Interference Management in OFDMAFemtocell Systems Using Frequency Reuse

8/3/2019 Interference Management in OFDMAFemtocell Systems Using Frequency Reuse

http://slidepdf.com/reader/full/interference-management-in-ofdmafemtocell-systems-using-frequency-reuse 1/5

f DA U q R

Taeyug Lee, Jsu Y Sagtae Lee, ebe ad Jtae Sh I EEE cl c vs K K

{gcaee ysOO bayag tsh}@su.edu

ba This paper presents an interference managementscheme for OFDMA Femtocell systems. Femtocell is recentlyintroduced for indoor coverage extension. However,interference problem between the femtocell and the macrocellshould be solved in advance. In this paper, we propose anecient interference management scheme in the OFDMfemtocell systems using Fractional Frequency Reuse (FFR) inorder to minimize the interference between both cells. Under thepre-allocated frequency band within a macrocell through the

FFR optimally, the proposed scheme allocates sub-bands thefemtocells eciently to consider macrocell having a priorityover femtocells and total/edge throughputs. Simulation resultsshow that proposed scheme enhances the throughputsperformance in overall network, especially for the macrocellusers and cell edge users.

NTRODUCTON

OOTHOGONAL equency Division Multipl Accss(ODMA) is one of major features for net genration

communication standars such as ong Tem Evolution (TE)

and IEEE 802.16 (Worldwide Interoprability for MicrowaveAccess, WiMAX). The equncy and time resources are

allocated to usrs in orthogonal maner. A key feature isquency euse factor of 1 for maimal resorce use in theODMA system. Rcently,femtocell has poposed for indoor

covrag tension. The femtocell is dend as very smallsize, low-powr home base station (S) that works in thlicensed equency bands, and it is connectd to broadbandInternet backhaul [1]. Th femtocell brings various benets to

both consumes and operators, such as enlarge indoorcoverage, enhanced system capacity, Quality of Service(QoS), and reduced capital and opration epense. De toincrasing indoor phone calls and data services but

insuciet macrocell coverage, the femtocell could be anattractive solution.

However, inteference problem between the femtocll and th maccll shl b slv n avanc, bcas thfemtocell is deployed over th eisting maco network,and ituses same spectrum with the macrocell. Dedicated channelapproach is oe of th easiest ways to solve this problem, but

th equncy rsources are not utilized eectively.Co-channel model is suitable for pactical deployment, but

the Co-channel intference (CCI) problem shold be solved[2]. The femtocell is oen tued on an o, and installd atunknown location, because it is managed by a persoalcustomr, not by a network operator. Therefoe, many

Manuscript received March 25, 2010. This work was suppored by the

National Research Foundation (NRF) grant funded by the Korea govement (MEST) (2010-0016896).

parameters shoul be automatcally adjusted to avoid theCCI.

Fractional requncy Ruse (FR) is discussed in theOFDMA based network to overcome th CCI problems [3-5].In th FR, whol equency ban is divided into severalsub-bands,and ach sub-band is dierently assigned to centerzone and edge zon of the cll. Whil reuse factor of thcenter zone is one, the edge zone adopts bigger reuse factor.

As a result, intra-cell interference is emoved, and inter-cellintrference is substantially reduced. At th same tim thsystem throughput is enhancd.

Dynamic requncy lanning (DP) algorithm wasanother proposal for interfrnce avoidanc [6]. Aerdividing the ODMA network into sveral sectors, theamount of sub-channels is estimatd considring usr

bandwidth demand in each sector. nterfernce among sectorsae calculated when the sectors transmi the same qency.Optimization function is run to minimize the overall networknefernce. Howve, his schem ollows a cnalizd ntwork structue, which is not appropriate for fmtocellmanage by a personal owne.

Sevral huristic algorithms for equency assignment wre investigated [7]. The recommended one is astInterfeence owr (I) algorithm, whee a powered-upfemtocell S chooses a frequency segment that minimizes theinterfeence lvl. In tu-o ordes algorithm [7], quencyallocation is conducted according to the oder of the femtocell

tued on. However, te fmtocells are deployd in the formof ectanglar matri, not in the random mannr. Also,interfernce between the macrocell and the femtocell is notevaluated.

Isolated and coupled modl considering user location wasdiscussed fo ODMA femtocells [8]. I the isolated model,macocll and femtocell usrs are allocat differnt resources split by time an quency slots. The coupled

model reuses macrocell resources for femtocells which arelca n th cll buna, wl h c n mtcllsus othogonal esources om th macrocll. However, theseschemes do not apply the FR concept for the ODMA

systm.reqency reus and pilot sensing scheme was proposd to

reduce the CCI [9]. Aer applying te RF of 3 o above to the macrocells, the femtocells use the remaining quencysub-bands. or eample, if the reuse factor is thre and themacrocell uss sub-band I among the three sub-bands I, II,an II, th femtocell chooss sub-band II and II. However,

the macrocell throughput is reduced, ven though the oveallcapacity is ehancd. Also the reuse facto in macrocll is

444//$ © EEE 7



against the OFDMA based network such as the TE and theWi MAX, where the target of the reuse factor is one.

The FFR is one of the solutions to reduce inter-cellinterference in macrocell system, especially for the cell edgeusers. Also it is helpl to achieve the reuse factor of one.

Under this condition, the interference om the femtocelldeployment should be minimized for the macrocell users. So,

we focus on the interference management between themacrocell and the femtocell using the FFR.

In this paper, we propose an efcient resource allocationsceme n the DMA emtocell systems usng te . Wedene optimization problem on allocating frequencysub-bands to femtocell in each zone and evaluate an efcientinterference management scheme using sectored-FFR andequency band allocation. Under the macrocell allocatesequency band using the FFR, the femtocell choosessub-bands which are not used in the macrocell sub-area toavoid interference.

II. FFR EMTOCELL LLOCA ON PROBLEM

A. OFDMA Femtocell System Model

The rst step in problem formulation in the OFDMAfemtocells is to allocate the equency sub-bands into amacrocell based on FFR. The other equency allocation forfemtocell users will be done aer allocating macrocell usersoptimally. This allocation order is acceptable becausemacrocell users have a priority over femtocell users. Thisode redes e omplexy of te otmal ato

problem in macro-/femto-cell mixed systems.The optimal equency sub-band allocation for macro cell is

assumed by referring previous works [5, 10]. In [5], authors provide an analysis of the interference coordination techniquein macrocell OFDMA systems. An optimization problem isformulated and solved via a linear interior point method. Theobjective of this problem is to determine the equency reusefactor of the cell eterior zone and the percentage of theoverall system bandwidth attributed to either interior orexterior zones. Simulation results show that the optimalinterior zone radius is 630m (cell radius 1000m) and theoptimal equency reuse factor of the eterior zone is equal to3. Ref. [10] shows more than thee sectorization can not makedistinct differentiation on performance.

From previous reseach results for macrocell equencyallocation, we can set the size of center zone (interior zone) to

0.63 of macrocell coverage and equency reuse factors of terr and exteio one to n 3 respectvely. Also ormacrocell, each cell is partitioned into three sectors anddifferent equency sub-band is allocated to the eachmacrocell sub-area according to the above FFR method. Asshown in Fig. (a), the set of interferers for a user in sector Eof macro cell 1 is the adjacent macrocells of cells numbered {2,

7,8,9,17,18,19} but a user in center zone of cell interferedom all 18 macrocells. By applying cell conguration of Fig., edge users can achieve higher throughput due to reduced number of interferers.

aFig. !. (a) 19 Multicell OFDMA structure, (b) The basic layout andallocation for macrocell users using sectored-FFR.

Fig. 1 (b) shows basic equency allocation for macocellusers when sectored-FFR used.

B Problem Formulations

CCI between the femtocell and macrocell users is

increased when using same sub-band at same sectored zone.Thus, the objective of allocating equency sub-bands forfemtocell is how many sub-bands are allocated to femtocellusers in each zone.

The achievable SIR value of macrocell user m in centerzones can be shown as follows:

Nk {l 2 'M. mM.

'm, "

N+L.'kGmM'.k +L.PF,kGm.F.kI F

(1)

where and is transmit power of serving macrocell M an negbor macrocell M' on sub-carer respectvely.G is channel gain between macrocell user m and servingmacrocell M on sub-carrier k Channel gain from neighbor

macrocells are denoted by G Similarly, is transmit power of neighbor femtocell F on sub-carrier k G ischannel gain between macrocell user m and neighborfemtocells F on sub-carrier k N is white noise power spectraldensity, and is sub-carrier spacing. Similarly, SR of macrocell users in exterior zone E on sub-carrier k is

"GM (2)k {Fl} m , o+ I \ 'G' + IPFGp

E [2 ,7 ,8 ,9,17 , 8 , 19}The other SR of macrocell users in E2 and E3 are similar toEq. (2) except which is different according to sector.

In case of a femtocell user in center zones, it is interferedom all 19 macrocells and adjacent femtocells. The received

SR of a femtocell user on sub-carrier k in center and edgezones can be expressed similarly as Eq. (1) and (2).The channel gain G is dominantly affected by Rayleigh

fading( H) log-normal shadowing( in dB), and

outdoor/indoor path losses. The path loss for outdoor ismodeled as PL 28+35gJ( dB, where d is thedistance om a BS to a user in meters. Otherwise, indoormodel is PL 38.5+20lg(+L dB, where L is 7,10, and 15 dB for light internal, inteal, and eteal walls, respectively [2]. So, the channel gain G of user i onsub-carrier k can be expressed as

G 10-+uIO ·IH 12 (3)

77



The practical capacity of macrocel user m or femtoceluseron sub-carrier k can be given by [3]

Cmor/,k =f j·10g2(+a S I NRor/,k) 4

where, a is a SR gap for target Bit Error Rate (BER), and

dened by a =IS / ( SBER) . Here, we set BER to .6The overal throughput of serving macrocel M and

femtocel F can be epressed as,

TM =LLP "kC"k S)1

z

} L,kCj,k (6)J k

where, m [ noti the sub-carrier assignment for

macrocel and femtocel users. When 1m or [ the

sub-carrier k is assigned to macrocel user m or femtocel userf Otherwise,1m or [ From the characteristics of the

OFDMA system, each sub-carrier is alocated only one

macrocel user or femtocel user in a macrocel or a femtocelin every time slot. This implies that �l m 1 V

where N is the number of macrocel users in a macrocell.Similar epression for femtocel users related to the practicalcapacity and the overal throughput is possible ecept [inx [ 1 on ' Where is available

sub-bands allocated to femtocel in a zone X i.e., X {C1, C2, C3, 1, 2, 3}

Eventualy, we can formulate centralized optimizationblem of alocating sb-bans t femtcells in eah zne sfollows.

subject to : 1 l m 1 for V2. [inx [ 1 for x3. SINR� � SINRthres .

Thus, the objective of optimization is that sub-bands

alocation for femtocell in each zone X should be decided. Inother words to nd available equency sub-bands for

femtocels in each zone X that is to nd optimal subset in

each zone X is our optimization goal. In the given centralized

optimization problem, to investigate overal possible

combination of subset takes much time so that can easily

not obtain optimal solution within each time slot. The

thinkabe cases in each zone are selecting n sub-bands among

4 su b-ba nds {A, B, C,D}, i.e., + � + + (!) = 15

cases arise in the zone. Since a macrocel has 6 zones, the

number of overal cases is156 Therefore, we proposeddistributed sub-optimal sub-bands alocation scheme having

relatively low interference between macrocel users andfemtocel users

Iuencing Factors Needed To Discuss

How to setup the relative ratio of equency sub-bands A, B,C,and D in macrocel needs to be discussed. In this paper,for

the sake of simplicity, we refer to the results in [5] [] But

there eist the difference between macrocel-only system andour femtocel system. In summary, the ratio of each sub-band has an effect on throughput lightly.

III. PROOSED NERFERENCE ANAGEMEN CHEME

In order to solve the formulated problem, rst we canalocate equency sub-bands in practicaly disjoint ways. So

the femtocel chooses sub-bands which are not used in themacrocel sub-area

,-

Mcr 0Feto AC

c

D

Fig. 2. roposed practical allocation scheme for femtocell users under givenmacrocell allocation distriution

We all ocate the frequency sub-bands into macrocel andfemtocel as depicted in Fig. 2. The macrocel coverage isdivided into center zone and edge region including threesectors per each,denoted by C,C2, C3, and E, E2, E3. The whole equency band is partitioned into two parts, and one part of them is classied into three pieces again, totalydenoted by A, B, C, and D.

For macrocel,different equency sub-band is alocated to the each macrocel sub-area according to the FFR. The reusefactor of one is applied in the center zone, while the edge

region adopts the factor of three. The sub-band A is used in the center zone (C ,C2,and C3),and sub-band B,C,and D isapplied in the E E2, and E3 regions, respectively.

Under this circumstance, the femtocel chooses sub-bands which are not used in the macrocel sub-area. Especialy when the femtocel is located in the center zone, the femtoceladditionaly ecludes a sub-band which is used by macrocelin the edge region of current sector. For eample, when afemtocel is located in region E it uses sub-band A, C, or D,

while the macrocel uses sub-band B. If a femtocel is positioned in zone C, sub-band C and D is applied. Thefemtocel avoids sub-band A which is used by macrocel inzone C. Also it avoids sub-band B which is used bymacrocel in region E

because the received signal power of

sub-band B is relatively strong for that femtocell.Due to the characteristics of he OFDMA, the macrocell is

interfered by inter-cel, and that interference is rther

mitigated by the FFR. The femtocel uses different sub-band to avoid interference om the macrocell. The sub-band is reused in the macrocel coverage as much as possible,

because transmitted power of the femtocel is very small.Therefore, the interference between the macrocel andfemtocel is greatly avoided. Also more sub-carriers arealocated to femtocel which is located in the edge region, inorder to improve the performance of the edge users.

78



V. MULATON ESULTS

A. Scenarios and Environments

In In this paper, we evaluate the proposed scheme in termsof throughput. We also focus on the performance of the

macrocell users and the cell edge users because macrocellusers have a priority over femtocell users.

We evaluated realizations and compared the proposedscheme with several comparison schemes: Femtocell 3-sector,-3, and -3 Femtocell 3-sector scheme is similar to the proposed method except sub-band A is excluded inedge zones. In FFR-3 scheme, FFR is applied to themacrocell users, and femtocell users are randomly assignedfrom ll equency band. On the other hand, FR is notadopted for macrocell users in No-3 scheme. n Femto3-sector the amount of sub-carriers allocated to femtocellusers is three times in {B, D}. Meanwhile, for FFR-3 andNo-3 the amount of total available sub-carriers for

femtocell users is three times of the ll band, as equal to total numbers of sub-bands like the proposed scheme for faircomparison. These features are summarized in Table TheAmount column implies the amount of used sub-carriersdenoted by the sub-carrier assignment parameterp

The simulation parameters are listed in Table . Theoverall network is composed of two-tier 9 macrocells, andfemtocells are randomly deployed over the macrocells. All

the BSs are operated by the OFDMA technology. We vary the number of femtocells om 3 to 8 in one macrocellcoverage to change simuation environment. The macrocell

TABLE SMMY O PROOSED D OMIO SHEMEMacrocell user F emtocell user

SchemesFrequency Amount Frequency Amount

Proposed FFRDivide center andedes Fi. 3

Femto Exclude

3sector FFR I sub-band A in 3ede

F-3 FFR RandomNoFFR3 Random Random

Note: Amount column implies value of L P m,k and LxLr inx P r,k for macrocell and femtocell users, resectivel

TABLE SIMTIO METEarameters

Macrocell Femtocell

Number of cells 19 (two-tier)30-180

macrocell

Cell coverae 280m 30m

BS transmit power FFR 1522 W

2mW w/o FFR: 20 W

Number of users in one macro cell180 180

coveraeChannel Bandwidth 5 MHzFFT size 512 Number of occupied sub-carriers 300Sub-carrier spacing 15 kHzWhite noise power density -174 dBmHzSize of center zone 0.63 of macrocell coverae

L = 28+35o(

Channel mode Rayleih fadin, Lj = 38.5+20 o'()+

path loss, shadow fading(8dB) = 7 dB, if in ( 0, I 0 = 10 dB, i f in (10,20" = 15 dB, if in (20,30

and femtocell users are randomly distributed in the overall network. The macrocell and femtocell users are randomlyallocated the available sub-carriers om the designated rangein each scheme. When the FFR is applied to macrocell user, a

half of the ll band is assigned for center zone, the other for

edge zone. An omni-directional antenna and three sectorantennas are installed at a macrocell BS,for center zone with

transmit power of 5 W and for edge zone with W, respectively.

On the other hand,if the FR is not used by macrocell user, the transmit power of a macro BS is W. All the femtocellsuse mW as the transmit power. The different channelmodel is used for indoor and outdoor, where the path loss is adominant factor. Then, we nd out the downlink receivedSIR values for each user and each sub-carrier. Using this

value, the throughput is calculated via users located in thecentral serving macrocell of 9 cells.

BNumerical Results and Comparison

Fig. 3 shows the throughput of macrocell users located in the macrocell coverage as the number of femtocells varies. n proposed scheme, the femtocell users can get the sub-bands which are not used by macrocell users at each zone. So, theinterference between macrocell users and femtocells isgreatly avoided. The femtocells interfere with macrocell usersless than FFR-3 and NoFFR-3 This result is proper forassumption, that is, macro cell users have a priority overfemtocell users. However, Femto 3-sector has slightly higher

thoughput than proposed scheme in macrocell users throughput, due to sub-band A was not used in edge regionsfor femtocells. This means that macrocell users suffer lessinterference om femtocell users.

In Fig. 4 it describes the throughput of total users locatedat the edge zone only. n the ODMA cellular network,currently, the performance of the edge zone is poor due to theinter-cell interference. n the proposed scheme, allocating themore sub-carriers to femtocells which are located in the edgezone, the throughput of the edge zone is improved. The

reason that Femto 3-sector has low throughput in edge zonesis due to the exclusion of sub-band A in the availablesub-bands x' To conclude, proposed scheme can eectively

reduce interference in the edge zones and can utilize moresub-bands for femtocell users.

Fig. 5 depicts total throughput of total users in both themacrocell and femtocells. Even though proposed scheme has

slightly lower throughput for macrocell users than Femto 3- sector in Fig. 3 it has the muh igher tota thoughput. FFR-3 shows similar performance, however, throughput of macrocell users is relatively low due to interference omfemtocell as depicted in Fig. 3

As shown in these results, our proposed scheme enhances the overall throughput with considering macrocell andfemtocells. Especially, the throughput of cell edge users andmacrocell users are much enhanced in proposed scheme.

V. C

Femtocell systems are promsg to provide higher throughput service with low cost in wireless cellular systems.

79



However, there is interference problem how to allocateequency bands to femtocell because of co-channel operation

between macrocell users and femtocell users. In this paper, we dene optimal sub-band allocation problem and proposean efcient resource allocation scheme in the OFMDA

femtocell systems using the 3-sectored FFR. The proposedscheme enhances overall throughputs, especially for bothmacro cell user and cell edge users. It is benecial for theOFDMA network such as the TE, where the 3-sectored FFRis applied. As a result, in case of using 3-sectored FFR formacrocell, the premise whch allocates equency su-bansin practically disjoint ways is quite usel for femtocell

networks.

12,-, , . ,10 : , . , , ,

B "--, -

, , - - - , ,

, 4 --�---:---;---T."="-�.

, , , , , , , , , , , , 2 - _ . - - - �_ .=�P -o -o- -- I

, , , , , , , Femto 3sectoFFR-NoFFR3

�0---60--- �0--�1�0=50==�80Nube of Feocells

Fig. 3. Total throughput of macrocell users located in center zones andedge zones as the number of femtocells varies caption.

90. 8 0 - - � - : .

, , , , , , , , , , , , 70 : � , , , , 60 , L • , , i , , ,

'' � , ,

, ' , , , , , , , ,

ProposedFeo 3 -sector

FFR3-NoFFR3 , ,

1�L---�0�--- �0---1�0-=1 50�80Nber of Fetoces

Fig. 4. Throughput of macro and femtocell users located only in the edgezones as the number of femtocells varies.

10 ..

100 -- -- --

, , , , . . .. .. . ,, ,, ,, ,

. : : :1 , , , ,

: : : r'popos.d- Fe o 3secto

2 0 ---------------- :----------..

6

FFR3

3 0

, , , , , , , , , 60 9 0 1 2 0

Nber of Feocels

NoFFR31 5 0 1 8

Fig. 5. Total throughput of macrocell and femtocell users in an entiremacrocell as the number of femtocells varies.

FRNCS

[ I ] V . Chandrasekhar, 1. Andrews, Femtocell Netwoks: A Survey, Commun. Mag., vol. 46, no. 9, pp. 59-67, Sept. 2008.

[2] L. Ho, H. Claussen, ffects of User-Deployed, Co-ChannelFemtocells on the Call Drop robability in a Residential Scenario, nteational Symposium on ersonal, ndoor and Mobile RadioCommunications (MRC), Sept. 2007.

[3] H. Lei, L. Zhang, X. Zhang, and D. Yang, A Novel Multi-cellOFDMA Sstem Structure Usin Fractional Frequency Reuse, nteational Symposium on ersonal, ndoor and Mobile RadioCommunications (MRC), Sept. 2007.

[4] M. Assaad, Optimal Fractional Frequency Reuse (FFR) inMulticellular OFDMA System, Vehicular TechnologyConference (VTC), Sept. 2008.

[5] S. Yeh et a., WiMAX Femtocel ls: A erspective on NetworkArchitecture, Capacity, and Coverage, Commnu. Mag., vol. 46,no. 10, pp. 58-65, Oc 2008.

[6] D. Lopez-erez et a., nterference Avoidance and DynamicFrequency lanning for WiMAX Femtocells Networks, nteational Conference on Communication Systems (CCS), Nov.2008.

[7] H. Zeng, C Zhu, and W. Chen, System erformace of Self-Organizing Network Algorithm in WiMAX Femtocells, ACM nteationalConference on Wireless nteet (WCON), Nov. 2008.

[8] K. Sundaresan, S. Rangarajan, fcient Resource Management inOFDMA Femto Cells, ACM nteational Symposium on Mobile AdHoc Networking and Computing (MobiHoc), May 2009.

[9] T. Kim, T. Lee, Throughput nhancement of Macro and Femto

Networks By Frequency Reuse and ilot Sensing, nteationalerformance, Computing and Communications Conference (CCC),D. 20.

[10] Lee, D., Xu, Mayekar, U, Mohile, M., Frequency reuse factor vs.pathloss exponent and sectorization, Wireless Applications Digest,1997., MTT-S Symposium on Technologies

80

Interference Management in OFDMAFemtocell Systems Using Frequency Reuse

Documents

Transcript of Interference Management in OFDMAFemtocell Systems Using Frequency Reuse