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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.