Explanations on reliable parallel operation of wireless radio solutions

Coexistence of Wireless Systemsin Automation Technology

P U B L I S H E R ‘ S D E T A I L S

Coexistence of Wireless Systems in Automation Technology

Explanations on reliable parallel operation of wireless radio solutions

Issued by:ZVEI - German Electrical and Electronic Manufacturers’ AssociationAutomation DivisionLyoner Strasse 960528 Frankfurt am MainGermanyInternet: www.zvei.org/automation

Contact partner:Carolin TheobaldPhone: +49 (0)69 6302- 429Fax: +49 (0)69 6302-319E-mail: theobald@zvei.org

This brochure has been produced by the „Wireless in Automation“ working group in the ZVEI Professional Association for Automation. Further participants:

Holger BentjeDr. Sven CichosMichael EchterhoffWolfgang FeuchtJens GrebnerDr. Reinhard HüppeJochen KochMarko KrätzigHermann KrauseRalf MedowGernot de MürAndreas Pape

Design:NEEDCOM GmbHwww.needcom.de

Print:Berthold Druck und Direktwerbung GmbHwww.berthold-gmbh.de

1st Edition, April 2009

Günther QuednauDr. Andreas RampeDr. Lutz RauchhauptUwe SchaeferDr. Guntram ScheibleSven SieberRolf-Dieter SommerCarolin TheobaldDr. Werner ThorenDr. Andreas VedralThorsten VerseJens Wienecke

© All rights reserved, ZVEI. Despite careful control, ZVEI does not assume any liability for the content.

Summary ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5

Motivation ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 5

1 Wireless systems in automation technology ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 6

1.1 Available frequency ranges ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 6

1.2 The 2.4 GHz ISM band ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 8

1.3 Distinguishing features of radio systems ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 8

2 Behaviour of wireless radio systems in parallel operation ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 10

3 The path toward coexistence ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 12

3.1 Spatial decoupling ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 12

3.2 Decoupling in the frequency domain ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 13

3.3 Time-based decoupling ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 13

3.4 Summary of coexistence management ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 15

4 Prospects ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 15

Typical wireless systems for automation technology in the 2.4 GHz ISM band ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 16

FAQ – Frequently Asked Questions ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 18

References ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 19

Contents

4

The following companies have jointly investigated the topic of coexistence of wireless solutions in industrial applications under the umbrella of the ZVEI – German Electrical and Electronic Manufacturers’ Association. In hundreds of practical test cases, measurements were carried out at the ifak - Institut für Automation und Kommunikation e. V. Magdeburg using currently available automation components. These measurements were then used as a basis for assessing radio influences and elaborating funda-mental points applicable in practice for the coexistence of radio systems operating in parallel.

Background

5

Several suppliers of automation solutions have started marketing automation products with wireless technologies. In order to guarantee the coexistence of wireless systems, i.e. their reliable parallel operation, the manufacturers listed opposite jointly investigated the topic of mutual radio influence. Comprehensive measurements under practice-oriented conditions have shown: Wireless works!

In summary, the following statements can be made:

• Duetothesmalldataquantitiescommoninautomationapplications,significantlylowerradioinfluencescan be expected than is generally the case.

• However,inordertoachievethereliabilityandavailabilityrequired,industrialcomponentsmustbeusedwhichdiffersignificantlyfromconsumerandofficeequipment.

• Coexistence ispossible,butmustbeplanned. Italwaysdependson theapplication,andcanalsochange during the life cycle of a plant, e.g. through the addition of further wireless systems.

• Planning requirements increasewith thenumberofwireless systemsoperating inparallel. In thiscontext,parallelmeans:Atthesamelocation,atthesamefrequency,andatthesametime.

• Indifficultcases,coexistencemanagementmustbecarriedoutwiththeassistanceofaspecialist,e.g.ifseveraldifferentsystemswithstrictreal-timerequirementsbutwithlowimmunitytoradiointerference have to be operated in parallel.

• Itisimportantthattherestrictedresourcesforwirelesscommunicationareusedwithconsiderationandthatforthevariousfieldswherewirelesstechnologyistobeused,interestsarebalancedandprioritizedsufficientlyearly.ThisappliesinparticulartoITdepartments,logistics,andthosewhoareresponsible for wireless automation. The latter are the center of attention in this document.

Summary

WirelesssystemssuchasWLANorBluetoothhavebeenusedinthehomeandinofficesformanyyears.They are also being increasingly implemented in automation systems, e.g. in plants and machines. How-ever,therequirementsassociatedwithsuchapplicationsareoftenbyfargreater.Forexample,applica-tionsmayrequiredefinedresponsetimeswithveryhighavailability.

Atrendtowardutilizationofthe2.4GHzfrequencybandcanbeobserved,sinceitislicense-freeandavailableworldwide.Furthermoreitoffersahighbandwidthandsufficientrange,andisnotinfluencedby the typical industrial sources of interference.

Duetotheextensiveuseofthisfrequencyband,itisnotpossibletocompletelyruleoutamutualinflu-ence of wireless systems operating in parallel, however. These radio influences may lead to a restriction intherequiredavailabilityofindividualsystems.Itisthereforerecommendedthattheuseofwirelessresources be carefully considered.

This brochure explains important characteristics for differentiating wireless systems, their behaviour during parallel operation, and measures for coexistence management. Users should be made aware of why and how they should consider the topic of coexistence of wireless systems. Above all, it will be shown how simple it is to avoid mutual wireless interference and to implement interference-free parallel operation.

Motivation

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1 W I R E L E S S S y S T E M S I n A U To M AT I o n T E C H n o Lo G y

Inaddition to savingsoncablesandconnectors, thebenefitsof radio-based systemscompared tocabled systems include increased mobility and flexibility as well as the wear-free transmission medium. These advantages are particularly useful with regard to human-machine communication and wireless sensors and actuators on moving plant components. For example, some tasks can be handled far more efficiently byusingwirelessnetworking for data acquisition terminals orAGVs (automatedguidedvehicles).

Ofcourse,eachoftheseapplicationshasdifferentrequirements.Sincenowirelessradiosystemcansatisfyallrequirementssimultaneously,itmaybenecessarytooperateseveralsystemsinparallelfordifferent tasks.

The same medium is used by all wireless systems for radio transmission: The free air space which sur-roundsthem.Thismediumisalimitedresourcesinceonlycertainlimitedfrequenciesareavailable.Coordinateduseofthisresourceisrequiredforthisreason.ThepersonsresponsiblefortheITandbuilding infrastructure, the logistics, and the automated production systems must therefore work together closely when planning wireless applications. It makes sense to obtain an overview of the variousfrequencyranges,wirelesssystems,andtheircharacteristics.

Further information on the use of wireless systems in automation technology can also be obtained from VDI Guideline 2185 [1].

1.1 Available frequency ranges

Internationalandnationalregulatoryauthoritiesdeterminehowthelimitedresourcefrequencyrangecanbeusedforradiotransmissionusingelectromagneticwaves(seeFig.1).Theyprovideafrequencyutiliza-tionplaninwhichlicense-freefrequenciesandthoserequiringalicensearedefined.Onlyafewfrequencyranges are used in the automation technology sector; they are presented briefly here (see also Table 1).

1 Wireless systems in automation technology

Fig. 1: License-free frequency bands in the electromagnetic spectrum

101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019

400 500 600 700 800 900 1000 2000 3000 4000 5000 6000 7000

433.

05 -

434.

79 M

Hz

863

- 870

MHz

2400

- 24

83.5

MHz

5150

- 57

25 M

Hz

LightRadio waves Microwaves Infrared UV

Frequency in MHz

Frequency in Hz

7

1 W I R E L E S S S y S T E M S I n A U To M AT I o n T E C H n o Lo G y

License-free ISM bandsSo-calledISMbands(industrial,scientificandmedical)havebeenapprovedbytheregulatoryauthoritiesforradioapplications.ThemainadvantageoftheISMbandscomparedtootherfrequencybandsisthatdevices using these bands can be used without any further limitation if they comply with the respective statutorydirectives.Thedisadvantageisquiteclear:thesefrequencybandsareoftenusedbecausetheyarelicense-free, and mutual influence can result from radio systems operated in parallel. Typically approved ISM bands are at 433 MHz, 2.4 GHz and 5 GHz. These are used in spatially limited networks such as e.g. WLAN, ZigBee or Bluetooth. But microwave ovens, wireless thermometers, and vehicle remote controls also usefrequenciesoftheISMbands.

Further license-free frequenciesInaddition,thefrequencybandbetween868and870MHzcanbeusedlicense-freeinEurope.Differentoutput powers, bandwidths and duty cycles sometimes apply to different applications. Typical applica-tionsareinthesafetysector(alarmsystemsandfiredetectionsystems)andbuildingautomation.RFIDsystemsalsofrequentlyusethisband.OutsideEurope,e.g.intheUSA,thefrequencybandfrom902to928MHzisavailableforsimilarapplications–hereasanISMband.Anexclusivefrequencybandfrom1880 to 1900 MHz is reserved for the DECT standard, whereas in the USA, for example, DECT is operated inthe2.4GHzISMband(referredtoasupbandedDECT).Thefrequencybandfrom5.1to5.7GHzcanbeusedlicense-freebyWLANsystems,butrequiresadditionaltechniquessuchasdynamicfrequencyselection and power regulation.

Frequency in MHz Type of use Utilization conditions / output power Properties

433 .. 434 License-free(ISM)

Output power max. 10 mW ERP*,max. 10% duty cycle**

Good penetration,reduced data rate

448 and 459 Licenserequired Output power max. 6 W, time-synchronized,limited duty cycle**

Good penetration,low data rate,wide ranges

410 .. 470 Licenserequired Outputpowerdependsonfrequency assignment, typically 6 W / 12 W for mobile devices, channel spacing typically 12.5 kHz / 25 kHz

Good penetration,wide ranges

863 .. 870

(USA: 902 .. 928 as ISM band)

License-free Output power 5 .. 500 mW ERP*, channels partially with 25 kHz bandwidth, duty cycle** partially only 0.1 %

Wide ranges

1880 .. 1900 License-freeaccording to DECT standard

Output power 250 mW peak ERP*, timeandfrequencydivisionmultipleaccess (TDMA / FDMA)

Good availability, high output power

2400 .. 2483.5 License-free (ISM) Output power 10 mW (100 mW when usingspreadspectrumtechniques,usewithin buildings without restrictions), no limitations regarding duty cycle**

Available almost worldwide, broad bandwidth, already widely used

5150 .. 53505470 .. 5725

License-free(partially ISM)

Output power partially up to 1 W, powercontrolanddynamicfrequencyselectionsometimesrequired

Low penetration of walls, quasi-optical propagation, high data rate

* ERP – the effective radiated power from an antenna** The duty cycle refers to the relative utilization duration of the medium which may be subject to certain limitationsinsomefrequencybands

Table 1: Frequencies used in Europe for wireless automation technology

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1 W I R E L E S S S y S T E M S I n A U To M AT I o n T E C H n o Lo G y

Licensed frequenciesIn the automation technology sector, additional frequencies can be used following licensing by theregulatory authorities. Examples are 448 MHz and 459 MHz to which limitations apply with regard to the duty cycle. Ranges of up to 10 km can be covered as a result of the higher transmitter power which is permissible.

1.2 The 2.4 GHz ISM band

Based on the development of cheap yet sophisticated radio components, and driven by the home and officesectors,radiotechnologiesinthe2.4GHzbandarebeingencounteredinautomationtechnologymoreandmorefrequently.Awiderangeofvarioussolutionsnowexistswithdifferentstandardsandproprietary solutions also being used. The most important ones are briefly presented in the table „Typical wireless systems for automation technology in the 2.4 GHz ISM band“ at the end of this brochure on pages 16 and 17. One reason for the keen interest in the 2.4 GHz band is that this ISM band is available license-free worldwide with only minor limitations and that it has a high bandwidth. This permits either a high data rate, or can be used to increase the immunity to influences from other radio systems.

Furthermore, the interference spectrum in an industrial environment, originating for example from arc weldingmachinesorpowerelectronics,doesnotextendup to frequenciesof2.4GHz.Thereforenointerference originates from such devices (see also Fig. 2).

However,difficultiesmayoccurifthetransmittingrangesofdifferentradiosystemsaresuperimposedinthe 2.4 GHz ISM band. As a result of the widespread utilization of the 2.4 GHz ISM band, this brochure will describe considerations for the coexistence of wireless radio applications pertaining to this band as representative.

1.3 Distinguishing features of radio systems

A diversity of radio technologies can now be encountered in automation technology. They differ with regardtothefrequencybandused,thebandwidthrequirement,andthenumberofchannels,amongother features. As far as users are concerned, the data rate, the cycle time, the maximum number of net-worknodes,andthecoexistenceperformancearealsosignificantforthechoiceofatechnology.Furtherinformation on the various radio technologies can be obtained from the table on pages 16 and 17. Whenchoosingaradiosysteminthe2.4GHzband,theapplicationshouldbeinitiallyclassifiedinordertoensure parallel operation with systems which may already exist.

• Thetransmissionofsafety-relateddata(e.g.emergencystop)placesmaximumrequirementswithregard to fail-safety and reliability.

• Closed-loopcontrolsandalsothecontrolofmachinesaremainlyassociatedwithhighdemandsonthetime response.

• Ifonlydataforvisualizationorrecordingistransmitted,thisislesstime-criticalbutmayrequireahigh data rate.

Conditions in an industrial environmentThechannelqualityforradiocommunicationinanindustrialenvironmentissubjecttointensevaria-tions.Adirectlineofsightfrequentlydoesnotexistbecauseofhindrances(sometimesmoving)betweenthe radio units. Multi-path reception resulting from reflections at surfaces results in superposition of the radiowavesandthereforegreatlydifferentstrengthsofthereceivedsignalatcertainfrequencies.Theradio systems designed for use in such environments therefore use appropriate technologies for media access(seealsobox„Techniquesformediaaccess“onpage9).

Fig. 2: Interference spectra of other typical devices in an industrial environment

Arc welding

Spot welding

Frequency converters

RFID

Switchgear

Relays

Drives

Induction heaters

MH

z10,0001,000

1001010.1

0.010.001

Basic frequency

Harmonics

2.4 GH

z ISM

9

1 W I R E L E S S S y S T E M S I n A U To M AT I o n T E C H n o Lo G y

Wireless networksGreat communication distances often have to be covered in large plants. Hence wireless networks with severalradionodes,eachwithasmallertransmittingpower,arefrequentlydesignedinsteadofpoint-to-point connections. In process automation, for example, less time-critical sensor data can be transmitted via several radio nodes to the actual receiver. The network management then not only supports the simple transmission of data, but also the search for alternative transmission paths within the network (referred to as routing).

AntennasIn addition to the transmitter output power, the receiver sensitivity and the ambient conditions, the communication ranges of a radio system are also dependent on the antenna. According to the an-tennadesign,itsefficiencyincertaindirectionsisincreasedorreduced.Everyantennatypehasitscharacteristicdirectivitydiagram.Themaximumgainisusuallyspecifiedforthedirectionofmaxi-mum sensitivity. This antenna gain applies to both the transmitter and receiver directions. Therefore the orientation of the antennas to each other must be observed when installing a radio system.

Techniques for media access:

Various techniques for media access are used with different properties regarding the sensitivity of a radio system to environmental influences and systems operated in parallel.

• In the simplest case, thedata ismodulatedonafixed frequency. This so-callednarrow-bandaccessonlyrequiresasmallbandwidth,butbecauseofthefixedtransmitterfrequencyismoresusceptible to changing propagation conditions or interferences.

• Withthefrequencyhoppingspreadspectrum(FHSS),thetransmitterfrequencyhopsinaccordancewith a pattern known to the receiver – interferences only affect a part of the transmitted data, so that only a small portion has to be retransmitted.

• Withthedirectsequencespreadspectrum(DSSS),thebandwidthrequiredfortheradiotransmissionis spread using a so-called chipping sequence by which the data signal is multiplied. This chipping sequence makes the transmission less sensitive to narrow-band interference.

• With the chirp spread spectrum (CSS), the energyof a transmittedbit is distributedover awide frequency range by sweeping the transmitting frequency rapidly during the transmission procedure for each bit to be sent.

• Withorthogonalfrequencydivisionmultiplexing(OFDM),thepropertiesofnarrow-bandmodu-lation are utilized to operate several channels bound close to each other. This allows a high data transfer rate; with narrow-band interferences, the total data rate is only reduced by the affected share.

Different directivity of antennas

Horizontal beam angle

Vertical beam angle

Horizontal beam angle

Vertical beam angle

omnidirectional antenna Directional antenna

As soon as several radio systems of the same or different types are used, it is possible that they will influence each other. But when and how exactly does this influencing occur in parallel operation?

Influencing of radio operation can only occur if several systems are transmitting

• atthesamelocation,• atthesametime,and• atthesamefrequency(seeFig.3).

Thecauseisthereforealwaysthesimultaneoususeofafrequency,butthepracticaleffectsdependontheimmunity to radio interference of the wireless systems involved.

Spatialandfrequencyoverlappingdependontheselectionofthetechnologyusedandtheassociatedradio planning. Overlapping with regard to time in addition depends on the application itself, i.e.: • Onthefrequencyofradiouse:Howandhowoftenaretelegramssent,cyclicallyorevent-controlled?• Onthedurationoftheindividualfrequencyuse:Howlongdoesthetransmissionofatelegramtake (dependsonbittransferrateanddataquantity)?

One refers to the coexistence of several systems if fault-free operation, relevant to the respective appli-cations, still exists despite mutual radio influences [1]. The following graphic shows the most important influencingfactorsofradiosystems,anddefinesthetermsused.

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2 B E H AV I o U R o F W I R E L E S S R A D I o S y S T E M S I n PA R A LL E L o P E R AT I o n

2 Behaviour of wireless radio systems in parallel operation

Fig. 3: Mutual radio influences only arise with simultaneous spatial,timeandfrequencyoverlapping

Radio influences

Influencing factors of radio systems in parallel operation

Coexistence is a state in which different radio systems each fulfill their expected functions with regard to applications, their requirements and boundary conditions (e.g. environ-ment), despite the existence of other radio systems.

Immunity to radio interference is the ability of a radio system to be resilient to influences from other radio systems and changes in environmental conditions.

Frequency utilization is the property of a radio system which describes the time, location and frequency-based occupation of the radio medium.

Targ

etApplication requirements

A

Application requirements

B

Coexistence = operation as

expected

Ambient conditions

B B B

Radio system BRadio system A

A A AMutual radio influences

„Immunity to radio interference“

„Frequency utilization“ „Immunity to radio interference“

„Frequency utilization“

11

2 B E H AV I o U R o F W I R E L E S S R A D I o S y S T E M S I n PA R A LL E L o P E R AT I o n

What are the effects of radio influences ?With regard to applications in automation technology, radio influences mainly have an effect on the transmission delay of the telegrams. It thus takes longer until a telegram has been completely and correctly transmitted.

This transmission delay is measured at interfaces accessible to the user, for example the duration from triggeringofasensoruntiltheavailabilityofthesignalonafieldbus,orfromscanningofabarcodeupto successful transfer to a network (see Fig. 4).

Measurementofthetransmissiondelaysoveralongerperiod,andananalysisofhowfrequentlydelaysoccur (see Fig. 5) therefore allow rapid assessment of whether a radio influence is present, and whether this radio influence is still permissible. What can still be considered as permissible depends on the application (see Table 2, column 3). The transmission delay and error or frame loss rate must be below a tolerable maximum value (see red bar in Fig. 5).

Thebiterrorratesaresignificantlyhigherinwirelessapplicationscomparedwithwiredtransmissions.Therefore all wireless systems use internal mechanisms for error correction and telegram repetition. Some wireless technologies initially listen to the radio medium and wait with their transmission if neces-sary. Radio transmissions therefore always have a certain jitter in their time response which can some-times amount to a multiple of the minimum transmission delay. This jitter resulting from radio influences ismoreorlesssignificantdependingontheimmunitytoradiointerferenceofthesystemconsidered.

Thetelegramlossratecanbeverydifferentinthevariousfieldsofapplication(seeTable2,column5).When using a wireless system to control a machine, for example, downtime through violation of a delay limitshouldneveroccur(verysmallerrorraterequired),whereaswhentransmittingthedatameasuredby a vibration sensor it is even possible to tolerate occasional data losses or the violation of a time limit (relatively high error rate permissible).

* Fortheuser,theviolationofthemaximumtransmissiondelayisequivalenttoaframelosssincetheinformationhas become irrelevant, for example.** Fast drive controls with time constants in the 1 ms range cannot be reliably covered by current wireless technologies.

Application area

Application Max. trans-mission delay in ms

Update time in ms

Telegram loss rate or timeout*

Factory automation

Control** of machine and production cell „local“

10 .. 20 20 .. 30 < 10-9

Control in production hall „global“

20 .. 30 30 .. 100 < 10-9

Monitoring and diagnostics

> 100 > 500 10-3 - 10-9

Mobile operators, safety 10 .. 20 10 .. 30 < 10-9

Process automation

Open-loop / closed-loop control

50 .. 100 100 .. 5000 < 10-4

Operation „local“ > 100 < 1000 < 10-3

Monitoring and diagnostics

> 100 > 10000 < 10-4

Fig. 5: Examples of time response without (top) and with radio influences (bottom)

Table 2: Typical applications in factory and process automation: Time response requirements and telegram loss rates

0 10 20 30 40 50 60 70

1000

100

10

1num

ber

of p

acke

ts

Jitter

Tole

rabl

e

max

imum

val

ue

0 10 20 30 40 50 60 70

1000

100

10

1num

ber

of p

acke

ts

Transmission delay (ms)

User interfaces

Tran

smis

sion

del

ay

Sens

or c

onne

ctio

n

Gat

eway

Com

mun

icat

ion

mod

ule

Com

mun

icat

ion

mod

ule

Fig. 4: The transmission delay is measured at interfaces accessible to the user

Tole

rabl

e

max

imum

val

ue

Jitter

Transmission delay (ms)

12

3 T H E PAT H To WA R D C o E x I S T E n C E

Theresultsofthemeasurementscarriedoutshowthatforallwirelesstechnologies,radiofrequencieshaveto be considered a scarce resource to be used carefully. This is simple to achieve without great effort using obviousmeasures.Furthermore,allinvolvedpersonsshouldbeintegratedintoplanningsufficientlyearlyso that a reconciliation of interests and a prioritization can be carried out for the various departments within a company that wish to use radio technology.

Experience has shown that this coordination of interests and priorities is not always carried out at an early enough stage, since radio systems are not introduced everywhere at the same time due to the dif-ferentavailabilityofvarioussolutionsfordifferentfieldsofapplication.Furthermore,theterm„wireless“isoftenconsideredbyITdepartmentsorotherusergroupstobeequivalenttoWLANandthenetworkconnections and data transmissions handled over this, sometimes resulting in problems of understanding. Only if these aspects are taken into consideration can the actual coexistence management of radio sys-tems be carried out.

Coexistence management mainly comprises the following steps: 1. The registration of all radio applications in all departments of the company according to the following criteria: • Whereiswhichradiosystemusedandinwhichfrequencyrange? • Whoisresponsible? • Whatistheexactapplication? • Howistheradiospectrumusedintime? 2. The assessment of the coexistence situation, and if necessary, 3. Minimization of radio influences. 4. Continuous checking following installation and conservation of the situation (regularcheckingforcompliancewiththerequirementsoffrequencymanagement).

The individual steps involved in coexistence management are described in more detail in the VDI Guideline 2185 [1]. These steps, in particular minimization of radio influences, should be carried out with the support of an expert.

Minimization of radio influences is considered in more detail below. The aim is to decouple the wireless systemsinatleastoneofthesectors–location,frequencyortime–(seeFig.3,page10)sothatdesiredoperation of the systems involved is guaranteed.

3.1 Spatial decoupling

The transmitted power of a wireless system determines the spatial coverage of the radiocell,sincetheradiatedsignalstrengthissignificantlyreducedthegreaterthedistancefromthetransmitterantenna.Thereforethefrequencybandasaresourcecan be used again by other systems at a safe distance, but this depends on the re-ceiver sensitivity of the other radio system. The transmitted power should therefore beselectedassmallaspossibletopermitreuseoffrequencybands.

Receivers with higher sensitivity result in expansion of the range, but also simulta-neously increase the potential for being influenced by others. The spatial coverage can be additionally influenced by selection of the antennas. Antennas are available with various directional characteristics and the associated antenna gains. Optimiza-tion of the wireless systems can also be achieved through appropriate positioning of antennas.

Userscannormallyplantheradiofieldusingplanningtools,e.g.forWLANorBluetoothsystems.Amongother factors, this includes setting of the transmit power, selection of antennas, and their location and orientation.

Fig. 6 shows an example of radio propagation planning in which radio cells 1 to 3 have each been assigned adifferent,non-overlappingfrequencybandAtoC.Radiocell4islocatedsufficientlyfarawayfromradiocell1suchthatradiocell4canusethefrequencybandAagainwithoutinfluencingradiocell1.

3 The path toward coexistence

Fig.6:Exampleofradiofieldpropagation planning with spatial decoupling of several radio cells and with reuse of frequencybands

FrequencybandB

FrequencybandA

FrequencybandC

FrequencybandA

Cell 2

Cell 1

Cell 3

Cell 4

13

3 T H E PAT H To WA R D C o E x I S T E n C E

3.2 Decoupling in the frequency domain

Thefrequencyoccupationofasystemisdefinedbyselectionofaradiotechnologyanditssettings.Abasicdifferentiationismadebetweenfixed-frequencysystems,towhichafrequencybandisassignedbyspecificconfiguration(e.g.WLAN)andvariable-frequencysystems (e.g. Bluetooth), which uniformly occupy various channels of the total available frequencybandbymeansofhoppingsequences.

Bothtypesoffrequencyoccupationfacilitatedecouplinginthefrequencydomain:•Fixed-frequencyradiosystemsremainseparatesimplybyexclusivelyreservingafrequencyband for each system (see Fig. 7).

• Variable-frequency radio systemsdonot require exclusive reservation. They permanently changethefrequencywithdifferenthoppingpatternssothat,intheeventofacollision,thetransmissionisrepeatedatadifferentfrequency.

• Inordertodecouplefixed-frequencyandvariable-frequencyradiosystemsoperatingatthesamelocation,variable-frequencysystemscanworkwithaso-calledblacklist, i.e.theyavoidthefre-quencieswhichareusedbythefixed-frequencysystems.

Measurements have shown that the generation of such a blacklist for automation applications should not be carried out autonomously by the radio system itself, but should be planned. Automatic mechanisms canoftenonlyrecognizeotherfrequencyusersifthesetransmitfrequentlyandlongenough,whichisnot necessarily typical for industrial automation applications. For example, Bluetooth only recognizes other radio systems if these load the radio channel by 10 % or more. Receiver limitations should also be

observed,whichinsomecasescannotsuppressadjacentfrequenciessufficientlysharply,for example.

Fig. 8 shows an example in the 2.4 GHz band used by a Bluetooth system. A blacklist has beentransferredtothesystem,causingthefrequencybandsofthreeWLANsystems(onchannels 1, 6 and 11) to be skipped and leaving the „gaps“ available for use.

As a result of the large number of wireless components and possible applications in the 2.4 GHz band, this band is a highly valuable resource. The bandwidth is comparatively large,butneverthelesslimited.Toachievedecouplinginthefrequencyband,itisthere-fore advisable to use the 5 GHz band as an alternative as far as possible, especially with largedataquantitiesorhighdutycycles.The5GHzband,whichinsomecountrieshasabandwidth many times that of the 2.4 GHz band, is not used very much at the moment.

3.3 Time-based decoupling

Ifvariousradiosystemsusethesamefrequenciesatthesamelocation,theycanneverthelessallfulfilltheir respective tasks reliably. This is possible if the use of spectrum in time by all systems is low and if appropriate error correction mechanisms are used. Data can then be transmitted by one system during the transmissionpauses in theother systems.Theuseof spectrum in time isusuallydefinedby theapplication, e.g. by the number of data transmissions per unit of time or the inactive interval between two data packets.

The use of spectrum in time also depends directly on the data rate of the system used. If the data rate ofasystemislarger,lesstimeisrequiredtotransmitadatapackagethanforasystemwithasmallerdata rate.

If the duty cycle of a typical data transmission for the control of a plant is examined, it can be seen that the radio medium is only loaded to a low extent by an individual system, and that capacity is available for further radio systems.

Fig.7:Frequencydecouplingwithdifferentfrequencybandsforfixed-frequencyradiosystems

Fig. 8: Example of blacklisting: a Bluetooth system leaves space for several WLAN channels

FrequencybandA FrequencybandB FrequencybandC

WLAN channels

Pow

er

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

Powe

r Tim

e

WLANchannels

Topographic View 0.0% 3,6%

14

3 T H E PAT H To WA R D C o E x I S T E n C E

„Actual“ use of spectrum in frequency and time:

The actual use of spectrum in frequencyandtimenotonlydepends on the amount of data for an application, but also on the number of wire-less systems and – with almost all wireless systems – on the number of respective partici-pants as well. However, the useofspectruminfrequencyand time usually increases over-proportionally above a certain degree when the number of radio systems and participants increases. The reasons for this are retrans-mits resulting from increased collisions, and the so-called protocol overhead which is transmitted in addition to the actual useful data.

Spectral power density

1 ms

BISM = ISM

Band

mW/MHz

10

1

2400 MHz

2480 MHz

3 ms

4 ms

2 ms

PROFINET IO for WLAN (100 mW / 22 MHz): 5 devices, bidirectional per 16 ms

WLAN scanner (100 mW / 22 MHz): 50 nodes every 2 s

WISA (1 mW / 1 MHz): 50 devices, 2 events / s

ZigBee (10 mW / 3 MHz): 20 devices, every 5 s

Bluetooth (10 mW / 1 MHz): 7 devices, 2 events / s

nanoLOC (100 mW / 22 MHz): 1 device / 50 ms

Frame destroyed repeated, OK

Fig. 9: Schematic view of the use of spectrum at one locationwithregardtotransmitpower,frequencyand time for typical wireless systems in automation applications

Wirelesssystemsinindustrialapplicationsfrequentlyhavealowuseofspectrumintime,e.g.duringtheevent-triggered transmission of sensor statuses or the wireless connection of scanners for logistics.

Atthemoment,applicationswithlowresponsetimerequirementsaremainlyencountered.Acceptablevaluescanthenbesignificantlymorethan100ms,similartoresponsetimeswithsimpledatatrans-missions in an IT network. With such systems it is primarily important to avoid an interruption in the connection resulting from excessively long delays or from permanent interference, even if short-lived.

However,notallindustrialapplicationshavesuchlowtimerequirementsorshortdutycycles.Forexample,thefastdataexchangebetweenacentralcontrolleranddistributedI/Omodulesmayrequireresponsetimes in the range of a few milliseconds, but still with a low duty cycle on average. On the other hand, the permanent video transmission of a camera occupies the radio medium for almost the whole time, and is also time-critical.

Topermitbetterunderstanding,thethree-dimensionalviewinFig.9clarifiestherelationshipbetweensignalstrengthortransmittedpower,frequencyutilization,andtimedependentbehaviour.Itshowstheoccupation of the 2.4 GHz band as encountered nowadays in industrial automation applications with a number of typical wireless systems in parallel operation. The volumes of the telegrams are shown in different colors and represent the actual radio resource usage of the various wireless technologies, illus-tratingtheterm„useofspectruminfrequencyandtime“.

Timedecouplingofseveralradiosystemswithmorethanonetime-criticalapplicationisverydifficulttoassess by a non-specialist, especially if one of the radio systems has a low immunity to interference. An expert should be consulted in such a case.

5 ms

Time

15

4 P R o S P E C T S

Positive experiences with current wireless technologies will lead to further distribution and establishment of radio systems.Coexistencemanagementwill also remainan importantprerequisite in the future.Because of the importance of the coexistence topic, manufacturers of industrial wireless systems are also working on improvement of immunity to radio interference.

New technologies will also be added as time passes, and will lead to an even greater utilization of the frequenciesratherthantoareductioninradioinfluences.

Newfrequencybandswhichcouldbeapprovedofbytheregulatoryauthoritiesatsometimeinthefuturecould result in more bandwidth in the long term. However, it can also be expected that these will be increasingly used and that a situation similar to the current one will result.

Therefore coexistence management is becoming increasingly important. It offers several measures for guaranteeing interference-free parallel operation of radio systems, and - as shown - these measures are simple to plan and implement.

4 Prospects

3.4 Summary of coexistence management

Althoughstatementsoncoexistencemustalwaysbeconsideredtogetherwiththespecificapplications,thefollowing statements can be derived from practice and from the measurements upon which this brochure is based:• The radio influences of the systems investigated are low in typical industrial applications, since

the data telegrams are typically very short in automation applications and also occur staggered in time.

• Coexistencemanagementshouldbecarriedoutwiththeassistanceofanexpertinthecaseofparalleloperation of different systems with more than one time-critical application.

The following procedure can be recommended: 1. Planningoftheradiopropagationshouldgenerallybecarriedoutintheplanningandconfiguration

phase.Evenatthisearlystage,positioningandselectionofantennas,andassignmentoffrequencieshave an influence on later integration into an existing plant.

2. However,ifspatialandfrequencyoverlappingneverthelessexist,itisnecessarytoconsiderthetimeutilizationandfrequencycharacteristicsofthesystemsaswellasthereliabilityrequirementsoftheapplications.

3. If wireless systems with low immunity to interference are used in time-critical applications, all associ-ated systems must be checked in order to guarantee coexistence. If it is impossible to separate such systemsspatiallyorinthefrequencybands,anexpertshouldbeconsulted(see[1]).

Coexistence is possible through decoupling in at least one of the sectors – location, frequency and time:

Spatial decoupling• Adaptationoftransmittedpower• Antennaselection• Antennapositioningandorientation

Decoupling in the frequency domain• Channelselection• Blacklistingoffrequencyrangesorchannels

Minimization of frequency utilization over time• Averageloadingofallindividualsystemsaslowaspossible

16

T y P I C A L W I R E L E S S S y S T E M S F o R A U To M AT I o n T E C H n o Lo G y I n T H E 2 . 4 G H z I S M B A n D

Media access DSSS/OFDMdirect sequence spread spectrum /

orthogonal frequency division multiplexing

FHSSfrequency hopping spread spectrum

DSSSdirect sequence spread spectrum

CSSchirp spread spectrum

Bandwidth 22 MHz 1 MHz 5 MHz 80 MHz / 22 MHz

Number of channels

14 (3 non- overlapping) 79 16 1 / 7 (3 non- overlapping)

Spectrum

Radio technology WLAN Bluetooth WISA WirelessHART ZigBee MeshScape nanoLOC

Channel selection Fixed Dynamic Dynamic Dynamic Fixed Dynamic Fixed

Description WLAN is a radio standard in accor-dance with IEEE 802.11 for design-ing wireless local area networks. WLAN is characterized by high data rates, a large number of network nodes, and average ranges. It permits roaming between access points for mobile nodes and, e.g. in association with PROFINET, also wireless, deter-ministic communication between auto mation devices, even for safety-relatedapplications.Thefrequenciesused within the license-free ISM bands are also defined in substan-dards. IEEE 802.11b and g use three non-overlapping channels with a bandwidth of 22 MHz each in the 2.4 GHz band with gross data rates of 11 and 54 Mbit / s respectively. IEEE 802.11h uses 19 non-over-lapping channels with a bandwidth of 22 MHz each in the 5 GHz band, also with a gross data rate of 54 Mbit / s. WLAN provides data security during radio transmission by pro-viding authentication and encryption features (WPA2, AES).

The Bluetooth radio technology is standardized in accordance with IEEE 802.15.1. The Bluetooth SIG (Special Interest Group) defines applicationprofiles,e.g.forvoicetransmission.Inindustrial applications, the Serial Port Profile (SPP)andPersonalAreaNet-work (PAN) application profiles fortransparent Ethernet transmission are available for control and parameteri-zation tasks. The technology features both authentication and encryption systems. The net data rate of approx. 700kbit/sissufficientfortheseauto-mation applications. Bluetooth uses anadaptivefrequencyhoppingspreadspectrum (FHSS) technology with 1600frequencyhopspersecondtoa maximum of 79 channels with a bandwidth of 1 MHz each, where many systems can be operated in par-allel. This frequencyhopping spreadspectrum is a particularly rugged technology for industrial environ-ments since it can cope well with ef-fects such as multi-path propagation in strongly reflecting environments.

WISA (Wireless Interface for Sensors and Actuators) has been designed for local use in control loops in fac-tory automation. It is based on the physical layer of the IEEE 802.15.1 standard (Bluetooth), and also uses 79hoppingfrequencieswithaband-width of 1 MHz. WISA uses a fre-quency hopping spread spectrumwith a time frame of 2 ms optimized for WLAN and ZigBee and also a large minimum hop width in order tohopreliablyoutoffrequencybandswhich are already used or faulty. Up to 120 participants can commu-nicate in the 2 ms time frame with-out time overlapping. WISA works withafixed,smalltransmitpowerof1 mW in order to apply many small radio cells to access any number of radio participants in a factory hall.Characteristics of WISA are its low energy consumption and a rugged and deterministic response, inde-pendent of the number of partici-pants.

WirelessHART is the expansion of the HART standard for wire-less communication, and has been specially developed for monitoring, diagnostics and slow control in process auto-mation. At the physical layer, WirelessHART is based on the IEEE 802.15.4 standard. In order to cover large plants with a small number of access points, the participants of a Wireless-HART network have a routing functionality, i.e. the data of other participants is passed on in the network. This results in alternative data paths so that no data losses occur even with local radio interferences. All aspects of this network are con-trolled by special software, the network manager. For example, the frequency occupation andthe timing are controlled such that information can be ex-changed simultaneously be-tween the participants on the different paths.

ZigBee supports the design of wide-coverage mesh and star topologies. Particular attention has been paid to low data transfer rates, low energy con-sumption, security and availa-bility. ZigBee is based on the IEEE 802.15.4 standard. Zig-Bee has been optimized with regard to energy consumption such that autonomous sensor nodes can achieve long opera-ting times of up to several years without battery replacement.

Although the ZigBee Alliance addresses the complete auto-mation sector, the standard is primarily directed at the home and building automation sec-tor. It has been designed for a gross data rate of 250 kbit / s in the 2.4 GHz band; further channels with lower data trans-fer rates are available in other bands.

MeshScape is based on IEEE 802.15.4, supplemented by a network protocol for mesh-to-pology, self-organizing networks which primarily feature high re-liability for data transmission in both directions as well as fault tolerance, and also enable auto-matic and fast adaptation follow-ing changes in the network, e.g. with a faulty transmission link or non-accessible network nodes. The technology offers a high energyefficiencywhichguaran-tees a long battery service life for routers, and also scalability which permits networks with a hundred or more nodes. Mesh-Scape offers event-triggered transmission of small data pack-ets ranging from several bits up to several dozen bytes with a delay extending from milli seconds up to a few seconds, depending on the network size. It is partic ularly suitable for general pro-cess automation applications as well as for moni toring and diag-nostics in factory and process automation.

The nanoLOC technology whose media access is standardized as IEEE 802.15.4a can be used to design point-to-point connections as well as networks. It is charac-terized by the rugged modulation technology Chirp Spread Spectrum (CSS). The transmitted signal runs through a selectable frequencyrange with a bandwidth of 22 MHz (7 overlapping channels available) or 80 MHz (1 channel available) for each transmitted bit in a short, definedtimeofe.g.1µs.Thesym-bol duration and the frequencyrange swept by the transmitted signal determine the transmission ruggedness, e.g. with a multi-path propagation. This allows a high availabilityandrangeindifficultenvironments without direct line of sight. The gross data transfer rate is between 250 kbit / s and 2 Mbit / s depending on the used mode. The properties of the CSS signal permit runtime-based de-termination of the distance be-tween the radio nodes and their location.

Typical wireless systems for automation technology in the 2.4 GHz ISM band

Media access DSSS/OFDMdirect sequence spread spectrum /

orthogonal frequency division multiplexing

FHSSfrequency hopping spread spectrum

DSSSdirect sequence spread spectrum

CSSchirp spread spectrum

Bandwidth 22 MHz 1 MHz 5 MHz 80 MHz / 22 MHz

Number of channels

14 (3 non- overlapping) 79 16 1 / 7 (3 non- overlapping)

Spectrum

Radio technology WLAN Bluetooth WISA WirelessHART ZigBee MeshScape nanoLOC

Channel selection Fixed Dynamic Dynamic Dynamic Fixed Dynamic Fixed

Description WLAN is a radio standard in accor-dance with IEEE 802.11 for design-ing wireless local area networks. WLAN is characterized by high data rates, a large number of network nodes, and average ranges. It permits roaming between access points for mobile nodes and, e.g. in association with PROFINET, also wireless, deter-ministic communication between auto mation devices, even for safety-relatedapplications.Thefrequenciesused within the license-free ISM bands are also defined in substan-dards. IEEE 802.11b and g use three non-overlapping channels with a bandwidth of 22 MHz each in the 2.4 GHz band with gross data rates of 11 and 54 Mbit / s respectively. IEEE 802.11h uses 19 non-over-lapping channels with a bandwidth of 22 MHz each in the 5 GHz band, also with a gross data rate of 54 Mbit / s. WLAN provides data security during radio transmission by pro-viding authentication and encryption features (WPA2, AES).

The Bluetooth radio technology is standardized in accordance with IEEE 802.15.1. The Bluetooth SIG (Special Interest Group) defines applicationprofiles,e.g.forvoicetransmission.Inindustrial applications, the Serial Port Profile (SPP)andPersonalAreaNet-work (PAN) application profiles fortransparent Ethernet transmission are available for control and parameteri-zation tasks. The technology features both authentication and encryption systems. The net data rate of approx. 700kbit/sissufficientfortheseauto-mation applications. Bluetooth uses anadaptivefrequencyhoppingspreadspectrum (FHSS) technology with 1600frequencyhopspersecondtoa maximum of 79 channels with a bandwidth of 1 MHz each, where many systems can be operated in par-allel. This frequencyhopping spreadspectrum is a particularly rugged technology for industrial environ-ments since it can cope well with ef-fects such as multi-path propagation in strongly reflecting environments.

WISA (Wireless Interface for Sensors and Actuators) has been designed for local use in control loops in fac-tory automation. It is based on the physical layer of the IEEE 802.15.1 standard (Bluetooth), and also uses 79hoppingfrequencieswithaband-width of 1 MHz. WISA uses a fre-quency hopping spread spectrumwith a time frame of 2 ms optimized for WLAN and ZigBee and also a large minimum hop width in order tohopreliablyoutoffrequencybandswhich are already used or faulty. Up to 120 participants can commu-nicate in the 2 ms time frame with-out time overlapping. WISA works withafixed,smalltransmitpowerof1 mW in order to apply many small radio cells to access any number of radio participants in a factory hall.Characteristics of WISA are its low energy consumption and a rugged and deterministic response, inde-pendent of the number of partici-pants.

WirelessHART is the expansion of the HART standard for wire-less communication, and has been specially developed for monitoring, diagnostics and slow control in process auto-mation. At the physical layer, WirelessHART is based on the IEEE 802.15.4 standard. In order to cover large plants with a small number of access points, the participants of a Wireless-HART network have a routing functionality, i.e. the data of other participants is passed on in the network. This results in alternative data paths so that no data losses occur even with local radio interferences. All aspects of this network are con-trolled by special software, the network manager. For example, the frequency occupation andthe timing are controlled such that information can be ex-changed simultaneously be-tween the participants on the different paths.

ZigBee supports the design of wide-coverage mesh and star topologies. Particular attention has been paid to low data transfer rates, low energy con-sumption, security and availa-bility. ZigBee is based on the IEEE 802.15.4 standard. Zig-Bee has been optimized with regard to energy consumption such that autonomous sensor nodes can achieve long opera-ting times of up to several years without battery replacement.

Although the ZigBee Alliance addresses the complete auto-mation sector, the standard is primarily directed at the home and building automation sec-tor. It has been designed for a gross data rate of 250 kbit / s in the 2.4 GHz band; further channels with lower data trans-fer rates are available in other bands.

MeshScape is based on IEEE 802.15.4, supplemented by a network protocol for mesh-to-pology, self-organizing networks which primarily feature high re-liability for data transmission in both directions as well as fault tolerance, and also enable auto-matic and fast adaptation follow-ing changes in the network, e.g. with a faulty transmission link or non-accessible network nodes. The technology offers a high energyefficiencywhichguaran-tees a long battery service life for routers, and also scalability which permits networks with a hundred or more nodes. Mesh-Scape offers event-triggered transmission of small data pack-ets ranging from several bits up to several dozen bytes with a delay extending from milli seconds up to a few seconds, depending on the network size. It is partic ularly suitable for general pro-cess automation applications as well as for moni toring and diag-nostics in factory and process automation.

The nanoLOC technology whose media access is standardized as IEEE 802.15.4a can be used to design point-to-point connections as well as networks. It is charac-terized by the rugged modulation technology Chirp Spread Spectrum (CSS). The transmitted signal runs through a selectable frequencyrange with a bandwidth of 22 MHz (7 overlapping channels available) or 80 MHz (1 channel available) for each transmitted bit in a short, definedtimeofe.g.1µs.Thesym-bol duration and the frequencyrange swept by the transmitted signal determine the transmission ruggedness, e.g. with a multi-path propagation. This allows a high availabilityandrangeindifficultenvironments without direct line of sight. The gross data transfer rate is between 250 kbit / s and 2 Mbit / s depending on the used mode. The properties of the CSS signal permit runtime-based de-termination of the distance be-tween the radio nodes and their location.

Typical wireless systems for automation technology in the 2.4 GHz ISM band

17

T y P I C A L W I R E L E S S S y S T E M S F o R A U To M AT I o n T E C H n o Lo G y I n T H E 2 . 4 G H z I S M B A n D

18

FA Q – F R E Q U E n T Ly A S k E D Q U E S T I o n S

FAQ – Frequently Asked Questions

Can the coexistence of industrial radio solutions be configured ?

Various parameters for the coexistence of radio solutions are supported depending on the manu-facturer and the radio solutions. Typical parameters are the transmitted power, channel, access procedure or blacklisting.

Does a coexistence indicator exist ?

Coexistence is not the property of a radio system but a state in which different wireless systems fulfilltheircorrectfunctionsdespitetheexistenceofanotherradioapplication(accordingtoVDIGuideline2185)andradioinfluences.Aspecificindicatororasimplemeasuredvalue,e.g.intheform of a number, is not available.

Does the type or orientation of the antenna play a role ?

Selection of the antenna influences the local propagation and reception of radio signals. Antenna manufacturers provide corresponding diagrams which describe the radiated signal strength of an antenna in free space. To achieve an optimum connection, all antennas should be used with opti-mum directivity and have the same orientation. It should be noted that the range of interference can be many times the operational range.

Who evaluates coexistence ?

VDI Guideline 2185 suggests, in addition to introduction of coexistence management, that a “fre-quencyrepresentative”benamedwhoisresponsibleforevaluationsintheapplicationenvironment.

Do typical industrial processes and devices have an influence on the radio transmission ?

Typical widely-used devices such as motors and power converters have no influence. Depending on the wireless solution used, some industrial applications such as open microwave processes (2.45GHz)orarcweldingprocesses(upto1GHz)maycauseinterferenceinthefrequencyrangeof the radio application. The manufacturers of the radio systems can provide information on potentialcausesofinterferenceinthefrequencybandofaradiosolutionandsuggestappropri-ate measures for guaranteeing correct functioning of the radio system.

Why is there no separate frequency band for industrial applications ?

The2.4GHzbandisoneofthefewlicense-freefrequencybandsforindustrialapplicationswhichis available worldwide. The establishment of a separate band for automation technology with global availabilitywouldrequiremanyyearsandahugenumberofapplicationstomakeitlegitimate.Furthermore, it can be expected that this band would also become increasingly used and that a situation similar to the current one would result.

FA Q – F R E Q U E n T Ly A S k E D Q U E S T I o n S

19

How can coexistence be guaranteed permanently ?

To guarantee the coexistence of radio solutions, it is advisable to organize and monitor all used radio solutions by means of coexistence management (e.g. in accordance with [1]).

Why is coexistence a topic for industry but not for home ?

Coexistence is actually a topic wherever several radio solutions are used. However, industrial radio solutionsareusuallysubjecttosignificantlyhigheravailabilitydemandsthanapplicationsintheprivatesector.Thereforecoexistenceinindustrialenvironmentsisofgreatersignificance.

Is IT-WLAn disturbed by the radio solutions in automation technology ?

The IT-WLAN function is always guaranteed for the current radio components of the involved auto-mation companies, as long as no time-critical applications are used in the ms range. The IT-WLAN must nevertheless be involved in the coexistence management.

Can mobile phones, laptops, walkie-talkies, hands-free units or measuring equipment be brought into halls with wireless automation solutions ?

If industrial applications in the 2.4 GHz ISM band are used, it is inadvisable to use mobile phones, laptops, PDAs etc. with active Bluetooth or WLAN uncontrolled in such halls, since the influences on some existing industrial solutions cannot always be predicted.

Is a „normal“ mobile phone a source of interference ?

Mobilephoneshavetheirownfrequencies,andnointerferenceforautomationapplicationscanbeexpected as long as Bluetooth and WLAN have been deactivated and a minimum distance of a few 10cm is respected.

What must be clarified with the IT department ?

The IT department must be integrated in the coexistence management if radio solutions are to be operated by it.

[1] Guideline VDI / VDE 2185 „Radio based communication in industrial automation“, Publisher: The VDI / VDE Society for Measurement and Automatic Control (VDI / VDE - GMA Gesellschaft Mess- und Automatisierungstechnik); Beuth Verlag, September 2007

References

ZVEI - German Electrical and Electronic Manufacturer‘s AssociationAutomation DivisionLyoner Strasse 960528 Frankfurt am MainGermanyPhone: +49 (0)69 6302-305Fax: +49 (0)69 6302-319E-mail: automation@zvei.orgwww.zvei.org