12301_digital comm overview
DESCRIPTION
SSSSSWFTRANSCRIPT
-
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
-
Digital comm overview.ppt 2
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.
-
Digital comm overview.ppt 3
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
-
Digital comm overview.ppt 4
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
-
Digital comm overview.ppt 5
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 comm overview.ppt 6
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.
-
Digital comm overview.ppt 7
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
-
Digital comm overview.ppt 8
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
-
Digital comm overview.ppt 9
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 10
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
-
Digital comm overview.ppt 11
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
-
Digital comm overview.ppt 12
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.
-
Digital comm overview.ppt 13
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 14
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)
-
Digital comm overview.ppt 15
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