1. EEC4113 Data Communication & Multimedia System Chapter 3: Broadband Encoding by Muhazam Mustapha, October 2011

2. Learning Outcome
By the end of this chapter, students are expected to be able to explain link level broadband encoding for transmission

3. Chapter Content Amplitude Shift Keying Frequency Shift Keying
Phase Shift Keying
Pulse Width Modulation
Quadrature Modulation
Spread Spectrum Technology

4. Amplitude Shift Keying
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5. Amplitude Shift Keying (ASK)
Values represented by different amplitudes of carrier frequency
It is similar to Amplitude Modulation (AM) in analog communication, but with only two levels of amplitude
Usually, one amplitude is zero – i.e. presence and absence of carrier
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6. Amplitude Shift Keying (ASK)
Susceptible to sudden changes in gain
Up to 1200bps on voice grade lines
Used over optical fiber
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7. Amplitude Shift Keying (ASK)1 1 1 1
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8. Frequency Shift Keying
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9. Frequency Shift Keying (FSK)
Different frequency used to represent data
Two types:
Binary FSK (BFSK)
Multiple FSK (MFSK)
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10. FSK on Voice Grade Line Signal strength Frequency (Hz) 1170 2125
Spectrum of signal transmitted in one direction
Spectrum of signal transmitted in opposite direction
Frequency (Hz)
1170
2125
Full Duplex FSK Transmission on a Voice Grade Line
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11. Multiple FSK More than two frequencies used
Each signaling element represents more than one bit
Example: 3 bits per signal element, 8 signal elements, 8 different frequencies
Advantage: More bandwidth efficient
Disadvantage: More prone to error
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12. Binary Frequency Shift Keying (BFSK)
Most common form of FSK
Two binary values are represented by two different frequencies
It is similar to Frequency Modulation (FM) in analog communication, but with only two frequencies
Less susceptible to error than ASK
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13. Binary Frequency Shift Keying (BFSK)
Up to 1200bps on voice grade lines
High frequency radio transmission (3 to 30 MHz)
Even higher frequency on LANs using coaxial cable
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14. Binary Frequency Shift Keying (BFSK)1 1 1 1
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15. Phase Shift Keying
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16. Phase Shift Keying (PSK)
Phase of carrier signal is shifted to represent data
Broadband signaling
ASK
FSK
PSK
QAM
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17. Binary Phase Shift Keying (BPSK)
Two phases represent two binary digits 1 1 1 1
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18. Differential Phase Shift Keying (DPSK)
Binary 1: Phase change
Binary 0: No phase change 1 1 1 1
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19. Pulse Width Modulation
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20. Pulse Width Modulation (PWM)
The width (duration) of the pulse is used to represent data
In analog communication, PWM needs continuous width values to represent the analog waveform at certain sampling instances
In data (digital) communication, the pulses can have discrete values of width
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21. Pulse Width Modulation (PWM)
An integrator or timing circuit can be used to decode the carried bits
Data 10 01 11 01 00
Pulse
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22. Quadrature Modulation
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23. Quadrature PSK (QPSK)
More efficient use by each signal representing more than one bit
e.g. for shifts of π/2 (90°), each element represents two bits
135°:01
45°:11
225°:00
315°:10
Constellation diagram for QPSK
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24. Quadrature PSK (QPSK) Alternative choice of phases: Dibit Phase 00 01
90°:01
Dibit
Phase 00 01 90 10
180 11
270
180°:10
0°:00
270°:11
Constellation diagram
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25. Quadrature PSK (QPSK) 00 11 01 00 10
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26. Phase Detector Signal Signal Ref Signal Ref Signal Ref Ref CO1
phase leading
Signal
phase lagging
Signal
Ref
phase leading
Signal
Ref
phase leading
phase lagging
phase leading
Signal
phase lagging
Ref
phase lagging
Ref
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27. OQPSK (Offset / Orthogonal)
QPSK sometimes causes a phase change of 180° (π).
Phase change of 180° sometimes means discontinuity jump, and it is the largest possible jump.
The jump causes a large amplitude of high frequency and small amplitude of low frequency in transmission, and if it is low pass filtered, then the signal will experience a large fluctuation during the phase change.
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28. Examples: QPSK & OQPSK Waveforms
bit number 1 2 3 4 5 6 7 8 9 10
value 1 −1 1 1 −1 −1 −1 1 1 1 I Q I Q I Q I Q I Q
input signal
I(t) 1 3 5 7 9
Q(t) 2 4 6 8 10
phase of output signal
−π/4
π/4
−3π/4
3π/4
π/4
Q(t−Tb)
phase of output signal
−π/4
−π/4
π/4
3π/4
−3π/4
−3π/4
3π/4
π/4
π/4
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29. OQPSK
OQPSK was designed to reduce this fluctuation by delaying one of the signal combined in the modulator.
Revisit the example: Transition between bit 3-4 & 5-6 causes 180° in QPSK, but OQPSK signal made a gradual 2 steps of 90°. This reduces the fluctuation.
The signal would be strobed at the right time to get the right binary combination.
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30. QPSK and OQPSK Modulators
Offset QPSK (orthogonal QPSK)
Delay in Q stream
I(t)
an = ±1
R/2 bps
carrier
frequency
binary input
2 bit
serial to parallel
conversion Σ
output
s(n)
π/2
phase shift
R/2 bps
Delay Tb
Q(t)
bn = ±1
OQPSK only
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31. OQPSK 1 3 5 7 9 2 4 6 8 10 −π/4 π/4 −3π/4 3π/4 π/4 −π/4 −π/4 π/4 3π/4
input signal
I(t) 1 3 5 7 9
Q(t) 2 4 6 8 10
phase of output signal
−π/4
π/4
−3π/4
3π/4
π/4
180° transition
Q(t−Tb)
phase of output signal
−π/4
−π/4
π/4
3π/4
−3π/4
−3π/4
3π/4
π/4
π/4
two 90° transitions
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32. Multilevel PSK More levels taking more than 2 bits at a time
e.g. Transmit 3 bits at a time by using 8 phase angles (8-PSK)
Further, each angle can have more than one amplitude (QAM)
Tribit
Phase
000
001 45
010 90
011
135
100
180
101
225
110
270
111
315
010
011
001
100
000
101
111
110
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Constellation diagram

33. Quadrature Amplitude Modulation (QAM)
QAM used on Asymmetric Digital Subscriber Line (ADSL) and some wireless standards
Combination of ASK and PSK
Logical extension of QPSK
Send different signals simultaneously on same carrier frequency
Signals are distinguished by phase and amplitude difference
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34. 8-QAM 2 amplitudes 4 phases
QAM Constellations
011 01 00
010
101
100
000
001 10 11
110
111
4-QAM 1 amplitude 4 phases
8-QAM 2 amplitudes 4 phases
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35. QAM Constellations CO1, CO2 2 amplitudes 8 phases
Standard 9600 bps modem use 12 angles, four of which have two amplitudes for a total of 16 different signal elements
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36. Spread Spectrum Technology
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37. Spread Spectrum Technology
Spread spectrum technology is a method whereby a signal is being transmitted over a bandwidth much wider than the required bandwidth to transmit the intended information.
The main reason for the technology is security.
The signal that has been scattered over a large bandwidth can easily be scrambled and become hard to detect or interpret
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38. Frequency Hopping Spread Spectrum (FHSS)
The communication is done over many carrier frequencies that is changed randomly
Sequence:
Initiating side sends a request via a predefined frequency
The receiving side sends a number, known as seed
The initiating side uses the number to calculate a sequence of frequencies to be used
The initiating side sends a synchronization signal through the first frequency in the sequence
Both sides would then continue communication via those frequencies at known timing
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39. Frequency Hopping Spread Spectrum (FHSS)
Signal strength
Carrier frequency hops from channel to channel
Frequency (Hz)
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40. Frequency Hopping Spread Spectrum (FHSS)
Carrier frequencies are chosen at random over times
Carrier Frequency 8 7 6 5 4 3 2 1
Time 1 2 3 4 5 6 7 8 9 10 11 12 13 14
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41. Direct Sequence Spread Spectrum (DSSS)
The communication is done over many phase modulations that is applied randomly
The original modulated signal is further phase modulated at random phases – called chips
The rate of chips are much higher than the data rate
The receiver should know the same sequence of chips
The original data can then be retrieved by removing the chips
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42. Direct Sequence Spread Spectrum (DSSS)
Signal strength
Narrow Band Spectrum
Spread Spectrum
Noise Level
Frequency (Hz)
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43. Other Spread Spectrum Technologies
Time Hopping Spread Spectrum (THSS)
Data is burst at random times
bursts
Time
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44. Other Spread Spectrum Technologies
Chirp Spread Spectrum (CSS)
A chirp is sinusoidal signal that increases or decreases over some period of time
CSS data is transmitted over a pulse that is modulated by chirp (time varying carrier frequency)
data spectrum at start of pulse
data spectrum at end of pulse
Frequency (Hz)
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