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A Nv Int-C Intfnc Cntn Scm f OFDMA Sytm

Hui Ba,Jiaia LiuElectronic and Communication Engineering

North China Electric ower UniversityBaoding, China

mengou@2cn.com

Abstract- this paper, a novel Inter-Cell InterferenceCoordination(ICIC) scheme to mitigate the Co-Channel

interference(CCI) in downlnk Orthogonal Frequency DivisionMultiple Access (OFDMA) cellular system is presented. Theproposed scheme combines a power control strategy, and usesthe fractional frequency reuse to reduce the CCI. Simulation

results show that the proposed scheme is superior to otherexisting schemes. It also shows that the scheme can greatly

improve the throughput of the cell-edge users while guaranteesystem performance.

KeywordsOFDMA;Interce Interference; CCI; ResourceSchedulng

I. NTRODUCTON

As the increasingly requirement of data ansmission foruture services, there wil e higher requirement fr moile

communication system of the performance and bandwidth.The FDMA system which has high spectral efciency isbecoming one of the key technologies in the ture mobile

comication systems. Due to inherent property ofFDMA the intra-cell interference is avoided, however theinter-cell interference(ICI) which limits the systemperformance remains the main source of interference. Thecell-edge area, especially, which sensitive to the interferenceconditions have weak date rates or even not receive anyservices while the trac is heavy [,2].ne possible way toreduce the inuence of ICI is to use a large frequency reusefactor. However this may result in low spectrum eciency.It is a new challenge for frequency reuse due to the seriousICI. The ICIC algorithms based on FDMA is given greatlyattention in B3G/4G system[3].

The concept of ICIC was rst introduced in [4,5] toimprove data rates experienced y cell edge users. But in this

way, the lly orthogonal frequency of the adjacent cell edgelimt the pectrum ecienc. hs paper prope a newICIC scheme for the downlink multicell system based onexisting research and the features of FDMA system. Thescheme use different frequency reuse strategies, according to

commications environment of users in the cell edge orcenter respectively[6]. In addition, the scheme introduces anew power allocation algorithm which based on thecalculated SIR value. The simulation analysis shows that thescheme has higher rates of the cell edge users than traditional

ICIC scheme, and balance the quality of communicationbetween users in the cell edge nd center to some extent.

9784446890$600 00

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The organization of this paper is as follows. Section IIoutlines the system model, followed by the formulation ofthe resource allocation problem. Then, a novel ICIC schemeis deduced and simplied in Section III. In Section IV,theproposed algorithms are simulated and compared with

existing nown approaches. Finally, we conclude in SectionV.

II. YSTEM ODEL

We assume that the system with M cells, and subcarriers, U active users and one ase station (BS) in each

cell in FDMA downlink cellular systems. To simpli theanalysis, we just consider CCI from the innermost layer ofadjacent cells, and the model is descried in Figre .Thesystem frequency reuse factor is nd the BS in each cell

only comunicate with onl one user in each moment. Wealso assume that the base station can get each user'ssubcarrier channel gain instntaneity, and the distriuted

information can be fed back to the user terminal with no

distortion.

Figure I. CC om the multi-cell

We suppose a subchannel is made up with S adjacentsu-caier, nd the subchannel number is N = S. Then,the CCI of the user k in subcarrier j, cell m is:

M

l Li=i

P O k I,j 1, } (1)

Where, .jis transmit power for cell i, subchannel j, is the chnnel gain for user k, cell i and sub channel j.

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We can get the signal to interference ratio(SIR) of theuser k in j subchannel:

G 'R = ,j , ,j,j, + 2,jhere, 2 is Gaussian white noise.

(2)

For a given bit error rate (Bit Error Rate, BER), themaximum number of bits can be sent for user k, subchannel jat each scheduling cycle is:( R

J

),,= log2 1 + k'J

" / sc Hz (3)In the Aitive White ssi Nise(WGN) channel,

kJm = n (BER) I 1 5 (4)' is a constnt with certain BER requirement, whichmeans the Signal to Noise Ratio margin between M-QAMmodulation and Shannon capacity.

Cell edge users generally refer to the user whose averageSIR falls below a certain threshold. ICIC plan the frequency

and power resources of adjacent cell to improve the SIR andrates of cell edge users.

III. HE ROPOSEDICIC CHEME

Traditional ICIC scheme which reduce inter-cell

interference and improve rates of the users in cell-edge areais an effective solution to avoid the interference betweencells[7]. The proposed algorithm is based on fractionalequency reuse plan, User equipments(UEs) c be dividedinto the cell-center user (CCU) and cell-edge user (CEU)according to fading of the cell. The algorithm usecompensated power distribution scheme, on the one hand,the algorithm coordinate the inter-cell interference to avoidthe major source of interference in each region, on the otherhand, increase the rates of cell-edge users while guarantee acertain amount of user throughput.

The detail steps of the proposed scheme as follows:For every TT clock=O,1,2 . ..

) C br fCC CAccording to each user channel quality indicator(CQI)

report we can calculate the number of CCU and CEU

Kccu and K 0 Sb .The subchannels in the cell-centre and cell-edge areas

will be allocated independently. We use proporional fair scheduling in order to improve the rate of cells.For all If

is CEU, carried out as the following distributionmodes.

313

The whole frequency resources f is divided into threenon-overlapped subchannel sets: X{1,4,7 ... f-2}, Y {2,,8, ...f-1}, Z {3, 6,9, ... .

The serial number in the set is the number of thesubchannels. The available subchannels in the cell-edge areacan be chosen by each cell in order as the above set. In thisway, it can avoid the subchannels of the neighboring cellsoverlapped to the utmost extent and reduce the CCI of thecell-edge areas.Else

V is CCU cell site allocates good other channels toUE;

3 Pwr Drb.System will allocate the power resource according to the

results of step 1 and step 2. The principle of inter-cellbalanced SIR among cells is used for mitigating ICI. By

means of the power control, a balanced SIR c be made upfor CEU and CC. This algorithm is introduced as follows.

Every base station calculate the transmit power of allthe subchannels. SIR value based power control method willbe adopted.

If U is CEU, power distribution will be carried outaccording to the SIR value as follow:

If R >

then = -else Pm=in(Pm+M,hreshold)

Where is the target value of the SIR. M is thepower control step length, is the power of allocating touser m, hresholdis power of threshold value.

Else UE; is CCU to send half of ll power.This method not only ensure the comication quality ofthe users near the base station, but also increase the equity ofthe users who farter away om the base station.

4) S .According to the results of subchannel and power

allocation in step 2 and step 3, system will send the datas toevery UE. nce scheduling ended.

IV. ERFOMANCE ULATON AND ESULT NALYSSWe evaluate the proposed schee in a FDMA cellular

systems with 19 cells, tree sectors per cell, and use theban macro-cellular model. The basic system parametersare summarized i Table 1 .The CCI and white noise to beseen as the main source of interferences and the rest aresmall enough to be ignored. At the same time, we assume

that the numbers of users in each sector are equal andconstant in a process of resource allocation, and eachsubchannel can only be assigned to one user in a scheduling.

The simulation compare with other two methods of resourceallocation scheme, the rst one is traditional ICIC scheme in[8]. The second one is a baseline algoritm(BASE) that not

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use any interference coordination scheme. The packetscheduling is performed using time domain and equencydomain F packet scheduler.

TABLE!. IMULATION ARAMETERS

Parameter ValueCell radius/m 500The interval of subcier/kz 15System Bandwidth/MHz 10Number of subcarier 600Number of users each cell 20BER 103Penetration loss/dB 20Shadowing standard deviation/dB 6TT/ms ISimulationlength 2000TTTotal BS transmitpower/dBm 46Trac model Full buffer

The proposed algothm whch troduces a frequencyreuse scheme would be complicated more or less than thebase one. The new power allocation algorithm (described inpart III), which has te computational complexity of (K)for a system of K users. The complexity has not increasedcompared to the scheme in[8].

The simulation result in Figre 2 shows that the totalcapacity of the system of te proposed algorithm is highercompared with the traditional ICIC algorithm, which isgenerally not available in the other ordinary algorithm. TheICIC scheme is a compromise of the performance betweenthe ege nd the whole system, some perormance o tesystem loss in exchange for the improvements of the edge

area. Therefore, the throughput of both of the ICIC

algorithms is lower thn the ASE algorithm.The rate of the CEUs is the most important issue we haveinvestigated. As the number of users increase, the edge ratein the proposed algorithm i the paper is higher than thetraditional ICIC algorithm, and both of them are greatlyhigher than the the ASE algorithm in Figure 3. It mensthat the problem of CCI has been signicantly improved dueto the proposed algorithm.H1-

3 ---------- , 2.5

1 ----.----------+-----------------

. . . . .

05 - ------ ; ----- . .

L56059 05number of uses

Figure 2. Average cell capacity

4

1614 t 1 ----------- ----------:------------f 08 --- -------- - ----- --- ------- , :06 ... .

04 . " 0.23' 0-6:0,-- 0- 0

nmbe of ses

Figure . The rates of CEUs

As can be seen form the above, the advantages of theproposed scheme compared with aditional ICIC scheme areshown below.

1) The larger peak rate at cell edge;2)The improvement of system capacity.3) Adjust the faiess among CEU and CCU to meet the

QoS need of users.

V. OCLUO

In this paper we proposed a novel interferencecoordination methods based on FDMA system. The

scheme use different frequency reuse strategies in differentregions, nd a SIR-based power control strategy is combined.Simulation results show tat the proposed interference

coordination algorithms improved the throughput ofcell-edge users signicantly and reduced the inuence ofCCI. Contributed to the proposed scheme, the overallperformance of te system improves.

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