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  • Fundamentals ofFundamentalsofCommunications

  • Communication SystemCommunicationSystem

    Transmitter ReceiverMedium

    Transmitter:originatesthesignal Receiver: receives transmitted signal after itReceiver:receivestransmittedsignalafterittravelsoverthemedium

    Medium: guides the signal from the Medium:guidesthesignalfromthetransmittertothereceiver

  • ElectricalSignal(Amplitude,Period,Frequency)

    Amplitude:theintensityofthesignalsvoltage PeriodT(s):timethewavetakestocompleteonecycle

    Frequencyf=1/T(Hz):numberofcompletecycledpersecond

  • ElectricalSignal(PhaseDifference)

    Phasedifferenceismeaningfulwhencomparing2signalsg

    Itdescribesthedegreeinwhichthe2signalsarealignedrelativetoeachother,inotherwordsitgmeasurewhetherthe2signalsareinsynchorelse

  • ElectricalSignal(Wavelength)

    Thewavelength()measuresthephysicallengthoftheelectricalsignalg

    =c/f(m),c=3X108 m/s(1)

  • Signal BandwidthSignalBandwidth

    Thebandwidthofasignalrepresentstherangeofitsfrequencycomponentsg q y p

    Acomplexsignalismadeofarangeoffrequencies called spectrumfrequenciescalledspectrum

    TheBandwidthofasignaliscalculatedbysubtractingthehighestfrequencycomponentfrom the lowest frequency componentfromthelowestfrequencycomponent

  • Electromagnetic WaveElectromagneticWave

    Transmitting the signal over the air requires Transmittingthesignalovertheairrequiresconvertingtheelectricsignalintoelectromagneticsignalsignal

    Antennaisthedeviceusedtoconvertelectrici l t l t ti i lsignalstoelectromagneticsignals

    Thelengthoftheantennadependsontheeclectici l l hsignalwavelength

    Thelengthoftheantenna(l)hastobeatleastonetenththelengthofthesignalwavelength;

    l=/10=3X108/(10Xf)..(2)( f) ( )

  • The Radio SpectrumTheRadioSpectrum

    Thefrequencyrangeoftheelectromagneticspectrumbetween3Hzand300GHziscalledptheradiospectrum

    The radio spectrum is further divided into Theradiospectrumisfurtherdividedintoregionscalledfrequencybands

    RadioFrequency(RF)systemsusebandswithin the radio spectrum for communicationintheradiospectrumforcommunication

  • Modulation/DemodulationModulation/Demodulation Modulationisatechniqueusedtoconvertalowf l ( f l) h hfrequencysignal(informationsignal)toahigherfrequencysignal(modulatedsignal)usingahigher frequency signal (carrier)higherfrequencysignal(carrier).

    Inmodulation,thecarrierismodulated(reshaped) according to the information signal(reshaped)accordingtotheinformationsignal

    Demodulationistheoppositetechniquewhereinformation signal is extracted from the carrierinformationsignalisextractedfromthecarrier

    AnalogueModulationisusedtotransmitanalogue signals and digital modulation is used toanaloguesignalsanddigitalmodulationisusedtoconvertdigitalsignals

  • a Analogue Modulationa.AnalogueModulation

    Themainideaofanaloguemodulationistocarrytheinformationsignalontothecarriersy gamplitude,frequencyorphase.

    Carrying the information signal onto the carriers amplitudeCarryingtheinformationsignalontothecarrier samplitude

    Carryingtheinformationsignalontothecarriersfrequency

    Carrying the information signal onto the carriers phaseCarryingtheinformationsignalontothecarrier sphase

  • 1 Amplitude Modulation (AM)1.AmplitudeModulation(AM)

    envelop

    Thecarriersamplitudeisvariedinproportiontothe amplitude of the information signaltheamplitudeoftheinformationsignal

  • Amplitude ModulationAmplitudeModulation

    InAM,theinformationiscarriedonthecarriersamplitudep

    Thedemodulationisachievedbydetectingthe variation of the modulated signalsthevariationofthemodulatedsignal samplitudecalledenvelop

    AMispronetonoiseinterferenceasnoisemay change the amplitude of the modulatedmaychangetheamplitudeofthemodulatedsignal

  • 2 Frequency Modulation (FM)2.FrequencyModulation(FM)

    Thecarriersfrequencyisvariedinproportiontothe amplitude of the information signaltheamplitudeoftheinformationsignal

  • Frequency ModulationFrequencyModulation

    InFM,theinformationiscarriedonthecarriersfrequencyq y

    Thedemodulationisachievedbydetectingthe variation of the modulated signalsthevariationofthemodulatedsignal sfrequency

    FMbetterperformsinthepresenceofnoisebecause noise dont change the frequency ofbecausenoisedon tchangethefrequencyofthemodulatedsignal

  • b Digital Modulationb.DigitalModulation

    Digitalinformationismadeof2levelsofamplituderepresentingtwologicallevels(0and1)

    In digital modulation the 0 and 1 are carried onIndigitalmodulationthe0and1arecarriedonthecarriersamplitude,frequencyandphase

    Carryingthe0sand1sontothecarriersamplitude

    Carryingthe0sand1sontothecarriersfrequency

    Carryingthe0sand1sontothecarriersphase

  • 1 Amplitude Shift Keying (ASK)1.AmplitudeShiftKeying(ASK)

    Theamplitudeofthecarriervariesaccordingtothebinaryamplitudeoftheinformationy psignal

    In ASK the modulated signal has 2 amplitude InASK,themodulatedsignalhas2amplitude Logic0ismodulatedwithamplitudeof0andlogic1ismodulatedwithmaximumamplitudevaluevalue

  • ASKASK

  • 2 Frequency Shift Keying (FSK)2.FrequencyShiftKeying(FSK)

    Thefrequencyofthecarriervariesaccordingtothebinaryamplitudeoftheinformationy psignal

    In FSK the modulated signal has 2 frequencies InFSK,themodulatedsignalhas2frequencies Logic0ismodulatedusingonefrequencyandlogic1ismodulatedusinganotherfrequency

  • FSKFSK

  • 3 Phase Shift Keying (PSK)3.PhaseShiftKeying(PSK)

    Thephaseofthecarriervariesaccordingtothebinaryamplitudeoftheinformationsignaly p g

    InPSK,themodulatedsignalhas2differentphasesphases

    Logic0ismodulatedusingthecarriersphaseandlogic1ismodulatedusingthesamecarrier with a phase shift.carrierwithaphaseshift.

  • PSKPSK

  • ASK/FSK/PSKASK/FSK/PSK

    ThebandwidthoftheFSKistwiceasmuchthebandwidthofthePSKandASK

    FSKandPSKhavebetterqualitythanASKPSK i d f bil di i l bil PSKisusedformobiledigitalmobiletelephony

    FSKisusedinfaxmachinesASK i d i t llit i ti ASKisusedinsatellitecommunications

  • AttenuationAttenuation

    Whenthesignaltravelsthroughamediumsuchasairorcable,itlosesenergygy

    ThelossofenergyiscalledattenuationA h di b h i d Asthedistancebetweenthetransmitterandreceiverincreases,signalattenuationincreases

    The signal reduction in air follow the inverse Thesignalreductioninairfollowtheinversesquarelaw

  • Attenuation in the airAttenuationintheair

    Attenuationinairisgovernedbytheinversesquarelaw

    InverseSquarelawstatesthatreceivedsignalpower(Pr)isinverselyproportionalthetransmitsignalrpower(Pt),therelationbetweenthereceivedandtransmitsignalpowersisgivenbythisequation:

    Pr=Pt /r2(3)

    where r is the distance between the transmitter andwhereristhedistancebetweenthetransmitterandthereceiver

  • Electric Signal PowerElectricSignalPower

    Thepowerofanelectricalsignalisthesquarevalueofitsvoltage;g

    P=V2 (W)(4)

    I i i lik h Incommunicationweliketoexpressthepowerindecibels;

    Thisrelationusedtoexpresssignalpowerindecibels:decibels:

    P[dB]=10log10 P(dB)(5)

  • Inverse Square law in dBInverseSquarelawindB

    P = Pt/r2 (W)Pr Pt/r (W)

    Pr[dB]=10log10 Pt 10log10 (r2)

    Pr[dB]=Pt [dB] 20log10 r(6)

  • Example 1Example1

    CalculatethereceivedpowerindBwhenthedistancebetweena100Wtransmitterandareceiveris10000mm?

    SolutionSolution

    Pr[dB]=Pt[dB] 20logr

    Pr[dB]=10log100 20log10

    P [dB] 20 20 0 dBPr[dB]=20 20=0dB

  • Attenuation in wired mediumAttenuationinwiredmedium

    Inwiredmedium,signalattenuationdependsonthetypeofthemediumassometypeofyp ypwiredmediumsaremorepronetoattenuationthan other types of wired mediumthanothertypesofwiredmedium

    Forexample,somefiberopticscabletransmitl h f ( /k )signalattenuatesattherateof(0.2dB/km)

    comparetocoaxialcablewhichattenuatesat(175dB/km)inthebestcase

  • Example 2 (using repeaters)Example2(usingrepeaters)

    (a)Calculatethereceivedpowerattheendofa30kmcablethathasattenuationrateof(10(dB/km)ifthetransmitpoweris100dB?(b)Using 2 Repeaters spaced at 10 km each whatUsing2Repeatersspacedat10kmeach,whatisthereceivedpoweriftherepeatercanamplify the received signal by 10 dB?amplifythereceivedsignalby10dB?

    10 dB/ kTxPt=100dB

    RxPr=?

    30 km

    10 dB/ km

  • SolutionSolution(a):Pr =100 30*10=200dB

    (b)

    TxPt=100dB

    RxPr=?

    10 dB/ kmR1 R2

    AtR1

    10 km 10 km 10 km

    Pr, 1 =100 10X10=0dB

    Pt,1 =0+10=10dB

    A R2AtR2

    Pr,2=10 10x10=90dB

    Pt 2 = 90 + 10 = 80 dBPt,2 90+10 80dB

    Pr =80 10X10=180dB

  • NoiseNoise

    Noiseisundesirableelectricenergythatfallswithinthebandwidthofthetransmittedsignal.g

    Assignalbandwidthincreases,noiseinterference increasesinterferenceincreases

    Noiseinterferencewillresultinpoorqualityofthereceivedsignal.

    Noise interference with the transmitted signal Noiseinterferencewiththetransmittedsignalisunavoidable.

  • Communication systemCommunicationsystem

    NoiseNoise

    Transmitter ReceiverMediumTransmitter ReceiverMedium

  • Noise CategoriesNoiseCategories

    A. ExternalNoise:Noisethatisgeneratedinthecommunicationmedium:1. AtmosphericNoise

    2. ExtraterrestrialNoise

    3. Manmadenoise

    B Internal Noise: Noise that is generated insideB. InternalNoise:NoisethatisgeneratedinsidetheReceiverhardware:1. ThermalNoise;temperaturedependantnoise

  • Signal To Noise Ratio (SNR)SignalToNoiseRatio(SNR)

    SNRisafigureofmeritthatexpressestheratioofthereceivedsignalpower(Pr)totheg p ( r)noisepower(PN)

    SNR = P / PSNR=P

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