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Transcript of 1 A first course in Telecommunications: a top-down approach Peter Driessen Faculty of Engineering...
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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
Digital modulation
Coding
Microwave components
Fiber optics
Antennas
Networks and protocols
Wireless systems
3rd year
4th year
Digital filters
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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 …– (Webster’s)
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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, …
A
C
D
B
node
channel
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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 > 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) cos[2pi f t + phi(t)] = x(t) cos[2pi f t] - y(t) sin[2pi f t]
Special casesAM: a(t) = 1 + m(t), phi(t) = constantSSB: x(t) = m(t), y(t) = hilbert[m(t)]FM: a(t) = constant, phi(t) = integral[m(t)]
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3-D signal representation
Side views: x(t), y(t) End view: a(t), phi(t)
x(t)
y(t)
t
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Demodulation - receivers General I-Q receiver yields x(t), y(t) Envelope a(t) = sqrt[ x^2(t) + y^2(t) ] Phase phi(t) = arctan[y(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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90f2
90f4
90f3
90f1
Bits inWaveformout
Demuxmap
General modulator - up to 8 orthogonal streams
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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) cos[2pi 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.