12301_digital Comm Overview

Digital comm overview.ppt 1

Enel571 Overview of Digital CommunicationsRead 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


What is Communications?

Physical separation

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

Source of information

Intendedreceiver

Voice, text, numerical dataPictures, video Etc.


Communications channel

Physical separation

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

Source IntendedReceiver

Noisedistortion


What is Analog Communications then?

Analog source

Analog modulatorFM, AM etc

Communication link

ReceiverAnalog signaldemodulator

Analog communications analog mapping of analog source directly into analog transmit signalNo intermediate mapping into a discrete finite set of symbols


What is Digital Communications?Analog source

Sampling, - discrete time samples- quantized amplitude

Digital source

Mapping to discrete symbol set (encoding)

Communication linkModulator mappingFrom discrete symbol Set to analog signal

ReceiverDatademodulator

Error control feedback link

Digital communications involves:mapping into a discrete finite set of modulated symbolsModulated symbols are analog in nature and sent over noisy communication channelDemodulation of analog symbols back into discrete finite setProviding error correction to decoded symbols


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, Marconis spark generator, etc.


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 GuglielmoMarconi:

1898 English Channel 1901 Cornwall to Newfoundland Spark transmission system - ~0.5 bit per second


1900 first voice wireless system (Fessenden in US)

1920s 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 1990s

2000 unprecedented telecom technology bubble, 2001 bubble burst, 2004 modest recovery beginning


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


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.

Digital signal processor and CPU

Generic analog to digital convertor

Low pass filter X

Fixed localoscillator

antenna Simple analog front end


Components of a digital communication link

Signal source

Source encoder

Channel encoder

Digital

Signal output

Source decoder

Channel decoder

Digital

Physical propagation channel

Digital or analog

digital

modulator demodulator

analog


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 downconvertorfrom 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.


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)


Types of communication channels

Guided Twisted pair telephone cable, PC cables Coaxial cable, optical fiber, waveguide

Radiated 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)


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

Enel571 Overview of Digital CommunicationsWhat is Communications?Communications channelWhat is Analog Communications then?What is Digital Communications?Digital came firstBrief history of digital communicationsWhy digital vs AnalogImpact of DSP on receiver architecturesComponents of a digital communication linkComponents of a digital communication linkNoise and interferenceTypes of communication channelsGeneral Theme of Digital Communications Engineering

Documents

12301_digital Comm Overview