Microwave Link Path Analysis
Transcript of Microwave Link Path Analysis
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EMClarity White Paper
BASIC PATH CONSIDERATIONS FOR A
MICROWAVE LINK
Date: May 2010
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BASICPATHCONSIDERATIONSFORAMICROWAVELINK
Date:May2010
Revised:
1.0SummaryThereareanumberoffactorstobecheckedwhenconsideringapossiblepathforamicrowave
link.Ideally,amicrowavelinkwouldoperatebetweentwopointswithoutpassingthroughany
materialsubstances,includingtheatmosphereandrain,andwithoutanyobjectsbeing
anywherenearthepathofpropagation.Suchalinkiscloselyapproximatedbyatypicallink
betweenanearthstationandageostationarysatellitewheretheonlyinterveningmaterialis
about12kmofatmosphereinthe36,000kmbetweentheearthstationandthesatellite.The
closerthesatellitelinklineofpropagationistoverticalthroughtheatmosphere,thesmallerthe
atmosphericeffectsonthesignalpropagation.
Aterrestrialmicrowavelinkusuallyhasapropagationpathapproximatelyhorizontallythrough
theatmosphereandthisaddsanumberofcomplicationstotheanalysisofthepath.Itisnotthe
intentionofthisnotetogointotheseissuesingreatdetailbutrathertooutlinethebasicpoints
foragoodunderstandingofhowterrestrialmicrowavelinksworkandwhattobeawareofwhen
consideringamicrowavelinkinstallation.
2.0NoiseandInterferenceTherearetwomainsourcesofnoisethatcanlimittheperformanceofmicrowavelinks.Thefirst
isthenoiseduetonaturalsourcesandthisisasignificantdifferencebetweenaterrestriallink
andasatellitelink.Asatelliteterminalbasicallylooksintodeepspacewheretheeffectivenoise
fromallsourcesisquitesmallinthemicrowavefrequencyrange.Typically,mostofthenatural
noiseforasatelliteterminalcomesfromthenoisefromthetemperatureoftheearththatis
pickedupintheearthstationantennasidelobes.
Aterrestrialmicrowaveantennatypicallylooksalongthehorizontalandsowillpickupnearlyall
thenoiseemittedbytheearthinthemicrowaveband.Thisgivesaterrestrialmicrowave
antennaaneffectivenoisetemperatureofabout300Ksothereislittlepointinusingexpensive
lownoisereceiverswithmicrowavelinks.
Noiseoriginatingfrommanmadesourcesisusuallyreferredtoasinterference.Thisisa
significantissuewithlinksthatoperateintheclasslicensebandwherethereareonlyverybasic
constraintsontheemittedpowerlevelsofthetransmitters.Thisissuewillnotbedealtwith
furtherinthisnote.
Forlinksinthelicensedbands,theregulationsarenominallysetuptoavoiddirect,highsignal
levelinterferencefromotherlinksoperatinginthesameoradjacentbands.Furthermore,the
generalnoiselevelfromothermicrowavelinksandsourcessuchasradarandsatellite
communicationsiscontrolledsuchthattheinbandnoisefromallsuchsourcesshouldnot
materiallyaffecttheperformanceofthelink.
InterferencethatisoutofthebandwillberemovedbythesharpfrontendfilteringinEMClarity
links.Theplanningshouldensurethatsignalsinthesamechannelorintheimmediateadjacent
channelsaresufficientlysmalltobenegligible.
Itisnotonlymicrowavesignalsthatcancauseinterference.Highlevelsignalsatlower
frequenciesmayleakinatvarioussitesbutinmostcasesthiscanbepreventedbyproper
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installationtechniquessuchasfullyshieldedcables,lowimpedanceearths,useofshielded
roomswheretheinternalequipmentishousedandwellregulatedandfilteredpowersupplies.
[SEEEMCLARITYNOTEXYZformoreinformationoninterference,itscauses,identificationand
resolution]
3.0PathAnalysisThefirstbasicrequirementforamicrowavelinkisthatthereisaclearlineofsight(optical)
betweenthetwopaths.Iftheantennaatoneendcannotbeseenfromtheotherend,eitherby
eyeorwithbinoculars,thenitisveryunlikelythatthelinkwillperformancesatisfactorily.
Thenextrequirementisthatnoobjectssuchashills,buildingsandtreesarewithinacertain
radiusofthenominallineofsight.ThisradiusisreferredtoastheFresnelzoneandisdependent
onthefrequencyandthedistancebetweenthetwoantennassinceitisdeterminedwherethe
pathlengthfromeachantennatotheobjectisonehalfwavelengthlongerthanthedirectpath
lengthbetweentheantennas.
TheFresnelradiusisamaximumatthemidpointandgraduallydecreasestowardseachantenna.
Theradiusinmetresisgivenby:
R=17.3(D1D2/(Fx(D1+D2))1/2(1)
WhereRisinm
D1andD2areinkmandarethedistancetotheobstaclefromeach
antenna
FisinGHz.
Closetoeitherantenna,theformulacanbeapproximatedby
R=17.3(D1/F)1/2 (2)
AndatthemiddlethemaximumvalueofRisgivenby
R=8.67(D/F)1/2 (3)
WhereD=D1+D2isthetotaldistancebetweentheantennas.
Theaboveformulaissuitableforhighgain,narrowbeamwidthantennaswithagaingreaterthan
25dBorso.
Example:Considera20kmpathwhereabuildingabout3kmfromoneendisseentobe
reasonablyclosetothelineofsightbetweentheantennas.Assumethatthelinkfrequencyis
8.2GHz.
Equation(2)givestheFresnelzoneradiusas9.6mwhichwouldbeabouttheminimumradial
spacingbetweenthebuildingandthelineofsight.
Atthe10kmpointtheFresnelradiusis13.5mwhereassay100mfromtheantenna,theradiusis
1.9m.
TheeffectoftheobstacleclosetoorwithintheFresnelzonedependsonthetypeofobjectandhow
closeitistothelineofsight.Smoothobjectssuchasgrassyhillsorabuildingroofcanhaveamuch
moresignificanteffectthanangularobstaclessuchasthecornerofabuildingordispersedobstacles
suchastrees.
Itmaybenecessarytouseprecisionopticalmeasurementsand/orGoogleEarthtodeterminehow
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stillperformquitewellwhenthisisincludedintheplanning.Ifallobstaclesarewellclearofthe
Fresnelzonethenthenextstepistoconsideratmosphericandearthcurvatureeffects.
4.0AtmosphericandEarthCurvatureEffectsMicrowavessignalsarenotsignificantlyaffectedbyfog,dustandpollutionlevels.Rainfallhas
littleeffectonfrequenciesbelowabout8GHzbuttheattenuationcausedbyrainincreasesquitequicklyandthefrequencyandrainfallrateincrease.Rainfallisthenamajorfactorforpaths
longerthan10kmandforfrequencieshigherthan10GHzorforshortpaths(5kmorso)at18GHz
andhigher.
Forpathlengthslongerthanabout10kmthecurvatureoftheearthandtheeffectofthe
atmosphereonthepropagationdirectionhavetobetakenintoaccount.Themicrowavesignal
typicallytravelshorizontallythroughtheatmosphereandthedielectricpropertiesofthe
atmospheremaychangewithheight.Thiscancausethemicrowavebeamtobendupordown
andsotochangetheeffectivecurvatureoftheearth.Thismayeffectivelymoveobjectsbelow
thepropagationpathintotheFresnelzoneorinsomecasesrightintothepathwhichcancause
largeincreasesinthepathattenuation.Thescienceofatmosphericpropagationhasbeenstudiedformanyyearsandtheengineeringof
linkstocompensateforsuchpatheffectshavebeencodifiedsothatstandardstatisticalbased
analysesareavailabletoestimatetheprobabilitythatthelinkwillfailduetotheatmospheric
andraineffects.
Forpathdistancesgreaterthanabout10kmorwherethereareanyobstaclesclosetoorwithin
theFresnelzone,EMClaritycanprovideacompletepathanalysis.
5.0DifficultPathsDifficultpathsarethosewherelargereflectionsmayoccurmainlyduetosmoothflatsurfaces
thatinterceptasignificantproportionoftheenergypropagatingbetweenthetwoantennas.
Suchpathstypicallyoccurwheretherearelongerdistancesof40kmormoreandflatsurfaces
suchaslakes,flatbaregroundortheseaalongsomeorallofthepath.Thepathisthensubject
tomultipathfadingwhichmayvarygreatlywiththetimeofdayastheatmosphericvariations
movethereflectionsinanoutofphasewiththemainsignal.
Sucheffectscanalsohappenovermuchshorterpathswhereoneorbothantennasmaybe
relativelyclosetoareflectingsurfacesuchastheroofofabuildingoronlyseveralmetresabove
thesealevel.
Thereareanumberofwaystoimprovethelinkavailabilityofsuchpathsbyproperpositioning
andalignmentoftheantennas,addingantennanullingalongthedirectionofthereflected
signal,increasingthetransmitpowerlevels,addingredundancybyusingextrafrequenciesor
extraantennasorsomecombinationoftheabove.
EMClaritycanassistwithdecidingontheoptimumconfigurationforanysuchpathsubjectto
costandavailabilityconstraints.