DiversityAntennas.docx

DiversityAntennas.docx

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DiversityAntennas

GSMProductsDivision

DiversityAntennaSystems

For

GSM900/GSM1800/PCS1900Networks

IssueA

JunXiang

ProductManagement

GSMProductsDivision

Emailxiangj@

Phone+44（0）1793565680

Fax+44（0）1793514215

Aboutthisissue

ThisdocumenthasbeenrequestedbyanumberofstaffinMotorolasystem/fieldengineeringgroups.

Itisatechnicalprimerintendedtogivearationalbackgroundtotheissuesofdiversityantennaapplicationandselection.Thecontentallowsforaverywidevarietyofexperienceandapplicationknowledge.

ItisintendedforthisdocumenttocoverallthefundamentalinformationthatMotorolaGSMsystem/fieldengineersneedtoimplementadiversityantennasystem.Thecompletepackagewillinclude:

basicprinciples,applications,systemspecifications,Motorolapreferredantennavendors,proceduresforthediversityantennatrialandanalysistool.

Thereishoweveranurgentneedforearlierreleaseofthedocument,hencesomepartshavenotbeencompletedinthisissue.

UltimatelythisdocumentwillincludepracticalrecommendationsderivedfromallMotorolaexperiences.Theserecommendationswillalsoincorporatethemostcommerciallyadvantageousequipmentselectionsbasedonglobalpurchasingadvantage.

Thisversioncoverssection1to6andAppendixD.

TheantennavendorslistedinAppendixAareapreliminaryselectionbasedonapurelytechnicalanalysis.ThisisbecausetheantennavendorsneedtobecommerciallyassessedbytheWirelessNetworkProductsDivision（WNP）.GPDandWNPareworkingtogethertofinalisethisprocesswithinthenextfewweeks.Thefinalisedantennarecommendationswillbeashorterlistandwillbepublishedinthenextissueofthisdocument.

AppendixCdescribestheproceduresforthediversityantennatrialandmethodfordataacquisitionandpost-processing.TheGSMresearchgroupofGPDandvariousofothergroupsarecurrentlyworkingonthisproject.

Oncethemeasurementtoolhasbeendeveloped,largescalemeasurementswillbeorganised.ThetypicaldiversitygainwiththeMotorolaBTSequipmentcanthenbederivedfromthemeasuredresultsandpublishedinAppendixB.

Contents

1Scope

2Introduction

2.1Theneeds

2.2Conceptofdiversityantennasystems

2.3Realisationofthediversityantennasystems

3Spatialdiversityantennasystems

3.1Basicprinciples

3.2Configurations

3.3Specifications

4Polarisationdiversityantennasystems

4.1Basicprinciples

4.2Typesofdualpolarisationantennas

4.3Configurations

4.4Specifications

5Summary

6References

AppendixAProvisionalrecommendeddualpolarisationantennasandvendors

AppendixBTypicaldiversitygain

AppendixCGenerictestproceduresandanalysistool

AppendixDGenericspecificationsfordualpolarisationantennas

1Scope

Thisdocumentdescribestheprinciple,applicationandpracticalconfigurationofdiversityantennasystemsforGSM900/GSM1800/PCS1900networks.

TheaimofthedocumentistoprovidefundamentalinformationregardingthediversityantennasystemasaguidelinetoMotorolaGSMsystem/fieldengineersintheprocessofselecting,implementingandtestingthediversityantennasystem.

Thisdocumentisintendedforinternaluseonlypurpose.

2Introduction

2.1Theneeds

AdiversityantennasystemisanessentialpartofthefrontendRFdistributionsystematacellularbasesite.Theuseofadiversityantennasystemenhancestheabilityofacellularnetworktocombatthemultipathfadingandincreasesthesignalstrengthorsignal-to-noise（S/N）ratio.Theultimateaimistoenablethenetworktoofferhightransmissionquality.

Thedistinctphenomenoninthemobilepropagationenvironmentismultipathfading.Overcomingthemultipathfadingtomaintainanacceptableradiotransmissionqualityisthereforeoneofthekeyelementsthatdrivethearchitecturedesignofanymobilecommunicationsystems.

Inatypicalcellularradioenvironment,thecommunicationbetweenthecellsiteandmobileisnotbyadirectradiopathbutviamanypaths.Thisisbecausethedirectpathbetweenthetransmitterandthereceiverisobstructedbybuildingsandotherobjects.Thesignalthatarrivesatthereceiveristhereforelargelybywayofscattering,eitherbyreflectionfromtheflatsidesofbuildingsorbydiffractionaroundmanmadeornaturalobstructions.

Whenvariousincomingradiowavesarriveatthereceiverantenna,theycombineconstructivelyordestructively,whichleadstoarapidvariationinsignalstrengthasshowninFigure1.Thesignalfluctuationsareknownas‘multipathfading’.Thefluctuationcanveryoftenbesolargethattheradiolinksuffersfromeitherseveredeteriorationoftransmissionqualityorlossofthecommunication.

Diversitytechniqueshavebeenrecognisedasaneffectivemeanswhichenhancestheimmunityofthecommunicationsystemtothemultipathfading.GSMthereforeextensivelyadoptsdiversitytechniquesthatinclude

∙Inthetimedomain-interleaving

∙Inthefrequencydomain-frequencyhopping

∙Inthespatialdomain-spatialdiversity

∙Inthepolarisationdomain-polarisationdiversity.

Figure1

Illustrationofthemultipathfading

Thefundamentalprincipleofthepreviouslymentionedtechniquesistocreatevariouscopiesofsignalsthatcarrythesameorconsecutivepiecesofinformationbutarriveatthereceiverthroughdifferenttransmissionmedia（Figure2）intermsoftime,frequency,locationandpolarisation.Thosediversifiedsignalsexperiencedifferentfadingandhencetheprobabilitythattheyallsufferfromadeepfadeareconsiderablyreduced.Optimaluseofthediversifiedincomingsignalswillthenresultinanimprovementoftheradiotransmissionquality.

2.2Conceptofdiversityantennasystems

Ofthosetechniquesdescribedearlier,spatialandpolarisationdiversitytechniquesarerealisedthroughantennasystems.

Ingeneral,adiversityantennasystemprovidesanumberofreceivingbranchesorports（typically2inthepresentnetwork,thiswillbeassumedintherestofthisdocument）fromwhichthediversifiedsignalsarederivedandfedtoareceiver.Thereceiverthencombinestheincomingsignalsfromthebranchestoproduceacombinedsignal（Figure3）withimprovedqualityintermsofsignalstrengthorsignal-to-noiseratio（S/N）.Theuseofthecombinedsignalfordemodulationwillthenleadtoalowertransmissionerrorrate.

Figure2

Independentfadingderived

fromdifferenttransmissionmedia

Theperformanceofadiversityantennasystemprimarilyreliesonthebranchcorrelationandsignalleveldifferencebetweenbranches.

Correlationbetweentworeceivingbranches

Thebranchcorrelationcoefficient（ρ）representsthedegreeofsimilaritybetweenthesignalsfromtwodifferentreceivingbranches.Thecorrelationcoefficientrangesfrom0to1.

ρ=1meansthesignalsfromtwodifferentbranchesbehaveexactlythesame.Inthiscase,thesignalsarecoherent.

ρ=0meansthesignalsfromtwodifferentbranchesbehavecompletelydifferent.Inthiscase,thesignalsareuncorrelated.

Toachievethebestperformance,adiversityantennasystemisrequiredtoprovideuncorrelatedsignals.Inthescenariothatthesignalsareuncorrelated,thetwosignalsfluctuatedifferentlyasshowninfigure3.Asaresult,theprobabilitythatbothsignalssimultaneouslyexperiencedeepfadesisconsiderablyreduced.Byappropriatecombiningthetwouncorrelatedsignals,thereceiverisabletoderiveacombinedsignalwithincreasedsignalstrengthorS/NandhenceanimprovedBit-Error-Rate（BER）performance.

Theimprovementofthetransmissionqualitydecreasesasthedecorrelationbetweenthetwobranchesincreases.Forρ=0,thediversityantennabecomesineffectiveincombatingthemultipathfading.

（a）

（b）

Figure3

Diversitycombining

Anexampleofselectioncombining

Theprocessbywhichareceivercombinesthetwoinputstoproduceasignalwithgoodqualityfordemodulationiscalleddiversitycombining.Thesimplestcombiningtechniqueistheselectioncombiningwhichselectsthebestsignalfromthetworeceivingbranches.ThetechniquewidelyusedinGSMishoweverthemaximalratiocombining,whichaimsatmaximisingS/N.

Inreality,however,itisnotalwayspracticaltohaveadiversityantennasystemwhichguaranteesρ=0.Extensiveresearchinthisfieldhasrevealedthatadiversityantennasystemcanperformsatisfactorilyprovidedthatρ≤0.7.

ρ≤0.7isthereforeoneofthefundamentalcriteriaforthedesignofadiversityantennasystem.

Signalleveldifference

Thesecondkeyparameterforagooddiversityantennasystemisthemeansignalleveldifference.Thedifferenceisastatisticalparameterwhichindicatesthebalanceofthesignalstrengthsfromthetworeceivingbranches.

Ithasbeenfoundthatabalanceofthesignalstrengthsbetweenthetwobranchesisessentialforadiversityantennasystemtooperatetoitsmaximumefficiency.

Inarealsystem,thestatisticalbalancecanbeverifiedbycomparingthemeanvaluesofthetwosignalsmeasuredoveralengthyperiod,asinFigure（4）.Iftheratiobetweenthemedianvaluesis0dB,thetworeceivingbranchesarestatisticallybalanced.Thisisanidealscenarioforadiversitysystemtooperate.Theperformanceofthediversitysystemwilldeterioratewhiletheratioincreasesordecreasesfrom0dB.

Figure4

Illustrationofthesignalleveldifference

Table1[1]liststhediversitygain（G）achievedbythemaximalratiocombiningversusthesignalleveldifference（∆）.Thetableshowsthatthesignalleveldifferencemustbelessthan4dBinordertoachieve3dBdiversitygain.

Sincethediversitygainisnormallyassumedtobe3dBinalinkbudgetanalysis,