Transmission performance for WLAN employing switched beamforming array

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Transmission Performance for WLAN Employing SwitchedBeamformingArray Hsueh-Jyh Li, Zong-Hua You, Ta-Chien Yeh

Graduate Institute of CommunicationEngineering,NationalTaiwanUniversity, Taipei, Taiwan,R.O.C., email:hili(a)ew.ee.ntu.edu.tw

Abstract

Inthis paper weproposeda vectorwideband channel model andusethischannel model to compare the transmission performance of the WLAN system using omni-directional antennas and switched beamforming arrays for different channel situations. Wefound thatthegreaterthedifferencebetween the amplitude of the direct path and themultipath, the faster the multipathpowerdecaying and the smaller theray arrival rate, the more improvement can be obtained by employment of switched beamformingarrays.

Introduction

The wireless local area network (WLAN) uses orthogonal frequency division multiplexing (OFDM) techniquetomitigateintersymbol interference (ISI). Itis known that the smaller the channel delayspread,the smoother thefrequencyresponseand the smaller probability that the subcarrier will encounterdeep fading. Using a directive antenna notonlycanincreasethe antennagain,but also can suppressmultipathscoming form the sidelobe directions.Hence,the channel delayspreadcanbe reduced.However, if thetransmitter and receiverpositions are not known to eachother, the mainbeam directionshouldbeabletoadaptivelyadjust.Usingtheswitchedbeamformingarrays is thesimplestway toachievethispurpose [1].

TheSaleh-Valenzuela (S-V) model only considers the amplitude and time delayof eachpathcomponent[2].Inthispaper wewilladd the information ofanangle ofarrival (AOA) andanangleof departure (AOD)toeachpathcomponentandusethis modelto study theswitchedbeamformingarrays on WLANtransmissionperformance for different channel situations.

Wideband Vector Indoor Channel Model

TheS-Vmodelassumesthatraysarrive in clusters.Thearrivaltimesof the clusters andtheraysin each clusteraremodeledasPoisson arrivalprocess. In themodified single clusterS-Vmodel, the channel impulseresponse(CIR) is expressed by

h(t) =₁₌₀F,,ej@'6(t-T,)=

fl80(t)

+₁₌₁Y

f8,ej6'6(t

-r,)

where /3,, 9, and rIaretheamplitude, phaseand time delay of thethpathcomponent

respectively. lisanindependentrandomvariableuniformlydistributed over [0,2 n4).

rIcanbedescribed by aPoissondistribution with ameanarrival rate A.,B is the

amplitude of the first path, and 6X is a random variable having the relationship

3,2

exp(-r,

/ ) where y is thepower

decay

constant.

The S-Vmodel implicitly assumes that the transmitter antenna and the receiver antenna are both omni-directional so that angular information of each multipath component can be ignored. When the antennas aredirective,we have tofurtherconsider theangular informationof the AOD and theAOA. To simplify the following discussion, we only consider the azimuthal pattern and neglect the elevationpattern. In indoor

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environments, multipaths areresulted fromreflections,diffractionsandscattering.Since the transmitterand receiver canbe placedatanyposition,itis reasonable to assume that the AOAand the AODof each path are uniformly distributed over [0,2ur).

Whenclosely-spaced multiple antennaelements areemployedin thetransmitterside and/or thereceiver side,wemake twoassumptions. First,the CIR of eachtransmitting andreceiving element pairhas the samemultipathcomponents,i.e.,the sameamplitude, time delay,AOA andAOD. Second, thewaveof eachmultipath componentis aplane wave. Under the aboveassumptions we cansimulate the channel characteristics obtained byusing multipleantennaelements.

Performance ofSwitchedBeamforming Arrayfor Indoor WLAN Now wecompare the channelcharacteristics and transmissionperformance when switched beamforming arrays and omni-directional antennas are employed in WLAN system whichutilizes theOFDMtechnology. The parameters of theWLANsystemare accordingtoIEEE802.1 la.

Inthis section both the transmitting array andreceiving array have four elements which uniformly spaced with aspacingof halfawavelength. The weight of the mt/ antenna element at either side can beexpressed by

W.=exp(j(m-l)a)

wherea can be450,1350,-1350 or -450 dependingonthe desiredbeamdirection [3]. The four beams of the switched beam array areshown in Fig.1. Sincethetransmitting and receiving arrays bothhave4different beams, therearetotallyl6 beam combinations.Wechoosethebeam combination withthelargest outputpower as thedesired beamcombination.

Givenasetof{ /Io,r,A}, we randomly generatealarge number of CIRs,andeach multipath component is randomly assignedanAOAandan AOD. ForeachCIR, the correspondingreceiving output of the switched beamforming array is calculated. The cumulative distribution functions (CDF) of the subcarrier power over the 20MHz bandwidthareshowninFig.2-4fordifferentsetsofparameters.

Theparameters shown in Fig.2 are

yr=21.5nsec,

A=0.4nsec' and different values ofJo,15dB, 5dB and-5dB.It is notedthat

o=0Il5dB

correspondstothecaseofastrong LOSwhile,I0=-5dB correspondstothecaseofahighobstruction loss.We can see that the greater the/ o, the more improvement of power distribution by using switched beamforming arrays.Notonly theCDF curveshifts toright, but also its slope is getting larger. Eventhough theLOShas beenseriouslyobstructed, the switched beamforming array stillhas beamforming gain.

The parameters shown inFig. 3 are,80=I0dB, A=0.4nsec' and different values ofy,17.2nsec,28.6nsecand 43nsec. This is a caseof having LOS

since,/O=lOdB.

Itis seen that thegreater theattenuation constant, the greater the power distribution for the omnidirectionalantenna.The beamforming gain is greateras y issmaller.

Theparametersshown inFig.4are/8o=lOdB, r=21.5nsecand different values ofi, 0.2nsec-',0.4nsec-'and lnsec-'. It is seenthat foromni directionalantennas, the power distribution isgreater when the meanarrival rate is higher. However, after switched beamforming, theyhavecomparable power distribution, which implies that channels with smallermeanarrivalrate can havemorebeamforming gain.

Next we employ the channel responses to simulate the WLAN transmission performance.Weplotthe BERcurves versus themean singalto noiseratio(SNR),which isobtainedby averagingallsubcarrier powersoverthe20MHz bandwidth forthe SISO

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(single input single output)case.Shown inFig.5-7 areplots of theBER curvesforthe SISO caseand afterswitched beamforming. The channelparameter sets arethesame as those used in thepreviousexamples. The BER curve forthe AWGN channel is also plottedforcomparison. Itis seen that after switchedbeamforming,the BER curves all shift to theleft. Ifwedefine theminimum required SNR,(S/N)m,,,astheSNR gives a specificBER(forexample,104),then we can obtain(S/N),,,,for each different parameter sets. Itis seen that therequired (S/N)m,,, have been reduced afterswitched beamforming. However, thedegreeofimprovementalsodependsonchannel parameters. Fromthese plots, we can conclude that the larger theps, the greater the attenuation slope of multipathcomponents and the smaller the mean arrival rate, the moreimprovementcan be obtained. This conclusion can be explained by the previous CDF plots of the subearrier power ofFig. 2-4. Beamforming techniques can increase powergain and reducedelay spread.Asmallerdelay spreadwill result in a smoother subcarrierfrequency responseoverthe 20MHz bandwidth and therefore theprobabilitythatsubcarrier power suffering deep fadingwill be much smaller.

Conclusion

Inthispaper the vector wideband indoor channel model which includes notonlythe parametersof theS-Vmodel but also the AOA and AOD of eachpathcomponent has beenpresented. We use this channel model to study the channel characteristics and transmission performanceobtained byusingswitchedbeamforming arrays at boththe transmitter and thereceiver. Wefoundthat a switchedbeamformingarray caneffectively increase the powergain, decrease the probabilitythat thesubcarrier encounters deep fading and reduce the required

(S/N)_,,,

evenwhen the LOS is seriously obstructed. Howeverthe degree ofthe improvement dependsonthevalues ofchannel parameters. We found that the grater the/6, the greater the attenuation slope of the multipath component and the smaller the meanarrivalrate,themoreimprovementin transmission

performance

canbeobtained

by employment

of switched

beamforming

arrays. Reference

[1] J.C. Liberti and T.S. Rappaport, "Smart Antennas forWireless Communications," Prentice-Hall PTR, 1999.

[2]A. Saleh, R. Valenzuela, "A Statistical Model for Indoor Multipath Propagation," IEEEJoumal on Selected Area in Communications, Vol. 5, No. 2, Feb 1987, pp.

128-137.

[3]

Robert S.

Elliott,

"Antenna

Theory

andDesign," Prentice-Hall,Inc,1981.

-45 ,=13e 120 120 s 0 30 10 210 3 1 240 270 240 270 .=An' a 4 ,20 270 a,020 30 FgIs 202ot c bd f

Fig. IIllustration ofthe switchedbeamformingarray

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-

v'

s

lX,

_r ' §t'

_: z 'j

Jr v: 21 J : e::

i

1e' S ,, 11 ; o1e Jl

X-,|IJsI

_,,r,

|o Y'.t

IIOI'1.#7.

,,

_*4aStr

ffi ,.,;

w i' .^ {, A! gr I v X s :, G'I ,#, _ ,c, ,/ af, $ I 4 w/, r g i , I g J r

.r|s{

^|S||w|

X//|||

.u )_.--l -S_-0 o₀ 5₀ 10₁₀ 15₁₅!,ffio₂₀ is₂₅ ₃₀ 1gu D t-S 0 S 10 15 20 2S 20 P- G,i0piSb00031 (dB)

Fig.2CDF of thesubcarrierpower for the different valuesof8Is

10o'

.9s ,=S

;(

!4

_

t]

!

/P

40.

z

.

-10 4`5 0 S 10 15 20) 25 Pow- GWi- p. SA (dB) '>0o lO 0 0 10 10 25 20 3 PaiGinp S*..Hne(dB)

Fig. 3. CDF of thesubcamrierpower for thedifferent values ofy

100 2' 4 6RS 1 SNR(dB)

Fig. 4CDF ofthesubearrierpower for the differentvalues ofA

so

-10 ~6_

-1(

_SNR_(dB)

Fig. 6Plots ofthe BER curves for the differentvalues ofy

Fig. 5. Plots ofthe BERcurvesfor the differentvalues of 60

0 2 4 6 _0NR0_(dB)10 *2

Fig. 7Plots ofthe BER curves for the differentvalues of A

726

Ic