EC2401- Wireless Communication Study Materials for All 5 Units

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EC2401-WirelessCommunication VIISemester ECE

Nomadicdevices:nomadicdevicesareplacedatacertainlocationforalimiteddurationoftime

(minutestohours)andthenmovedtoadifferentlocation.Thismeansthatduringonedrop

(placingofthedevice),thedeviceissimilartoafixeddevice.However,fromonedroptothenext,theenvironmentcanchangeradically.Laptopsaretypicalexamples:peopledonotoperatetheirlaptopswhilewalkingaround,butplacethemonadesktoworkwiththem.

Lowmobility:manycommunicationsdevicesareoperatedatpedestrianspeeds.Cordlessphones,aswellascellphonesoperatedbywalkinghumanusersaretypicalexamples.Theeffectofthe

lowmobilityisachannelthatchangesratherslowly,andinasystemwithmultipleBSs.

Highmobility usuallydescribesspeedrangesfromabout30to150km/h.Cellphonesoperatedbypeopleinmovingcarsareonetypicalexample.

Extremely high mobility is representedbyhigh-speed trains and planes, which cover speedsbetween 300and 1000km/h.These speedsposeuniquechallenges bothfor thedesignof thephysicallayer(Dopplershift,seeChapter5)andforthehandoverbetweencells.

EnergyConsumptionEnergyconsumptionisacriticalaspectforwirelessdevices.Mostwirelessdevicesuse(one-wayorrechargeable) batteries, as they should be free of any wires both the ones used forcommunicationandtheonesprovidingthepowersupply. Rechargeablebatteries:nomadicandmobiledevices,likelaptops,cellphones,andcordlessphones,areusuallyoperatedwithrechargeablebatteries.Standby times as well as operating times are one of the determining factors for customer

satisfaction.Energyconsumptionisdeterminedononehandbythedistanceoverwhichthedatahavetobetransmitted(rememberthataminimumSNRhastobemaintained),andontheother


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PreparedByA.DevasenaAsso.Prof.,&R.Sankari Asst.Prof. Page10

, bit).One-waybatteries:sensornetworknodesoftenuseone-waybatteries,whichofferhigherenergydensity at lowerprices.Furthermore, changing the battery isoftennot anoption; rather, thesensorincludingthebatteryandthewirelesstransceiverisoftendiscardedafterthebatteryhas

runout.

Powermains:BSsandotherfixeddevicescanbeconnectedtothepowermains.Therefore,energyefficiencyisnotamajorconcernforthem.Itisthusdesirable,ifpossible,toshiftasmuchfunctionality(andthusenergyconsumption)fromtheMStotheBS.

UseofSpectrumSpectrumcanbeassignedonanexclusivebasis,oronasharedbasis.Thatdeterminestoalarge

degreethemultipleaccessschemeandtheinterferenceresistancethatthesystemhastoprovide:

Spectrumdedicatedto serviceandoperator: inthis case,acertainpart ofthe electromagneticspectrum is assigned, onan exclusive basis, toa service provider. A primepoint incase is


cellulartelephony,wherethenetworkoperatorsbuyorleasethespectrumonanexclusivebasis

(oftenforaveryhighprice).Duetothisarrangement,theoperatorhascontroloverthespectrumandcanplantheuseofdifferentpartsofthisspectrumindifferentgeographicalregions,inorder

tominimizeinterference.

Spectrumallowingmultipleoperators: Spectrum dedicated to a service: in this case,the spectrumcan beusedonly for a certainservice(e.g.,cordlesstelephonesinEuropeandJapan),butisnotassignedtoaspecificoperator.

Rather, users can set up qualified equipment without a license. Such an approach does notrequire(orallow)interferenceplanning.Rather,thesystemmustbedesignedinsuchawaythat

itavoidsinterferingwithotherusersinthesameregion

Freespectrum:isassignedfordifferentservicesaswellasfordifferentoperators.TheISM

bandat2.45GHzisthebestknownexampleitisallowedtooperatemicrowaveovens,WiFiLANs,andBluetoothwirelesslinks,amongothers,inthisband.Alsoforthiscase,eachuserhastoadheretostrictemission limits, inordernot tointerferetoomuchwithothersystemsand

users.However,coordinationbetweenusers(inordertominimizeinterference)becomesalmostimpossible different systems cannot exchange coordination messageswith each other, andoftenevenhaveproblemsdetermining theexactcharacteristics(bandwidth,dutycycle)of the

interferers.

Di rectionofTransmissionSimplexsystems sendtheinformationonlyinonedirectione.g.,broadcastsystemsandpagers.

Semi-duplexsystems cantransmitinformationinbothdirections.However,onlyonedirection

isallowedatanytime.Walkie-talkies,whichrequiretheusertopushabuttoninordertotalk,areatypicalexample.Notethatoneusermustsignify(e.g.,byusingthewordover)that(s)hehasfinishedhis/her transmission; thenthe other userknows thatnow (s)hecan transmit. Full-duplexsystems allowsimultaneoustransmissioninbothdirectionse.g.,cellphonesandcordless

phones.

Asymmetricduplexsystems:fordatatransmission,weoftenfindthattherequireddatarateinonedirection(usuallythedownlink)ishigherthanintheotherdirection.However,eveninthis

case,fullduplexcapabilityismaintained.


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Servi ceQuali ty

Therequirements forservicequalityalsodiffervastlyfordifferentwirelessservices.Thefirstmainindicatorforservicequalityisspeechquality forspeechservicesandfiletransferspeedfordataservices.SpeechqualityisusuallymeasuredbytheMeanOpinionScore (MOS).Itrepresents

theaverageofalargenumberof(subjective)humanjudgments(onascalefrom1to5)aboutthe

quality of received speech . The speed of data transmission is simply measured in bit/s obviously,ahigherspeedisbetter.Anevenmoreimportantfactoristheavailabilityofaservice.


For cellphones and other speech services, the service quality is often computed as thecomplementoffractionofblockedcalls.plus10timesthefractionofdroppedcalls.

NoiseandInterferenceLimitedSystems.

Noise-LimitedSystems

Wirelesssystemsarerequiredtoprovideacertainminimumtransmissionquality.Thistransmissionquality in turnrequiresa minimum Signal-to-Noise Ratio (SNR) at thereceiver(RX).ConsidernowasituationwhereonlyasingleBStransmits,andaMobileStation(MS)

receives;thus,theperformanceofthesystemisdeterminedonlybythestrengthofthe(useful)signal and the noise.AstheMSmovesfurtheraway fromtheBS, the receivedsignalpowerdecreases,andatacertaindistance,theSNRdoesnotachievetherequiredthresholdforreliable

communications.Therefore,therangeofthesystemisnoiselimited;equivalently,wecancallitsignalpowerlimited .Dependingontheinterpretation,itistoomuchnoiseortoolittlesignalpowerthatleadstobadlinkquality.

PRX= PTXGRXGTX (/4d)2

.----------------------------------------------- (1)

where GRX and GTX are thegains of thereceiveandtransmitantennas, respectively, isthewavelength,andPTXisthetransmitpower

Thenoisethatdisturbsthesignalcanconsistofseveralcomponents,asfollows

Thermalnoise: The power spectral density of thermal noise depends on the environmentaltemperatureTethattheantennasees.ThetemperatureoftheEarthisaround300K,whilethetemperatureofthe(cold)skyisapproximately Te 4K(thetemperatureinthedirectionoftheSun is of course much higher). As a first approximation, it is usually assumed that theenvironmentaltemperatureisisotropically 300K.Noisepowerspectraldensityisthen

N0=kBTe -------------------------------------------------------------------- (2)

wherekBisBoltzmannsconstant,kB=1.381023J/K,and

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= 0 --------------------------------------------------------------------------------(3)whereB isRXbandwidth(inunitsofHz).ItiscommontowriteEq.(.2)usinglogarithmicunits

(powerPexpressedinunitsofdBmis10log10(P/1mW))N0=174dBm/Hz----------------------------------------------(4)


Thismeansthatthenoisepowercontainedina1-Hzbandwidthis 174dBm.Thenoisepower

containedinbandwidthB is

174+10log10(B) dB.--------------------------------------------------------------(5)ThelogarithmofbandwidthB,specifically10log10(B),hastheunitsdBHz.

Man-madenoise:Wecandistinguishtwotypesofman-madenoise:

(a)Spuriousemissions:Manyelectricalappliancesaswellasradiotransmitters(TXs)designedfor other frequency bandshave spurious emissionsover a largebandwidth that includes the

frequency range in which wireless communications systems operate. For urban outdoorenvironments,carignitionsandotherimpulsesourcesareespeciallysignificantsourcesofnoise.Incontrasttothermalnoise,thenoisecreatedbyimpulsesourcesdecreaseswithFrequency At

150MHz,itcanbe20dBstrongerthanthermalnoise;at900MHz,itistypically10dBstronger.At UniversalMobile Telecommunications System (UMTS) frequencies, Note that frequency

regulators in most countries impose limits on spurious or out-of-band emissions for all

electricaldevices.Furthermore,forcommunicationsoperatinginlicensedbands,suchspuriousemissionsaretheonlysourceofman-madenoise.Itliesin thenatureofthelicense(forwhichthelicenseholderusuallyhaspaid)thatnootherintentionalemittersareallowedtooperatein

thisband.Incontrasttothermalnoise,man-madenoiseisnotnecessarilyGaussiandistributed.However,asamatterofconvenience,mostsystem-planningtools,aswellastheoreticaldesigns,assumeGaussianity anyway.

(b) Otherintentionalemissionsources: Severalwirelesscommunications systems operate inunlicensed bands. In these bands, everybody is allowed to operate (emit electromagnetic

radiation)aslongascertainrestrictionswithrespecttotransmitpower,etc.arefulfilled.Themostimportantofthesebandsis the2.45-GHzIndustrial,Scientific,andMedical(ISM)band.Theamountofinterferenceinthesebandscanbeconsiderable.

Receivernoise: TheamplifiersandmixersintheRXarenoisy,andthusincreasethetotalnoisepower.Thiseffectisdescribedbythenoisefigure F,whichisdefinedastheSNRattheRXinput(typicallyafterdownconversiontobaseband)dividedbytheSNRattheRXoutput.Asthe

amplifiershavegain,noiseaddedinthelaterstagesdoesnothaveasmuchofanimpactasnoiseaddedinthefirststageoftheRX.Mathematically,thetotalnoisefigure Feqofacascadeof

componentsis

Feq=F1+F21 +F31+

G1 G1G2whereFi andGi arenoisefiguresandnoisegainsoftheindividualstagesinabsoluteunits(notindecibels(dB)).Notethatforthisequation,passivecomponents,likeattenuatorswithgainm

EC2401- Wireless Communication Study Materials for All 5 Units

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