mimo wlan perf whitepaper

8/8/2019 MIMO WLAN Perf Whitepaper

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Itwasdow

Getting the Most out of MIMO:

Boosting Wireless LAN Performance withFull Compatibility


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OVERVIEW

Multipleinput/multiple

output

(MIMO)

technology

offers

tremendous

performance

gains

for

wireless

LANs(WLANs)atrelativelylowcost.Anysystemwithmultipleinputsintothereceiverandmultiple

outputstothetransmitterisaMIMOsystem,butimplementingsuchasysteminvolvesseveraldistinctly

differentradiotechniques.Someofthesetechniquesarebeneficialandfullycompatiblewithtodays

standardWLANequipment,whileothersdonotimproveperformancewhenusedwithexisting

equipment.

BecauseindustrystandardsforMIMOtechniqueswilltakemanyyearstofinalize,WLANusersand

equipmentvendorswhowantMIMOsbenefitsnowmustchoosebetweenMIMOtechniquesfully

compatiblewithtodaysequipmentandproprietaryMIMOtechniquesthatworkwithasinglevendors

equipment.BytakingadvantageofMIMOtechniquesfullycompatiblewithtodaysstandards,userssee

immediaterange/throughputbenefits,evenwithexistinglaptopcomputersandotherWLANenabled

devices.

AtherosVLocityMIMOtechnologyusesfullycompatibleMIMOtechniques.MIMOproductsbased

onthistechnologyprovidebetterperformanceatafarloweroverallcostthanproductsusingproprietary

MIMOtechniques.AfullycompatibleMIMOapproachoffersstraightforwardchoicesforbothWLAN

usersandequipmentvendors.

Thesechoicescomeatatimewhengreaterthroughputandrangearevitalforuserswhowanttostream

multimediacontentwirelesslythroughoutlargehomesandotherbuildings.Streamingmultimegabit

HDTVsignalsacrossalargehomewithoutdropoutsordeadspotsdemandsthataWLANkeep

throughputhigheveninchallengingareasandatrangesnormallybeseenasextreme.MIMOtechniques

areidealformeetingsuchperformancegoals.

Thispaper

explains

the

background

and

tradeoffs

involved

in

using

MIMO

techniques

and

shows

how

AtherosVLocitytechnologyachievestodaysbestMIMOperformancewhilemaintainingfull

compatibilitywithexisting802.11standards.

THE ORIGINS OF MIMO

MIMOconceptshavebeenunderdevelopmentformanyyearsforbothwirelessandwirelinesystems.

OneoftheearliestMIMOtowirelesscommunications applicationscamein1984withgroundbreaking

developmentsbyJackWintersofBellLaboratories.ThisMIMOpioneerdescribedwaystosenddatafrom

multipleusersonthesamefrequency/timechannelusingmultipleantennasatthetransmitterand

receiver.Sincethen,severalacademicsandengineershavemadesignificantcontributionsinthefieldof

MIMO.

ManyWLAN,WiMax,andcellularcompaniesoffer(orareplanningtooffer)MIMObasedsolutions.Existingapplicationsincludemultipleantennasystems,CodeDivisionMultipleAccess(CDMA)systems

usedin3Gcellularsystems,andevenDigitalSubscriberLines(DSL)withmultipletelephonelines

experiencingcrosstalk.

2005 Atheros Communications, Inc. All rights reserved. June 2005 - 3 -


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MIMO MULTIPLIES POWER

InMIMO,multipleinmeansaWLANdevicesimultaneouslysendstwoormoreradiosignalsinto

multipletransmittingantennas.Multipleoutreferstotwoormoreradiosignalscomingfrommultiple

receivingantennas(seeFigure1).

Figure 1 Any system with multiple inputs into the receiver and multiple outputs to the transmitter is a multiple-input/multiple-output (MIMO) system. On a practical level, several techniques are involved in implementing a MIMO system.

Theseviewsofinandoutmayseemreversed,butMIMOterminologyfocusesonthesystem

interfacewithantennasratherthantheairinterface.Whatevertheterminology,MIMOsbasicadvantage

seemssimple:multipleantennasreceivemoresignalandtransmitmoresignal.

Infact,thereismuchmoretoMIMOthanthat.Onthereceiveside(seeFigure2),forexample,havingmultiplereceiversnotonlyincreasestheamountofreceivepower,butalsoreducesmultipathproblemsby

combiningthereceivedsignalsforeachfrequencycomponentseparately.

Figure 2 Maximal receive combining takes the signals from multiple antennas/receivers and combines them in a way thatsignificantly boosts signal strength. This technique is fully compatible with standard 802.11abg devices.

Thisprocessiscalledsubcarrierbasedmaximalreceivecombining.Itsignificantlyimprovesoverallgain,

especiallyinmultipathenvironments.Insuchenvironments,signalspassthroughandreflectfrom

variousobjectssothatdifferentsignalcharacteristicsreachthetworeceivingantennas.Somefrequencies

tendtobeattenuatedatoneantennabutnottheother,asshownbychannelmeasurementsinamultipath

environment(seeFigure3).



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Figure 3 Different antennas usually see different signal characteristics at various frequencies due to multipath effectsbetween transmitter and receiver. Subcarrier-based maximal receive combining intelligently combines the strongest signal ateach frequency for best overall signal strength.

Bycombiningsignalsfromtheantennasateachfrequency,maximalreceivecombiningincreasessignal

power.Atfrequencieswheresignalshavesimilarstrength,thereceiverselectivelycombinestheirsignalstrength,thusmorethandoublingthesignalpowerevenwhenusingonlytwoantennas.

Receivecombiningshouldnotbeconfusedwithantennadiversity.Thelatterdoesnotinvolveselecting

signalcomponentsbasedonstrengthatdifferentfrequenciesortheadditionofthesignalstrengthfrom

multipleantennas.Areceiverwithantennadiversitychoosesoneantennathatprovidesthebestaverage

performanceandignorestheotherantenna(s).Itispossibletoimplementthistechniqueusinganantenna

arraywithcontrolalgorithmsthatselectthebestperformingantennafrommomenttomomentbasedon

sophisticatedperformancecriteria.Systemsmayusethisformofantennadiversity,likebasiceither/or

diversity,inadditiontomaximalreceivecombining.

NotethatthetransmitterdoesnotneedMIMOtechnologytotakeadvantageofareceiversreceive

combiningcapability.

Transmitters

in

existing

devices

such

as

hot

spot

access

points

and

home

gateways,

laptop,anddesktopcomputersarefullycompatiblewiththisMIMOtechnology,whichappliesequally

wellto2.4 and5GHzbandstoimproveperformanceforallstandard802.11a,802.11b,and802.11g

devices.Unlikesomeothertechniques,receivecombiningincreasesperformanceonevenoneendofa

wirelessconnection.

MULTIPLYING TRANSMIT PERFORMANCE

UsingmultiplepoweramplifiersandantennasimplementstheotherhalfoftheMIMOequation(see

Figure4).Aphaseshiftingalgorithmallowsthetransmittertodrivemultipleantennasinawaythat

focusesmostoftheradiospowertowardsthetargetreceiver.Thisapproachcanincreaseeffective

transmitpowerbythesquareofthenumberoftransmitantennas.Withtwotransmitantennas,for

instance,thesystemeffectivelyquadruplestransmitpower.



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Figure 4 Using multiple transmitters and antennas is a MIMO technique that improves performance in two ways. First, itputs more signal power into the air. Second, it enables transmit beamforming to focus the radio energy toward the intendedreceiver. Both of these techniques are fully compatible with standard 802.11abg devices.

Thisincreasehastwocomponents:powergainandarraygain.Thepowergainresultsfrommultiple

transmitantennasdeliveringmorepowerintotheair,thusincreasingthetotalamountofenergybythe

numberofantennas.AtwotransmitterMIMOsystemdeliverstwicethepower,forexample.Thearray

gainresultsfromfocusingthedeliveredenergyinthedirectionofthereceiverbeamformingsothat

lessiswastedinotherdirections.Withtwotransmitantennas,thefractionoftotalenergysentinthe

desireddirectiondoubles.Thusthecombinationofthesetwoeffectsresultsinanetincreaseineffectivetransmitpowerofuptofourtimes.

Thisfocusingprocesscanbehighlyadaptivetothefrequencycharacteristicsofthetransmissionchannel.

Ifthebeamformingsystemhasreceivedatleastonepacketfromthetargetdevice,thesystemknowsthe

channelresponse,asillustratedinFigure3.(Thesamechannelresponseappliestobothtransmitand

receivedirections.)Thebeamformingsystemcanthereforeadjustthephaseofthesignalssentfromthe

twoantennassuchthattheyconstructivelyaddatthereceiveantenna(s)foreachsetoffrequencies.This

techniquereducestheeffectsofmultipath,evenwhentransmittingtononMIMOdevices.

TheMIMOtransmitterdramaticallyimprovestherangeoverwhichareceivercanobtainahigh

bandwidthsignal,soMIMOoffersbettercoverageforlargehomesoroffices.TheMIMOtransmitteralso

makes

WLAN

setup

easier

because

users

can

pick

up

a

usable

signal

even

at

extreme

ranges.

COMPATIBILE MIMO

Obviously,themostpowerfulMIMOapproachistouseallthetechniquesdiscussedsofarmaximal

receivecombining,multiplepoweramplifiersandtransmitbeamformingtomultiplythegainofa

WLANlinkseveralfold(seeFigure5).ThisMIMOapproachusesthethreetransmitterandreceiver

techniquestosendthesame,coherent802.11signalusedbyallexistingwirelessLANdevices.Together,

thethreecompatibletechniquesachievedramaticimprovementsinrange/throughputwhilealso

improvingperformancewithWLANdevicesthatdonotuseMIMOtechniques.

Figure 5 Using the techniques illustrated in Figures 2 and 4 on both sides of the radio link results in a WLAN system that improves

performance dramatically. At the same time, these techniques offer significant performance gains with existing 802.11abg devices.



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PROPRIETARY MIMO

TheinformationonVLocityMIMOdescribedinthispapershowsexactlyhowsuccessfulthecompatible

MIMOtechniquesareatimprovingperformance.Beforelookingatthisperformancedata,however,

consideraMIMOtechniquethatdoesnotimproveperformancewithstandard802.11abgdevices.

ThepreviouslydescribedMIMOtechniquessendandreceivethesamedatastreamthroughmultiple

antennas.Somesignalgainachievedbythisapproachcomesfromthefactthatthesignalsfromthe

multipleantennastraveloverdifferentspatialpaths.ThemultipathsignalsshowninFigure3indicatethiseffect.Thesamedatastreamtransmitsfrommultipleantennas,andmultipleantennasreceivethese

streamswithslightlydifferentfrequencycharacteristics.

Itturnsoutthatmultipleantennascansendentirelydifferentdatastreamsoverseparatespatialchannels,

andmultiplereceivingantennascanrecoverthesedatastreams(seeFigure6).

Figure 6 Spatial multiplexing is a MIMO technique in which multiple data streams transmit at the same frequency but overdifferent spatial channels. Standard 802.11abg devices cannot demultiplex these data streams, and thus cannot improvethroughput with this technique.

Thisschemeiscalledspatialmultiplexingbecauseonedatastreamismultiplexed(divided)between

multipletransmittingantennasandsentoverseparatespatialchannels.Thereceiverthendemultiplexes

the

spatial

streams

to

recover

the

original

data.

The

multiplexed

signal

can

only

be

demultiplexed

by

a

deviceusingthesameproprietaryMIMOimplementation.

ThisMIMOapproachholdsgreatpromiseforincreasingfuturechannelbandwidth,buttheapproachis

incompatiblewithtodays802.11abgdevices.Thatis,aspatialmultiplexingtransmittercanstill

communicatewithastandard802.11abgreceiver,butnotusingspatialmultiplexingtechniques.Such

productsmustfallbacktosingle transmitter/singlereceivermodewhenusedwithstandard802.11

devices.Inthesecases,thebenefitsofspatialmultiplexingMIMOarelost.

SpatialmultiplexingproductsarethereforebeneficialonlyforuserswhobuyallnewWLANdevices

basedonthesameproprietaryMIMOtechnology.ManydevicesthathaveintegratedWLANcapability

willnotbeabletotakeadvantageofthisMIMOtechnology,includingnotebookcomputers(Centrino

basednotebooks,forexample)andconsumerelectronicsequipmentsuchasdigitalmediaplayersand

PDAs.Giventhepriceofnewequipment,userswhoinstalltheproprietaryMIMOproductsarelikelytobedisappointedwiththeperformancewhenusedwithmost802.11products.

MIMO STANDARDS

SpatialmultiplexingcouldbeusefulifallWLANvendorsimplementedthetechniqueinacompatible

way.IEEE802.11nwilleventuallyprovideanindustrystandardforsuchimplementations.TheIEEE

802.11TaskGroupNisstillintheprocessofselectingabaseproposalthatthetaskgroupwillthenneed

torefine.Astandardwillprobablynotberatifieduntillate2006orearly2007.



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Spatialmultiplexingproductsintroducednowthushavetwodrawbacks:Theycannotprovidespatial

multiplexingbenefitstolegacydevicesandwillmostlikelybeincompatiblewiththeeventualMIMO

standardspecifiedin802.11n.

RushingtoimplementspatialmultiplexingnowmakeslittlesensebecausethereisanalternativeMIMO

approachthatoffersfullcompatibility,withdramaticperformancegainsforall802.11abgdevices.Ona

practicallevel,AtheroscompatibleMIMO(combiningtransmitbeamformingwithmaximalreceive

combining)plusSuperGtechnologymeetsorexceedstheperformanceofspatialmultiplexingproductsinmostcases,asshownlaterinthispaper.

InadditiontoMIMOtechniques,802.11nwillundoubtedlyincludeseveralotherperformance

enhancementtechniques.AlloftheproposalsthathavebeenunderconsiderationbytheIEEE802.11Task

GroupNincludesupportforpacketburstingandfastframes,alongwith20/40MHzmode(knownas

TurboModeinAtherosproducts).AllthreeofthesetechniqueshavelongbeenpartofAtheros SuperG

andSuperAGtechnology.

THE POWER OF VLOCITY MIMO

AtherosVLocityMIMOtechnologyimplementsallofthecompatibleMIMOtechniquesdescribedhere.

Asnotedpreviously,thesetechniquesboostperformancemanytimesover,evenwhenonlyoneVLocity

MIMOdevicecommunicateswithanonMIMO802.11abgdevice.

PlacingaVLocityMIMOdeviceonbothsidesofthelinkachievesmoredramaticimprovements.Measuring

theseimprovementsinvolvestestsintwodifferentenvironments:afreespaceenvironment(measuringthe

lineofsightperformancebetweenthetwodevices)andanenvironmentthatincludesthemultipath

characteristicsofthetypicalhomeoroffice.ThetestresultsshowthebenefitoftheVLocityMIMO

technologycomparedtononVLocityMIMOdevices,withlinkimprovementvaluesgivenindecibels.A

valueof3dBequatestoadoublingofapparentpower(i.e.a100mWtransmitterwouldappeartoprovide

200mWofsignalpower).Table1summarizestheVLocityMIMOtestresults.

Table 1: VLocity MIMO Performance Gains (dB)

ScenarioTwo-Power-Amplifier-

GainBeamforming/ Receive

Combining GainTotal

VLocity MIMO VLocity MIMO(line-of-sight)

3 6 9

VLocity MIMO VLocity MIMO(home/office)

3 6-9 9-12

VLocity MIMO Legacy(line-of-sight)

3 3 6

VLocity MIMO Legacy(home/office)

3 3-6 6-9

Legacy VLocity MIMO(line-of-sight)

0 3 3

Legacy

VLocity MIMO(home/office) 0 3-6 3-6

WhenVLocityMIMOdevicesareatbothsidesofthelinkinafreespaceenvironment,measuredresults

showthattheeffectisadditivewhilemaintainingfull802.11abgcompatibility.Thetotallink

improvementis9dB.Ofthistotal,3dBcomesfrommaximalreceivecombining,3dBfrombeamforming,

and3dBfromtheuseoftwopoweramplifiersthatcoherentlydelivertwicethepowerinasinglesignal.



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Inhomeorofficeenvironmentswithvariableamountsofmultipath,thelinkimprovementisthesameor

evengreater.VLocityMIMOstillgets3dBofimprovementfromthetwopoweramplifiersplus69dB

improvementfrombeamformingandmaximalreceivecombining.Thus,usingVLocityMIMOtechnology

onbothsidesofatypicalhomeorofficelinkshowsimprovementsasgreatas12dBmorethan1000mW

ofequivalenttransmitterpower.

COMPARING VLOCITY MIMO WITH SPATIAL MULTIPLEXING

TocomparetheperformanceofVLocityMIMOproductsandcurrentspatialmultiplexingdesigns,

AtherosmeasuredTCPdownlinkthroughputforvariouslocationsinandneara3000squarefoothome

(seeFigure7).

Figure 7 Throughput measurements of Atheros VLocity MIMO devices and spatial multiplexing devices show that theAtheros devices provide superior performance for almost all locations in and around a large home. The Atheros devices useMIMO techniques that are fully compatible with non-MIMO 802.11abg devices.

TCP Downlink

0.00

10.00

20.00

30.00

40.00

50.00

60.00

Living Breakfast Kitchen Office Bedroom 3 Bedroom 2 Bedroom 1 Car Street

Location

Throughput(Mbps)

Pre-n Spatial Mult iplexing Access Point VLocity MIMO/CB63

TheVLocityMIMOthroughputwassuperiorinmostlocationsandonlyexceededbythespatial

multiplexingsysteminonelocation.Currentspatialmultiplexingproductsfailtoprovidejustificationfor

usingproprietarytechnologytogainasignificantperformanceadvantage.

Inbothtestsandwiderangingsimulationsthatcovermanypossibleconditions,VLocityMIMOs

maximumTCP/IPthroughputexceedsthatofspatialmultiplexingproductsby20percentormore,and

VLocityMIMOproductscostsignificantlyless.

COMPATIBLE MIMO NOW

ForusersofbusinessandhomeWLANswhoneedadditionalrangeand/orthroughput,MIMOcanoffer

thedesiredimprovementsimmediately.ToseethoseimprovementswithexistingWLANstationsstillin

place,however,usersmustleveragethefullycompatibleMIMOtechniquesmaximalreceive

combining,multiplepoweramplifiersandphasedarrayadaptivetransmitbeamforming.

AtherosVLocityMIMOproductsimplementtheseMIMOtechniquesusingsophisticatedmethods

toobtainahighperformance,costeffectivesolution.LongbeforeMIMOstandardsbecomeavailable,

Atherosisprovidingfullycompatibletechnologythatimprovesperformancewithany802.11abg

device.



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Atheros Communications, Inc.

529 Almanor Avenue

Sunnyvale, CA 94086

t: 408/773-5200

f: 408/773-9940

www.atheros.com

Subject to Change without Notice.

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