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• Fundamentals ofFundamentalsofCommunications

• Communication SystemCommunicationSystem

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

TheBandwidthofasignaliscalculatedbysubtractingthehighestfrequencycomponentfrom the lowest frequency componentfromthelowestfrequencycomponent

• Electromagnetic WaveElectromagneticWave

Transmitting the signal over the air requires Transmittingthesignalovertheairrequiresconvertingtheelectricsignalintoelectromagneticsignalsignal

Antennaisthedeviceusedtoconvertelectrici l t l t ti i lsignalstoelectromagneticsignals

Thelengthoftheantenna(l)hastobeatleastonetenththelengthofthesignalwavelength;

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

• 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

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

Carryingthe0sand1sontothecarriersamplitude

Carryingthe0sand1sontothecarriersfrequency

Carryingthe0sand1sontothecarriersphase

Theamplitudeofthecarriervariesaccordingtothebinaryamplitudeoftheinformationy psignal

• 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

FSKandPSKhavebetterqualitythanASKPSK i d f bil di i l bil PSKisusedformobiledigitalmobiletelephony

• 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

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

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)

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

Noise interference with the transmitted signal Noiseinterferencewiththetransmittedsignalisunavoidable.

• Communication systemCommunicationsystem

NoiseNoise

• Noise CategoriesNoiseCategories

A. ExternalNoise:Noisethatisgeneratedinthecommunicationmedium:1. AtmosphericNoise

2. ExtraterrestrialNoise

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

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

SNR = P / PSNR=P

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