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Itwasdow
Getting the Most out of MIMO:
Boosting Wireless LAN Performance withFull Compatibility
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8/8/2019 MIMO WLAN Perf Whitepaper
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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.
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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.