2. Fundamentals of Communication Chapter 3:
Analog Transmission 2
Chapter Outline
3.1 Introduction
3.2 Signal Conversion
3.3 Analog Data, Analog Signal
3.3.1 Amplitude Modulation (AM)
3.3.2 Frequency Modulation (FM)
3.3.3 Phase Modulation (PM)
3.4 Digital Data, Analog Signal
3.4.1 Amplitude Shift Keying (ASK)
3.4.2 Frequency Shift Keying (FSK)
3.4.3 Phase Shift Keying (PSK)
3.4.4 Quadrature Amplitude Modulation (QAM)
3.5 Sideband
3.6 Spread Spectrum
3.6.1 Basic Principle
3.6.2 Direct Sequence Spread Spectrum (DSSS)
3.6.3 Frequency Hopping Spread Spectrum (FHSS)
3.6.4 Time Hopping Spread Spectrum (THSS)
3.7 Key Points
3.8 Exercises
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3.1 Introduction
“Modulate” means to regulate or adjust. In communication, it
means to regulate some parameter of a high-frequency carrier
wave with a lower frequency information signal
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3.2 Signal Conversion
Encoder Decoder
Digital
or
Analog
Digital
g(t)
(a) Encoding onto a digital signal
x(t)
S(f)
fc f
t
x(t)g(t)
Modulator Demodulator
Digital
or
Analog
Digital
m(t)s(t)m(t)
(b) Modulation onto an analog signal
Figure 3.1: Encoding and modulation techniques
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Analog Transmission 5
Types of Data to Signal Conversion
1. Digital data to digital signal
2. Analog data to digital signal
3. Digital data to analog signal
4. Analog data to analog signal
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3.3 Analog Data Analog Signal
Principal reasons for analog modulation of analog signals:
1. A higher frequency may be needed for effective transmission.
For unguided transmission, it is virtually impossible to
transmit baseband signals; the required antennas would be
many kilometers in diameter.
2. Modulation permits frequency division multiplexing.
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Analog Transmission 7
The mathematical expression for a sinusoidal carrier wave is -
Obviously the waveform can be varied by any of its following
three factors or parameter-
Ec - the amplitude
fc - the frequency
φ - the phase
Mathematical Expression of wave:
( )ϕω += tEe cc sin
( )ϕπ += tfEe cc 2sin
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Analog Transmission 8
3.3.1 Amplitude Modulation (AM)
Figure 3.2: Amplitude modulation
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Analog Transmission 9
Percent Modulation
Percent modulation, m, indicates the degree to which the AF
signal modulates the carrier wave
100
max
max
×=
wavecarrierofvalueimum
wavesignalofvalueimum
m
100×=
amplitudecarrier
amplitudesignal
100×=
A
B
( ) ( )
( ) ( )
100
minmax
minmax
X
EE
EE
m
cc
cc
+
−
=
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Analog Transmission 10
Effect of modulation Index
Figure 3.3: Amplitude modulation of various indexes
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Analog Transmission 11
Amplifier Analog Modulation
Figure 3.4: Methods of amplitude modulation
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Analog Transmission 12
Amplifier Analog Modulation
Figure 3.5: Block diagram of a typical AM transmitter
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Analog Transmission 14
AM Bandwidth
fc
BWm BWm
BWt= 2 x BWm
BWm= Bandwidth of the modulating signal (audio)
BWt= Total bandwidth (radio)
fc= Frequency of the carrier
Frequency
Amplitude
Figure 3.7: Amplitude modulation bandwidth
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Analog Transmission 15
Amplitude modulation band allocation
Figure 3.8: Amplitude modulation band allocation
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Analog Transmission 16
3.3.2 Frequency Modulation (FM)
Figure 3.9: Frequency modulation
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Analog Transmission 17
Mathematical Analysis of FM Signal
The instantaneous frequency f for a frequency modulated signal
is given by
( )tKEff mmc ωcos1+=
where, K = proportionality constant
fc = unmodulated carrier frequency
fm = carrier frequency of the modulating signal
Em = maximum value of the modulating voltage
ωc = unmodulated angular frequency of the carrier
ωm = angular frequency of the modulating signal
Considering mf (modulation index) the a frequency modulated signal can
be defined as -
( )tmtEe mfcc ωω sinsin +=
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Analog Transmission 18
FM Bandwidth
fc
5 BWm 5 BWm
BWt= 10 x BWm
BWm= Bandwidth of the modulating signal (audio)
BWt= Total bandwidth (radio)
fc= Frequency of the carrier
Frequency
Amplitude
Figure 3.10: Frequency modulation bandwidth
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Analog Transmission 19
FM Bandwidth
Figure 3.11: Frequency modulation band allocation
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Analog Transmission 20
3.3.3 Phase Modulation (PM)
Phase modulation and frequency modulation are very closely
related, and infact frequency modulation can be very easily
obtained from phase modulation by the so called Armstrong
method. Phase modulation and frequency modulation are
basically two types of angle modulation.
The expression for a phase modulated wave will be-
( )ttEe mmcc ωϕω sinsin +=
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Analog Transmission 21
3.4 Digital Data, Analog Signal
Figure 3.12: Digital to analog modulation
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Analog Transmission 22
Digital to analog modulation is the technique to convert digital data to
an analog signal.
Note
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Analog Transmission 23
Types of digital data to analog modulation
Figure 3.13: Types of digital data to analog modulation
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Analog Transmission 24
3.4.1 Amplitude Shift Keying (ASK)
Figure 3.14: Amplitude shift keying
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Analog Transmission 25
Noise usually affects the amplitude; therefore, thus ASK is most
affected by noise.
Note
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Analog Transmission 26
Amplitude Shift Keying (Cont.)
On/Off Keying (OOK) is a popular ASK technique. In OOK, logic 0 is
represented by the absence of a carrier. This can save the required
energy to transmit information.
In mathematical terms the ASK modulated signal can be expressed as:
( )
( )
=
0,0
1,2cos
bitfor
bitfortfA
ts cc π
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Analog Transmission 27
Bandwidth for ASK
Figure 3.15: Relationship between Baud rate and bandwidth in ASK
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Analog Transmission 28
3.4.2 Frequency Shift Keying (FSK)
Figure 3.16: Frequency shift keying
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Analog Transmission 29
Frequency Shift Keying (cont.)
In FSK system, two sinusoidal waves of the same
amplitude but different frequencies f1 and f2 are used to
represent binary bits 1 and 0 respectively.
In mathematical terms the FSK modulated signal can be
expressed as:
( )
( )
( )
=
0,2cos
1,2cos
2
1
bitfortfA
bitfortfA
ts
c
c
π
π
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Bandwidth for FSK
Figure 3.17: Relationship between Baud rate and bandwidth in FSK
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3.4.3 Phase Shift Keying (PSK)
Figure 3.18: Phase shift keying
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Phase Shift Keying (cont.)
In PSK system, two sinusoidal waves of the same
amplitude and frequency fc but phases are 0 and π are
used to represent binary bits 1 and 0 respectively.
In mathematical terms, the PSK modulated signal can be
expressed as .
≤≤−
≤≤
=
0,0,2cos
1,0,2cos
)(
forTttfA
forTttfA
ts
cc
cc
π
π
Figure 3.19: PSK constellation
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Quadrature Phase-shift Keying (QPSK)
Figure 3.20: The 4-PSK method
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Analog Transmission 34
Quadrature Phase-shift Keying
In mathematical terms, the QPSK modulated signal can be
expressed as-
≤≤
+
≤≤
+
≤≤
+
≤≤
+
=
01,0,
4
7
2cos
00,0,
4
5
2cos
10,0,
4
3
2cos
11,0,
4
2cos
)(
forTttfA
forTttfA
forTttfA
forTttfA
ts
cc
cc
cc
cc
π
π
π
π
π
π
π
π
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Quadrature Phase-shift Keying
The constellation diagram for the signal will be-
Figure 3.21 The 4-PSK characteristics
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QPSK and 8-PSK are 2 and 3 times as efficient as 2-PSK respectively.
Note
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π/4 Phase shift Keying (8-PSK)
Figure 3.22: The 8-PSK characteristics
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Analog Transmission 38
Bandwidth for PSK
Figure 3.23: Relationship between Baud rate and bandwidth in PSK
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3.4.4 Quadrature Amplitude Modulation
(QAM)
Quadrature amplitude modulation is a combination of ASK and PSK
such that a maximum contrast between each signal element (bit,
dibit, tribit, and so on) is achieved.
Figure 3.24: The 4-QAM and 8-QAM constellations
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Analog Transmission 40
3.4.4 Quadrature Amplitude Modulation
(QAM)
Figure 3.25: Time domain representation of 8-QAM signal
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Various 16 QAM Constellation
Figure 3.26: 16-QAM constellations
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Analog Transmission 42
3.5 Sideband
In Amplitude Modulation (AM), a band of frequencies higher than or
lower than the carrier frequency, contains energy as a result of the
modulation process. The frequencies above the carrier frequency
constitute the upper side band (USB); and those below the carrier
frequency constitute the lower side band (LSB).
Figure 3.27: Frequency domain representation of DSB-AM
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Analog Transmission 43
DSB-SC
Figure 3.28: Frequency domain representation of DSB-SC
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Analog Transmission 44
SSB-AM
Figure 3.29: Frequency domain representation of SSB-AM
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Analog Transmission 45
SSB-SC
Figure 3.30: Frequency domain representation of SSB-SC
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Analog Transmission 46
Vestigial Side Band (VSB)
Figure 3.31: Allocated frequency range for picture carrier and sound carrier
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Analog Transmission 47
3.6.1 Spread Spectrum Basic Principle
Figure 3.32: General model of spread spectrum digital
communication system
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3.6.2 Direct Sequence Spread Spectrum
(DSSS)
Figure 3.33: Direct Sequence Spread Spectrum
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Analog Transmission 49
3.6.3 Frequency Hopping Spread
Spectrum (FHSS)
Figure 3.35: Time Hopping Spread Spectrum