On the Performance Comparison of VSF-OFCDMAand OFDMA

Chia-Chin Chong, Fujio Watanabe and Hiroshi InamuraDoCoMo Communications Laboratories USA, Inc.

3240 Hillview Avenue, Palo Alto, CA 94304Email: {cchong.watanabe.inamura}@docomolabs-usa.com

Dandan Wang, Hlaing Minn and Naofal Al-DhahirDept. of Electrical Engineering,

University of Texas at Dallas, Richardson, TX 75083Email: {dxw053000, hxm025000, aldhahir} @utdallas.edu

I Abstract-In this paper, we compare the downlink capacityand throughput between the variable spreading factor-orthogonalfrequency and code division multiple access (VSF-OFCDMA)with one-cell frequency reuse, and the orthogonal frequencydivision multiple access (OFDMA) with conventional three-cellfrequency reuse. Since the multiuser diversity gain in OFDMAsystems can be achieved by approximately designing its accessschemes, while it is non-trivial for the VSF-OFCDMA systems,the effect of both access schemes on the link capacity are investi­gated. Our simulation results reveal that although the multiuserdiversity gain achieved by OFDMA systems can compensate theloss of low frequency reuse at low to mid range of signal­to-interference-plus-noise ratio (SINR) values, the achievablethroughput of VSF-OFCDMA systems is much higher than theOFDMA systems. Furthermore, the impact of user's location onthe inter-cell-interference is also reported.

I. INTRODUCTION

The orthogonal frequency division multiplexing (OFDM)technology is well known for its capability to cope withfrequency-selective fading channel due to multipath and itslow complexity equalizer. Therefore, it becomes a popularcandidate for various high data-rate wireless systems such asIEEE 802.11n WLAN, IEEE 802.16 WiMAX, 3GPP LTE,3GPP2 UMB, etc. However, OFDM suffers from high peak­to-average-power-ratio (PAPR). Furthermore, in multi-cell sce­narios, the OFDM based access scheme, namely, orthogonalfrequency division multiple access (OFDMA) can suffer severeco-channel interference from the adjacent cells which makesone-cell frequency reuse non-trivial for OFDMA based cellularnetworks. On the other hand, code division multiple access(CDMA) is a technology being used in the 3G cellular net­works such as 3GPP WCDMA and 3GPP2 CMDA2000. Themost significant advantage of CDMA is that it can achieve theone-cell frequency reuse among adjacent cells by deployinga properly designed set of codes or pseudo-noise sequence.Later, several technologies combining the benefits of OFDMwith CDMA are reported in the literature, mainly in the formof multi-carrier CDMA (MC-CDMA) [1]-[3]. In particular,the variable spreading factor orthogonal frequency and code

IThis work was done when D. Wang was an intern in DoCoMo USA Labs.

978-1-4244-2644-7/08/$25.00 ©2008 IEEE

division multiplexing (YSF-OFCDM) which belong to thefamily of MC-CDMA was first proposed by NTT DoCoMo[3]. It was shown in [4] and [5] as a very promising broadbandpacket wireless access scheme in order to achieve very highdata-rate using an approximate bandwidth of 50 to 100 MHz.The main advantage of YSF-OFCDM is due to its flexibilityin which the spreading factor can be varied according tothe cell structure and radio link conditions, which makesit more suitable to be used from multi-cell to isolated-cellenvironments while maintaining high link capacity.

The superiority of YSF-OFCDM over OFDM in a multi­cell environment, such as a cellular system has been shownin [6] and [7]. However, [6] and [7] mainly consider the peakthroughput of a cell without considering the effects of theaccess scheme. Researchers in [8] and [9] shown that multiplediversity gain can be achieved in a system if the access schemeis properly designed. The basic idea of multiuser diversity isto exploit the randomness of fading channels among differentusers and to allocate the channel to the user with the largestchannel power gain. The larger the dynamic range of channelfluctuations and the number of users, the larger the achievablemultiuser diversity gain.

In OFDMA systems, users are scheduled to transmit onorthogonal frequency bands which enables the exploitationof multiuser diversity in the frequency domain since usersare scheduled to transmit on their frequency fading peaks asproposed in [10] and [11]. On the other hand, in basic settingof YSF-OFCDMA systems, users are scheduled to transmit onorthogonal spreading codes over all frequency bands, whichmakes it difficult to achieve the multiuser diversity gain. Dueto these reasons, it is important to investigate the performanceof YSF-OFCDMA and OFDMA under the effects of themultiuser diversity gains.

The remainder of this paper is organized as follows: In Sec­tion II, the main features of YSF-OFCDMA and OFDMA arereviewed. In Section III, the system simulation configurationis described. Simulation results are presented in Section IY.Finally, Section Y concludes the paper.

Fig. 1. Illustration of the YSF-OFCDM symbol.

B.OFDMA

In OFDMA systems, the sub-carriers are divided into severalgroups and each group, namely, sub-channel is allocated toone user. Different techniques have been proposed in theliterature to group these sub-carriers such as adjacent grouping(i.e., adjacent sub-carriers are grouped into one sub-channel),distributed grouping (i.e., distributed sub-carriers are groupedinto one sub-channel), etc. In this paper, we focus on theadjacent grouping technique so that it is more pronouncedto see the effect of multiuser diversity gain and the benefit ofutilizing AMC.

There are rich literature about the scheduling schemes inOFDMA systems in order to achieve the multiuser diversitygain such as [10]-[12] and the references therein. Here, inorder to perform fair comparison with VSF-OFCDMA, weconsider the case in which the number of users is equal to thenumber of sub-channels, and each sub-channel is allocated to

II. COMPARISON OF VSF-OFCDMA AND OFDMA

A. VSF-OFCDMA

In VSF-OFCDMA systems, users access the system with or­thogonal spreading factor using VSF-OFCDM. VSF-OFCDMspreads the data in both the time and frequency domains usingorthogonal short channelization codes as shown in Fig. 1. Thetotal spreading factor, 8F is 8F == 8Ft x 8F.r, where 8Ft

and 8 F.r are the spreading factors in the time and frequencydon1ain, respectively. The time domain spreading is superiorover the frequency domain spreading in order to maintainthe orthogonality among the code-multiplexed channels withless inter-code interference. In particular, this property is ad­vantageous for the application of adaptive modulation coding(AMC) scheme since the channel does not vary too muchin the time domain. On the other hand, in order to reducethe effects of co-channel interference from adjacent cells inthe case of one-cell frequency reuse (see Fig. 2(a)), a largerspreading factor can be used in addition to a cell-specific longscrambling code which is used to whiten the interference.

one user. In the rest of the paper, we will use the followingthree scheduling schemes to illustrate the impact of schedulingschemes on the system performance:

1) Ideal Scheduling: Each user is scheduled to transmit onits best sub-channel on each frame. Note that, this schemeis too optimistic and impractical since more than one usercould choose the same best sub-channel.

2) Fixed Scheduling: Each user is scheduled to transmit ona specific sub-channel, which is fixed over all frames.Note that, no multiuser diversity gain is achieved in thisscheduling scheme.

3) Random Scheduling: Each randomly selected user isscheduled to transmit on its best available sub-channel.For example, the first randomly selected user is scheduledto transmit on its best sub-channel which will not bereused by other users. Then, the second randomly selecteduser (excluding the user which has been scheduled a sub­channel) is scheduled to transmit on its best available sub­channels. The same process continues until all the usersare being scheduled with their respective sub-channels.

III. SIMULATION CONFIGURATION

A. System Level Simulation Setup

In the system level simulation, a 19-hexagonal cell layoutwith one sector per cell is assumed for both wireless accessschemes. In practice, usually a fixed and limited amount ofbandwidth are available. Therefore, in order to have a faircomparison between VSF-OFCDMA and OFDMA, the sameamount of bandwidth will be used by both schemes. Here,one-cell frequency reuse will be deployed in VSF-OFCDMAand while three-cell frequency cell reuse will be deployedin OFDMA as illustrated in Fig. 2. Base stations (BSs) arelocated at the center of each cell, and the mobile stations (MSs)are distributed uniformly over the whole cell. Here, we assumethat each MS is only connected to one target BS, from whichthe MS's received the strongest signal power among all theBSs. The 3GPP Spatial Channel Model (SCM) is used in thesystem level simulations. This model has a distance-dependentpath loss and lognormal shadowing as reported in [13]. In thispaper, only the urban-micro scenario is being considered. Thesystem level simulation parameters are summarized in TableI. The average received signal-to-interference-plus-noise-ratio(SINR) at each MS is defined as the ratio of the received signalpower from the target BS to the sum of the received signalpower from other cells and the background noise.

B. Link Level Simulation Setup

In the link level simulation, the 3GPP SCM multipathchannel model is used and only one cell is considered [13].The throughput achieved by a user in this cell is measuredas a function of the received SINR. The link level simulationparameters are summarized in Table II. Note that since thethree-cell frequency reuse is deployed in OFDMA scheme,the bandwidth of each cell (i.e., the number of sub-carriers)is 1/3 of that in VSF-OFCDMA scheme that deployed one­cell frequency reuse. In VSF-OFCDMA, the total spreading

Frequency

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Target Gell

(a) YSF-OFCDMA: one-cell frequency reuse

(b) OFDMA: three-cell frequency reuse

Fig. 2. Cell layout for (a) YSF-OFCDMA with one-cell frequency reuse,and (b) OFDMA with three-cell frequency reuse.

TABLE ISYSTEM LEVEL SIMULATION PARAMETERS

Cellular layout 19 cells, 1 sector/cell

Distance dependent path 3GPP SCM [13], Urban-loss and shadowing micro scenario

Radius, r 500 m, 1000 m, 1500 m

Carrier frequency 2 GHz

Background thermal noise -98 dBm

BS transmit power 43 dBm

BS antenna height 32 m

MS antenna height 1.5 m

MS velocity 10 m/s

MS distribution in a cell Uniformly distributed

factor is 8F == 16 in which only the time domain spreading isapplied i.e., 8Ft == 16 and 8F.t == 1. The applied modulationand channel coding scheme are summarized in Table III. Here,we assumed that perfect channel knowledge are available atthe receiver.

TABLE IILINK LEVEL SIMULATION PARAMETERS

System YSF-OFCDMAa OFDMAb

Bandwidth 101.5 MHz 34.0 MHz

No. of sub-carriers 768 256

Spreading factore 16 --

No. of subchannels 768 256/16 == 16

Spreading Scrambling Pseudo randomd --Code

Short Walsh-Hadamard

Symbol duration 9.259 f-LS

(Data + guard interval) (7.585 + 1.674) f-LS

Frame length 0.481 ms

(48 data + 4 pilot symbols)

Channel coding/ Convolutional coding

decoding Yiterbi decoding

Number of active users {I, 2, ... ,16} uniformly

distribution in each cell

Channel model 3GPP SCM, urban-micro scenario

aOne-cell frequency reuse.hThree-cell frequency reuse.cSF = SFt x SFf = SFt x 1.d768 chips/sub-carriers

C. Impact of Users' Location

In one-cell frequency reuse YSF-OFCDMA systems, theinter-cell interference (ICI) can be reduced by deploying thecell-specific long scrambling code. However, the residual ICIwill still have impact on the system performance. In this paper,we investigate the effect of the ICI when a MS is at a differentlocation within the cell. In particular, we consider a MS moveswith 10 m/s along the horizonalline from the center of a targetcell to the cell edge. Here, we define the target cell as thecenter cell of the 19-hexagonal cell layout (see Fig. 2). Thecell radius, r is 1000 m and QPSK modulated symbols withchannel coding rate R == ! are transmitted. The distances tothe target cell are O.lr, 0.3r, 0.5r, 0.7r, and 0.9r, respectively.

IY. SIMULATION RESULTS

A. Throughput Comparison of VSF-OFCDMA and OFDMA

Fig. 3 compares the cumulative distribution function (CDF)of the average received SINR for YSF-OFCDMA andOFDMA systems. It can be seen that YSF-OFCDMA haslarger probability of small SINR value than OFDMA espe­cially when r is small. This is due to the fact that three­cell frequency reuse is deployed in the latter and thus, itexperiences much smaller ICI than the former which deployone-cell frequency reuse. Interestingly, when the r increases,the differences between YSF-OFCDMA and OFDMA becomeinsignificant. For example, at r == 1500 m, both schemes havealmost identical performances. This is because when the r is

Fig. 3. CDF of the average received SINR for YSF-OFCDMA and OFDMA.

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Fig. 4. Throughput comparison of YSF-OFCDMA and OFDMA withdifferent scheduling schemes.

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V. CONCLUSION

In this paper, we have compared the performance of twowidely used broadband multiple access schemes for highdata-rate applications, namely, VSF-OFCDMA and OFDMA.Different from works already reported in the literature, here,

performance degrades with increased in PER and decreased inthroughput for both schemes. When the user is located closeto the center of the target cell, VSF-OFCDMA achieves muchhigher throughput than OFDMA due to its three times largerbandwidth utilization. However, when the user is close to thecell edge, OFDMA performs better than VSF-OFCDMA dueto three-frequency reuse. Note that for the analysis in thispaper, we fixed the spreading factor to SF == 16. We believethat by using longer spreading factor, the performance of VSF­OFCDMA for users at the cell edge could be further improvedby reducing the effect of ICI. However, this investigation willleave open as our future work.

TABLE IIIMODULATION AND CHANNEL CODING SCHEMES

large, only users at the cell edge will experience significantICI. On the other hand, when T is small, even users locatedclose to the center of the cell might also experience severeICI.

Fig. 4 shows the throughput comparison for VSF-OFCDMAand OFDMA with different scheduling schemes as describedin Section II. The throughput is achieved by using the mod­ulation and coding schemes given in Table III. As we cansee, from low to mid range of SINR values (i.e., < 25dB), VSF-OFCDMA performs better than OFDMA with fixedscheduling, but perform slightly worst when ideal and randomscheduling schemes are used in OFDMA scheme. This ismainly due to multiuser diversity gain brought by the latter twoscheduling schemes which compensate the effect of low fre­quency reuse. However, when the received SINR is high (i.e.,> 25 dB), VSF-OFCDMA achieves much higher throughputthan OFDMA irrespective of the scheduling schemes used asillustrated in Fig. 4.

The CDF of the achievable throughput as shown in Fig.5 can be obtained from Figs. 3 and 4. Different from theprevious works reported in [6] and [7] in which only thepeak throughput of a cell are shown, here, the "throughput" isdefined as the achievable throughput for a user using only onesub-channel (i.e., 1/16 of the total available resources). FromFig. 5, we can see that when T == 500 m, the probability thatthe throughput is less than 1 Mbps for VSF-OFCDMA andOFDMA (with ideal, fixed, and random scheduling schemes)are approximately 66%, 25%, 36%, and 30%, respectively.When T increases, the difference between VSF-OFCDMAand OFDMA at low achievable throughput become smaller(see Fig. 5(c)). However, irrespective to the value of T, ingeneral, VSF-OFCDMA achieves much higher throughputthan OFDMA. For example, the highest throughput achievedby VSF-OFCDMA is about 20 Mbps while it is only about 7Mbps for OFDMA i.e., approximately three times better.

B. Performance Comparison at Different Users' locations

Fig. 6 shows the packet error rate (PER) and Fig. 7 showsthe throughput comparison for VSF-OFCDMA and OFDMAwhen a user is at different locations. It can be seen that whenthe user moves away from the center of the target cell, the

VSF-OFCDMA OFDMAModulation & Data Modulation & Datacoding rate coding rate

(Mbps) (Mbps)QPSK, R=1/2 4.8 QPSK, R=1/2 1.6QPSK, R=3/4 7.2 QPSK, R=3/4 2.416QAM, R=1/2 9.6 16QAM, R=1/2 3.216QAM, R=3/4 14.4 16QAM, R=3/4 4.864QAM, R=1/2 14.4 64QAM,R=1/2 4.864QAM, R=2/3 19.2 64QAM, R=2/3 6.464QAM, R=3/4 21.6 64QAM, R=3/4 7.2

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106

107

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(b) r = 1000 m

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Achievable throughput (bps)

(c) r = 1500 m

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--+- OFDMA: Ideal scheduling-e-- OFDMA: Fixed scheduling~ OFDMA: Random scheduling-e- VSF-OFCDMA

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