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A first course in Telecommunications: a top-down approach

A first course in Telecommunications: a top-down approach. Peter Driessen Faculty of Engineering University of Victoria. Outline. Introduction Traditional course curriculum New course curriculum Systems Link budget Modulation Spectra Discussion. Introduction.

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A first course in Telecommunications: a top-down approach

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  1. A first course in Telecommunications: a top-down approach Peter Driessen Faculty of Engineering University of Victoria

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

  3. 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

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

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

  6. 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

  7. Definition of telecommunications • Science and technology of communications at a distance by electronic transmission … • (Webster’s)

  8. 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

  9. 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

  10. 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

  11. 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, …

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

  13. 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

  14. 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

  15. Analog and digital messages • Sine wave message may be • Fourier component of analog message • Filtered one-zero data pattern 10101010….

  16. 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 cases AM: a(t) = 1 + m(t), phi(t) = constant SSB: x(t) = m(t), y(t) = hilbert[m(t)] FM: a(t) = constant, phi(t) = integral[m(t)]

  17. 3-D signal representation • Side views: x(t), y(t) • End view: a(t), phi(t) t y(t) x(t)

  18. 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

  19. 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

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

  21. 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

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

  23. 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

  24. 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.

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