Distributed Power Control Mechanisms forHSDPA Femtocells

Naveen Arulselvan, Vinod Ramachandran, Suresh KalyanasundaramMotorola India Private Limited, Bangalore

Email: {naveen.a,vinodkumar,suresh.kalyanasundaram}@motorola.com

Guang HanMotorola Inc, USA

{guang.han}@motorola.com

Abstract—Femtocells are low-cost, miniature base-stations intended to improve indoor coverage in 3Gnetworks and beyond. One of the main issues in adopt-ing femtocells en masse is the surge in interferenceto the mobile users served by the macrocell arisingfrom unplanned networks and private access. Therefore,distributed power control mechanisms for femtocells areessential to shield the existing users of the macrocellas well as to enable scalable femtocell deployments.This paper studies several such power control schemesthat strike an effective balance between the throughputof the femto users and the degradation in macrocellperformance.

I. INTRODUCTION

Recent years have witnessed price-wars amongvoice operators, and inspite of increasing number ofsubscribers, the average revenue per user (ARPU)for voice is declining [1]. Operators are starting tofocus on home services, through bundled offeringswith voice, television, data as the triple play, andmobile access adding a potential fourth dimension.But scarce spectrum has pushed 3G and 4G wirelesstechnologies to operate at 2 GHz and above. This, inturn, will lead to spotty coverage for indoor users assignals tend to fade out faster at higher frequencies.Femtocells are low-cost cellular base-stations that canprovide improved home coverage and increase thecapacity for user traffic. Using an IP-based backhaulsuch as the user’s existing DSL connection, femtocellscan provide cost-effective high-bandwidth wireless ser-vices. In combination with a macrocellular network forcoverage, femtocells can significantly reduce the totalnetwork costs [2]. For mobile operators, femtocellsalso provide an opportunity to compete directly withfixed-line and Voice-over-IP (VoIP) service providers.

The two most important deployment characteristicsof femtocells are related to (a) frequency of operationand (b) access privileges to the user [3]. Femtocells canshare a carrier with the existing macro network, whichis referred to as co-channel deployment, or operatein their own frequency band as in dedicated channeldeployments. Femtocells could be deployed in a ClosedSubscriber Group (CSG) fashion in private offices and

homes, which implies that only registered users mayestablish connection with these femtocells. Alterna-tively, femtocells could have open access, where allsubscribers of the operator can access these femtocells.Hot-spots such as cafes and hotel lobbies may opt foropen-access deployments.

Operating femtocells on a dedicated frequencyand/or allowing open access remains a possibility;interference management is arguably simpler in suchscenarios [4]. But co-channel operation with an exist-ing macrocellular network is more rewarding for theoperator due to increased spectral efficiency throughfrequency re-use. Moreover privacy issues of individ-ual subscribers may preclude open access deploymentat all times. As a result, we study the co-channeloperation of femtocells with restricted access. Not onlydoes this present more challenging problems, we alsoexpect this to be the de facto deployment mode.

Previous work in [5] and [6] have proposed methodswhere femtocells locally calibrate their DL transmis-sion powers. Femtocell transmit power is computed sothat minimum coverage is guaranteed for an imaginarymacro user in the vicinity and the maximum datarate for a nearby femto user is fixed to control theinterference to other femtocells. Such hard assumptionsand errors in estimating physical locations will causethese calibration schemes to perform conservatively.

In this work, we investigate two categories ofpower control algorithms for femtocells that can beperformed locally with minimal network intervention.These power control schemes differ in their time-scaleof operation. In the geo-static power control scheme,the transmit power of the femtocell is based on asimple function of the distance from the macrocell. Thekey idea behind the adaptive power control scheme isto adjust the transmit power of the femtocells so asto just achieve their target data rates. The target rate,in turn, is computed by the network to balance thedegradation in macrocell performance and increase infemto user throughput. Using system-level simulations,we present coverage and capacity results for usersserved by both the macrocell and the femtocells.

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The paper is structured as follows. In section II, westudy the effect of fixed transmit powers of femtocellson users in the macrocell. In section III, we discuss aclass of power control schemes that operate on a slowtime-scale. In section IV, we analyze a dynamic powercontrol scheme.

II. FIXED TRANSMIT POWER FOR FEMTOCELLS

In this section, we investigate the adverse impact offixed femtocell powers on the macro users’ capacityand coverage. We observe a near fifty-percent reduc-tion in capacity of the macrocell for even nominal fem-tocell densities. The results also show that femtocellswith unregulated transmit power are bound to causelarge dead zones (i.e., out-of-coverage areas) for usersserved by the macrocell.

We consider a High-Speed Downlink Packet Access(HSDPA) system deployed in a sub-urban environmentwith parameters indicated in Table I. Two tiers ofneighboring macrocells are considered for modelinginterference, but the statistics for the center-cell aloneare considered for the results. We note that the resultspresented in this section can be easily extended to otheradvanced 4G systems. The macro users are all assumedto be outdoors, whereas the femtocells and the usersthey serve are indoors. Each femtocell is assumed tohave one static user randomly placed in a cell radiusof 15 meters. The center macrocell contains 50 userswith full buffer FTP traffic, while the femtocells serveone mobile user each. The path-loss values for macro-cell transmissions as a function of the distance d tothe user is given by LM (d), and the path-loss valuesfor femtocell transmissions are given by Li

F (d) orLo

F (d), depending on whether the mobile user is indooror outdoor. The fraction of femtocell control channelpower constituting total transmit power is generallyhigher than for macrocells to ensure better coverage.The macro users will experience interference from thetransmissions of the neighboring femtocells and othermacrocells. The macrocell employs a proportionally-fair scheduler.

To study the impact of control channel power offemtocells on the macrocell performance, we modelthe activity level of femtocells via an activity factor.Active femtocells have data transmissions, while inac-tive femtocells simply transmit control channel power.

A. Impact on Macrocell Capacity

Consider 200 femtocells uniformly distributed inthe center macrocell, each with an activity factorof 0.5. This corresponds to 100 effective femtocellstransmitting at a fixed transmit power of PF and100 other inactive femtocells transmitting only con-trol signaling, which in turn is taken to be 0.25PF .Figure 1 gives the corresponding average macrocell

TABLE ISYSTEM PARAMETERS

Parameter V alueHexagonal gridCell Layout19 cell sites, 3 sectors/site

Inter-site dist 1732 mCarrier frequency/ Bandwidth 2 GHz/ 5 MHzTotal Macro Tx power 20 WControl channel power 2 WData channel power 14 WFemto total Tx power PF

Femto control channel power 0.25PF

Wall penetration loss (WP) 10 dBσ=8db (towards macro)Log-normal shadowingσ=10 dB (towards femto)

UE noise figure 9 dBSpeed 3 km/h (for macro users only)Macro to indoor UE (LM ) 128.1 + 37.6log10 d(km)+WPFemto to outdoor UE (Lo

F ) 7 + 56log10 d(m)+WPFemto to indoor UE (Li

F ) 37 + 20log10 d(m)Scheduler Proportional fairMin inter-femto distance 25 mMin femto-macro UE distance 15 m

and femtocell throughputs. We can observe that themacrocell throughput suffers significantly for evenmoderate femtocell transmit powers. On the otherhand, femtocell throughputs are reasonably large evenwhen their transmit power levels are low.

Fig. 1. Effect of fixed femtocell power

B. Impact on Macrocell Coverage

Common Pilot Control Channel (CPICH) is a down-link channel broadcast by the macrocell with constantpower and a known bit sequence. The macrocell’sCPICH strength indicates the extent of macro UE cov-erage. In this section, we study the impact of femtocellson macro CPICH signal strength. The probability of afemtocell k being active is given by the activity factor

2

ak and the corresponding transmission power is PF,k.The inactive femtocells are assumed to transmit controlchannel power P control

F,k . Then for a macro UE i, theCPICH signal-to-noise ratio Ec

N0is given by

Ec

N0=

Pcpich

LMi

∑Nn=1

PM,n

LM,n+

∑Kk=1

akPF,k+(1−ak)P controlF,k

LoF,k

+ W,

(1)where Pcpich is the CPICH power, W is the noisepower, and N and K are the number of macrocellsand femtocells respectively. The total transmit poweris taken to be 20 dBm for all the femtocells. The CDFsof CPICH Ec/N0 are shown in Figure 2 for differentactivity factors. Here the femtocells are assumed toswitch off their control channels when inactive. Fig-ure 3 gives the CDFs of CPICH Ec/No for the casewhere femtocells continue to transmit control channelpower when inactive. As expected, the macrocell cov-erage further deteriorates and is almost independentof the activity factor. These results corroborate thefact that number of active femtocells and the controlchannel of femtocells have a strong negative impact onmacrocell performance.

Fig. 2. Macro signal strength CDF for different activity factors (Nocontrol channel)

III. CALIBRATION TECHNIQUES

In the next few sections, we propose several powercontrol schemes that do not require the femtocells tochange their transmit powers at a fast time-scale.

A. Femtocells with Varying Data/Control Power Ratios

Based on the results so far, it is evident that thefemtocells are able to achieve a significantly largerthroughput than the macrocell. However, they reducethe macrocell capacity, especially when operating athigher transmit powers. While reducing the transmitpower of femtocells can help macro users, the femto-cell coverage can be affected significantly. Therefore,

Fig. 3. Macro signal strength CDF for different activity factors

instead of scaling down both the control channel andthe data channel powers by the same factor, we fix thefemtocell control channel power and reduce the datachannel power alone. This ensures optimum coveragefor femto users and minimizes effect on macro users.

Figure 4 shows the average macrocell and femto-cell throughput when femtocells operate with a fixedcontrol channel power of 25 mW while the datachannel power is varied from 75 mW to 25 mW. Weobserve that this leads to approximately a ten percentimprovement in the macrocell throughput. On the otherhand, the femtocell throughput is large even with thereduced data channel power. Moreover, the femtocellcoverage will not be impacted as the control channelpower remains constant.

Fig. 4. Average cell throughput vs. femtocell data/control powerratio

B. Geo-static Transmit Power

First, we study the geographic impact of femtocelltransmit power on macro layer coverage by looking atthe CPICH coverage. For a macro user i, the CPICHSINR Ec/No takes the form as in (1) with activityfactor aj = 1 for all j, i.e., all femtocells are active.

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We declare a macro user is in x percent-outage if itsCPICH Ec/No is below a threshold value to for afraction of time larger than x. For to and x values of -25 dB and 2%, respectively, Figure 5 shows macrocellmobiles that are in coverage and outage for femtocelltransmit power settings of 0.1 mW and 100 mW(only extremes shown due to space constraints). Thepositions shown in green and red indicate mobiles thatare in coverage and outage, respectively. The outageprobability of a macro user is influenced by boththe proximity to the serving macrocell as well as thedistance to the nearest interfering femtocell. From thefigure, we observe that (a) the coverage probabilityof macro users close to the serving macrocell isnot influenced by presence of femtocells, even forhigh transmit powers of the femtocells, and (b) Thecoverage probability of macro users in the cell edgeis sensitive to the distance to its nearest femtocellinterferer. This coverage probability will deterioraterapidly as the femtocell transmit power increases.

The network can set the transmit power of thefemtocells according to the geographical zone in whichthey lie. Because femtocell deployments will be un-planned, the power zones of the femtocell can besegregated simply based on the distance to the macro-cell. The zone and the power level of the femtocells canbe pre-configured and/or modified over the broadcastchannel or the DSL backhaul. The power setting willnot change, except in the case of rare events likemacrocell-splitting or femtocell relocation. An exam-ple of such a power-zone division is shown in Figure 6.Macro users in outage will be much fewer than whenall femtocells transmit at the maximum power level.

Fig. 5. Femto transmit power vs User-outage positions

Fig. 6. Power Zone segregation for Femtocells

IV. ADAPTIVE TRANSMIT POWER CONTROL

For a given macro user i, we assume there are Nmacrocells and K femtocells. The downlink Signal toInterference and Noise ratio (SINR) for user i is thengiven by

SINRi =Pi

LM,i∑Nn=1

PM,n

LM,n+

∑Kk=1

PF,k

LoF,k

+ αPres

LM,i+ W

,

(2)where Pi is the power in the shared data channel re-served for user i, PM,n is the power used by macrocelln, PF,k is the power used by femtocell k. Here α isa factor that arises from non-orthogonality betweendifferent channelization codes in the HSDPA systemand Pres is the residual power of the serving cell aftersubtracting the power used for user i. The downlinkbit-rate at time t is then given by the modified Shan-non’s equation [7]:

RM (t) =∑

i∈S(t)

log2(1 + 0.5 ∗ SINRi) bps/Hz, (3)

where S(t) is the set of the users scheduled at time tand the factor of 0.5 is to account for the gap betweenthe theoretical Shannon capacity and the capacity thatcan be achieved in practice with HSDPA. When allthe femtocells use equal power, it is easy to see thatSINR and hence macrocell throughput RM decreaseswith increasing femtocell density.

The power allocation problem for multiple femto-cells is similar to the non-cooperative game formula-tion studied in [8] for Digital Subscriber Lines (DSL).The authors developed an iterative algorithm with nocentralized control and show that Nash Equilibrium(NE) is reached for the two-user case. We employa similar approach where the network provides eachfemtocell k with a target bit-rate Tk. The network maytake into account factors such as macrocell load, num-ber of active femtocells, distance of the femtocell tothe macrocell, and fading environment to determine Tk

for a given femtocell. The femtocells collect the SINRmeasurements from the mobile users they serve anduse this to schedule in each transmission opportunity.

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The femtocells can determine the effective data rateRk

F in the downlink using (3). Then the femtocellscan regulate their transmit power so that their targetbit-rates are just met. The power control algorithmexecuted in every femtocell k = 1, 2, · · · ,K at everyscheduling instant is as follows:

• Femtocell k checks to see if the DL bit-rate RkF

is in the range [Tk, Tk + ε], for some constant ε• If not, the femtocell adjusts its transmit power

level Pk accordingly• The adjusted power level Pk is then constrained

to lie in the permissible range [Pmin, Pmax].

For the numerical studies, we assume identical targetrates are set for all femtocells. Figure 7 gives the re-lationship between the total macrocell throughput andthe number of femtocells. Small target bit-rate settingsfor femtocell give as much as 15 % macrocell through-put enhancement even when the femtocell deploymentis relatively sparse. This benefit increases to around33% as femtocell deployments get denser. Figure 8shows the throughput distribution of all macro usersand the bottom 50% macro users for 300 femtocellsand a fixed target bit rate, when (a) femtocells performno power control, (b) femtocells perform the adaptivepower control described, and (c) there are no femto-cells. This demonstrates that adaptive power controlbenefits apply fairly to all macro users and not justusers with good channel conditions. We also note thatthe total cell throughput (i.e., the sum of throughputsof all femtocells and the macrocell) increases linearlywith the number of femtocells. This is because, inspiteof power control, the femtocells operate at much higherspectral efficiencies than their macro counterpart. Asa result, the contribution from macrocell to the totalthroughput is negligible.

Fig. 7. Macrocell throughput vs Femtocell density

Fig. 8. Macro user throughput distribution (300 femtocells)

V. CONCLUSIONS

We have studied the impact of femtocell interfer-ence on HSDPA macrocell capacity and coverage.We discussed the need for distributed transmit powercontrol solutions for femtocells to enhance macro userperformance and enable scalable deployments. Wedemonstrated the benefits of simply varying the ratioof femtocell data channel and control channel powersto improve macro users experience and maintain fem-tocell coverage. In the geo-static scheme, the transmitpower of the femtocell is regulated as a function ofits geographical location alone. In the adaptive powercontrol scheme, femtocells locally achieve a targetdata rate that is centrally computed by the network.All these power control mechanisms provides efficienttrade-offs between femtocell and macrocell perfor-mances, while varying in implementation complexity.

VI. ACKNOWLEDGMENTS

The authors would like to thank their colleagueRajeev Agrawal for several useful discussions.

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