Figure 3. It is observed that anincrease in HeNB transmit power improves downlink user



0 50 100 150 200 250 3000

0.2

0.4

0.6

0.8

1

User Capacity [Mbps]

CD

F

Open DL; Tx pow. −30 dBm

Closed DL; Tx pow. −30 dBm



Open DL; Tx pow. 20 dBm

Closed DL; Tx pow. 20 dBm

Figure 3. Downlink capacity for Set A.

capacity for both open and closed-access. This is expected,since the increased transmit power results in an increasedSINR at the receiver. Furthermore, closed-access systemsoutperform open-access systems. This is attributed to thefact that in case of open-access, macro UEs that are locatedwithin the coverage of the HeNB will get assimilated intothat femto-cell. In this case, the HeNB must distribute theavailable resources among the femto UEs and the assimi-lated UE, thus resulting in each UE getting allocated fewerRBs compared to closed-access.

Figure 4(a) shows the interference experienced by Set Bfor open and closed-access systems as well as for the bench-mark system. The cumulative distribution function (CDF)for the benchmark system shows three distinct sections. Thelowest interference regime spanning between −180 dBmand −120 dBm corresponds to the interference experiencedby indoor users. The middle portion corresponds to non-LoS interference experienced by outdoor users and the last

portion represents LoS interference experienced by outdoorusers. The systems with femto-cell deployment are exposedto significantly more interference, since, unlike the bench-mark system, most macro UEs are located outdoors, unpro-tected by building walls. An interesting trend is observedas depicted in the inset. Since indoor macro UEs in closed-access systems are not served by the HeNB, at −30 dBmHeNB transmit power, such UEs experience very low in-terference from the eNB due to wall shielding. In the caseof open-access systems, such UEs are assimilated into thefemto-cell and thereafter join Set A UEs. Therefore, macroUEs in open-access systems strictly lie outdoors and experi-ence higher interference for low HeNB transmit power. Thesituation reverses as the HeNB transmit power is increased.

Since also the intended signal received from an eNB isstrongly attenuated indoors and the amount of assigned re-sources per user differs, the user capacity should be consid-ered for a fair comparison. Figure 4(b) depicts the CDF ofuser capacity for Set B in the downlink. Here, the lower theHeNB transmit power, the better the downlink performanceof the UEs. This is expected since the amount of interfer-ence originating from femto-cells is directly proportional tothe HeNB transmit power. For this reason, the best macroperformance is achieved when the HeNB transmit power isat its lowest, i.e. −30 dBm. It is interesting to note that thesystems with femto-cell deployment significantly outper-form the benchmark system. This is due to the lower spatialreuse in the benchmark system, which means that the eNBhas to share the available resources with a higher numberof users. In this setting, with 15 femto-cells per macro-cell on average, each femto-cell serving one UE, eNBs inthe benchmark system, need to serve more than twice thenumber of macro UEs, giving rise to poor benchmark usercapacities.

−180 −160 −140 −120 −100 −80 −600

0.2

0.4

0.6

0.8

(a) (b)1

Interference [dBm]

CD

F

Open DL; Tx power −30 dBm

Closed DL; Tx power −30 dBm



Open DL; Tx power 20 dBm

Closed DL; Tx power 20 dBm

Benchmark DL

−99 −98 −970.02

0.04

0.06

0 1 2 3 4 5 60

0.2

0.4

0.6

0.8

1


CD

F





Open DL; Tx power 20 dBm

Closed DL; Tx power 20 dBm

Benchmark DL

0 0.005 0.010

0.2

0.4

Figure 4. (a) Downlink interference per RB and (b) capacity statistics for Set B.



−150 −140 −130 −120 −110 −100 −90 −80 −70 −600

0.2

0.4

0.6

0.8

(a) (b)1

Interference [dBm]

CD

F

Open ULClosed ULBenchmark UL

ThermalNoise

0 0.5 1 1.5 20

0.2

0.4

0.6

0.8

1


CD

F

Open ULClosed ULBenchmark UL

0.4 0.45 0.5 0.55 0.6 0.65

0.6

0.65

0.7

Figure 5. (a) Uplink interference per RB and (b) capacity statistics for Set B.

3.2. Uplink

Figure 5(a) shows the uplink interference experienced bySet B UEs. It is seen that that the trends of uplink inter-ference are substantially different from those of downlinkinterference. All macro UEs are served by eNBs regard-less of whether femto-cells are deployed or not. Since alleNBs lie outdoors and interference is measured at the eNB,the corresponding CDFs do not exhibit such pronouncedmulti-modal distributions. There is no significant differencebetween open and closed-access, because UEs that get as-similated into open-access femto-cells are classified as SetA UEs, and the uplink interference their HeNBs experi-ence is not reported in this figure. Similar to Figure 4(a),the benchmark system exhibits the least interference, dueto the lack of femto-cells.

Figure 5(b) shows the uplink capacity CDF of Set B.The performance of the benchmark system is plotted forcomparison. As expected, open-access outperforms closed-

access, since macro UEs lying within femto-cells get assim-ilated. This has a double impact on macro performance: (i)these UEs get offloaded from the macro-cell, thus freeing upresources, and (ii) this user experiences a boost in capac-ity because the wall penetration losses are circumvented,transmission distances are reduced and strong femto-cellinterference is avoided. However, the actual performancedifference is small. Interestingly, the low benchmark inter-ference seen in Figure 4(a) does not translate to a systemwith high capacity as seen in Figure 5(b) since the availableRBs must be shared by all (indoor and outdoor) users.

3.3. Set C: Affected macro user performance

Figure 6(a) and (b) show the uplink and downlink capacitiesof Set C in closed-access and open-access systems (noticethe difference in scale of the x-axes), respectively. In thedownlink, for any HeNB transmit power, the downlink ca-pacity of Set C UEs is significantly higher for open-access

0 0.2 0.4 0.6 0.8 1 1.2 1.40.85

0.9

0.95

1(a) (b)


CD

F

DL; Tx power −30 dBm


DL; Tx power 20 dBm

UL0 0.01 0.02 0.030.85

0.9

0.95

1

0 20 40 60 80 100 120 1400

0.2

0.4

0.6

0.8

1


CD

F



DL; Tx power 20 dBm

UL

Figure 6. Capacity statistics for Set C in (a) closed-access and (b) open-access.



−200 −150 −100 −50 0 500

0.2

0.4

0.6

0.8

1

Interference [dBm]

CD

F





Open DL; Tx pow. 20 dBm

Closed DL; Tx pow. 20 dBm

Benchmark DLThermalNoise

96.62 dB

Figure 7. Downlink interference statistics for Set C.

than for closed-access systems. For closed-access, wall pen-etration losses and longer transmit distances drastically re-duce capacity. Figures 7 and 6(a) reveal a very importantproblem that is expected to be encountered in systems withfemto-cell deployment: extremely severe downlink inter-ference originating from the HeNB, which renders the vul-nerable macro UE incapable of achieving a useful downlinkSINR. In Reference [22], the authors propose a method ofmitigating this interference.

For comparison with the overall case depicted in Figure4, the interference experienced in the downlink by Set Cis presented in Figure 7. This figure highlights the bene-fits of open as compared to closed-access in the context ofdownlink interference. It is clearly seen from this figure thatwhen an UE gets assimilated by a femto-cell, it undergoesa drastic reduction in interference. In fact, the level of inter-ference experienced by assimilated UEs approaches that ofthe benchmark system, which can be considered to be theideal case in terms of interference, since it does not con-tain any femto-to-macro interference. It is seen from thisfigure that in the case when the HeNB transmit power is atits highest, i.e. 20 dBm, the difference in interference be-tween open and closed-access systems is greater than 96 dB.For open-access, since the interference is below the thermalnoise floor, such a system is no longer interference-limited,but noise-limited.

4. SUMMARY AND CONCLUSION

Femto-cell deployment poses a viable complement to cel-lular networks. Operators need to bear low cost in theirdeployment since they are installed directly by the usersthemselves. Furthermore, since they share both the radio

access scheme and the frequency band with eNBs, theyare compatible with legacy UEs. Aside from these bene-fits, a cellular network stands to significantly gain in over-all system throughput through the widespread deploymentof HeNBs. Not only do HeNBs improve indoor coverage,bringing wireless broadband-like experience directly to in-door environments, but they also offload resources from theeNB, which can be utilised to improve coverage to outdoorusers. The key contribution of this work shows that despitethe benchmark system imposing lesser interference than thesystem with femto-cell deployment, the achieved capaci-ties are still lower because the limiting factor, i.e. the set ofavailable RBs, N , are reused less often and wall penetra-tion losses cannot be avoided. This shows that even withoutany interference coordination in place, capacities are sig-nificantly boosted through femto-cell deployment, wherethe advantage of open and closed-access systems is high-lighted not only due to the reduced number of users sharingthe eNB resources but also to the lower average path lossof the considered UEs.

In terms of overall throughput, under the assumptionof the femto-cell density used in this work, the differencebetween closed-access and open-access systems is almostnegligible because of the relatively low probability that amacro UE lies inside a femto-cell. However, if a macro UElies within a femto-cell, it might not be able to receive adownlink transmission from its eNB because of high signalattenuation and, more importantly, very high interferencefrom the nearby HeNB. If a closed-access femto-cell hap-pens to contain a macro UE, there has to be a method ofmitigating the downlink interference experienced by thismacro UE, or else, it will almost certainly go into out-age. In this context, smart resource allocation techniquesor power control for femto-cells is an avenue open to fur-ther research. Furthermore, it has been shown in this paperthat from the macro UE’s viewpoint in closed-access sys-tems, the downlink direction is the critical traffic direction.Similarly, from the femto UE’s viewpoint in closed-accesssystems, the uplink direction is critical. Therefore, depend-ing on whether priority lies with the macro or femto-cell,research on closed-access systems must be devoted to miti-gate uplink interference imposed on femto UEs, or downlinkinterference caused by closed-access HeNBs.

ACKNOWLEDGEMENTS

Initial parts of this work were supported by DFG grant HA 3570/2-1 as part of program SPP-1163 (adaptability in heterogeneouscommunication networks with wireless access – AKOM) while



some latter parts of this work have been performed within theframework of the CELTIC project CP5-026 WINNER+.

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Throughput enhancement through femto-cell deployment

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