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Enel571 Overview of Digital Communications
Read “Background and Preview” section in Haykin PP 1-29









What is digital communications?
Short History of Digital Communications
Why digital vs analog
Impact of DSP technology trend
Components of a communication system
Common channel types
General theme of digital communication engineering

Digital comm overview.ppt

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What is Communications?
Physical separation

Source of information Intended receiver Voice, text, numerical data
Pictures, video Etc.

Analog sources of information
• Voice, video, are analog continuous time signals
• No mapping of analog signal into a discrete alphabet of symbols
• Eg. AM, FM etc.

Digital sources of information
• Text, data files etc.
• Source data consists of symbols which are members of a finite discrete set. eg text
• Digital source may have originated as an analog signal that was mapped into a discrete set of symbols. eg DVD

Digital comm overview.ppt

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Communications channel
Physical separation

Source

Noise distortion Intended
Receiver

• Channel is always analog continuous time in nature
• Regardless of whether source is analog or digital
• Challenge for communications engineer is:




that channel resources are limited, power, bandwidth etc.
Interference noise in channel
Distortion effects

Digital comm overview.ppt

3

What is Analog Communications then?
Analog source

Analog modulator
FM, AM etc

Communication link
Receiver
Analog signal demodulator Analog communications analog mapping of analog source directly into analog transmit signal
No intermediate mapping into a discrete finite set of symbols

Digital comm overview.ppt

4

What is Digital Communications?
Analog source
Sampling,
- discrete time samples
- quantized amplitude

Digital source

Mapping to discrete symbol set (encoding)

Modulator mapping
From discrete symbol
Set to analog signal

Communication link

Receiver
Data
demodulator

Error control feedback link
Digital communications involves:
•mapping into a discrete finite set of modulated symbols
•Modulated symbols are analog in nature and sent over noisy communication channel
•Demodulation of analog symbols back into discrete finite set
•Providing error correction to decoded symbols

Digital comm overview.ppt

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Digital came first
•In antiquity, capacity of communication channels were very small
(capacity = how much information the channel can propagate per unit time)

•Not sufficient for analog signals ie voice
•Used a simple discrete alphabet to encode signals
•Hence digital communications preceded analog communications
•Various forms, drum beats, smoke signals, Marconi’s spark generator, etc.

Digital comm overview.ppt

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Brief history of digital communications


Ancient tribes used drums and smoke signals to communicate. Date of invention is unknown. Modulation method and encoding scheme is ….



1794 semaphore digital communication



Telegraph along railway lines about 1837 (Wheatstone in Britain)



Commercial telegraph 1851 in Europe



1858 first trans-Atlantic telegraph cable



1876 Alexander Bell and the voice telephone



The first radio frequency wireless digital communication system was by Guglielmo
Marconi:




1898 English Channel
1901 Cornwall to Newfoundland
Spark transmission system - ~0.5 bit per second
Digital comm overview.ppt

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1900 first voice wireless system (Fessenden in US)



1920’s radio stations



1926 first practical TV



1956 first voice quality transatlantic cable



1962 satellites for TV broadcast



1971 ARPANET – forerunner for internet



1977 first fiber optic cable in California



Wireless revolution 1990’s



2000 unprecedented telecom technology bubble, 2001 bubble burst, 2004 modest recovery beginning
Digital comm overview.ppt

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Why digital vs Analog


Presently moving away from analog towards all digital end to end networks and links



Reasons
– More digital sources of information than analog. Computer networking, internet, voice communication at saturated level
– Potentially less bandwidth per unit of information (example voice encoding)
– Effective error correction coding, message control etc.
– Regenerate signal along path between tx and rx
– Analog circuitry is finicky and therefore expensive
– Advancement in DSP-cheaper to integrate, sophisticated algorithms

Digital comm overview.ppt

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Impact of DSP on receiver architectures



”Moores Law” – continued advancement of DSP hardware, another quantum leap in performance “in the near future”
Slowly moving towards a complete SW radio or modem with negligible analog component count.

antenna

Simple analog front end

X

Low pass filter Generic analog to digital convertor

Digital signal processor and CPU

Fixed local oscillator Digital comm overview.ppt

10

Components of a digital communication link
Signal
source

Signal output Source encoder Source decoder Channel encoder Channel decoder Digital modulator Digital demodulator Digital or analog

digital

analog

Physical propagation channel

Digital comm overview.ppt

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Components of a digital communication link
Signal source – source of the signal to be communicated. It may be a data file, voice video picture etc. If the source is analog then it is converted to digital by direct sampling of the signal. Hence the output is a stream of digital bytes.
Source encoder – converts the raw digital bytes into something more palatable for transmission. Usually involves compression and encryption.
Channel encoder – adds coding for error detection/correction and translates the coded signal into the specific format required for the modulator. Also adds any framing and sychronization bits and associated physical layer messages at this stage
Digital Modulator – Takes the completed encoded signal and modulates the signal appropriate for transmission. This may involve analog upconversion to an
RF carrier etc.
Channel – The channel is the physical link between the transmitter and the receiver. It is typically non-ideal in that the signal becomes distorted and noisy before it appears at the receiver end.
Digital Demodulator – this block undoes the processing applied to the signal in the digital modulator. It may for example include an analog downconvertor from an RF carrier to baseband.
Channel decoder – this block unpacks the received frames and extracts the coded payload content. It checks/corrects any errors that may have occurred in the digital demodulator due to the signal noise and distortion
Source decoder - converts the stream of received and corrected bytes and generates the appropriate output. This could be an analog voice or video signal or a data file.
In this course we will deal primarily with the digital modulation and demodulator blocks as well as how the channel affects the signal.

Digital comm overview.ppt

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Noise and interference









Wide bandwidth independent additive noise (eg thermal)
“Lower frequency” interference from lightning, electrical machines etc
Other user interference from adjacent bands
Hostile jamming
Controlled interference from multiple users on the same channel eg CDMA
Receiver self jamming – malfunction, out of tune, not properly synchronizing etc.
Variable channel conditions, fading shadowing etc. (mobile radio communications)
Ionosphere interference, time variable plasma effects (eg Northern lights)

Digital comm overview.ppt

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Types of communication channels


Guided





Radiated








Twisted pair telephone cable, PC cables
Coaxial cable, optical fiber, waveguide
Acoustic
Point to point wireless
Mobile wireless
Satellite wireless
Wireless modem eg 802.11

Somewhere in between




free space optical house electrical wiring leaky coax etc)

Digital comm overview.ppt

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General Theme of Digital Communications
Engineering
Communicate information from a transmitter to a receiver at a rate which is commensurate with the information type and user requirements.







Minimize channel resources required to do this (eg bandwidth)
Minimize interference to other users (eg tx power, filtering)
Maximize robustness to sources of interference
Robustness also wrt varied operating conditions (eg fading mobile wireless)
Minimize cost/complexity of components (eg maximize $ for manufacture)
Comply with standards for universal adaptation of communications equipment

Digital comm overview.ppt

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