Femtocell/Macrocell Interference Analysisfor Mobile WiMAX System

Yu-Suk Sung∗, Nam-Ryul Jeon∗, Byung-Wan Yoon†, Jong-Sik Lee†, Seong-Choon Lee† and Seong-Cheol Kim∗∗Institute of New Media and Communications, Seoul National University

599 Gwanangno, Gwanak-gu, Seoul, 151-742, KoreaTel : +82-2-880-8481, Fax : +82-2-888-3633

E-mail : {sfamily, jeon3115, sckim}@maxwell.snu.ac.kr†Mobile Internet Development Department of KT Infra Laboratory

17 Woomyeon-dong, Seocho-gu, Seoul 137-792, KoreaTel : +82-2-526-6243, Fax : +82-2-526-5200

E-mail: {yoonbw, lowe, lsc}@kt.com

Abstract—Femtocell can provide better indoor coverage andcapacity of cellular systems. In order to operate femtocellappropriately, RF interference analysis between femtocells andmacrocells is needed. In this paper, we discussed the impact ofinterference by the introduction of femtocell in Mobile WiMAXsystem. Performance degradation was evaluated for the downlinkcase by system level simulation. Simulation results shows thatthe impact of interference from macrocell to femtocell is moresevere than from femtocell to macrocell. By allocating differ-ent frequency bands for femtocell and macrocell, performancedegradation can be negligible.

Index Terms—Femtocell, Mobile WiMAX, Interference Anal-ysis.

I. INTRODUCTION

As the demand for higher data rates and reliable wirelessservices in homes and buildings increases, the femtocell con-cept has been proposed recently. A femtocell base station(BS) is a low-power wireless access point that operates inlicensed spectrum to connect standard mobile devices to themobile operator’s network using residential DSL or cablebroadband connections [1]. Femtocells allow service providersto extend indoor service coverage, especially where access tothe wireless networks would be limited or unavailable. Theadvantages of the femtocell are summarized as follows: 1)better indoor coverage and capacity, 2) improved marcocellreliability, 3) cost benefits, 4) reduced subscriber turnover [2].

With increasing interest to femtocell, many manufactur-ers are developing femtocell chipsets, and some femtocellprototypes are already unveiled. Several mobile operatorsthroughout the world also announced their plans for the intro-duction and trials of femtocell technologies. In standardizationcommittees like 3GPP and 3GPP2, femtocell-related studieshave just been started [3, 4]. The study items include thefollowing: system architecture, basic functionality, mobility,RF-related issues, operation and maintenance, self optimiza-tion, security, network synchronization, handover procedure,

This work was partly supported by the Brain Korea 21 project and the ITR&D program of MKE/IITA. [2007-S-029-02, Development of Indoor WiBroSystem for Home and Enterprise]

interference control, and so on. But most of these are openissues still.

In order to deploy and operate femtocell networks appro-priately, all of the above mentioned issues should be consid-ered [2, 5]. In this paper, we focus on the RF interferenceproblem only. We discuss the effect of interference betweenfemtocell and macrocell for the OFDMA/TDD based MobileWiMAX system by system level simulation (SLS).

The rest of this paper is organized as follows. Severalsystem models for simulations are described in section II, andthe performance metrics which we consider are presented insection III. The simulation results and their interpretation arepresented in section IV. Finally, the paper ends with someconcluding remarks and the consideration for future works.

II. SYSTEM MODEL

A. System Layout

The system layout for simulation is depicted on Fig. 1. Thetotal number of macrocells is 19 so that interferences fromother cells are considered up to two-tier. For the reductionof simulation time, wrap-around technique is not used in thistime. The performance of a macrocell is evaluated only forthe center cell, while femtocells are distributed throughout thecenter cell.

Macrocell inter-site distance (ISD) is 1.5 km, and femtocellradius is assumed as several tens meters. The number ofmobile stations (MSs) which are connected to the macrocellBS is 15, and up to four femtocell MSs can be connected to thefemtocell BS. Handover between a macrocell and a femtocellis not considered, so the performances of macrocell MSs andfemtocell MSs are evaluated independently.

The target system which operates in TDD mode is assumedto be in perfect synchronization between femtocell and macro-cell networks. We consider the downlink case only.

B. OFDMA Frame Structure

Orthogonal frequency division multiple access (OFDMA) isa multi-carrier technology that extends OFDM to a multiple

Figure 1. Macrocell/Femtocell system layout

access technology. OFDMA supports the assignment of indi-vidual groups of subcarriers to specific users. Each subcarriergroup is denoted as a ‘subchannel’, and each user is assignedone or more subchannels for transmission depending on eachuser’s specific traffic requirements [6].

With PUSC (partial usage of subcarriers) scheme, subchan-nels are realized using a distributed subcarrier permutationmethod that pseudo-randomly draws subcarriers to form asubchannel. It is primarily used to achieve frequency diversitywhen it is difficult to track frequency-selective channel vari-ations and to allocate resources adaptively according to thesevariations [7]. A subchannel consists of two pilot subcarriersand 24 data subcarriers.

In the time domain, OFDMA frame duration is equivalent to48 OFDMA symbol durations. Each frame is partitioned intodownlink (DL) subframe and uplink (UL) subframe. DL/ULratio is equal to 32:15. Fig. 2 is DL subframe structure formacrocell users. As shown in the figure, we assume thatthe control overheads such as preamble, FCH, DL-MAP,and UL-MAP are fixed to two OFDMA symbols. For theease of the analysis, the remaining data region is partitionedequally into 15 DL bursts, so all of the macrocell users havesame radio resource allocation. One DL burst corresponds to10 subchannel × 6 OFDMA symbols. The number of datasubcarriers allocated to a macrocell user for a frame durationis 1,440(= 10× 24× 6). In case of femtocell, the data regionis partitioned into four equal DL bursts, and the burst size is15 subchannel × 15 OFDMA symbols.

Other OFDMA parameters are summarized in Table I.

C. Path Loss Model

Path loss between a macrocell MS and the macrocell BS ischaracterized by ITU-R vehicular test environment model [8].With the typical macrocell BS height of 32 m and the average

Figure 2. OFDMA downlink subframe structure for macrocell user

TABLE IOFDMA PARAMETERS

Parameter ValueSystem channel bandwidth 10 MHz

Sampling frequency 11.2 Mhz

FFT Size (NFFT ) 1024

Subcarrier frequency spacing (∆f) 10.94 kHz

Useful symbol time (Tb = 1/∆f) 91.4 µs

Guard time (Tg = Tb/8) 11.4 µs

OFDM symbol duration (Ts = Tb + Tg) 102.9 µs

Frame duration 5 ms

Number of OFDM symbols per frame 48

Number of DL data subcarriers 720

rooftop height of 15 m, the path loss can be calculated asfollows:

PLmacro(dB) = 130.68 + 37.6 log10(R) (1)

where R is the distance from the transmitter (Tx) to thereceiver (Rx) in kilometers.

For a femtocell MS which is connected to the femtocell BS,Home NodeB (HNB) apartment building model is used [9].HNB is the 3GPP terminology for the femtocell BS. Thismodel was proposed for studying interference scenarios be-tween HNBs, and is characterized as follows:

PLfemto (dB) = 20 log10

(4πdλ

)+ qW + F · n(n+2

n+1−0.46)

(2)The first term is the free space path loss, the the second

is partition loss, and the third is the floor loss. We ignorethe floor loss in this case. W is the partition loss due to thewall, and the value is fixed to be 5 dB. q is a random variablerepresenting the total number of walls between Tx and Rx. The

value of q is chosen from the set{

0, 1, ...,⌊d

dw

⌋}with equal

probability, where dw represents the minimum wall separationand is assumed to be 2 m.

In case of the other propagation scenario such as outdoor-to-indoor, the path loss is obtained from the combination of

Figure 3. Snapshot of frequency response of multipath channel realization

above two models and additional outdoor penetration loss Low.It is assumed that Low is 10 dB with probability 0.8 and 2 dBwith probability 0.2 which accounts for windows.

D. Multipath Model

Mobile WiMAX system exploits instantaneous channel con-ditions for the performance enhancement such as channeldependent scheduling and adaptive modulation and coding(AMC). For the realistic system performance evaluation, short-term time-varying channel characteristics should be consideredduring simulation as well as geometric path loss and long-termshadow fading.

Time-varying short-term fading effect can be generatedby power delay profiles. Conventional six-tap ITU models[8] which were originally developed for 5 MHz bandwidthchannels are not adequate to the target system which has10 MHz system bandwidth. As the bandwidth increases, thetime resolution in the delay domain increases so that moretaps are required [10]. Modified ITU power delay profileswith 24-taps proposed in [11] were used. Fig. 3 shows asnapshot of frequency response of modified ITU Pedestrian Band Vehicular A model.

By considering user mobility, different mixed user chan-nel models were chosen for macrocell and femtocell usersrespectively. In case of macrocell, typical mixed user channelmodel [12] was used—Ped-B 3km/h: 60%, Veh-A 30km/h:30%, Veh-A 120km/h: 10%. In case of femtocell, the channelmix model for indoor small office scenario in [10] is used—Ped-B 0km/h: 80%, Ped-B 3km/h: 20%. The multipath channelgain between the MSs and the BSs was generated once perframe.

Additional simulation parameters are presented in Table II.

III. PERFORMANCE METRICS

A. Per-subcarrier SINR

The instantaneous SINR of the nth subcarrier of a MS whichis connected to the BS 0 is given by

TABLE IIADDITIONAL SIMULATION PARAMETERS

Parameter ValueCarrier frequency 2.5 GHz

Frequency reuse (1, 1, 1)

Maximum Transmit power(macro BS / femto BS / MS) 43 dBm / 20 dBm / 23 dBm

Antenna configuration SISO, Omnidirectional

Antenna gain(macro BS / femto BS / MS) 15 dBi / 2 dBi / 0 dBi

Noise figure(macro BS / femto BS / MS) 4 dB / 5 dB / 7 dB

Lognormal shadowingstandard deviation(macrocell / femtocell)

8 dB / 7 dB

SINR(0)(n) =

P(0)TxG

(0)TxGRx

PL(0)

∣∣∣H(0)(n)∣∣∣2

J∑j=1

P(j)TxG

(j)TxGRx

PL(j)

∣∣∣H(j)(n)∣∣∣2 + PN

(3)

• P(j)Tx : transmit power per subcarrier from the jth BS,

• G(j)Tx : transmit antenna gain of the jth BS,

• GRx : receive antenna gain of the target MS,

• PL(j) : distance dependent path loss including shadow-

ing from the jth BS,

• H(j)(n) : multipath channel gain of the desired MS for

the nth subcarrier and the jth BS,

• J : total number of interferers,

• PN : thermal noise per subcarrier.For each MS, all of the instantaneous SINRs of data

subcarriers within the DL burst are collected.

B. Effective SINR

The effective SINR for each DL burst is calculated. Owingto the frequency selectivity of broadband channels, the av-erage SINR over all the subcarriers that constitute a givenburst is not a good indicator of the effective SINR, sinceaveraging fails to capture the variation of the SINR over allthe subcarriers. Several metrics, such as EESM (exponentiallyeffective SINR map), ECRM (effective code rate map), andMIC (mean instantaneous capacity), are widely accepted asbetter representatives of the effective SINR to capture thevariation of SINR in the subcarrier domain [13]. We chosethe MIC metric, and the effective SINR γeff is calculated asfollows:

γeff = 2

(1N

N∑n=1

log2(1+γn)

)− 1 (4)

where γn is the per-subcarrier SINR, N is the total numbersubcarriers within the burst.

TABLE IIIDOWNLINK MCS LEVEL

Modulation CodeRate

SINRThreshold Modulation Efficiency

QPSK 1/2 5.0 dB 1.0 bit/symbol/subcarrier3/4 8.4 dB 1.5 bits/symbol/subcarrier

16QAM 1/2 10.7 dB 2.0 bits/symbol/subcarrier3/4 14.8 dB 3.0 bits/symbol/subcarrier

64QAM 1/2 15.9 dB 3.0 bits/symbol/subcarrier2/3 19.0 dB 4.0 bits/symbol/subcarrier3/4 20.1 dB 4.5 bits/symbol/subcarrier5/6 22.0 dB 5.0 bits/symbol/subcarrier

C. Average Cell Throughput

From the effective SINR, the modulation and coding scheme(MCS) for each user is selected. The MCS level for thedownlink is described in Table III. Information bits that eachuser received are calculated by producting the modulationefficiency and the total data subcarriers allocated. For example,for a macrocell user whose MCS level is 16QAM 1/2, thereceived information bits are 2 × 1,440 = 2,880 bits. Userthroughput is calculated by dividing the received informationbits by the frame duration. Cell throughput is the summationof all user throughputs within the cell. Finally, cell throughputsare averaged over several thousands of Monte Carlo trials.

D. Spectral Efficiency

The average cell spectral efficiency is defined as

SE =R

BWeff(5)

where R is the average cell throughput, BWeff is the effectivechannel bandwidth.

The effective channel bandwidth is defined as

BWeff = BW × TR (6)

where BW is the used channel bandwidth, and TR is timeratio of the link. In case of downlink, TR = 32/48, soBWeff = 10MHz× 32/48 = 6.67MHz.

IV. SIMULATION RESULTS

We first analyzed the impact of the femtocell BS to macro-cell MSs. Theoretically, OFDMA uses orthogonal subcarriersso that there is no intra-cell interference within a macrocell butthe inter-cell interference between macrocells. Signal from thefemtocell BS can be regarded as intra-cell interference. Theperformance of macrocell without femtocell interference wasevaluated for the reference. In case of uniformly distributedmacrocell MSs, the average macrocell downlink throughputwas about 7.0 Mbps and corresponding spectral efficiency was1.05 bps/Hz. Table IV is the macrocell downlink performancewith additional femtocell interferences. As femtocell densityincreases, the macrocell MS perceives more intra-cell interfer-ences, so the degradation of spectral efficiency of macrocell

TABLE IVMACROCELL DOWNLINK PERFORMANCE WITH FEMTOCELL

INTERFERENCE

Number offemtocell BSsper macrocell

MacrocellThroughput

SpectralEfficiency

10 6.84 Mbps 1.03 bps/Hz

20 6.81 Mbps 1.02 bps/Hz

30 6.69 Mbps 1.00 bps/Hz

40 6.56 Mbps 0.98 bps/Hz

Figure 4. Spectral efficiency degradation of macrocell due to the femtocellinterferences

TABLE VFEMTOOCELL DOWNLINK PERFORMANCE RESULTS WITH OR WITHOUT

MACROCELL INTERFERENCE

Num. of Femtocell Throughput Spectral EfficiencyfemtocellBSs permacrocell

WithoutMacrocellBS

WithMacrocellBS

WithoutMacrocellBS

WithMacrocellBS

10 21.54Mbps 16.99Mbps 3.23bps/Hz 2.55bps/Hz

20 21.46Mbps 16.98Mbps 3.22bps/Hz 2.55bps/Hz

30 21.36Mbps 16.97Mbps 3.20bps/Hz 2.55bps/Hz

40 21.26Mbps 16.91Mbps 3.19bps/Hz 2.54bps/Hz

due to femtocell interferences also increases. This is shown inFig. 4.

Next, the impact of the macrocell BS to femtocell usersand their simulation results are presented in Table V. Due tothe small coverage and the relative high transmit power ofthe femtocell BS, femtocell throughput and spectral efficiencyshows the almost theoretical maximum value in the absence ofmacrocell BS interference. As the femtocell density increases,the inter-cell interference between femtocells also increases,but the performance degradation is negligible. When consid-ering interference from the macrocell BS, the degradation ofspectral efficiency of femtocell is about 20%. This value ismuch higher compared to the prior case where the degradationis about 2–6%. We can say that if the femtocell is located closeto the macrocell BS, the performance of femtocell severely

TABLE VIMACROCELL DOWNLINK PERFORMANCE WITH RESPECT TO THE

DISTANCE FROM THE TARGET BS

MS location Cell Throughput Spectral Efficiency0 – 0.1R 21.36 Mbps 3.20 bps/Hz

0.1 – 0.2R 18.12 Mbps 2.72 bps/Hz

0.2 – 0.3R 12.88 Mbps 1.93 bps/Hz

0.3 – 0.4R 8.56 Mbps 1.28 bps/Hz

0.4 – 0.5R 5.52 Mbps 0.83 bps/Hz

0.5 – 0.6R 3.56 Mbps 0.53 bps/Hz

0.6 – 0.7R 2.30 Mbps 0.35 bps/Hz

0.7 – 0.8R 1.50 Mbps 0.23 bps/Hz

0.8 – 0.9R 0.91 Mbps 0.14 bps/Hz

0.9 – 1.0R 0.57 Mbps 0.09 bps/Hz

degrades.To reduce the impact of interferences, different frequency

bands can be allocated to femtocell and macrocell respectively.Then the interferences between the other networks are loweredby the value of the ACIR (Adjacent Channel InterferencePower Ratio). ACIR is defined as the ratio of the total powertransmitted from a source (BS or MS) to the total interfer-ence power affecting a victim receiver, resulting from bothtransmitter and receiver imperfections. With the assumption ofACIRmacro = 37.5 dB and ACIRfemto = 25.5 dB, the degradationof spectral efficiency was almost negligible for both cases.

So far, we assumed that the femtocell BS operates in ‘closedaccess’ mode. Closed access means that the femtocell BS onlyserves the MSs which are a member of a particular ClosedSubscriber Group. In contrast, the femtocell BS which operatesin ‘open access’ mode can serve any MS [3]. That is, the MSserved by the macrocell BS can be connected to the nearbyfemtocell BS as it moves. Table VI is the downlink throughputof macrocell with the distance from the target BS. Obviously,as the MS moves towards the cell edge, the performance isseverely degraded due to reduced signal level and increasedinter-cell interferences. The overall system capacity would besignificantly improved by deploying the open access femtocellBSs to the place where the the signal from the macrocell BSis limited such as cell edge or where the traffic is very high.

V. CONCLUSION

In this paper, interference analysis between femtocell andmacrocell for the Mobile WiMAX system was presented. Fromthe simulation results, we observed that if the femtocells andthe macrocells are operated at the same frequency band, theperformance degradation is not as small as we can ignore. Andthe performance degradation is more severe when the femtocelMSs are victims and the macrocell BSs are interferers. If thefemtocells are operated at adjacent frequency band, the impactof interferences between two networks can be significantlyreduced. To maximize the overall system capacity, more judi-cious transmit power control and frequency planning schemeshould be considered.

We assumed perfect synchronization between macrocell andfemtocell networks. In reality, femtocell BS is hard to achievesynchronization to the networks because it is installed indoorsso the signal for synchronization from GPS is not easilydetected. In such a case, there may be more interferencescenarios, and these will be considered in the future.

REFERENCES

[1] Femtocell Forum, http://www.femtoforum.com[2] V. Chandrasekhar, J. Andrews, and A. Gatherer, “Femtocell Networks: A

Survey,” unpublished. [online]. Avaliable: http://aps.arxiv.org/abs/0803.0952

[3] 3GPP, “3G Home NodeB Study Item Technical Report,” 3GPP TR25.820, ver. 8.0.0, Mar. 2008.

[4] 3GPP2, Femto Cell workshop, http://www.3gpp2.org/public html/FEMTO, Oct. 2007.

[5] Webpage, Wikipedia:Femtocell, http://en.wikipedia.org/wiki/femtocell[6] Alvarion White Paper, “Understanding the Radio Technologies of Mo-

bile WiMAX And their effect on network deployment optimization,”2006

[7] K. Balachandran et al., “Design and Analysis of an IEEE 802.16e-BasedOFDMA Communication Sysetem,” Bell Labs Technical Journal, Issue4, pp. 53–73, Mar. 2007.

[8] ITU–R recommendation M.1225, “Guidelines for evaluation of radiotransmission technologies for IMT–2000,” 1997.

[9] Qualcomm Europe, “HNB and HNB–Macro Propagation Models,” 3GPPR4–071617, Oct. 2007.

[10] IEEE 802.16m–07/037r2, “Draft IEEE 802.16m Evaluation Methodol-ogy,” Dec. 2007.

[11] IEEE C802.16m–07/181, “Wideband Extension of the ITU profiles withdesired spaced-frequency correlation,” Sep. 2007.

[12] WiMAX Forum, “Mobile WiMAX – Part 1: A Technical Overview andPerformance Evaluation,” Feb. 2006.

[13] J. G. Andrews, A. Ghosh, and R. Muhamed, Fundamentals of WiMAX:Understanding Broadband Wireless Networking, Prentice Hall, 2007.