Download - Digital communication methods
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DIGITAL COMMUNICATION METHODS
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Amplitude Shift-Keying (ASK)
ASK is a simple version of amplitude modulation used for digital modulation
Two binary values (1 and 0) are represented by two different amplitudes of the carrier frequency (normally, ‘on’ and ‘off’)
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Frequency Shift-Keying (FSK)
FSK is the simplest (binary) form of frequency modulation used for digital modulation
The two binary values are represented by two different frequencies near the carrier frequency
Normally, the carrier is shifted high for a 1 and low for a 0
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Phase Shift-Keying (PSK)
PSK is a form of modulation in which the phase of the carrier signal is shifted to represent digital data
Binary Phase Shift-Keying (BPSK) is PSK between two phase states, normally 180° apart
Quadrature Phase Shift-Keying (QPSK) is a form of PSK using four phase states, normally 90° apart
Quadrature Amplitude Modulation (QAM) is a modulation technique in which both the phase and amplitude of a carrier are varied by the symbols of the message
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M-ary Encoding
M represents a digit that corresponds to the no. of conditions, levels or combinations possible for a given no. of binary variables
For example, an M-ary system with M=4 is a digital signal with four possible conditions
No. of bits necessary to produce a given no. of conditions is
Where N = no. of bits M = no. of conditions/levels/combinations
MN 2log
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No. of conditions possible:
For example, With one bit, only 21 = 2 conditions
possible With two bits, 22 = 4 conditions possible With three bits, 23 = 8 conditions
possibleand so on…
MN 2
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Amplitude Shift-Keying (ASK)
Uses logic levels in the data to control the amplitude of the carrier wave Data = 1 Amplitude HIGH (switch ON) Data = 0 Amplitude LOW (switch OFF)
ASK modulator block diagram:
Carrier input
Modulation input
Output
Carrier sine wave
Data stream
ASK waveform
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Example ASK waveform:
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ASK Receiver
Rectifier
Low pass filter
Voltage comparat
or
Voltage reference
Binary data
ASK waveform
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ASK Receiver
Rectifier Rectifies the input ASK waveform to contain
only positive signal However, signal still contains unwanted
carrier wave components, and waveform is too rounded and of unreliable amplitudes
Low pass filter Remove remnants of carrier wave
Voltage comparator Signal passes through a voltage level to
output a true copy of the original data stream
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Frequency Shift-Keying (FSK)
Uses logic levels in data to control the frequency of the carrier wave Data = 1 frequency HIGH Data = 0 frequency LOW
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Example FSK waveform:
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There are many ways to generate an FSK waveform. One way is to treat it as combining two different ASK waves
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FSK modulator block diagram
Carrier input
Carrier input
Modulation input
Modulation input
Output
Output
Carrier sine wave
Carrier sine wave
Inverted data stream
Data stream
Summing amplifier
FSK waveform
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Generating FSK waveform
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Advantage of FSK over ASK: Higher reliability in terms of data
accuracy at the receiver Disadvantage of FSK over ASK:
FSK uses 2 different frequencies, hence larger bandwidth
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Phase Shift-Keying (PSK)
PSK uses levels in data to control the phase of the carrier wave
Since sine wave is symmetrical, it is not possible for receiver to know whether signal is in inverted form or not. So the demodulator will create two different possibilities for the received signal, one is the inverse of the other
NRZ (non return-to-zero) code is used to detect the logic levels Data = 1 change in phase Data = 0 no change in phase
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PSK waveform:
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PSK modulator
Carrier input
Modulation input
Output
Carrier sine wave
Bipolar data stream
PSK waveformUnipolar
-bipolar convert
er
Unipolar data stream
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PSK Receiver block diagram
PSK demodulat
or
Low pass filter
Voltage comparat
or
PSK input signal
Differential bit
decoder
NRZ data output
Voltage reference
•PSK demodulator demodulates PSK input, resulting in a waveform containing the wanted dc level and ripple at the carrier frequency
•Low pass filter smoothens the ripple, resulting in the rounded version of the data
•Voltage comparator cleans up the wave shape
•Differential bit decoder extracts the NRZ data
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Binary Phase Shift Keying (BPSK)
Two phases are possible for the carrier
One phase represents a logic 1, the other represents logic 0
As input digital signal changes state (from 1 to 0, or 0 to 1), the phase of the output carrier shifts 180°
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BPSK truth table, phasor diagram and constellation diagram
Binary input
Output phase
Logic 0 180°
Logic 1 0°
Truth table: (+90°)cos ωc t
(-90°)-cos ωc t
(0°)sin ωc tLogic 1
(180°)-sin ωc tLogic 0
180°Logic 0
0°Logic 1
-cos ωc t
cos ωc t
Phasor diagram:
Constellation diagram:
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Quadrature Phase Shift Keying (QPSK)
QPSK is an M-ary encoding scheme Four output phases are possible for a
single carrier frequency There must be four different input
conditions: 00, 01, 10, 11 Binary input data are combined into
groups of two bits called dibits In the modulator, each dibit code
generates one of the four possible output phase (+45°, +135°, -45° and -135°)
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QPSK truth table, phasor diagram and constellation diagram
Binary input
Output phase
Q I
0 0 -135°
0 1 -45°
1 0 +135°
1 1 +45°
Truth table:
cos ωc t
-cos ωc t
sin ωc t-sin ωc t
Phasor diagram:
10
00
11
01
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QPSK truth table, phasor diagram and constellation diagram
cos ωc t
-cos ωc t
sin ωc t-sin ωc t
10
00
11
01
Constellation diagram:
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Quadrature Amplitude Modulation (QAM)
QAM is a form of digital modulation that is similar to PSK, except that the digital information is contained in both the amplitude and phase of the transmitted carrier
Amplitude and phase-shift keying are combined
Reduces probability of error
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For example, 8-QAM is an M-ary coding technique where M = 8
The phasor diagram for 8-QAM is shown below: cos ωc t
-cos ωc t
sin ωc t-sin ωc t
101
000
011
100
110
111
001
010
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Constellation diagram for 8-QAM:
cos ωc t
-cos ωc t
sin ωc t-sin ωc t
101
000
011
100
110
111
001
010