forSoftwareDefinedRadio
VincentJ.Arkesteijn,EricA.M.KlumperinkandBramNauta
MESA+ResearchInstitute,IC-DesignGroup
UniversityofTwente,P.O.Box217,7500AEEnschede,theNetherlandsv.j.arkesteijn@utwente.nl,http://icd.el.utwente.nl/
Abstract—ASoftwareDefinedRadio(SDR)isaradioreceiverand/ortransmitter,whosecharacteristicscantoalargeextentbedefinedbysoftware.Thus,anSDRcanre-ceiveand/ortransmitawidevarietyofsignals,supportingmanydifferentstandards.
Inourresearch,wecurrentlyfocusonademonstratorthatisabletoreceivebothBluetoothandHiperLAN/2.ThishelpsustoidentifyproblemsassociatedwithSDR,andwillprovideatest-bedforpossiblesolutionstotheseproblems.Thetwostandardsdiffersignificantlyincharacteristicslikefrequencyband,signalbandwidthandmodulationtype.Combiningtwodifferentstandardsinonereceiverappearstoposenewdesignchallenges.Forexample,inthewidefre-quencyrangethatwewanttoreceive,manystrongsignalsmayexist.Thisleadstoseverelinearityrequirementsforwidebandreceivers.
Thispaperdescribessomereceiverarchitectures.Onedesignhasbeenselected.Thisreceiverhasbeenbuilt,andsomemeasurementresultsareincluded.
bandch.bandwidthch.spacingnom.bitratemodulationmult.accessduplex
Bluetooth[4]2.4–2.48GHz∼600kHz1MHz1Mb/sGFSKFHSSTDDHiperlan/2[3]5.15–5.725GHz∼16MHz20MHz6–54Mb/sQAM+OFDMTDMATDD
TABLEI
SOMECHARACTERISTICSOFBLUETOOTHANDHIPERLAN/2
I.INTRODUCTION
ASoftwareRadioisaradioreceiverand/ortransmitterimplementedfullyinsoftware.Becausesoftwarerunsondigitalhardwareandradiowavesareanaloguebynature,ananalogue-to-digitalconverterisusuallyincluded.Duetotechnologyconstraintshowever,thisapproachisinfea-sible.
Inrecentyears,interestforSoftwareDefinedRadio(SDR)hasbeenincreasing,asindicatedforexampleby[2].InaSoftwareDefinedRadio,allrelevantfunctionsoftheradiocanbedefined(controlled,programmed)bysoftware.Thisdoesnothowevernecessarilymeanthatallfunctionsareimplementedinsoftware,asinaSoftwareRadio.
SoftwareDefinedRadiocanbringmanyadvantages.Oneadvantageistheconveniencefortheuser.Havingamulti-standardterminal(mobiletelephone,laptopwithwirelessLANinterface)enablesglobalroaming,withoutcarryinganabundanceofhardware.
Asecondadvantageisashorterdevelopmenttimeandcostforthemanufacturer.Assumingthatsoftwarecanbedevelopedfasterthanhardware,aSoftwareDefinedRadio
canbeupgradedtoanewstandard,anewversionofthestandardorfittedwithabetterfiltermuchfasterthanaconventionalradio.
AlastadvantageofSoftwareDefinedRadiomentionedhere,isitsadaptabilitytoadynamicenvironment.ASoft-wareDefinedRadiocandynamicallymakeatrade-offbe-tweenperformanceandenergyconsumption.Bymini-mizingtheperformance(whilestillmaintainingarequiredqualityofservice),batterylifecanbemaximized.Inourproject[1],weaimatSDRfront-endhardware.Twogroupsareinvolved;theIC-Designgroupconcentratesontheanaloguepartofthefront-end,theLaboratorySignalsandSystemsonthedigitalpart.Thispaperfocussesontheanaloguepart.
InordertolocatetypicalSDR-relatedproblems,andtohaveatest-bedforpossiblesolutionstotheseproblems,itwasdecidedtobuildademonstrator.Thisdemonstra-torshallbecapableofreceivingBluetooth[4]andHiper-LAN/2[3]signalsandofdemodulatingthemcorrectly.SomecharacteristicsofthesetwostandardsareshownintableI.Ascanbeseen,thesestandardsdifferconsiderably,whichshouldhelpinidentifyingtypicalSDR-problems.Inthenextsection,threearchitecturesarepresented,andoneisselected.Theselecteddesignhasbeenbuilt,andsectionIIIdiscussessomeresults.Finally,conclusionsaredrawnandsomeideasforfurtherresearcharepresented.
165
AD
2G-6G
LNA
Fig.1
ASOFTWARERADIOFRONT-END
TorelaxtherequirementontheADC,asecondarchitec-tureisproposed.Thisisshowninfigure3.SincetheADCisprecededbyadownconverterandalow-passfilter,sam-plerateandresolutionrequirementsarerelaxed.
Aproblemstillremains,however.BoththeBluetoothandtheHiperlan/2-standardspecifyout-of-bandsignallevelsatwhichcompliantreceivershavetomaintainacer-tainbiterrorrate.Theselevelsaresuchthatinasingle-bandreceiver,thesesignalscanbeattenuatedbyasimplesecondorfourthorderbandpassfilter,andthereforedonotpresentaproblem.Inthisreceiverhowever,theseout-of-bandsignalsarenotattenuated.Thisresultsinextremelyhighlinearityrequirements.Itwascalculatedforinstance,thatanIIP2of+82dBmandanIIP3of+36dBmwerere-quired.Thiswasdeemedunfeasible,basedonaliteraturestudyofstade-of-the-artintegratedfront-ends.
TorelaxlinearityrequirementsontheLNAandmixer,athirdarchitectureispresented.Seefigure4.InsteadofoneRFfilter,twoarenowpresent.Thesefiltersattenuatestrongout-of-bandunwantedsignals.Thisleadstofeasi-blelinearityrequirements.
Ofcourse,thislimitstheflexibilityofthearchitecture.Butsinceoneantennacoveringthewholefrequencyrangewouldalsobeproblematical,especiallywhenalsotrans-mitting,aswitchwouldberequiredanyway.Anoptionwouldbetointegrateeverythingononechip,excludingtheantennasandfilters.Thisway,developmentofare-ceiverforanewstandardwouldstillbespedup,becauseonlytheantennaandfilterwouldhavetobedesigned.Thepresentedarchitectureisalow-IFreceiverwhenusedforBluetoothreception,andazero-IFreceiverwhenusedforHiperLAN/2reception.
III.
IMPLEMENTATION
Fig.2
POWERCONSUMPTIONSOFADC’SASAFUNCTIONOF
SIGNALBANDWIDTHANDRESOLUTION
II.
ARCHITECTURECONSIDERATIONS
ThissectiondescribessomeofthedesignchallengesindesigningaSoftwareDefinedRadio.Thisisdonebystart-ingwithaverysimpleandflexiblereceiver,andgraduallychangingthisintoanarchitecturethatisfeasiblewithcur-renttechnology.
Thefirstarchitecturetobeconsideredisanidealsoftwareradio.Thisisshowninfigure1.Theantennasignalisfiltered,amplifiedbythelownoiseamplifier(LNA)andconvertedtodigitalbytheanalogue-to-digitalconverter(ADC).Asthebandwidthis4GHz,thiswouldrequireanADCwithasamplerateofatleast8GHz.
Furthermore,therequiredresolutionwouldbeveryhigh,ascanbeseenasfollows.Signalsofupto0dBmmaybepresentatthereceiverinput[3].Atthesametime,themaximuminputnoisetothedemodulatorisaround−164dBm/Hz,or−68dBm/4GHz.ThisrequiresanSNRof68dB,correspondingto12bitsofresolution.Thiscom-binationwouldleadtoexcessivepowerconsumptionwhenfeasible,ascanbeseeninfigure2.Thisisnotexpectedtochangesignificantlyinthenearfuture[5].
Thearchitecturepresentedinfigure4haslargelybeenbuilt.Theantennas,RFfiltersandbandswitchhavebeenomitted.Therestofthereceiver(LNA,powersplitters,mixers,filters)hasbeenbuilt.Allcomponentsareonseparateboards,connectedtogetherusingcoaxialconnec-tors.Thisfacilitateseasyexperimentingwithotherarchi-tectures.Thefollowingcomponentshavebeenused.LNA
powersplittermixers
90°powersplitterlowpassfilters
Mini-CircuitsERA-2Mini-CircuitsZN2PD-9GMini-CircuitsMBA-671Mini-CircuitsZN2PD-9G+adjustabledelayline
discrete7thorderButterworth,10MHzcut-offfrequency
Forthetimebeing,asignalgeneratorisusedasalocal
166
AD
0°
2G-6G
LNA
90°
LO
AD
0-10M
Fig.3
ASOFTWAREDEFINEDRADIOFRONT-ENDWITHONEWIDERFFILTER
AD
ant1
2G4-2G5
0°
LNA
ant2
5G1-5G7
0-10M
AD
90°
÷2
4G8-5G7
Fig.4
ASOFTWAREDEFINEDRADIOFRONT-ENDWITHSWITCHABLERFFILTERS.THETWOSWITCHES,THELOFREQUENCY
ANDTHEGAINOFTHEIFAMPLIFIERSARESOFTWAREDEFINED.
oscillator,andadigitaloscilloscopeasADC.
Aphotographofpartofthesetupcanbeseeninfigure5.Ontheleft,theLNAcanbeseen,mountedonaRogerssubstrate(white).Itisconnectedtoapowersplitter.Thisisfollowedbythetwomixers,againmountedonRogerssubstrates.AnotherpowersplitterprovidestheLOsignaltothetwomixers.Onecanclearlyseethedifferentlengthoftheconnectingcables,resultinginaphaseshiftof90°(modulo180)betweenthetwochannels.Themixersarebothfollowedbyalow-passfilter,whichcanjustbeseenonthetopandtoprightofthephotograph.
Thereceiverhasbeentested.Thiswasdonebyapply-ingtestsignalstotheinputofthefront-end,andusingtheoutputdataoftheADC’sinMatlab.InMatlab,onecancomputetheaveragenoisepowerinvariouspartsofthe
spectrumtodeterminethenoisefloorofthereceiver.Thisisusedtocalculatethenoisefigure.TheSSBnoisefigureat2.4GHzis5.4dB,at5.5GHz14.5dB.Thisincludestheentirereceiver,fromLNAuptoandincludingtheADC’s.Someothertestshavebeenperformedaswell.Blue-toothandHiperlan/2signalswerepresentedtothereceiver,andtheoutputsignalscanbeseeninfigure6.Thesesig-nalswerealsodemodulatedonageneralpurposecom-puter.Moreinformationonthesedemodulationtestscanbefoundin[6]and[7].
IV.CONCLUSIONSANDFURTHERRESEARCHASoftwareDefinedRadioFront-Endtest-bedhasbeendesigned.ItworksbothasaBluetoothandaHiperLAN/2receiver.Animportantbottleneckforwidebandreceivers
167
Fig.5
PHOTOGRAPHOFPARTOFTHEFRONT-END
in: −70 dBm Bluetooth @ 2402.5 MHz−60−80)z−100Hk/mBd−120( G/tuoP−140−160−180−25−20−15−10−50510152025f (MHz)
(a)Bluetoothinputsignal
in: −68 dBm 64QAM HiperLAN/2 @ 5500 MHz−60−80)z−100Hk/mBd−120( G/tuoP−140−160−180−25−20−15−10−50510152025f (MHz)
(b)HiPerLAN/2inputsignal
Fig.6
MEASUREDOUTPUTSPECTRAOFTHERECEIVER.
appearstobethelinearityrequirements,causedbystrongout-of-bandsignals.Thiscanbesolvedbyusingswitch-ablefilters.
Asswitchablefiltersimpairflexibilityofthereceiver,animportantsubjectoffurtherresearchwillbethefront-endlinearityofwidebandreceivers.
V.ACKNOWLEDGEMENT
WethankourcolleaguesfromtheSignalsandSystemsgroupfortheirworkonthedigitalpartofthefront-endandforinterestingdiscussions.
ThisresearchissupportedbythePROGramforRe-searchonEmbeddedSystems&Software(PROGRESS)oftheDutchorganizationforScientificResearchNWO,theDutchMinistryofEconomicAffairsandthetechnol-ogyfoundationSTW.
REFERENCES
[1]http://nt5.el.utwente.nl/sdr/
[2]JosephMitolaIII,“SoftwareRadioArchitecture”,JohnWiley&
Sons,2000.
[3]ETSI,BroadbandRadioAccessNetworks(BRAN);HIPERLAN
Type2;Physical(PHY)layer.2001.ETSITS101475V1.2.2(2001-02).
[4]BluetoothSIG,SpecificationoftheBluetoothSystem-Core.Tech-nicalSpecificationVersion1.1,BluetoothSIG,February2001.[5]RobertH.Walden,“Analog-to-digitalconvertersurveyandanaly-sis”,IEEEJournalonSelectedAreasinCommunications,vol.17,no.4,pp.539–550,Apr.1999.
[6]L.F.W.vanHoeseletal.,“FrequencyOffsetCorrectioninaSoft-wareDefinedHiperlan/2DemodulatorusingPreambleSectionA”,MMSA2002,acceptedforpublication.
[7]LarsvanMouriketal.,“PerformanceEvaluationofaCombined
HiperLAN/2-BluetoothDigitalFront-End”,ProRISC2002,ac-ceptedforpublication.
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