通信物理层安全保密-方向调制

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IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.58,NO.5,MAY20101545

DemonstrationofDirectionalModulationUsingaPhasedArray

MichaelP.Daly,GraduateStudentMember,IEEE,EricaLynnDaly,andJenniferT.Bernhard,Fellow,IEEE

Abstract—Afour-symbolmodulationiscreatedbyrepeatedswitchingofphaseshiftersinaphasedarray,inatechniqueknownasdirectionalmodulation(DM).Thephaseshiftsarechosentominimizethebiterrorrate(BER)inaline-of-sightchannelinadesireddirectionwhilemaximizingtheBERelsewhere.ADMtransmitterisdemonstratedinananechoicchamber,andresultsarecomparedwithatraditionalbasebandQPSKmodulationusingthesamephasedarray.ExperimentsindicatethattheDMtransmittercreatesanarrowerregionoflowBERsaroundthedesireddirectionthanthetraditionalphasedarraywhilemain-taininghighBERsinthesideloberegions.

IndexTerms—Directionalmodulation,Phase-shiftKeying(PSK),phasedarray,securecommunication.

I.INTRODUCTION

T

HEtraditionalmethodofsendingdigitalinformationusingaphasedarrayinvolvessynthesizingthedigitalsignalatbasebandandthenupconvertingtothecarrierfre-quencybeforesendingthesignalthroughtheRFportionofthetransmitter.Phaseshiftersareusedtosynthesizearadiationpat-ternthatmeetscertaincriteria,suchasmaximizingthepowerradiatedinthedesireddirectionandminimizingitelsewhere.

Onedrawbackofthismethodisthatthesameinformationis

transmittedinthesidelobes,andthatinformationcanstillberecoveredwithasufficientlysensitivereceiver.Ontheotherhand,directionalmodulation(DM),alsocallednear-fielddirectantennamodulation(NFDAM),synthesizesthemodulationintheRFportionofthetransmitterratherthanbaseband,causingthetransmitteddigitalsignaltobedirection-dependent[1]–[6]. WithDM,thesynthesisofadigitalmodulationcanbeim-plementedviaparasiticelementsofanantennaarray[1],[2],[5],phaseshifters[4],ordrivenreconfigurablearrayelements[3],[6].DMallowsmorecontroloverthetransmittedmodula-tion,includingtheabilitytosendmultipleindependentsignalsindifferentdirectionswiththesameRFchainandtheabilitytoscrambleaconstellationinundesiredtransmitdirections.ThedistortionofconstellationsviaDManditssecuritybenefitsareexplainedin[4],butuntilnowDMhasnotbeendemonstratedwithreal-timetransmissionofdata.Thepresentworkdemon-stratesaworkingDMtransmitterusingaphasedarrayandcom-paresitsperformancewithatransmitterusingthesamearraybut

ManuscriptreceivedAugust28,2009;revisedNovember06,2009;acceptedNovember30,2009.FirstpublishedMarch01,2010;currentversionpublishedMay05,2010.TheworkofbothM.P.DalyandE.L.DalywassupportedbyNDSEGFellowships.

TheauthorsarewiththeElectromagneticsLaboratory,DepartmentofElec-tricalandComputerEngineering,UniversityofIllinoisatUrbana-Champaign,Urbana,IL61801USA(e-mail:mpdaly@;edaly@;jbernhar@).

DigitalObjectIdentifier10.1109/TAP.2010.2044357

withtraditionalbasebandmodulation.SectionIIprovidesde-tailontheimplementationofeachtransmitterandthecommonreceiver.SectionIIIshowsthemeasuredperformanceofbothtransmittersinthepresenceofnoiseanddiscussessomedesigntradeoffs.

II.EXPERIMENTALSETUP

TocomparetheperformanceofDMversusbasebandmodula-tion,threeexperimentsareconductedforeachtransmitterwhereadesiredreceiverislocatedinaline-of-sightchannelatbroad-side,,and,relativetothetransmitarray.Eavesdrop-pingreceiversmaybelocatedinanyotherdirectionbesidesthatofthedesiredreceiver,andtheirlocationsarenotknowntothetransmitter.Afour-elementmicrostrippatcharrayisusedforbothtransmitters.Thearrayelementsarespacedone-halfwave-lengthapartattheiroperatingfrequency,7GHz.Thereceiveantennaisastandardgainhornorientedtoreceivethedominantpolarizationofthemicrostrippatcharray.Signalstransmittedinthecross-polarizationarenotconsideredintheanalysisofthedesiredreceiveroranyeavesdroppingreceiver,andareasubjectforfuturework.

A.TraditionalBasebandArraySetup

Theexperimentalprocedureofthetraditionalphasedarraytransmitterwillbeexplainedfirst.AblockdiagramoftheentirearrangementisshowninFig.1.Thefirststepforthetraditionalphasedarrayistocalculatethenecessaryphaseshiftstosteertowardthethreereceiverdirections.Thecalculatedphaseshiftsarestoredinacomputerlocatedinsideananechoicchamberalongwiththetransmitandreceiveantennas,andfourfive-bitMiteqdigitalphaseshifters[7].Thephaseshiftersareactuallysix-bitbutthenumberofanalogoutputsfromthecomputerlimitstheamountofcontrolbitstofive.Thephaseshiftswerecalculatedassumingisotropicelementpatterns.Thus,somebeamformingerrorisintroducedbecausethemicrostrippatchpatternsarenotentirelyconstantovertheanglesofinterest,whileothererrorisduetothequantizationofthephaseshifts.Still,themeasuredpatternswhenphasedtothethreedesireddirectionsallhavemainlobesofapproximatelythesamemag-nitude,showninFig.2.Sincethemainlobessteeredoffofbroadsidearenotsignificantlylowerthanthemainlobewhenallphaseshiftersaresetto0,thissuggeststhatthephasingisclosetoideal.Oneothersourceoferroristhepresenceofacomputerinsidetheanechoicchamber,whichslightlydistortsthepatterns,causingoneofthesidelobesinthebroadsidepatterninFig.2tobeabout5dBhigherthantheother.

ThebasebanddigitalmodulationisgeneratedbyanAgilentE4438Cvectorsignalgenerator.Apseudorandombinaryse-quence(PN15)issentbythetraditionalandDMtransmitters

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Fig.1.Experimentalconfigurationofthedirectionalmodulationtransmitterandreceiver.

Fig.2.Normalizedmeasuredpatternswhenthetransmitarrayissteeredtobroadside,—30frombroadside,and+20frombroadside.

[8].TheseinformationbitsareusedtocreateGray-codedQPSKmodulationwithabitrateof200kbpsthatispassedthrougharoot-raised-cosinefilter.Thevectorsignalgeneratorupconvertsthemodulationtoanintermediatefrequency(IF)of500MHz,anditisthenexternallymixedto7GHz.TheRFsignalisampli-fiedbyabroadbandamplifierwith21dBgainandthenpassesthroughafour-waypowerdividerbeforepassingthroughthephaseshiftersandfinally,theantennaarray.

Afterreceptionbyastandardgainhorn,root-raised-cosinebandpassfiltering,downconversiontobaseband,anddigitalsamplingisaccomplishedbyanAgilentE4440Aspectruman-alyzer.NoiseisaddedtoachieveadesiredSNRandthesignalisdemodulatedinMatlab[9].A10MHzreferencesignalbetweenthelocaloscillator(LO)andthespectrumanalyzermakesaphaselockloop(PLL)unnecessary.

B.DirectionalModulationArraySetup

ThearrangementoftheDMtransmitter,showninFig.3,dif-fersfromthetraditionaltransmitterbecausethemodulationisnowsynthesizedintheRFportion.Thesignalsentintothephaseshiftersisasinusoidatthearrayoperatingfrequency.Thesignalleavingthephaseshiftersismodulatedduetothefast,repeatedchangesofthephaseshifters,andthesemodulatedsignalsarenotsimplydelayedcopiesofeachother.Rather,thesignalsleavingthephaseshiftersaremodulatedinawaysothattheycombineinthefar-fieldtocreatethedesired4-arymodulationonlyinthedesireddirection[4].

Insteadofcalculatingasinglesetofphaseshifts,asetiscal-culatedforeachdigitalsymbol(inthiscase,four).Thisrequiresknowledgeoftheactiveelementpatterns,whicharemeasuredbeforehand.Ageneticalgorithm(GA)(explainedin[4])calcu-latesthefoursetsofphaseshiftsbasedontheactiveelementpatternswiththegoalofminimizingthefollowingratio:

(1)

Inotherwords,thegoaloftheGAistominimizetheBERto-wardthedesiredreceiverwhilemaximizingitelsewhere.Thereisa“don’tcare”regionof5oneithersideofthedesireddirec-tionthatisnotpartofthe“undesireddirections”in(1)becauseitisatransitionregionfromlowtohighBERs.ThesolutionsfromtheGAarealsorestrictedtothosethatarepossibletobeproducedbythequantizedfive-bitphaseshifters.Inordertoin-creaseaccuracy,theactualphaseshiftsofthephaseshiftersweremeasuredandusedintheGA.Forexample,switchingthemostsignificantbitinoneofthephaseshiftersproducesa175.3shiftinsteadof180.AsafinalstepintheGA,thesetsofphaseshiftswereassignedtothefoursymbolsbasedonGraycoding.TableIshowsthesetofphaseshiftsusedforcommunicationtowardbroadside.

DALYetal.:DEMONSTRATIONOFDIRECTIONALMODULATIONUSINGAPHASEDARRAY1547

Fig.3.Experimentalconfigurationofthetraditionalphasedarraytransmitterandreceiver.

TABLEI

SETOFPHASESHIFTSFORDMTOPRODUCEFOURSYMBOLSWHENTHE

DESIREDRECEIVERISATBROADSIDEFROMTHETRANSMITARRAY

Afterthephaseshiftsarecalculated,theyareusedtocon-structatextfilethatgovernsthereal-timeswitchingofthephaseshifters.Foreachsymbolconsistingoftwobitsofthepseudo-randombinarysequence,controlvoltagesarerecordedtopro-ducethecorrespondingphaseshiftsforthatsymbol.Twope-riodsofthebinarysequence(32767symbols)areloadedintoacomputercontaininganalogcontrolvoltagesforthefivebitsofeachphaseshifter.Thecomputerrepeatedlyreadsthroughtheentiresequencechangingthephaseshiftcontrolbitsatarateof100kSymbols/sec,yieldingabitrateof200kbps.

ThereceiverforDMisnearlythesameasthereceiverfortraditionalQPSKmodulation.Anormalbandpassfilterisusedinsteadofaroot-raised-cosinefilter,sincenopulseshapingisdoneontransmit.ThetransmittedCWsignalstillsharesacommonreferencewiththedownconverterinthereceiver,soaphaselockloopisnotneeded.However,thesymboltimingintheDMtransmitterisnowregulatedbythecomputercontrollingthephaseshifters,whichdoesnotshareacommonreferencewiththereceiver’ssamplingclock.Therefore,thereceivedsignalisoversampledbyafactoroffourabovethesymbolrateandadelaylockloopisimplementedtodeterminethebestsamplingpoints.

Thebitrateislimitedbythespeedofthecomputerpro-ducingtheanalogoutputs,sinceitmustproduceoutputsfor20controlbitseachtimetwobitsaretransmitted.Theswitchingspeedofthephaseshiftersisactuallymuchfaster,ontheorder

Fig.4.Measureddownconvertedoutputofaphaseshifterfedwitha7GHzCWsignalandswitchedbetween0to180atarateof100kHz.

ofnanoseconds[7].ThetransienteffectsofswitchingaphaseshifterareshowninFig.4.Here,asinglephaseshifteriscon-nectedbetweenasignalgeneratoroperatingat7GHzandthereceiverbyawire.Themostsignificantbit(0to180)isre-peatedlychangedatarateof100kHz.Thereceiverthendown-convertsthesignalandcreatescomplexbasebandsamples.Tenperiodsofswitching(100)areshowninFig.4.Ittakesabouthalfofthesymbolperiodforthephaseshiftertotransition,andthereforeoversamplingbyafactoroffourguaranteesthatatleastonesampleshouldoccurwhenthetransmittedsymbolhasreachedsteadystate.Thediscontinuouspartsofthecurvesarelikelyduetoadisallowedbiasvoltage.Whenthebiasvoltagetransitionsbetween0Vand5V,thereisapointaround2.5Vwhereboththe0and180modesinthephaseshifterareoff.Thispointinthemiddleofthetwobiasvoltagesiswhatwecallthedisallowedbiasvoltage.Atthispoint,thephaseshifter’sin-sertionlossincreasesbyabout20dB,suppressingthesignal.

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Fig.5.(a)MeasuredBERswhenbothtransmittersaredirectedtobroadside.AlsoshownisthepredictedBERofthetraditionaltransmitterbasedonthemeasuredradiationpatternandthepredictedlowerboundoftheBERofDMbasedonthemeasuredactiveelementpatterns.(b)ThenoisepowerintheDMcaseisdecreasedby0.6dBsothatbothtransmittersachievethesameBERtowardthedesiredreceiveratbroadside.

III.EXPERIMENTALRESULTS

Intheanechoicchamber,theantennaarrayforbothtransmit-terswasrotatedfromtowhilethereceiverhornan-tennawasstationary,tosimulatereceiversatthesedirections.

Between1.9and2.0millionbitsweresentateachdirection

in10incrementsandwhiteGaussiannoisewasaddedwith

anoisepowerof52dBmoverthefrequenciesofinteresttoachieveanSNRof12dBinthedesireddirection.Incomparison,thereceivedsignalshavereceivedpowerlessthan40dBm.Theinputpowerforbothtransmitterswas7.5dBm.

Fig.5(a)showstheBERsofadesiredreceiveratbroadsideandothereavesdroppingreceiversfromto.AlsoshownarepredictedBERcurvesbasedonmeasuredradiationpatterns.ThepredictedBERfortheDMtransmitterisalowerboundcalculatedfromtheGAusingtheactiveelementpat-terns[4].ThepredictedBERforthetraditionaltransmitteriscalculatedusingthemeasuredpatterndatafromFig.2.There-lationbetweentheradiationpatternpowerandBERforQPSKisgivenin[4].ThepredictedBERforthetraditionaltransmitteragreeswellwiththemeasuredBER,andthemeasuredBERof

Fig.6.Averagereceivedsymbolpowerbybothtransmitterswhendirectedto-wardbroadside.

theDMtransmitterisalwaysslightlyaboveitscalculatedlowerbound.ThecloseagreementbetweenBERsestimatedfromra-diationpatternsandtheBERsmeasuredfromtransmittingadig-italmodulationisimportantbecauseitmeansperformancecanbeaccuratelyassessedwhendesigningaDMtransmitter(forexample,usingtheGAin[4],givenmeasuredorsimulatedra-diationpatterns).

OneimportantfeatureinFig.5(a)isthattheBERofthetradi-tionaltransmitterinthedesireddirectionislessthantheBERof

theDMtransmitter.Thisistobeexpectedsincethephasedarraymaximizesthepowerinthebroadsidedirectionasitssolepri-ority.Ontheotherhand,theDMtransmittertradessomeofthepowertransmittedinthedesireddirectionforanarrowerregionoflowBERsandhighBERsinallotherdirections.ThisisalsoevidentinFig.5(a)inthe20regionaroundbroadsidewheretheBERofaneavesdropperissometimesanorderofmagni-tudelowerifthetraditionalarrayistransmittingcomparedtotheDMarray.

However,inordertofairlycomparethenarrownessoftheBERregions,theBERinthedirectionofthedesiredreceivershouldbeequalforboththeDMandtraditionaltransmitters.Inthecaseofthedesiredreceiveratbroadside,thisisaccom-plishedbyraisingthesignaltonoiseratio(SNR)oftheDMtransmitter0.6dB(byloweringtheaddednoisepoweraftersignalreception),whichlowerstheBERinalldirections.ThisnewBERcurveisshowninFig.5(b)alongwiththesamemea-suredBERsofthetraditionalarrayfromFig.5(a).TheDMtransmitterisabletotransmitalowBERinanarrowerregionthanthetraditionaltransmitter,confirmingtheresultsfirstcal-culatedin[4].

ThereasontheDMtransmitterproducesanarrowerlowBERregioncanbefoundfromthereceivedpowerandthereceivedconstellations.Fig.6showstheaveragereceivedsymbolpowercalculatedfromtheradiationpatternofthetraditionaltrans-mitterandtheactiveelementpatternsoftheDMtransmitter.ThisreceivedsymbolpowerwasusedtocalculatethepredictedBERcurvesinFig.5(a).SinceallconstellationpointshavethesamemagnitudeinthetraditionalarraywithQPSK,theaveragesymbolpowerequalstheinstantaneoussymbolpower.Onthe

DALYetal.:DEMONSTRATIONOFDIRECTIONALMODULATIONUSINGAPHASEDARRAY1549

Fig.7.Receivedconstellationsfrombothtransmittersbyaneavesdroppingre-ceiverat+50whenbothtransmittersdirectedtowardbroadside.

otherhand,theDMarraycreatesarbitraryconstellationswithdifferentpowerfordifferentsymbols,soaveragesymbolpowerisusedtocomparethetwomethods.

Towardthedesiredreceiveratbroadside,thetwotransmit-terssendaboutthesamepower(afterincreasingtheDMtrans-mitterpowerby0.6dB).Butoffbroadside,theDMarraytendstosendmorepowerthanthetraditionalarray.Yet,themeasuredBERsareeitherlowerfortheDMarrayoraboutthesameasthetraditionalarray.Thereasonforthiscanbegleanedfromthereceivedconstellation.Forexample,thefirst200receivedconstellationpointsthatwouldbeseenbyaneavesdropperat whentheDMandtraditionaltransmittersareintendingtotransmitto0isshowninFig.7.FromFig.6,thesymbolpowercalculatedfromradiationpatternsis7.7dBhigheratfortheDMarraythanthetraditionalarray.Whenactuallymea-suredbysendingdigitalsignals,thereceivedpowerwas7.6dBhigherfortheDMarraythanthetraditionalarray.TheBERmeasuredatwasapproximatelythesameforbothtrans-mitters(0.20forthetraditionalarrayand0.16fortheDMarray).ThereasontheDMarrayachievesthissamehighBERtowardtheeavesdropperwhiletransmittingatahigherpowerlevelisevidentfromtheconstellationdiagram.Threeoftheconstella-tionpointsaregroupedclosetogether,eventhoughtheyarefarfromtheorigin.Thisindicatesthreesignalswithhigherpowerthatlookapproximatelythesame,andthusaredifficulttode-modulatecorrectly.Thetraditionalbasebandconstellationsarethesameshaperegardlessofwherethereceiverislocated,sotheonlywaytoincreaseBERandreducethechanceofdemodula-tionbyaneavesdropperistoreducethepowerofeachsymbol,orequivalentlyreducethesidelobelevelintheradiationpattern. Figs.8and9(a)showthepredictedandmeasuredBERwhenthedesiredreceiverisatand,respectively.ThesefigureshavethesamecharacteristicsasFig.5(a).ThelowBERregionisnarrowerfortheDMtransmitterthanthetraditionaltransmitter,whiletheBERsareapproximatelyequalbetweenthetwotransmittersinthesideloberegion.Inthecasewhenthedesiredreceiverisat,bothtransmittersproducethesameBERatwithequalinputpower,duetothefactthatthe

Fig.8.MeasuredBERswhenbothtransmittersaredirectedto—30.AlsoshownisthepredictedBERofthetraditionaltransmitterbasedonthemeasuredradiationpatternandthepredictedlowerboundoftheBERofDMbasedonthemeasuredactiveelementpatterns.

Fig.9.(a)MeasuredBERswhenbothtransmittersaredirectedto+20.AlsoshownisthepredictedBERofthetraditionaltransmitterbasedonthemeasuredradiationpatternandthepredictedlowerboundoftheBERofDMbasedonthemeasuredactiveelementpatterns.(b)ThenoisepowerintheDMcaseisdecreasedby0.1dBsothatbothtransmittersachievethesameBERtowardthedesiredreceiveratbroadside.

traditionalarray’smaximumoftheradiationpatternoccursatratherthan.

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Inthecasewhenthedesiredreceiverisatfromarraybroadside,theDMtransmitterproducedthesameBERasthetraditionaltransmittertowardwhentheSNRoftheDMtransmitterwasincreasedby0.1dB,showninFig.9(b).There-gionoflowBERonceagainisnarrowerfortheDMtransmitter.

IV.CONCLUSION

Thisworkpresentsthefirstexperimentaldemonstrationofdi-rectionalmodulationbytransmittingdatainreal-time.There-sultsagreewellwiththecalculatedresultsfrom[4],indicatingthataDMtransmittermanipulatesadirection-dependentsignalsothatitishardertodecodeinmoreundesireddirections.Inad-dition,theDMarraysendsasignalthatwillbedecodedbythedesiredreceiverwiththesamelowBER(withsomesmallin-creaseintransmitpowerpossiblynecessary)withnoadditionalworkneededbythereceiver.

FutureworkconsistsofotherimplementationsofDM,in-cludingusingvectormodulatorssothatbothmagnitudeandphaseofeachantennaelementcanbemanipulated.Anotherareaofresearchisthereal-timedemonstrationofDMusingreconfig-urableradiatingelements.Thesynthesisofmorecomplexmod-ulations,theuseofradiatedcross-polarizedfields,andincorpo-rationintonon-line-of-sightchannelsarealsobeingexplored.

REFERENCES

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[2]A.Babakhani,D.B.Rutledge,andA.Hajimiri,“Transmitterarchitec-turesbasedonnear-fielddirectantennamodulation,”IEEEJ.Solid-StateCircuits,vol.43,no.12,pp.2674–2692,Dec.2008.

[3]M.P.DalyandJ.T.Bernhard,“Reconfigurablearrayformulti-direc-tionalandsecurecommunication,”inProc.AllertonAntennasSymp.,Monticello,IL,Sep.2008,pp.116–131.

[4]M.P.DalyandJ.T.Bernhard,“Directionalmodulationtechniqueforphasedarrays,”IEEETrans.AntennasPropag.,vol.57,pp.2633–2640,Sep.2009.

[5]A.Chang,A.Babakhani,andA.Hajimiri,“Near-fielddirectantennamodulation(NFDAM)transmitterat2.4GHz,”inProc.IEEEAntennasPropag.Soc.Int.Symp.,Jun.2009,pp.1–4.

[6]M.P.DalyandJ.T.Bernhard,“Beamsteeringinpatternreconfigurablearraysusingdirectionalmodulation,”IEEETrans.AntennasPropag.,acceptedforpublication.

[7]DigitalPhaseShifters,MITEQ,Inc.[Online].Available:http://amps./datasheets/MITEQ-DPS.PDF

[8]B.P.Lathi,ModernDigitalandAnalogCommunicationSystems,3rded.NewYork:OxfordUniv.Press,1998.

[9]MATLABVersion65(R14)ServicePack2,TheMathworks,Inc.,Natick,MA,2005.

MichaelP.Daly(S’09)wasborninSanJuan,PuertoRico,onOctober31,1984.HereceivedtheB.S.degree(withhighesthonors)andM.S.degreeinelectricalengineeringattheUniversityofIllinoisatUrbana-Champaign(UIUC),in2007and2008,respectively,whereheiscurrentlyworkingtowardthePh.D.degree.

Hisresearchinterestsincludereconfigurablean-tennas,arrays,anddigitalcommunications.

Mr.DalyistherecipientofanNDSEGfellowship.

EricaLynnDalywasbornin1985inChicago,IL.Since2003,shehasstudiedelectricalengineeringattheUniversityofIllinoisatUrbana-Champaign,wheresheiscurrentlyworkingtowardthePh.D.degree.

Herresearchinterestsincludesignalprocessingandappliedcommunicationtheory.

Mrs.DalyistherecipientofanNDSEGfellowship.

JenniferT.Bernhard(S’89–M’95–SM’01–F’10)wasbornonMay1,1966,inNewHartford,NY.ShereceivedtheB.S.E.E.degreefromCornellUniversityin1988andtheM.S.andPh.D.degreesinelectricalengineeringfromDukeUniversityin1990and1994,

respectively,withsupportfromaNationalScienceFoundationGraduateFellowship.

WhileatCornell,shewasaMcMullenDean’sScholarandparticipatedintheEngineeringCo-opProgram,workingatIBMFederalSystemsDivision

inOwego,NewYork.Duringthe1994–95academicyearsheheldthe