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WirelessIntroduction
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HCF_LIT-131
Rev. 1.0
Date of Publication: March1,2010
Contributors: Gerrit Lohmann Pepperl+Fuchs GmbH
Document Distribution / Maintenance Control / Document Approval
To obtain information concerning document distribution control, maintenance control, and
document approval please contact the HART Communication Foundation (HCF) at the address
shown below.
Copyright 2010 (2008) HART Communication Foundation
This document contains copyrighted material and may not be reproduced in any fashion without
the written permission of the HART Communication Foundation.
Trademark Information
HART is a registered trademark of the HART Communication Foundation, Austin, Texas, USA.
Any use of the term HART hereafter in this document, or in any document referenced by this
document, implies the registered trademark. WirelessHART is a trademark of the HART
Communication Foundation. All other trademarks used in this or referenced documents are
trademarks of their respective companies. For more information contact the HCF Staff at the
address below.
Attention: Foundation Director
HART Communication Foundation
9390 Research Boulevard, Suite I-350
Austin, TX 78759, USA
Voice: (512) 794-0369
FAX: (512) 794-3904
http://www.hartcomm.org
Intellectual Property Rights
The HCF does not knowingly use or incorporate any information or data into the HART Protocol
Standards which the HCF does not own or have lawful rights to use. Should the HCF receive
any notification regarding the existence of any conflicting Private IPR, the HCF will review the
disclosure and either (a) determine there is no conflict; (b) resolve the conflict with the IPR
owner; or (c) modify the standard to remove the conflicting requirement. In no case does the
HCF encourage implementers to infringe on any individual's or organization's IPR.
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Introduction
Wireless communication is emerging from the office world to the industrial
world. Since industrial communication underlies stronger conditions than
office communication, some know-how is important for users who applywireless communication in industrial environments.
This document will introduce to
RF and radio basic terminology
Boundary Conditions for RF in industrial environment
Modern Radio technology and standards
Media Access and Networking
Conditions for using wireless technologies in industrial applications
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RadioFrequencyTheradiowavesarepartoftheelectromagneticspectrum,coveringseveralotherpartsofradiationlike
light,gammaraysetc.
Therefore,the
propagation
of
radio
waves
can
generally
be
compared
with
the
propagation
of
light.
Light
movesstraightthroughspaceandcan
Penetratematerial(glass)
Bedamped(eitherthroughmateriallikefogorbysimplybydistance)
Bereflected(onmirrors)
Beabsorbed
Radiowavesalsocanpenetratematerial,bereflectedorabsorbed.
Duetotheotherfrequency(radiowavesareofmuchlowerfrequencyoflight),thepenetration
ofmaterial
is
better.
Radio
waves
can
even
penetrate
walls
if
not
too
massive
Radiowavesaredampedbythematerialandbydistance
Radiowaveswillbeabsorbedbymassivematerialanddonotpenetratethemanymore
theyarereflectedonthegroundorobstacles
Generally,inanopenspace,thepropagationgoesundisturbedstraightinalldirectionsandisonly
dampedbydistance.
ThedampingisdescribedindB
RadioTechnologyToenablethereceiverandsenderoftheinformationtocommunicate,bothhavetoagreeonwhat
frequencytheysend.Thisfrequencywillbeinoneofthebandsreservedforradiocommunications.The
bandsareregulatedbyofficialagenciesandtheusageofthesebandsisthereforerestricted.Inmost
bands,alicensemustbeobtainedtoallowusageofit.Theregulationsaredifferentfromcountryto
country.
Theoretical free space propagation
-110,0
-105,0
-100,0
-95,0
-90,0
-85,0
-80,0
-75,0
-70,0
0 500 1000 1500 2000
Distance [m]
PathLoss[dB]
900MHz
2.4GHz
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TheexceptionsforlicensedusageareISMbands(Industrial,scientific,medical).Inthesebandsanyoneisallowedtosendandreceive,followingsomerestrictionlikesendingpower.TheonlyworldwideusablebandforISMis2.4GHz
Now,inthisbandsinglechannelsareestablished.Thiscaneitherbeasinglefrequency(like2.45GHz)orafrequencyrange,e.g.2.407to2.447GHz.Thisisadifferenceof40MHz.Thisrangeiscalledbandwidth.Ashigherthebandwidth,asmoredatacanbetransmitted.Usingthehigherbandwidth,onespeaksalsoofspectrumusage.SpectrumusagecaneitherbeinsequencelikeFHSS,orsimultaneouslyas.FSSSusesasinglefrequencyatatimeandanotherfrequencyduringthefollowingtransmission.So,thewholespectrumisused,butnotforonetransmissionbutforconsecutive.Sincethesequenceofthesinglechannelsisunknown,itishardtotapthesetransmissionsandduetotheshortpeaksoftransmissions,thedisturbanceofotherradiosystemsisreduced.
DSSSincontraryusestheentirefrequencyspectrumduringonetransmission.Therefore,thesendingpowercanbereduced,thesignalishiddeninthebackgroundnoiseandcannotbetappednorjammednoritisjammingotherradiotransmissions
Channel
0
2
4
6
8
10
12
0 20 40 60 80 100 120
Time [ms]
Channel
Channel
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ThesendingpowerisonerestrictionintheISMbandof2.4GHz.Itisonlyallowedtosendwithanoutput
powerof10mW,insomecountriesorregionsitisallowedtosendwith100mW.Asmoresendingpower
isallowed,aslongerthepossiblerangeis.
ThesendingpowerissometimesalsodescribesasdBinrelationtoEIRP.EIRPmeans@.Anisotropic
radiatorradiatesequallyinalldirections(likethesun).Butrealworldantennasradiateinadifferent
pattern,e.g.adonutlikeshape.Sothepowerisnotwastedtothetopandbottombutmoreinthe
horizontalsurface.Sothe10mWarenotdistributedequally,butorfocusedwhichincreasesthesending
powerofe.g.2dB(inrelationtothe10mW)
Anotherfactorfortherangeisthereceiversensitivity.Evenifthesendingpowerisrestricted,the
sensitivityisnot.Soasbetterthereceivingsensitivityis,aslongertherangewhichcanbeobtained.
BounderyConditionsforRFinindustrialcommunicationThegeneralpropertiesofradiofrequencydoofcoursealsoapplyinindustrialenvironment.Threemain
effectshavetobetakenintoaccountinindustrialenvironments
Interference
Movingequipmentandpeople
Multipathfading
Interferencehappenswhereverawaveisreflectedorissuperimposedbyanotherwave.Thiscaneither
haveapositiveeffectoranegativeeffect:
DSSS
0
1
2
3
4
5
6
1 2 3 4 5 6 7 8 9 10 11 12 13 14
frequency
amplitude
Message
Spread Message
Positivly reflected wave
-2
-1,5
-1
-0,5
0
0,5
1
1,5
2
Amplitude
Wave
Reflected Wave
Resulting Wave
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Thesecondeffectisthatindustrialenvironmentsarenotstaticbutdynamic.Vehicles,movingequipment
liketanks,peoplewalkingthroughwillchangetheenvironment.Therefore,thepropagationofradio
wavesisaltered.
Thethirdeffectismultipathfading.Duetoreflections,onewaveismovingondifferentpathsfromthe
sendertothereceiverandarrivestherethenlightlytimeshifted.Thiscandistortthewavesothereceiver
doesnotrecognizeitanymore.
Asresultofthosethreeeffectstwomainconsequencesmustbetakenintoaccountwhenapplying
wirelesscommunicationsinindustrialenvironments
Range
Reliability
Therange
is
decreased
compared
to
the
theoretical
free
space
propagation.
A
practical
example
for
industrialenvironmentisshownbelow:
Themovingequipmentandchangingenvironmentdecreasesthereliabilityoftheradiocommunication.
Aconnectionworkingatonetimedoesnotworkanotherbecauseatruckisstandingintheway.This
decreasesthereliability.
Negativly reflected wave
-2
-1,5
-1
-0,5
0
0,51
1,5
2
Amplitude
Wave
Reflected Wave
Resulting Wave
Practical propagation
-130
-120
-110
-100
-90
-80
-70
0 50 100 150
Distance [m]
PathLoss[dB]
900MHz
2.4GHz
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ModernRadioTechnologyandStandardsToenableradiocommunication,acommonstandardismandatory.Thestandardmakesdevicesof
differentvendorscompatibletoeachotherendenableeasytousetechnology.
Currently,in
the
2.4GHz
Band
there
are
three
major
known
standards
available.
They
have
been
defined
byIEEEinthefamilyofcommunicationprotocolsandareworldwideusable.
IEEE802.11(WiFi)
IEEE802.15.1(Bluetooth)
IEEE802.15.4(ZigBee)
OneofthewidestusedisIEEE802.15.11,commonlyknownasWLANorWiFi(evenifthisisnot100%
correct).IEEE802.11providesalocalinfrastructureforfastwirelesstransmissionofrelativeamountof
dataoversomedistancelikeofficeareasoryourhomenetwork.
IEEE802.15.1isknownasBluetooth.Thisprovidesapersonalareanetworkforcablereplacementof
auxiliarydevicesandisabletotransmitsomelessdatathanWLANoveraverylimitedrange.Thisisalso
calledWPAN(WirelessPersonalAreaNetwork)
IEEE802.15.4isthebasisforNetworkslikeZigBee,whereverylimiteddataistransmittedoveramedium
distancethroughanetworkofknotssothecoveredareaisextended.
MediaAccessandNetworkingAllthestandardsdescribedabovehavetoaccessthesamespace:theair.Ifallparticipantsaccessthisat
thesame
time,
collisions
of
communication
will
occur.
Therefore,
the
media
access
must
be
handled.
Therearetwoprinciplesusedtocoordinatemediaaccess
TDMA(TimeDivisionMultipleAccess)
CSMA(CarrierSenseMultipleAccess)
WithTDMA,thedataaretransferredatagiventimeslot.Soalloftheparticipantsinanetworkknowthe
timeslotwhentosendandtoreceiveandavoidcollisionswiththis.
CSMAavoidscollisionswithrandomdelaytimesafterafreechannelisrecognized.Soifachannelis
recognizedto
be
free,
every
participant
waits
another
random
time
and
it
is
very
unlikely
that
the
next
sendpackagescollide.
Theotherthingwhichisrelevantintermsofthestandardsdescribedaboveishowthenetworkis
organized.Therearethreebasiclayoutsofnetworks
Star
Mesh
StarMeshorHybrid
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Usingstartopology,eachwirelesssensorendpointsendsdatadirectlytothegateway.Fromtheredata
issentontoothersystems.Starnetworksofferthefastestdatagatheringspeed,butallparticipantsmust
beinthecommunicationrangeofthegateway.Thistopologysuitsinstallationsthatneedthelowest
powerconsumptionoverlimitedgeographicrange
Inmeshnetworks,eachwirelesssensoractsasarouter,sendingandreceivingdatafromothersensorsor
thegateway.
Self
configuring
networks
automatically
determine
the
best
path
for
data
to
take
from
sensortogateway.Dataisautomaticallysentaroundfailedsensorrouters.Thislayoutisgoodforwide
areanetworkswithhighredundancy,butenoughpowerforallparticipantsisavailabletoroutethe
messages.
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Starmeshnetworkscombinestarandmeshtopologiestogainthespeedofthestarnetworkwiththe
selfrepairingcapabilityofthemeshnetwork.Sensorsmaybeeitherendpointsorrouters,dependingon
wheretheyareusedinthesystem.
ConditionsforusingwirelesstechnologiesinindustrialapplicationsIfanofficeapplicationwirelesssystemloosesapackageonceinawhile,itisresendandthereforethe
lossisnotrecognizedbytheuser.Incontrarytothis,Industrialapplicationsrelyonsecuredata
transmission.Forsomeusecasescompromisescanbemade,e.g.toparameterizedevoicesorforasset
managementapplications.
But
closed
control
loops
rely
on
astrict
data
transmission.
ThereliabilityofadatatransmissionisdefinedinBitErrorRate,meanshowmanyBitsarenot
transmittedcorrectlyinanumberofBits.ThebettertheBER,thebetterthereliabilityofthedata
transmission. AwiredtransmissionusuallyhasaBER
Todepicttherelations,aProfibushasaBERof@.,awirelesssystemwithouterrorcorrection.@
SowithcorrectionalgorithmsabetterBERispossible,butthealgorithmneedstimetofindoutand
correctthefaultymessage.Ifthemessageistoodisrupted,themessagecannotbereconstructedanda
resendingmightbenecessary.Thisdelaysthedatatransmissionandaclosedloopcontrolcouldgetout
ofphase.
Todefinethesingleapplication,6applicationclasseshavebeendefined:
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SummaryandConclusionUsingwirelesstechnologiesinindustrialapplicationissurelypossible,butsomerestrictionsmustbe
considered.
Radiotechnologyusesasharedmediumanddoesnotguaranteetransmissionreliabilityasawire
Radiowaves
are
damped
absorbed
and
mirrored
by
obstacles
and
therefore
not
everywhere
is
a
connectionpossible
Licensefreebandshavejustlimitedrange,alsootherwirelesssystemsmightmakeuseofthe
bandandcausecoexistenceissues