The Introduction to telecommunications.ppt

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1. Introduction to
Telecommunication on System

2. Telecommunication Signals
Telecommunicationsignalsarevariationover
timeofvoltages,currentsorlightlevelsthat
carryinformation.Foranalogsignals,these
variationsaredirectlyproportionaltosome
physicalvariablelikesound,light,
temperature,windspeed,etc.

3. Telecommunication Signals
Theinformationcanalsobetransmittedby
digitalsignals,thatwillhaveonlytwovalues,
adigitaloneandadigitalzero.Anyanalog
signalcanbeconvertedintoadigitalsignalby
appropriatelysamplingit.

4. Telecommunication Signals
Thesamplingfrequencymustbeatleasttwice
themaximumfrequencypresentinthesignal
inordertocarryalltheinformationcontained
init.Randomsignalsaretheonesthatare
unpredictableandcanbedescribedonlyby
statisticalmeans.

5. Telecommunication Signals
Noiseisatypicalrandomsignal,describedbyits
meanpowerandfrequencydistribution.

6. Telecommunication Signals
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7. Telecommunication Signals
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8. Telecommunication Signals
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9. Low-passfilter–lowfrequenciesarepassed,
highfrequenciesareattenuated.
High-passfilter–highfrequenciesarepassed,
lowfrequenciesareattenuated.
Band-passfilter–onlyfrequenciesina
frequencybandarepassed.
Band-stopfilterorband-rejectfilter–only
frequenciesinafrequencybandareattenuated

10. Oneimportantapplicationoffiltersisin
telecommunication.Manytelecommunicationsystems
usefrequency-divisionmultiplexing,wherethe
systemdesignersdivideawidefrequencybandinto
manynarrowerfrequencybandscalled"slots"or
"channels",andeachstreamofinformationis
allocatedoneofthosechannels.

11. Telecommunication Signals
WhyDigital?
Noisedoesnotaccumulatewhenyouhaveachainof
deviceslikeithappensinananalogsystem:CD
VersusVinyl,VHSVsDVD.Thesamegoesforthe
storingoftheinformation.Detectionofadigital
signaliseasierthanananalogsignal,sodigitalsignal
canhavegreaterrange.

12. Telecommunication Signals
Digitalsignalscanuselessbandwidth,asexemplified
bythe“digitaldividend”currentlybeingharnessedin
manycountries.Digitalcircuitsareeasiertodesign
andcanachievegreaterintegrationlevelsthananalog
circuits.Digitalsignalscanbeencodedinwaysthat
allowtherecoverfromtransmissionerrors,albeitat
theexpenseofthroughput

13. Telecommunication Signals
ElectricalNoise
Noiseposestheultimatelimittotherangeofa
communicationssystem•Everycomponentofthe
systemintroducesnoise•Therearealsoexternal
sourcesofnoise,likeatmosphericnoiseandman
madenoise•Thermalnoisepower(alwayspresent)
isfrequencyindependentandisgiven(inwatts)by
k*T*B.

14. Telecommunication Signals
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15. Telecommunication Signals
Interference
Anysignaldifferentfromtheonethatour
systemisdesignedtoreceivethatiscaptured
bythereceiverimpairsthecommunication
andiscalledinterference.

16. Telecommunication Signals
Intra-channelinterferenceoriginatesinthe
samechannelasoursignal.Co-channel
interferenceisduetotheimperfectionofthe
filtersthatwillletinsignalsfromadjacent
channels.

17. Telecommunication Signals
InformationMeasurement
I=log2(1/Pe)Theinformationcarriedbyasignalis
expressedinbitsandisproportionaltothelogarithm
oftheinverseoftheprobabilityoftheoccurrenceof
thecorrespondingevent.Themoreunlikelyanevent
tohappen,themoreinformationitshappeningwill
carry.

18. Telecommunication Signals
InformationMeasurement
Transmittingamessageofaneventthatthe
receiveralreadyknowscarriesnoinformation.The
amountofinformationtransmittedinonesecondis
thecapacityofthechannel,expressedinbit/s

19. Telecommunication Signals
Redundancy
Sendingtwicethesameinformationisawasteof
thesystemcapacitythatreducesthethroughput.
Nevertheless,ifanerroroccurs,theredundancy
canbeusedtoovercometheerror.Everyerror
correctingcodemustusesomesortofredundancy.

20. Telecommunication Signals

21. Telecommunication Signals

22. Telecommunication Signals
Thecommunicationsystemmustovercomethe
noiseandinterferencetodeliverasuitablereplica
ofthesignaltothereceiver.Thecapacityofthe
communicationchannelisproportionaltothe
bandwidthandtothelogarithmoftheS/Nratio.

23. Telecommunication Signals
Modulationisusedtoadaptthesignaltothe
channelandtoallowseveralsignalstosharethe
samechannel.Higherordermodulationschemes
permithighertransmissionrates,butrequirehigher
S/Nratio.Thechannelcanbesharedbyseveral
usesthatoccupydifferentfrequencies,different
timeslotsordifferentcodes