Digital communication systems

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Digital communication systems

Block Diagram of Digital Comm. systems

Analog InputSignal

 Continuous time and amplitudesignal
 Fromvoice, video orimage
 Goalis tominimize distortionof analogsignal
 Notethat digital transmission is used evenfor informationthat is inherently


 Sampling makes signal discrete intime
 Samplingtheorem says thatband limitedsignal canbe sampledwithout introducing


 Quantizermakes signal discrete inamplitude
 Quantizerintroduces somedistortion
 Goodquantizersare able to usefew bits andintroduce small distortion
 Wewill study optimum scalar and vectorquantizers


 After quantization, data is in digital (0,1)form
 Inherentlydigital information (e.g. computer files)donotrequire sampling or


 Compression of digital data to eliminateredundant information
 Sourcecoding is like quantization because its goalistoreduce bitrate
 Sourcecoding is unlike quantization because itdoes notintroducedistortion
 Wewill not cover this topic.


 Encryption techniques can ensure dataprivacy
 Encryptionis what we think of when we think ofspies andsecret decoder rings
-Communications engineers usethe word "coding" for otherideas

 Verygood "public key" encryption algorithm exist- thisworries the folks atNSA
 Wewill not talk about encryption in detail


 Provides protection against transmission errorsby selectivelyinserting

 Notethatquantizerand source encoder worktosqueezeoutredundantinformation.
Thechannelencoderinserts redundant informationin avery selective manner

 Wewill study the role thaterror correctioncodingplaysin system design,
including trellis andturbo codes


 Converts digital data to a continuouswaveformsuitablefor transmission over
channel -usuallya sinusoidal wave

 Informationis transmitted byvarying one ormore parametersof waveform:
 Amplitude
 Phase
 Frequency

Examples ofModulation

 Amplitude Shift Keying (ASK)

 FrequencyShift Keying (FSK):

 PhaseShift Keying (PSK):

Notes onModulation

 Choice of modulation greatly effectssystem performance
 Moderncommunications theory views channelcoding andmodulation asa single
operation. Thisresultsin"trellis codes" which yieldoutstandingperformance.

 Wewill study modulation extensively


 Carries signal - could be a telephone wire, freespace
 Presentsdistorted signal to demodulator.Effects includeattenuation,n...
Digital communication systems
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