to the sensitivity/SINAD parameters performance Simpliﬁed estimation of ISM-band, OFDM-based WSNsaccording Technology Journal of Applied Researchand

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Availableonlineatwww.sciencedirect.com

Journal of Applied Research and Technology

www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology15(2017)1–13

Original

Simplified performance estimation of ISM-band, OFDM-based WSNs according to the sensitivity/SINAD parameters

Pierre van Rhyn

^∗

, Gerhard P. Hancke

DepartmentofElectrical,ElectronicandComputerEngineering,UniversityofPretoria,PrivateBagX20,Hatﬁeld0028,LynnwoodRoad,Pretoria,SouthAfrica Received13January2016;accepted4October2016

Availableonline13January2017

Abstract

Anovelmethodisproposedtoestimatecommittedinformationrate(CIR)variationsintypicalorthogonalfrequencydivisionmultiplexing (OFDM)wirelesslocalareanetworks(WLANs)thatareappliedinwirelesssensornetworks(WSNs)andoperatewithintheindustrial,scientific andmedical(ISM)frequencybands.Themethodisbasedontheobservation ofaphenomenon ofwhichthe significancehasnotpreviously beenrecognizednordocumented;heretermedtheserviceleveldifferentialzone(SLDZ).Thismethod,whichconformstotheITU-TY.1564test methodology,providesthemeanstosetacommittedinformationrate(CIR)referenceforIEEE802.11a/g/nOFDMsystemsintermsofcommitted throughputbandwidthbetweenatestnodeandanaccesspoint(AP)ataspecificrange.Ananalyticalapproachispresentedtodeterminethe relationshipbetweenthemaximumoperatingrange(inmetres)ofawirelesssensornetworkforaspecificcommittedthroughputbandwidth, anditslinkbudget(indB).Themostsignificantcontributionsofthispaperaretheanalyticaltoolstodeterminewirelessnetworkcapabilities, variationsandperformanceinasimplifiedmethod,whichdoesnotrequirespecializedmeasurementequipment.Withtheseitbecomespossiblefor industrialtechniciansandengineers(whoarenotnecessarilyinformationtechnology(IT)networkexperts)tofieldanalyzeOFDMWLANsand soqualifytheirperformanceintermsofY.1564specifiedservicelevelagreement(SLA)requirements,aswellasintermsofthewidelyacceptable sensitivity/SINADparameters.

Keywords:Mobile;Wireless;Sensornetworks;Datathroughput;Committedinformationrate(CIR);Serviceleveldifferentialzone(SLDZ)

1. Introduction

WSNsconsistofsensornodesthatareplacedincloseprox- imityto, orwithin thephenomenatheyobserve,accordingto specificmeasurementrequirements,whilstcommunicatingwith othernodesornetworkaccesspoints(Hou&Bergmann,2010).

Atrue-to-typeWSNsensornodeconsistsofasuitablesensor ortransducer,amicroprocessortoexecutetriviallogicopera- tionsorcomplexprogrammes,anRFtransceivertoaccessthe networkandasuitablepowersupply(Gungor&Hancke,2009).

WSNshavebeensuccessfullyappliedinenvironmentaland temperaturemonitoring(Carullo,Corbellini,Parvis,&Vallan, 2009), machine monitoring (Flammini, Marioli, Sisinni, &

∗Correspondingauthor.

E-mailaddress:[email protected](P.vanRhyn).

PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.

Taroni, 2009), energy monitoring (Lu &Gungor, 2009),and increasinglyinthefieldsofopticalcharacterrecognitionOCR (Korber, Wattar, & Scholl, 2007) and thermography (Fluke, 2007).

Thermographymeasuresthetemperatureoftheentirepicture thatthecharged-coupleddevice(CCD)hascaptured.Forexam- pletheTi-300infraredthermographyimagermanufacturedby Flukecanbeappliedinatypicalindustrialsolutiontomonitor andinspectfurnacesandboilerswirelesslyusingitsproprietary

‘SD-wireless’,on-board802.11g/ninterface(Fluke,2008).The imager provides a real-time video image and thermographic analysisof thetemperaturedistributioninsidethecombustion chamberorboilertoAndroid/Linuxnodes.Thisinformationis crucialfor processcontrolandpreventativemaintenancepro- cedures, but it requires broadband capability for networked transmission.

ISM band, OFDM WLANs are the proposed solution for WSNs that require broadband operation with committed

http://dx.doi.org/10.1016/j.jart.2016.10.002

1665-6423/©2016UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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information rate and predetermined class of service. Two industrymeasurementstandardsfordatathroughput,RFC2544 (Giguère,2008)andITU-TY.1564(ITU,2016),areapplicable.

Thetermscommittedinformationrate(CIR),excessinformation rate(EIR)andclassofservice(CoS)aredefinedbytheMetro EthernetForum(MEF,2006)andbytheITU-TinY.1564.

CIRandEIRarespecificallydefinedbyY.1564,intermsof measurementphilosophyandmethodologies,astheacceptable parametersinthemethodtodetermineserviceperformancelev- elsthat conformtomodernindustrialandcommercialservice levelagreements.ItthereforeappearslogicalthatmodernIEEE 802.11a/g/nsystemsthattransportTCP/IPEthernetservicesfor industrialsolutionsshouldbeevaluatedaccordingtothesame ITU-TY.1564standards.Itshouldbenotedherethat802.11bis notanOFDM-basedsystem,butthat802.11a/g/nOFDMsys- temsarebackwardcompatiblewith802.11b(Paulraj,Nabar,&

Gore,2003).

Multiple-input multiple-output orthogonal frequency- division multiplexing (MIMO-OFDM) wireless technology appearstofulfiltherequirementsofindustrialWLANs(Giguère, 2008).MIMO-OFDM isconsideredamostinteresting devel- opmentinthefieldofbroadbandwirelesstechnologybecause it is extremely tolerant to multi-path propagation and fre- quencyselectivefading.AmajoradvantageofanOFDM-based 802.11a/g/ngatewayrouter,forWSNuse,isits3G/LTEInternet connectioncapability that facilitates aglobal communication connection(Liu&Li,2005),seamlesslyconnectiondissimilar operating systems across dissimilar networks. A far-reaching applicationinmindisthatAndroid/Linux-basedIWSNscanbe deployed,operatedandcontrolledfromanywhereontheglobe.

PerformancegainsmaybemadeinOFDMsystems bythe applicationofmorethanoneantennaateitherorbothconnecting pointsofthewirelesslink(Bolcskei,2006).Multipleantennas canbeusedtoeffectinterferencecancellationandmayalsobe usedincoherentcombiningtechniquestoobtaindiversityand arraygain.Anadditionalfundamentalchannelgainisrealized by the useof multipleantennas in aMIMO configuration at either/bothsidesofthelinkthatimprovesspectralefficiency.

PerformancegainsmayalsobemadeinOFDMsystemsby theapplicationofahighergainantennaateitherorbothcon- nectingpointsofthewirelesslink(Ide,Kingsley,O’Keefe,&

Saario,2005).Highergainantennaslikelogperiodictypesare usuallyhighlydirectionalwithabeamwidthof15^◦–20^◦insec- torspecificdirection(azimuth).Attentionisdrawntovertically polarizedomnidirectional antennas withhigher gain, such as themultiple-elementcollineararray,(Karlsen,2009)whichwill onlyoperatesatisfactorilyinthehorizontalplane.

Exceptionalperformancegainsmaybemadebyselectingthe appropriatetechnologythatisavailabletotheindustry(National Instruments,2016).Forexample,atrendobservedinboththe automotiveandmilitaryindustriesistomakeuseofcommercial off-the-shelf(COTS)technology,repackagedtoIP65standards, asopposedtodevelopmentofturnkeysystemsfromtheground up.

Thisapproachholdsmanyadvantagesforindustrialelectron- icsandinformaticspractitionersbecauseoftimeandcostsavings relatedtothemeasurementplatform.Forthesamereasons,the

applicationofAndroid/Linux-basedplatformshasbecomepro- lific (National Instruments, 2016). These devices incorporate mostly802.11a/g/ncompatibleWLANcapabilities,aswellas multipleconnectivityoptions.Theircapabilitiestoconnectdis- similaroperatingsystemsacrossdissimilarnetworksplacethem functionallyonparwithOSImodel,level7gateways.

Thelatestcommercialgateway/routersoperateinbothISM frequency bands of 2.4GHz and 5.8GHz in the WLAN, so providingfrequencydiversity.Eachfrequencybandisutilized bytwoomnidirectionaltransceivers,henceprovidingadegree of space diversity, as well as noise cancellation. The gate- wayemploysaso-called‘hotstick’USBmodemtoconnectthe WLANtotheInternetserviceprovider,typicallyinalicenced frequencybandsuchas3.8GHz.

However,arelatedproblemistodefinethedegreetowhich the network can beimproved interms of its service level as a result of performance optimization attempted by the user (Akerberg, Gidlund,& Bjorkman,2011; Bello, Mirabella,&

Raucea, 2007; Sun, Akyildiz, & Hancke, 2011). Advantages forindustrialelectronicsandinformaticspractitionersfromthis approach are time andcost savingsrelated tothe set-upand operationofthemeasurementplatform.

1.1. Researchproblem,objectiveandcontributions

Theresultsofbit-ratetestingbypatterntransmissionmaybe misleadingifoneconsidersaphenomenonweshallrefertoasthe

‘serviceleveldifferentialzone’(SLDZ)ofanOFDMWLAN.

The SLDZ of an OFDM WLAN was observed inan indus- trialenvironmentduringpreliminarytransmissionteststhatwere conducted bytheauthorsin2007,inpreparationofanengine test benchfor the purposeof (destructive) dieselenginetest- ing. Thisform of enginetesting requires remote temperature monitoringoftheenginecomponentsbecauseofthemechani- calrisksofenginefailurewhichincludeflyingshrapnelandhot oilejection.

ItwasobservedthattheOFDMnetworkwillmaintainbroad- bandwidth transmission (video andaudio soundtrack)witha node,uptoacertaindistanceawayfromtheaccesspoint(AP), astheseparatingrange(r)isincreased.

At a range rne (never exceeded) from the AP, the signal is suddenly compromised with little or no warning by video transmissionquality.Thenodenowhastomovebackacertain distancetowardstheAPtormaxinordertoregaintransmission.

Thisdistancemoving backwasobservedtobeaconsiderable percentageofthetotalapparenttransmissionrangerne.Itisthis considerablerangebetweenrmaxandrnethatweshallreferto astheSLDZ.

The qualityofservice (QoS)levelof anetworkcannotbe accuratelydeterminedwhennodesaredeployedwithin(orout- side)theSLDZofthenetwork.TheSLDZappearstoconform tothemagnetichysteresisphenomenonandunderlyingphysi- callawsthatdescribeit,forexampletheJilesandAtherton(JA) modelofhysteresis(Carpenter,1991).

Thispaperformulatesthehypothesisthat connectedband- width(BW)inMbpsisdirectlyproportionaltothepowerdensity inW/m²oftheradiofrequency(RF)wave,orconverselytothe

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electricfieldstrength(E)oftheRFwaveinV/m,atthereceiv- ingnode.Connectedbandwidthreferstodigitalbandwidth,as opposedtoanaloguebandwidth,whichisthefrequencyrange between the half power pointsof an electromagnetic energy wave.ThisimpliesthattheconnectedBWwilldecreaseatthe samerateasthepowerdensityortheelectricfieldstrengthofthe RFwave,untilthedeclineinsignal-to-noiseratio(SNR)results intheterminationofstreamingwirelesscommunication.

Testing the above-mentioned hypothesis positively is a researchgoalthatmotivatestheapplicationofthewell-known inverse-squarelawofradiatedpower,inconjunctionwitheffects ofthephenomenontermedhereastheserviceleveldifferential zone.The consideration of the service level differential zone isanovelapproachthatprovidesthemethodsandmathemat- ical buildingblocks, which are required to solve range, and capacity-related issues of wireless systems (Girs, Uhlemann,

&Björkman,2012).

Thispaperproposesamethodtodeterminetheexcessinfor- mationrate (EIR) and the committed informationrate (CIR) bydetection,usingtheSLDZdynamichysteresisphenomenon.

Thistechniqueiscombinedwithscaledvideostreamingtorep- resenttheuserdefinedthroughputbandwidth.Previousmethods usedthefiletransferprotocol(ftp)methodwhichwasnotaccu- ratebecausenotallthekeyperformanceindicators(KPIs)are considerednamelylatencyandjitter.Thed-CIR(detectedCIR) providesaqualityofservicereference,whichconformsmostly totheITU.Y-1564testphilosophy.

Thispapercontributesthescientificequationsandmethods thatarerequiredtoestimatewirelessnetworkcapabilities,vari- ationsandperformanceduring industrial deployment or field tests.WiththeseitbecomespossibletoqualifyOFDMWLANs interms oftheir serviceperformanceaccordingtoacceptable standardssuchasY.1564,whichisakeyrequirementforindus- trialapplications.

Anextendedresearchgoalofthisstudyistodeviseamethod tocomparedifferentOFDMWLANdatalinksintermsoftheir performance. An acceptable range performanceparameter is required when considering improvement to the link quality, becauseitwilldeterminethemaximumextenttowhichthelink budgetcanbeincreased.

Thestudyproposesthetypicalradiorangeparametersnamely sensitivityandSINAD (signaltonoiseanddistortionratio) to expressOFDMWLANdynamicrangeperformance.Theseare mostlyused inthe specializedfieldofradiocommunications.

Theseparameters can be derived from the d-CIR and d-EIR usingtheSLDZdetectionmethodpreviouslydescribedinthis paper.

Amethodisexploitedtodeterminetheexcessinformation rate(EIR)andthecommittedinformationrate(CIR)according tothe(ITU)and(MEF)bydetection,usingtheSLDZdynamic hysteresisphenomenon,andtoequatethesetotheradiorange characteristicsofthephysicallayersundertest.Thistechnique iscombinedwithscaledvideostreamingtorepresenttheuser definedthroughputbandwidth.Thed-CIR(detectedCIR)providesaqualityofservicereference,whichconformsmostlyto theITU.Y-1564testphilosophy.

1.2. Analyticalapproach:thefreespaceradiationmodel In ordertodeterminethecharacteristictransmissioncurve ofthewirelesslinkundertest,samplesofconnectedbandwidth BWaredrawnatdiminishingrangeintervals.Thesesamplesare thenprocessedwithMSExceltoproducealogarithmictrend line,depictingconnectedBWasafunctionofrangeinmetres.

The stepped curve response of the transmission characteristics makes more accurate reading intervals difficult and pointless. The connected bandwidth level at each respective range section is maintained until the firmware switches over tothenextlower(orhigher)BWsetting.TheBWswitchingcri- teriaarenotpublishedbythemanufacturersothattheeffects canonlybeobserved,butnotclearlyexplained.

Asimplesolutionisamethodtosmooththesteppedcurves byusinglogarithmictrendmodellingsuchasExcelorMatLab functions.Moremeaningfulreadingscanthenbemadegraphi- cally,andtheresultinggraphfunctionmaybeusedtocalculate range-correspondingBWvariations.

Thepointsourceradiatororisotropicantennahastobecon- sideredinthestudyoftheradiatedfieldfromanyantenna,when line-of-sighttransmissionisconsidered.Practicalantennasand their performance are oftenreferenced interms of thisbasic radiator.

1.3. Derivationoftherangeequations

Energyradiates equallyinalldirectionsfromtheisotropic antenna;therefore,thesphericalradiationpatterninanyplane iscircular,forexampleinthegroundplane.Thetypicalisotropic radiatorissuppliedwithapowerofPwattsattheoriginO.The powerflowsoutwardfromthispointandthroughthespherical surfaceS,whichhasaradiusr.Theintensityofradiation,i.e., thepowerdensityPDatpointIcanbedefinedas:

P_Di= P

4πr²W/m²

Thisequationillustratestheeffectsoftheinversesquarelaw ofirradiance:attwicetherange(2×r),withPataconstant, thepowerdensityPDidecreasesbyafactoroffourorby−6dB.

Also, toobtain thesame PDi at doublethe range(2×r), the power P would have toincrease fourfold or by +6dB, irre- spective ofthenumberof antennasused(Wolnicki, 2005),as confirmedbyWavion’sMIMOenterpriseAPequippedwithsix +6dBiantennas.

Maxwell’stheoriesdefinetherelationshipbetweenthepower density PDi and the E-field and H-field vectors as follows (Maxwell,2011):

P_Di=Ei×HW/m²

Theabsolutemagnitudeofthepowerdensityorintensityof irradiationisthus:

|PDi|=EiH = Ei² 120π

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(whereEiistheelectricfieldstrengthofirradianceandthe impedanceoffreespaceis120␲ohm,whichisapproximately 377).

Accordingtotheinversesquarelaw,theratiooftwopower densitiesattheirrespectedrangesfromthesourceofradiation canbeexpressedasthelinkbudgetvariation,indB:

Linkbudgetvariation(dB)=10Log

(PDi1/r₁)

(PDi2/r₂) (1)

Manipulationyields:

Linkbudgetvariation(dB)=10LogP_Di₁ P_Di₂ ×r₂

r₁

SincePDi=Ei²/120␲(Maxwell,2011):

∴Linkbudgetvariation(dB)=10Log

Ei²₁/120π

Ei²₂/120π ×r₂ r₁

Ei₁ Ei₂

2

×r₂ r₁

Butaccordingtothehypothesis,theelectricfieldstrengthEi isdirectlyproportionaltotheconnectedbandwidthBW:

BW₁ BW₂

2

×r₂ r₁

∴Linkbudgetvariation(dB)=20LogBW1

BW2 ×r2

r1

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Whenthebandwidthremainsaconstant(BW1=BW2),then theratioofthetworespectiverangesmaybeexpressedas:

Linkbudgetvariation(dB)=20Logr2

r₁ (3)

(Thisistheequationproposedfortheanalyticalprocessing ofdatasamples)

Also,whentherangeremainsaconstant(r1=r2),thenthe ratiooftwobandwidthsmaybeexpressedas:

Linkbudgetvariation(dB)=20LogBW₁

BW₂ (4)

WithEqs.(3)and(4)itbecomespossibletopredictthrough- putvariationsintermsofrelativeRFlinkbudget(indB),andto qualifyOFDMWLANsintermsoftheirserviceperformance accordingtoacceptablestandards such asY.1564, whichisa keyrequirementforindustrialapplications.

1.4. Researchobjective

The mainresearch objectiveof this study wasto devisea simplemethodtoanalyzeandestimatetheperformanceofan ISM-band,OFDM-based,WSNdatalink.Wecomparethetrans- missioncharacteristicsbeforeandaftersignalintensificationor attenuationisconfigured,andrelatethedifferencesintermsof theRFlinkbudget(indB),bytheapplicationofEq.(3).Results arethenpresentedfordiscussion.

2. Methodsandtestenvironment

Thissectiondescribestheexperimentalmethodsandthecon- ditionsunder whichthetestresultswere obtained sothat the experiments may be easily replicated for continuous review and investigation.Then, theanalytical approaches are briefly reviewedbeforethedataanalysismethodisdescribed.

Testingisinaccordancewiththeneural black-boxmethod combinedwiththemathematicalapproachoffunctionalapprox- imation. Thedigital inputstothe entiresystemarecompared with the analogue video and audio output obtained in terms of observed quality. The inner hardware of the node or the physicallayercannotbemanipulatedbythe user,thereforeit remainsirrelevanttothisstudy.However,differentphysicallay- erscannowbecomparedwithoneanotherintermsoffunctional performance.

The test topology is shown in Figure 1.1(a)–(c). The test rangeisshowninFigure1.2.ThetypicalOFDMtestnetwork consistsoftwomobilecomputersandanISM802.11.gaccess point(AP)router.The802.11gsystemwasusedas anexam- plebecausethissystemversion(g)wasalreadywelldeveloped intermsofindustrialpackagingandantennas/accessoriesavail- able.Wealsoused802.11gasanexamplebecauseofinterestin theresultsobtainedfromsingletransceiversystems,asopposed tomultipletransceiversasinMIMOoperation.Thefactofthe matteristhatthesimilarresultswill(predictably)beobtained whenrepeatingthesetestswithanyoftheIEEEratifiedsystems 802.11a/g/n.

Figure1.1depictsthenetworkoptionsfortesting.Theserver nodeusedoption(a)forExperiment1,option(b)forExperiment 2andoption(c)forExperiment3.TypicalISMbandOFDMnet- workvariationsweredeployedforstreamingvideotransmission.

Performance resultsweremeasuredandarepresentedhereas connectedbandwidth(BW)inMbps,asafunctionoftransmis- sionrange(r)inmetres.Theservernodeisdirectlyconnected totheAPviaCat5cable,andalsocontrolstheAPadapterset- tingswithabrowser-basedguideduserinterface(GUI).Forthe mobilenode’sadapter,IntelPROSetWirelessadaptersoftware isusedwiththemobilenodebecauseitisfreelyavailableand easytouse.Theinstallationofthisadditionalsoftwareenhances thefunctionalityanduserperceptionofthemeasurementexpe- rience.

Radiationmeasurementsnormallyhavetobeconductedinan anechoicchamber oropen-spaceantenna measurementrange.

Resultsotherwiseobtainedwillberegardedasconductionmea- surementsratherthanradiationmeasurements,andaretherefore notconsideredhere.

Sincethemethoddescribedhereisintendedforactualfield useandreferstotheSINADparameter(ratioofsignaltonoise anddistortion),theresultswererequiredtoincludetheeffects introduced byexternalnoiseanddistortion.Thetransmission testswerethereforeconductedonanoldcricketpitch,asshown inFigure1.2,providingarelativelylargeopenspacewithan evenlyflatgroundplane.

Figure1.2showstheplanareaofthetransmissiontestrange.

The approximate latitudeand longitude of the test station is indicated,aswellastheimaginaryrangelineandmarkers.The

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Fig.1.1.(a),(b)and(c)Experimentaltestnetwork.

Fig.1.2.Transmissiontestrange.

d-CIR and d-EIR ranges for the three transmissionseries in Figure3arealsoindicated.

Videotransmissioncommenceswhenthemobilenoderuns thefileintheservernode’ssharefolderwithWindows’media player.Theuseofapredeterminedvideobit-streamisappliedin atestdetectiontechnique:Ifthevideostreamataspecificpre- scaledratehasnoperceptualimpairmentintermsofsoundor picture,itconformscompletelytotheagreedCIR.Ifanynoise orvideodistortionisperceived,itmeansthekeyperformance indicators(KPIs)arecompromisedandthevideostreamdoes notconformtotheagreedCIR.

Thevideobit-streamratewaspredeterminedtorepresentthe committedinformationrate(CIR)oragreedQoSthroughput,set atnominally600kbpsmaximumfortheexperimentalrequire- ment.Thiswasachievedby selectingasuitable length video clip and scaling the frame rate down inH.264 with Apple’s

video editingsoftware until the desired bitrate was obtained.

Theresultingaudio/videointerleaved(AVI)filewasthensaved ontheservernodeinasharedfolder.

2.1. Experimentalmethods

Thefirstsetofexperimentsfocusedonpassivetesting.This meansthedeterminationofthetransmissioncharacteristicsfora throughputof600kbpswithanyvariationsimposedinapassive (i.e., inactive) manner, as compared with the standard low- complexity (+1dBi)antenna characteristics as inExperiment 1.

Thesecharacteristicsarecomparedtotransmissioncharac- teristicswhenusinganantennawithahighergaincharacteristic (Willig,2008),forexampleavertical(+6dBi)collineardesign for2.4GHzinExperiment2.Thetestwasrepeatedusingtwo

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inlineresistiveattenuatorsof6dBeachinconjunctionwiththe +1dBiantennasinExperiment3.

Thesecondsetofexperimentsfocusedonactivetesting.This meansthatthetransmissioncharacteristicsforathroughputof 600kbpsweredeterminedfordifferentsettingsoftransmitted power levels, whichwere electronically controlledby Intel’s PROSetWirelesssoftware.Thetransmissioncharacteristicsof atypical802.11gsingletransceivernetworkoperatingat20%

ofmaximumpowerweredeterminedinExperiment4.Thetest wasrepeatedforapowersettingof80%ofmaximumpowerin Experiment5.

2.1.1. Experiment1:Testnetworkservicereference

Experiment1determinedtheservicereferencefortheelec- tronics that was utilized. The electronics used for this test employedOFDMMIMOtechnology,manufacturedbyBelkin, AP model 802.11g+ MIMO with fixed transmit power at +17dBm.With the mobilenode equippedwiththe matching Belkin USB g+ MIMO modem, the two transceiver system enabledanearrangedataconnectionrateof108Mbps.

Theset-upforExperiment1isshowninFigure1(a).After startingthevideotransmission,theBWwasrecordedatsuitable distance intervalswhilst increasing the transmissionrange to the point of total signal deterioration, as shown inFigure 2.

Thispointismarkedasthedetectedexcessinformationrate(d- EIR)range.Thetransmissionrangewasthenreduceduntilvideo transmissionrestarted.Thispointwasmarkedasthe detected committed information rate(d-CIR) range. The SLDZ is the rangebetweend-CIRandd-EIR.

Figure2containsthedatacollectedduringExperiment1,in agraphgeneratedwiththeaidof Excel.The indicatedd-CIR andd-EIRareforatestbit-rateof600kbpsthatiscontinuously monitoredintermsoffourkeyperformanceindicatorsnamely bandwidth,latency,jitteranderrors.Notetherangeatwhich transmissionfails(72m)correspondstoapproximately6Mbps.

When reducing the transmission range, the transmission restartsat55matapproximately18Mbps.Ourmethodtouse atrend-lineisappliedsothat moreaccurateinformationmay beextractedfromthesteppedtransmissioncurves,orbyusing theequationy=−26ln(x)+125.6alsogeneratedwiththeuse of Excel.Thesesteppedcurvesare duetothemanufacturer’s (Intel)internalalgorithmsthatarecontainedinthefirmwareof theOFDMWLANphysicallayers,whichcanbesmoothedwith theuseofexternalsoftwarepackageslikeExcel.

TheSLDZmanifestsitselfinthesamemannerasatypical hysteresis curve (Carpenter, 1991); transmission failing sud- denlyatd-EIRandrestartingbackatd-CIR.Thispatternwill repeatasthemobilenodeapproachesorrecedesfromtheaccess pointAPacrosstheSLDZ.

2.1.2. Experiment2:Testnetworkwithimprovedservice reference

Experiment2determinedtheimprovementintheserviceref- erencefortheelectronicsthatwasutilized.However,insteadof the standard+1dBiantennas,the APwasequippedwithtwo +6dBiantennasasshowninFigure1(b).Themobilenodewas equippedwiththematchingBelkinUSBg+MIMOmodem.The

datacollectedduringExperiment2isdepictedinFigure3as(b) Transmission+6dBi.

Figure 3(b) indicates thetest results obtained with+6dBi antennas insteadof +1dBi, whichmeans thelinkbudget was improvedby5dB.

2.1.3. Experiment3:Testnetworkwithreducedservice reference

Experiment 3 indicates the reduction in the service refer- encefortheelectronicsthatwasutilized,asacontrolprocedure.

However,insteadofthestandard+1dBiantennas,theAPwas alsoequippedwithtwo6dBattenuatorsfittedin-linewitheach antennaasshowninFigure1.1(c).Thismeansthelinkbudget wasreducedby6dB.

Theset-upforExperiment3isshowninFigure1.1(c)and thedatacollectedduringtheexperimentisdepictedinFigure3 as(c)Transmission−5dBi.

2.1.4. Experiment4:TestnetworkwithnodeRFpowerat +11dBm

Experiment4determinedtheservicereferencefortheelec- tronics that was utilized when operating a node at 20% of maximum power. The electronics used for this test was an 802.11gAProuterwithsingletransceivermanufacturedbyD- Linkoperatingat+18dBm.

The wireless adaptercontrol software on the mobilenode madeitpossibletosetRFpowerin20%decrementsfrommaxi- mum(+18dBm/63mW)tozero.ForthistestthemobilenodeRF powerwassetto12.6mWor+11dBm.Thesingletransceiver systemenabledanearrangedataconnectionrateof54Mbps.

2.1.5. Experiment5:TestnetworkwithnodeRFpowerat +17dBm

Experiment5determinedtheservicereferencefortheelec- tronics that was utilized when operating a node at 80% of maximumpower.ForthistestthemobilenodeRFpowerwasset to50mWor+17dBm.Thesingletransceiversystemenableda nearrangedataconnectionrateof54Mbps.

Theset-upforExperiment4andExperiment5isessentially indicatedinFigure1.1(a)butwithasingleantennaontheAP andusingtheinternalRFmodemsituatedon-boardthemobile node.TheresultsoftheactivetestsareindicatedFigure4.

InFigure4,thefirstseriespresentsthetransmissionresults withRFpowersetto+11dBm(12.6mW),andthesecondseries ofresultsareforRFpowersetto+17dBm(50.4mW).Figure4 thus indicates the transmission range variation when the RF outputpowersettingisadjustedby+6dB,whichisfourtimes originalsetting.

2.2. Comparativetesting

Thissectionbrieflyreviewsthemethodsandconditionsunder which thecomparative test resultswere obtained, so that the experimentsmayeasilybereplicatedforcontinuousreviewand investigation.

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Fig.2.Transmissioncharacteristicsfortestnetworkwith+1dBiantennas.

Fig.3.TransmissioncharacteristicsforMIMOtestnetworkwith(a)+1dBiantennas,(b)+6dBiantennasand(c)6dBattenuatorsin-linewith+1dBiantennas.

ThetesttopologyisshowninFigure5(a)and(b).Thetypical OFDMtestnetworkconsistsoftwomobilecomputersandan ISM802.11.gaccesspoint(AP)router.

Theserver nodeused option (a)for Experiment6 andfor Experiment7.ThemobilenodeinExperiment6usedanexternal USBmodem.Nevertheless, adifferentmanufacturer’s mobile node was used for Experiment 7, equipped with an internal 802.11a/g/nphysicallayer,asinFigure5(b).

Experiment 6 focused on the determination of the trans- missioncharacteristicsforathroughputof600 kbpswithtwo standardlow-complexity (+1dBi) antennas in aMIMO con- figurationasdepictedinFigure5(a).Thesecharacteristicsare compared tothe transmissioncharacteristics obtained during Experiment7,whenusing asingletransceiver(Txcvr)APas indicatedinFigure5(b).Notethat,theRFpowerlevelforboth tests,weresettoequallevelsat+17dBmeach.

2.2.1. Experiment6:MIMOtestnetworkservicereference Experiment6determinedtheservicereferencefortheelec- tronics that was utilized. The electronics used for this test employedOFDMMIMOtechnology,manufacturedbyBelkin, AP model 802.11g+ MIMO with fixed transmit power at +17dBm.With the mobilenode equippedwiththe matching Belkin USB g+ MIMO modem, the two transceiver system enabled anear range dataconnectionrate of 108 Mbps. The set-upforExperiment6isshowninFigure5(a).

Figure6containsthedatacollectedduringExperiment6,in agraphgeneratedwiththeaid ofExcel.Theindicatedd-CIR andd-EIRareforatestbitrateof600kbpsthatiscontinuously monitored at the receiver node in terms of four key perfor- manceindicatorsnamelybandwidth,latency,jitteranderrors.

Notetherangeatwhichtransmissionfails(72m)correspondsto approximately6Mbps.Whenreducingthetransmissionrange,

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Fig.4.Singletransceivertransmissioncharacteristicswithactivelycontrolledtransmitpower,indicatingtypicalsteppedcurvesfortwoseries.

Fig.5.(a)and(b)Experimentaltestnetworkforcomparativetesting.

thetransmissionrestartsat55matapproximately18Mbps.A methodtouseatrend-lineisappliedsothatmoreaccurateinfor- mationmaybeextractedfromthesteppedtransmissioncurves.

2.2.2. Experiment7:Singletransceivertestnetwork servicereference

Experiment 7 determined the service reference for the electronicsthatwasutilizedwhenoperatingalinkwithasingle- transceiver node.The electronicsused for thistest employed OFDMMIMOtechnology,manufacturedbyBelkin,APmodel 802.11g+MIMOwithfixedtransmitpowerat+17dBm.

Nonetheless,themobilenodeforthistestwasequippedwitha single-transceiversystemthatenabledanearrangedataconnec- tionrateof54Mbps.ThetransmissionresultsforExperiment7 areshowninFigure7.

2.3. Analyticalapproachandmethodologytocomparative testing

The analytical approach is mathematically simplified, in order to accommodate a much wider spectrum of industrial andscientificuserswhomaynotallbeexpertmathematicians.

AlldatawasfirstcapturedintoMSExcel,andthenprocessed

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Fig.6.TransmissioncharacteristicsforMIMOtestnetwork.

Fig.7.Transmissioncharacteristicsforsingletransceivertestnetwork,indicat- ingsteppedcurvewithcorrespondinglogarithmictrend-line.

toproducelogarithmictrend-lineequivalentcurveswithcorre- spondingmathematicalexpressions.Theresultingtrend-line’s y-intersect wasthen adjusteduntil the curveintersected with thelastminimumrecorded(70m)atthedistanceboundary,as showninFigure6fortheMIMOtestnetworkandFigure7for thesingletransceiver.

With the aid of the logarithmic trend graph indicated in Figures6and7,theBWvaluescorrespondingtothed-CIRand d-EIRcanbemoreaccuratelyextractedfromthedatacollected.

Asindicated,thetransmissioncharacteristicsweremeasuredin decreasingsteps as determined by the wireless adapter’son- board firmwareand algorithms. The result is arather erratic steppedcurvewithmorethanonerangevalueforeachBWvalue below54Mbps.

AccordingtothetrendgraphinFigure6(whichcanalsobe describedbyy=−26.6ln(x)+125.6forthesectionofinterest), theBWvalueatarangeof55m(d-CIR)is19Mbps.Thetrend

graphextendsinrangeuptothelastmeasuredBWvaluebefore transmissionterminated.

For the section of graph beyond 70m, the interpolation method used here to estimate the BWat 72m(d-EIR) is by assumingastraightlinegraphsectionbetween70m(9Mbps) and75m(0Mbps).If72.5mcorrespondsto4.5Mbps,thenthe interpolatedbandwidthat72misapproximately5Mbps.

Considering the single transceiver’s characteristics and resulting logarithmic trend graph as depicted in Figure 7, y=−14.17ln(x)+71.356,thed-CIRrangecorrespondstoacon- nected bandwidth valueof 14.5Mbps andthe d-EIRrange a valueof2.5Mbps.

3. Resultsanddataanalysis

Inthemethodfollowedtotestthehypothesis,sampleswere drawnfromthedatacollectedduringexperimentationpresented inSection2.Afterstartingthevideotransmission,theBWdata wasrecordedatsuitabledistanceintervalswhilstincreasingthe transmissionrangetothepointoftotalsignaldeterioration,as showninFigure3.Thesedatasampleswerethenprocessedwith anEq.(3)derivedinthepreviousSection1.3,whichisbasedon theinversesquarelawofradiatedpower.

Theequationrevealstherelationshipbetweenrangevariation (r1/r2)andsignal strength(dB) when the bandwidth (BW)is constant (at 600kbps). Each of the processed results is then comparedwiththeexpectedvalue.

3.1. Passivetestdataanalysis

ConsideringFigures2 and3,Table1comparesthe results obtained with the standard +1dBi antennas to the results obtainedwiththe+6dBiantennas,thusatotallinkbudgetvari- ationof+5dB.

ConsideringFigures2 and4,Table2comparesthe results obtained with the standard +1dBi antennas to the results

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Table1

Transmissionresults+1dBiantennavs.+6dBiantenna.

Measuredresult Calculatedresult dB=(20Logr2/r1)

Expectedresult dB=(+6−1dB) Datarate r1 r2

36 25 45 5.11 5

36 32 57 5.01 5

36 40 70 4.86 5

18 45 75 4.43 5

18 50 90 4.81 5

18 55 100 5.19 5

CIR 57 102 5.05 5

9 65 115 4.95 5

9 70 125 5.03 5

EIR 72 127 4.93 5

Standarddeviation 13% –

Table2

Transmissionresults+1dBiantennavs.−6dBattenuator.

Expectedresult dB=(–6dB) Datarate r1 r2

36 40 20 −6 −6

18 50 25 −6 −6

CIR 55 27 −6.1 −6

9 60 30 −6 −6

9 70 35 −6 −6

EIR 72 37 −5.7 −6

Standarddeviation 5.5% –

Table3

Transmissionresults+17dBmrf-outvs.+11dBmrf-out.

36 17 37 6.7 6

36 20 40 6 6

18 25 50 6 6

CIR 27 54 6 6

9 32 65 6.1 6

9 35 70 6 6

EIR 37 73 5.9 6

obtainedwith6dBattenuatorssetfittedin-line,thusatotallink budgetvariationof−6dB.

3.2. Activetestdataanalysis

ConsideringFigure5,Table3comparestheresultsobtained withtheRFtransmitpowersetto50.4mW(+17dBm);tothe results obtained withthe RF transmit power set to12.6mW (+11dBm).Thisrepresentsa6dBdifferenceinpowerlevel,or a1–4powerratio.

Table4

Transmissionresults+1dBiantennavs.+6dBantenna.

36 36 62 4.72 +5

18 56 100 5.03 +5

CIR 55 102 5.36 +5

9 70 125 5.03 +5

EIR 72 127 4.99 +5

3.3. Standarddeviation

The standard deviation for each of these three series in Tables1–3,arerespectivelyestimatedat13%,4.2%and16.6%.

Thisisregardedasexcessiveinthefirstandlastcasesandcan beattributedtothesteppedtransmissioncurves.

3.4. Processingdatatoimprovestandarddeviation

Inasecondmethodofdataanalysis,collecteddatasamples are processedtoproduce alogarithmic trend-line,inorderto reducethestandarddeviationi.e.accuracyofthemeasurements.

ThefollowingFigure8indicatesalogarithmictrend-lineforthe series+6dBishowninFigure3(b).

Using the logarithmic trend-line, specific values can be extracted for the respective data rates 36Mbps, 18Mbps, 9Mbps, d-CIR and d-EIR. Similarly, specific values were extractedfromtheseriesinFigure2,andthetwosetsofpro- cessedvaluescanagainbecomparedintermsoftheexpected results,asshowninTable4below.

4. Discussion 4.1. Thehypothesis

Thehypothesissuggeststhatconnectedbandwidth(BW)in Mbpsisdirectlyproportionaltotheelectricfieldstrength(E)in V/m,ortothepowerdensity(PD)inW/m²,oftheRFwaveat thereceivingnode.

Totestthehypothesis,severalexperimentswereconducted.

The hypothesis was confirmed by the experimental results obtained andpresentedinthisstudy,sincetheycorrelatewith expectedvalues.Thepositivetestingofthehypothesissuggests thatconnectedbandwidthBWinMbpsisdirectlyproportional to therelative signalstrength indBm,whichis thepreferred expressionofsignalpowerlevel.

Positivetestingofthehypothesismotivatesthe application oftheinversesquaredlawofradiatedpower(orirradiation)asa mathematicaltooltoestimateinformationratevariationsinISM bandOFDMwirelessEthernetnetworks,asappliedinindustrial wirelesssensornetworks.

Byusingavideobitstreamofapre-determinedratetospecify therequiredcommittedthroughput,theserviceleveldifferential zone SLDZcaneasilybemeasured andthe CIRandEIR for

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Fig.8.Transmissioncharacteristicsfor+6dBiwithlogarithmictrendline.

eachnetworkoption canbedetected.Oncethed-CIR andd- EIR transmissionranges for the network options are known, thesevaluescanbeusedinconjunctionwithasimplesetofEqs.

(3)and(4)toestimaterangeorbandwidthvariationsinnetwork performance,intermsofrelativelinkbudgetindB.

Theobservationofavideostreaminordertoevaluatenet- workkey performance indicators (KPIs),namely bandwidth, latency,jitter anderrors, provides afast,intuitive andeasily interpretedmethodtoconfirmtheCIRunderactualoperational conditions.ItappearseasiertouseBWtoexpressrelativevari- ations insignal strength in dB, because the measurement of RSSindBm requiresspecial testequipment andanin-depth knowledgeoftestandmeasurementw.r.t.resolutionbandwidth measurements. BWreadings maybe obtained from on-board instrumentationsuch as Window Task Manager (Networking pane)orPROSetWirelessfreewarefromIntel.

4.2. ThedigitalSINADratio

AninterestingphenomenoninitiallyobservedisthattheCIR (inthe example’scase600kbps max)requires aminimumof approximatelytentimes theconnectedbandwidth,toamaxi- mumofapproximately thirty timesthe connectedbandwidth, toreconnectorreset.Theselimitsaredeterminedbytheman- ufacturer’sfirmware,situatedontheradiochipset.Thismeans the600kbpsCIRfailsataconnectedbandwidthvaluebelow 6Mbps,andreconnectsorrecoversfromresetatapproximately 18Mbpsconnectedbandwidth.

With specific reference drawn to the observation above, thesignal-to-noise-and-distortionratio(SINAD)isconsidered because it is an indicator of the quality of the signal that is transmitted from a communications device, and is often definedas:

SINAD= Psignal+Pnoise+Pdistortion

Pnoise+Pdistortion

By functional comparison, the d-EIR is the transmission rangeatwhichthesignallevelof theCIRhasdeterioratedto thepointthatitisovercomebythenoiseanddistortionlevel.

Conversely, thed-CIR isthe transmissionrangeat whichthe CIRsignallevelisatasufficientlyhigherratioelevatedabove thenoiseanddistortionlevelforacceptablevideotransmission.

ThissituationrelativelyrepresentsthedigitalSINADratiowhich canbecalculatedfromtheproposedequation:

SINAD(dBcir)=10LogEffective bandwidth

CIR (5)

With reference to the results of the passive tests in Figures2–4,theSINADforeachcharacteristicwascalculated with valuesobtained from the Excel trend-line method as in Figure2.Forexample,forthecurveTx+1dBi:

SINAD(dBcir)=10LogBWd−CIR−BWd−EIR

CIR (6)

∴SINAD(dB)=10Log19×10⁶−5×10⁶ 600×10³

∴SINAD=13.35dBcir

ForthecurveTx+6dBi,usingthesamemethod,theSINAD iscalculatedat13.18dB,andforthecurveTx−5dBitheSINAD is15.44dB.AcceptableresultswerenotedforTx+1dBiandTx +6dBi,sincetheSINADrequiredformicrowavefrequencyis expectedtobeslightlyhigherthanthe12dBrecommendedfor VHF.

AhigherthanexpectedSINADof15.44dBwasnotedforthe Tx−5dBicurvetoproduceacceptableresults.Theadditional signallevelthatisrequiredtoproduceacceptableresultsisprob- ablytoovercomeadditionalreturnlossbecauseof mismatch.

The mismatch probably occurs as a result of the additional connectorsintroducedtothesystembytheattenuatorpads.

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Table5

Comparisonofexperiment6–mimodualtransceiverandExperiment7–single transceiver.

MIMO Experiment6

Singletransceiver Experiment7

SINAD 13.35dB 13.01dB

Sensitivity 19Mbpsfor

approx.13dB SINADat CIR=600kbps

14.5Mbpsfor approx.13dB SINADat CIR=600kbps

4.3. Comparativetestresults

The sensitivity specification of the first mobile node in Figure5(a)asaradioreceiver,forthetransmissioncharacteris- ticsshowninFigure6,canalsonowbeestimatedas19Mbpsfor a13.35dBSINADatthroughputCIRof600kbps.(18Mbpsmay beusedbeingthe closeststandardBWvaluethat thenetwork canconnect,foranapproximatedvalueof13dBSINAD).

Forthesecondexample,theSINADcanbecalculatedforthe curvedepictedinFigure7bythesamemethod:

SINAD(dBcir)=10LogBW_dCIR−BW_dEIR

CIR (7)

∴SINAD=10Log14.5×10⁶−2.5×10⁶ 600×10³

∴SINAD=13.01dBcir

Again,thesensitivityspecificationofthesecondmobilenode in Figure 5(b) as a radio receiver can now be estimated as 14.5Mbpsfora13dBSINADatathroughputCIRof600kbps.

Theresultsobtainedbythestudy,fortwodissimilarphysical layerstransportingthesameCoStraffic,cannowbetabulated forbriefdiscussion(Table5):

It is noted with interest that the single transceiver has a muchimprovedsensitivityrating.Thiswasnotclearfromini- tialmeasurements,becausethesamed-CIRrangewasmeasured.

However,withtheuseofthelogarithmictrendgraphitbecame possible to estimate the corresponding data rate with closer accuracyandclearlyrevealthedifferenceinphysicallayerchar- acteristicfeatures.

5. Conclusion

ThestudyintroducesWSNstoISM-bandOFDMWLANs, with specific reference to thermography and Android-based computertechnology.

A novel method to estimate a specified committed information rate in terms of range and link budget is positively testedagainstexpectedvalues.ThesecapabilitiesrenderOFDM WLANsoperatingwithinISMbandsanextremelyusefulcom- munications transport medium for industrial wireless sensor networks,andspecificallyforcuttingedgeAndroidnodeswith Linux-basedoperatingsystems.

AhypothesisisproposedthatconnectedbandwidthinOFDM WLANs is directly proportional to the relative RF signal

strength. The concept is mathematically proved by a heuris- tic statistical analysis approach,whereby datacollected from transmissioncharacteristicsarecomparedwithexpectedresults, when the signalsare intensified or attenuated to specifically knownvalues.Amethodisrevealedwherebycollecteddataare processedinordertosubstantiallyimprovetheaccuracyofthe measurements.

Comparative studiesby physicalexperimentation revealed that mobilenodescanbeindividuallytestedintermsof their respectiveSINADparameters,aswellastheirsensitivityparam- eters, withoutconnectingexternaltestequipment. Withthese parameters available, mobileAndroid/Linux computer nodes can infuture be qualified for industrial use interms of their throughputperformance,whichisakeyrequirementforservice levelagreements(SLAs)whichincorporatetheITU-TY.1564 andEtherSAMtestmethodologies.

Conﬂictofinterest

Theauthorsdeclarenoconflictofinterest.

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