EDX WP UseofClutterDatainRFPlanning Aug08 Web

8/8/2019 EDX WP UseofClutterDatainRFPlanning Aug08 Web

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Technology White Paper

TheApplicationofLandUse/LandCover(Clutter)

DatatoWirelessCommunicationSystemDesign

The Power of Planning 12008EDXWireless www.edx.com

TechnologyWhitePaper

TheApplication

of

Land

Use/

LandCover(Clutter)Datato

WirelessCommunication

SystemDesign


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HarryAnderson,TedHicks,JodyKirtnerEDXWireless,LLC

Eugene,OregonUSA

Introduction

Landuse/landcover(LULC)datacanplayasignificantofroleindesigningwirelesscommunicationsystems. Itcan

beusedtoimprovepredictionsofsignalattenuationandotherradiopropagationeffectsandtoassistinfinding

theoptimallocationofnetworkbasestationsandotherwirelesssystemtransmitters. Itcanalsobeutilizedwhen

projectingusage(traffic)trendsinanytypeofmobileornomadicsystem.

Withliterallybillionsofdollarsbeingspentannuallyonbuildingwirelesscommunicationsystems,thereis

significantincentivetouseengineeringtoolsthatcanaccuratelyandefficientlydesignandplansuchsystems.

Therecurrentlyareanumberofsoftwaretoolsonthemarketintendedforjustthatpurpose. Consideringthata

wirelessnetworkforasingleurbanareacanbecomprisedofhundredsofbasestations,anefficientsystemdesign

processcan

easily

justify

the

expense

of

the

design

tool

and

the

effort

using

it.

As

the

wireless

system

grows

to

meetincreasingandchangingdemandsforservice,thedesigntoolisagainavaluableassetinplanningoptimum

modificationstothesystemtoaccommodategrowth.

Offundamentalimportancetothewirelesssystemdesigntoolistheabilitytoaccuratelypredictthestrengthof

radiosignalsfromthevarioustransmittersinthesystem.Themathematicalalgorithmsusedforthesepredictions

aregenerallyknownaspropagationmodels. Originally,propagationmodelsreliedonterrainelevationdataasthe

soleenvironmentparameteronwhichtobasepredictions. Substantialefforthasbeeninvestedoverthelast2030

yearsindevelopingaccurateDTEMs(digitalterrainelevationmodels)forallpartsoftheworld. Whileterrainhasa

profoundeffectonthepropagationofradiosignals(especiallyathigherfrequencies),morelocalizedfeaturesof

theenvironmentsuchastreesandstructures(buildings,houses,etc.)alsohaveasubstantialimpacton

propagation. Animportanttrendinwirelesssystemarchitectureistousesmallercellcoverageareas(socalled

microcells)and

much

higher

radio

frequencies

where

significantly

greater

transmission

bandwidth

is

available.

For

suchshortrangesystemarchitectureswherethecoverageradiusofthetransmittermayrangefrom0.5to5

kilometers,theterraincanoftenberegardedaslocallyflat. Undersuchconditions,signalpropagationis

dominatedbylocalobstructions("clutter")ratherthanbyterrain,makingthedescriptionandaccurate

classificationoflanduse/landcoverdataofprimaryimportance.


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WirelessSystemPropagationModeling

Radiowavepropagationisaphysicalphenomenonthatcanbedescribedusingelectromagneticwaveequations.

Likewavestravellingawayfromthespotwhereastonehasbeentossedintoapond,radiowavestravelawayfrom

thetransmittingantennainalldirections. Theoreticallyitispossibletoexactlypredictthestrengthofthesignal

fromanytransmitteratanyotherlocationifalltheelementsofthepropagationenvironmentarecorrectlytaken

intoaccount. Insocalledfreespace(actuallyavacuum),therearenoelementsinthepropagationenvironment

andthesignalstrengthatsomedistancefromthetransmittercanbeexactlycalculated. Radiowavetransmission

throughouterspaceisoneregionwheresuchasimpleformulationapplies. Fortransmitterslocatedonthe

earthssurfacetheproblemismuchmorecomplicated. Everyphysicalentityaradiosignalencountersafterit

leavesthetransmittingantennaaffectsthestrengthanddirectionofthesignal. Thephysicalentitiesthataffect

thesignalcanbegroupedintofourbroadcategories:

1. Theatmosphere(orothergaseousmedia)refracts(bends)anddiffracts(scatters)theradiowaves;

bendingchanges

the

direction

of

the

radio

wave

while

scattering

generally

weakens

the

wave.

2. Terrainfeatures(hillsandmountains)blocktheradiowaves,requiringthemtodiffractoverthetopor

aroundthesides,weakeningthesignalontheotherside. Radiowavesalsoreflectandscatteroffof

terrainsurfacescausingachangeinthedirectionoftheradiowave.

3. Muchliketerrain,structuressuchasbuildings,houses,towers,etc.blocktheradiowaves.Thewaves

diffract,reflect,scatterandtransmitthroughstructures.

4. Theleavesandbranchesoftreesandothertypesoffoliagealsoweakenradiowavesbyscatteringthem,whichhasasimilareffectasthatoftheblockingthemasterrainorbuildingscando

Asmentionedintheintroduction,theatmosphereandterrainhavebeenincludedformanydecadesinthe

propagationmodelswhicharedesignedtopredictthestrengthofradiosignals. Figure1showsarepresentation

ofaradiowavepropagatingfromatransmitter(ontheleft)toareceiverlocationintheopen(ontheright).

Figure1 RadioPathwithTree

Althoughthesignalarrivingatthereceiverwillincludesomereflectionsfromtheterrain,thedominantsignalis

theonethatarrivesdirectlyfromthetransmitter. Atreeislocatedalongthispathnearthereceiver. Becauseof

thetree,thesignalissome1020dBweakeratafrequencyof2.5GHz(atypicalWiMAXfrequency). Thisisdueto

theeffectsdescribedaboveinitemfourandisasimpleexampleoftheeffectofgroundcoveronsignal


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propagation. Onewaytoincludetheseeffectsistouseheuristicpropagationmodels. TheOkamuraHata,COST

231,IEEESUI,and"Pointslope"modelsareexamplesofthese. Thesedesignersofthesemodelsattemptto

integratethe

effects

of

clutter

by

making

many

measurements

of

the

signal

in

typical

environments

and

then

creatingsimpledistancevs.pathlossformulasthatincludenotonly"freespace"lossbutalsotheclutterloss

foundinthatenvironment. Forexample,theOkamuraHatamodelisbasedonmeasurementsdoneinandaround

Tokyo,Japan. Therefore,themodelisrepresentativeoflanduseenvironmentssimilartothatarea. Thesemodels

canbequiteusefulwhenonedoesnothaveaccesstogoodqualitydigitizedgroundcoverdata.

Figure2 ReceivedPowerusingRayTracingPropagationModel

However,itisobviousthatasimplemodelcannotfullyconsidertheactuallocalconditionsofeverystudy

situation. Asanexample,Figure2showsatypicalurbanbuildingenvironmentinwhichatransmitterhasbeen

located. Inreality,theradiosignalfromthetransmitterwillbepropagatedtothereceiverbydiffractingand

reflectingfromthevariousbuildingsshowninthisfigure. Becausethesignalsarriveatthereceiverviaanumber

ofradiopaths,thesignalatthereceiverisoftendescribedasamultipathsignal. Whenusingasingleantennaat

thetransmitterlocationandoneatthereceiveramultipathsignalcanhaveasignificantimpactonthesignal

qualityorintegrityofdatawhichissentbecausemanyversionsofthesamesignalarearrivingatthereceiverat

differenttimes. Ascanbeseen,thesignalatlineofsightreceiverlocationsalongthestreetstotheNorthandthe

Eastofthetransmitterareconsiderablydifferent. However,inthisstudyasimplepropagationmodelwouldshow

thesamesignalalongbothstreetsbecausethemodelwouldconsiderallstudypointsintheareahashavingonly

oneclass

of

clutter

rather

than

seeing

the

effect

each

of

the

individual

buildings

on

the

signal.

As

the

environment

becomesmorecomplex,asimplepropagationmodelisincapableofaccountingcompletelyforhowthe

environmentaffectsthesignal.


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ClassifyingGroundcoverData

Araytracingpropagationmodelhasbeenusedheretoshowtheeffectthatabuildingenvironmenthasonradio

waves. However,formanysystemslikeWiMAXorCellular/PCS,adetailedanalysisofanentiremarketwithray

tracingmaynotcomputationallyviable. Moreover,acquiringdatabasesdescribingthelocationandcharacteristics

ofeverytreeorbuildingmaybetimeorcostprohibitive. Therefore,tosimplifytheproblemofaccountingfor

cluttereffectsinpropagationprediction,itisusefultobroadlyclassifythedatainsomeway.Forexample,instead

ofconsideringeverytreeinaforest,awiderareacouldbeclassifiedasforestandanyreceiverlocatedwithin

thatareawouldexperienceanadditionalsignalstrengthlossbasedonfrequency. Ifsignallossdatawerealso

availablefordifferenttypesofforest,e.g.evergreenversusdeciduous,ortalltreesversusshorttrees,then

multiplegroundcoverclassificationsforforestswouldbeappropriate.

ManyGIS(GeographicalInformationSystem)landuseclassificationschemesprovideforalargenumberof

categoriesthatmaybeappropriateforlanduseplanningorzoning. However,forthepurposeofwirelesssystem

design,unless

there

are

calculations

or

measurements

that

demonstrate

astatistically

significant

difference

betweencategoriesintermsoftheirrelativeimpactonradiosignals,havingtheadditionalcategoriesdoesnot

necessarilyprovidemoreaccuratesignalpredictions. Theheight,extent,andlocationofgroundcoverelements

arethefactorswhichprimarilygoverntheimpactonradiosignals. Differentclassificationtypessuchas

industrialandcommercialmaybeusefulinurbanplanning,butifbothindicaterelativelylowbuildingsof

broadextent,thenthedistinctionisunimportantforwirelessengineeringbothwillaffectthesignalinsimilar

ways. Inthesamemanner,thegridpointspacingresolutionofthesedatabasesneedstobesmallenoughonlyto

allowtheRFplanningtooltoreasonablylocatetheboundarybetweenadjacentcluttertypes. 30mistypical.

ApplyingClutterDatatotheCalculation

Clutterdatacanbeusedinseveralwaystoenhancetheresolutionofthesignallevelcalculationtoreturnmore

accuratepointspecificresults. Themoststraightforwardapproachistousethemethodalludedtoabovewhere

theattenuationataparticularreceiverlocationisadirectfunctionofthecluttertypeatthatlocation. Iffor

example,aremoteunitislocatedinasuburbanareacontainingsinglestoryhousesandmaturetreesonemight

applyanadditionalattenuationof20dBat2.5GHz;forrelativelyopenParklands10dBmightbeused. Theongoing

challengeinusingclutterisdeterminingtheappropriateattenuationvaluesforeachcluttercategory. Oneoption

istousetheTelecommunicationsIndustryAssociationresources. Thisgrouphastypifiedclutterlossesbasedon

tencluttercategoriesappropriatetoRFplanninganddocumentedthemaspartoftheTSB881document.

Ultimatelythemostaccuratevaluewillbebasedonreceivedsignalmeasurements(i.e.drivetest)withineach

marketareathattakeintoaccountthetypeofvegetationandmanmadestructurespresentinthatarea.

Asecondapproachistoassumethattheclutterrepresentsahard,nontransparentblockagetotheradiosignal.

Theclutter

data

is

then

used

to

effectively

raise

the

terrain

elevation

used

by

the

planning

tool

so

that

the

propagationcalculationwillseetheclutterasobstaclesalongthepathratherthana"bareearth"condition. Here,

theclutterdataiscategorizedashavinganabovethegroundheightvaluerepresentingtheaverageheightfor

eachtypeofclutter. Thismethodismosteffectiveatthehigherfrequencies,especiallyabove5GHzwherealmost

anycluttertyperepresentsahighlossobstacletotheradiosignal. Thiscanbeextremelyusefulinurbanareas

wherebuildingsrepresenttheprimaryobstaclestothesignal. Ratherthaninvestinginahighcostvectordatabase


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describingeachindividualbuildingasa3Dpolygonwithindividuallydefinedwallsurfaces,onecancreateaclutter

databaseusingaveryfinegridpointspacing(15meters). Thisallowsthetransitionbetweenbuildingclutterand

othertypes

of

clutter

(such

as

streets)

to

be

sharply

defined.

Then,

by

organizing

the

buildings

into

anumber

of

clutterclassesbasedonaveragebuildingheightareasonablethreedimensionalrepresentationofthebuilding

environmentcanbecreated. Table1belowcontainsrepresentativevaluesfortypicalbuildingheights.

Finally,athird

method

acknowledges

that

for

most

clutter,

the

signal

propagation

mechanism

is

of

a"pass

through

attenuation"type. So,unlesstheclutteriscomposedofbuildingsmadeofreinforcedconcreteorsomeother

extremelyhighlossmaterial,arelationshipcanbemadebetweensignalattenuationandthedistancethatthe

signaltravelsthroughaclutterarea(i.e.dB/m). Thisismostusefulwheremuchifnotallofthesignalpasses

horizontallythroughtheclutterenvironment;anurbanWiFimeshnetworkisagoodexample. Here,theMesh

AccessPointsaretypicallyonlightpolesatabout8metersabovethegroundwhichisusuallylessthanheightof

thetreesand/orbuildingsinthearea. Thenodetonodesignalpathswillpassthroughtheclutterandby

determiningthelengthofpassagewithineachtypeofclutteragoodvaluecanbehadforthetotalattenuation

effect.

SignalStrengthStudies

Toillustratetheroleofgroundcoverdatainwirelesssystemdesign,threesignalstudiesareincludedtoshowthe

threemethodsforusingclutteraspreviouslydescribed.

Figure3showsthecoveragefromafoursiteWiMAXsystemoperatingat2.5GHz inamediumsizedurbanarea.

Thebaseantennaisat30mabovethegroundandtheremoteunitsareat1.5m. Inthisstudy,noclutterwas

used;terrainwastheonlyfactorindeterminingsignalstrength. Theareainthetophalfofthecoverageareais

relativelyflatsonotmuchvariationinsignalstrengthisseen freespacelossisthedominateattenuationfactor

here. Terrainbecomessignificantinthebottomhalfoftheareaasseenbythesharpreductionofsignalalongthe

southernedgeofthecoverage.

Table 1. Structure Height Classification

Classification Average block height

Residential < 2 meters

Low density urban 5 10 meters

Moderate density urban 10 25 meters

High density urban > 25 meters


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Figure3 ReceivedPower(TerrainDataonly)

ClutterdatawasthenaddedtothisareaandFigure4isamapindicatingthedifferenttypesofclutterbasedon

color. Theurbanandcommercial/industrialusesareshowninredandpinkrespectively;residentialisshownin

yellow. Asignalstudywasthenrunusingthefirstmethoddescribedabovewhereasimpleattenuationfactor(a

margin)wasappliedtothereceivedsignalstrengthbasedonthecluttertypeatthestudylocation. Thisisshown

inFigure

5.

The

correlation

between

additional

attenuation

and

clutter

type

can

be

clearly

seen.

Note

how

the

signalstrengthisnowshownasmuchlessinthebuiltupareaswhilethecoveragealongtheriver(lightbluearea

ontheClutterCategorymap)hasnotbeenaffected.


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Figure4 ClutterCategoryAreas


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Figure5 ReceivedPower TerrainplusClutterCategoryAttenuation

Figure6shows

the

second

way

to

use

clutter

data

where

the

height

of

each

clutter

type

is

taken

into

account

whenestablishingthepropagationpathbetweentransmitterandreceiver. Here,insteadofapplyingafixed

attenuationatthereceiverlocation,thesignalpropagationisinsteadaffectedbyshadowingfromthehigher

cluttertypes. Inthisinstance,thecommercial/industrialareas(Pink)aresetat20mheightandtheurbanarea

(Red)issetto15m. Themapshowshowthesignalisreducedintotheseareasduetotheirheights.

Figure6 ReceivedPowerwithTerrainplusClutterHeight

Finally,Figure7isanexampleofanumberofWiFiMeshnodeslocatedonlightpoles. Ahighresolutionclutter

database(3mpointspacing)wasusedheretobeabletodefinethestreetsfromothercluttertypes. Road

informationisalsodisplayedonthismaptoshowhowthesignalfromthemeshnodestravelsfurtherdownthe

streetcanyonsthanthroughtheclutteroneithersideofthestreets.


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Figure7 WiFiMesh ClutterPassThroughAttenuation

Fromtheseexamplesitisclearthatclutterdataplaysanimportantroleinwirelesssystemdesign. Addingthis

informationprovidesacriticalrefinementtoasignalstudybytakingintoaccountamoreaccurateandrealistic

characteristicof

the

service

area

where

the

wireless

system

will

be

used.

UsingLandUseDatatoPredictServiceDemand(Traffic)

Thecoreofanywirelesscommunicationsystemisthenetworkoftowers,transmittersandantennasthatconvert

informationtoradiosignalsthatcanbereceivedandunderstoodbytheuser. Thenetworkmustbeconfiguredso

thatenoughsectorsandchannelsareallocatedtohandlethedemand. Considerationsinsystemdesignare:

locatingtransmitterstoprovideadequatesignallevelsthroughouttheservicearea,judgingwheresystemcapacity

maybeinsufficient(orunderutilized),andinthecaseofmultiusersystems,gauginghowthesystemmayhaveto

evolvetoaccommodatechangingusercallanddatatrafficpatterns.

Thesedynamictrafficpatternsrefertotimeandlocation. Peopleareusingthesystematvarioustimesthroughout

theday,

and

because

these

users

are

mobile,

they

may

be

located

anywhere

in

the

system

service

area.

For

example,duringmorningandeveningrushhour,trafficdensityforcellular/PCSphonesishighestalong

transportationroutes. Large,temporarygatheringsofusersconventioncenters,concerthalls,sportingevents

mustalsobeconsidered. Duringworkhourstrafficishighestinthecitycenters. Todeterminethecapacitythat

mustbeprovidedbyeachsector/siteinordertohandlethetraffic,itisnecessarytosomehowestablisha


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EDX WP UseofClutterDatainRFPlanning Aug08 Web

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