A first course in Telecommunications: a top-down approach

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A first course in Telecommunications a top-down
approach

• Peter Driessen
• Faculty of Engineering
• University of Victoria

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Outline

• Introduction
• Traditional course curriculum
• New course curriculum
• Systems
• Link budget
• Modulation
• Spectra
• Discussion

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Introduction

• The traditional first course in
  telecommunications
• Analog modulation AM, SSB, FM
• Noise, threshold effect, capture effect
• New top-down approach
• Baseband digital
• Link budget
• General amplitude/phase modulation
• AM and FM as special cases

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Telecommunications courses
Signals, spectra, AM, SSB, FM
3rd year
4th year
Digital modulation
Networks and protocols
Microwave components
Antennas
Digital filters
Coding
Fiber optics
Wireless systems
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Traditional course curriculum

• First course in telecommunications
• Signals and spectra
• Linear filtering
• Analog modulation AM, SSB, FM
• Noise, threshold effect, capture effect

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Top down course curriculum

• Definition of telecommunications
• Idea of carrier wave
• Link budget
• Baseband message signals
• General amplitude/phase modulation
• General demodulation
• AM, FM, PSK etc as special cases

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Definition of telecommunications

• Science and technology of communications at a
  distance by electronic transmission
• (Websters)

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Telecommunications system

• Convert from human readable form
• Speech, music, image, video, text, data)
• To electronic form
• Transmit over a distance (between points A and B)
  via some channel (electronic pathway)
• Convert back to human readable form

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Channel

• The electronic pathway between points A and B may
  be
• Wire (twisted pair)
• Coaxial cable
• Fiber optics
• Free space (wireless)
• A carrier wave is needed (in most cases) to carry
  the message over a distance via the channel

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Networks

• Networks consist of nodes and channels
• Messages may be sent from node A to node B via
  intermediate nodes C, D,

node
D
A
B
channel
C
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Carrier frequencies

• The radio spectrum from DC to daylight
• Long wave, AM broadcast, shortwave, TV, FM
  broadcast, two-way radio, more TV, cellphones,
  GPS, more cellphones, microwave ovens, wireless
  LANs, police radar, infrared, lightwave,
  ultraviolet, xrays,

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Link budget

• To find out how much distance we can cover with
  the carrier wave
• Available resources
• Transmit power
• bandwidth
• Obstacles
• Noise
• interference

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Link budget 2

• P_r,o
• Receive power needed for acceptable quality
• P_r,n
• Receive power obtained via the channel
• For the link to work
• M P_r,o - P_r,n gt 0

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Link budget 3

• P_r,o P_T G_T G_R - L_0
• P_r,n (S/N) W F - k
• Examples
• Range of cellphone from tower
• Data rate of images from Saturn
• Transmit power of FM and TV broadcast
• Size of antenna needed for one-mile wireless LAN
  link

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Analog and digital messages

• Sine wave message may be
• Fourier component of analog message
• Filtered one-zero data pattern 10101010.

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Modulation

• General amplitude/phase modulation
• s(t) a(t) cos2pi f t phi(t)
• x(t) cos2pi f t - y(t) sin2pi f t
• Special cases
• AM a(t) 1 m(t), phi(t) constant
• SSB x(t) m(t), y(t) hilbertm(t)
• FM a(t) constant, phi(t) integralm(t)

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3-D signal representation

• Side views x(t), y(t)
• End view a(t), phi(t)

t
y(t)
x(t)
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Demodulation - receivers

• General I-Q receiver yields x(t), y(t)
• Envelope a(t) sqrt x2(t) y2(t)
• Phase phi(t) arctany(t)/x(t)
• Frequency f(t) d phi(t)/ dt
• Traditional analog demodulation circuits
  implement these equations
• Digital demodulators program these equations in
  software or firmware

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General orthogonal modulator structure

• QAM on 4 carriers
• 8 - dimensional signalling space
• In each dimension during each symbol time, can
  send
• 0
• 0 or 1
• 1 or -1
• Multilevel 3/1/-1/-3
• Mapper takes 1,2,4,8 or 16 bits per symbol

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General modulator - up to 8 orthogonal streams
f1
90
f2
Demux map
90
Bits in
Waveform out
f3
90
f4
90
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General orthogonal modulator structure 2

• Mapper takes 1,2,4,8 or 16 bits per symbol
• 1 bit
• binary FSK, ASK, PSK
• 2 bits
• 4 level ASK, 4-PSK (QPSK)
• Binary ASK or PSK on two carriers
• FSK (two carriers at one time, choose (f_1 or
  f_2) and (f_3 or f_4)
• MFSK (choose one out of 4 carriers)
• 4 or more bits many combinations

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Signal spectra

• Compute spectra using sine wave messages m(t)
• Illustrate concept of sidebands with audio demo
• 220 Hz 440 Hz
• AM
• FM

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AM/FM spectra

• Bell sound using combined AM/FM
• s(t) a(t) cos2pi fc t b(t) sin 2pi fm t
• a(t) exp(-t/t1)
• b(t) b0 exp (-t/t2)

short
long
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Discussion

• Top-down approach creates motivation, context and
  structure
• Link budget provides intuition about tradeoffs
  between power, bandwidth and distance
• General modulator unifies AM, FM, PSK etc.