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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

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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

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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

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