basics of electronic communication
TRANSCRIPT
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EEE 309
Communication Theory
Lecture 3Dr. Md. Forkan Uddin
Assistant ProfessorDept. of EEE, BUET, Dhaka 10000.
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Types of Transmission Two types: Baseband and Carrier Modulation/Passband
Baseband Transmission: Baseband - frequency band of the original message
signal from the source or input transducer
Baseband transmission refers to transmitting the signal
directly, without any modification to the spectral content Most baseband signals (audio, video) contain significant
low-frequency component
Cannot be effectively transmitted over the wireless
channel Can be transmitted with copper or coaxial cable for
dedicated transmitter-receiver pair
Baseband signals have overlapping band - results in
severe interference if sharing a channel 2
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Amplitude Modulation (AM)
Consider the sinusoidal carrier c(t), with carrier
frequency fc,
Assume, baseband signal/modulating signal/inteligence
signal m(t) M(f), with bandwidth B, and fc > B
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In AM, amplitude of the carrier c(t) is varied about a
mean (A) linearly with m(t)
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AM (Contd..)
In other words, we transmit an unmodulated carrier in
addition to the modulated message
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The carrier
component oscillates
between the
envelope |A + m(t)|
and itsnegative image |A +
m(t)|
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AM (Contd..)
The envelope is an accurate representation of themessage, provided
a. the carrier frequency fc is much greater than the
highest frequency component of m(t), which is the
message bandwidth : fc >> Bb. the message amplitude satisfies A + m(t)>= 0; for all t
Condition arelates to overlap of the frequency spectrum
components
Condition bensures that the message can be recoveredfrom the envelope.
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AM (Contd..) Suppose m(t) has zero offset, and mpand mp denote
the maximum and minimum values of m(t), respectively.
That is m(t) >=- mpfor all t and condition for envelope
detection is A >=mp
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AM (Contd..)
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AM (Contd..)
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AM (Contd..)
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AM Detection/ Demodulation
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AM: Summary
AM is simple to generate and to demodulate.
AM is wasteful of transmitted power
AM is wasteful of channel bandwidth: twice as much
bandwidth as required.
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AM: DSB-SC
Double sideband suppressed carrier (DSB-SC) AM usessimple multiplication of modulating signal and carrier
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AM:DSB-SC
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Both lower and upper sidebands are transmitted
Bandwidth of the modulated signal, BT=2B
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AM:DSB-SC Demodulation
Power efficiency in DSB-SC increases However, the simple envelope detection/demodulation is
not possible
Need coherent detection/ synchronous detection
What is coherent/synchronous detection?
The same carrier frequency signal must be generated
and multiplied the received signal at receiver
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AM:DSB-SC Summary
Modulation technique is easy
DSB-SC is wasteful of channel bandwidth due to
transmission of both sidebands takes up twice as much
bandwidth as required
Need costly coherent detection
Power efficiency is better
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AM vs DSB-SC
AM DSB-SC
Modulation Simple Simple
Demodulation Envelope detection
Low cost and simple
Coherent detection
Costly and complex
Power Efficiency Max 33 % Better
Bandwidth Uses Double of the
required BW
Double of the
required BW
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SSB-SC Modulation
Graphical presentation in frequency domain is easy
How to do modulation and demodulation?
How to express mathematically in time and frequency
domain?
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SSB-SC Modulation: Filter out a side band
There are a number of ways to generate SSB signals
(1) filter out the lower/upper sideband
(2) apply a Hilbert Transform
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In theory we can remove one of the sidebands by passing it
through a band-pass filter H(f) - one that passes either the lower or
upper sideband only
However practical construction of filters with steep transition
band is difficult
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Filter out method can be applied for signals that have little or
no energy at low frequencies
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SSB-SC Modulation: Filter out a side band
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Hilbert Transform
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d
t
m
ttmtmh
)(1
1)()(
jsgn(- ))M(
)(M)H(
)sgn()M()(M
H
H
jH()
M() Mh()
1)H(
)H(2
2
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Frequency Domain of SSB-SC
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Frequency Domain of SSB-SC
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)]()([2
1
)](1[2
1
)()()()( hjMMsignMuMM
)sgn()M()(Msince H j
)]()([2
1)](1[
2
1)()()()( hjMMsignMuMM
)]()([
2
1)]()([
2
1
)]()([2
1)]()([
2
1
)()()(
chchcc
chcchc
ccUSB
MMjMM
jMMjMM
MM
)]()([
2
1)]()([
2
1
)]()([2
1)]()([
2
1
)()()(
chchcc
chcchc
ccLSB
MMjMM
jMMjMM
MM
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Time Domain of SSB-SC
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)]()([2
*)()]()([2
1*)(
)]()([2
1
)]()([2
1
)(
cchcc
chchccUSB
jMM
MMjMM
ttmttmt chcUSB sin)(cos)()(
)]()([2
*)()]()([2
1*)(
)]()([2
1)]()([
2
1)(
cchcc
chchccLSB
jMM
MMjMM
ttmttmt chcLSB sin)(cos)()(
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SSB-SC Modulation: Hilbert Transform
Implementation of an SSB modulator using the phase-shiftmethod
Ideal phase shifter is not realizable - we can only approximate
it over a finite frequency range.
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SSB-SC Demodulation
It is simple to verify that SSB (suppressed carrier) can be
coherently demodulated
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Upon low-pass filtering the double frequency componentsresults in the desired signal
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AM vs DSB-SC vs SSB-SC
AM DSB-SC SSB-SC
Modulation Simple Simple Costly and
Complex
Demodulation Envelope
detectionLow cost and
simple
Coherent
detectionCostly and
complex
Coherent
detectionCostly and
complex
Power
Efficiency
Max 33 %
Inefficient
Better
Efficient
Better
Efficient
Bandwidth
Uses
Double of the
required BW
Inefficient
Double of the
required BW
Inefficient
Equal to the
required BW
Efficient
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QAM
Quadrature amplitude modulation (QAM) or quadrature
multiplexing can be used to transmit two modulating signals
Both modulating signals use DSB-SC and they will occupy as
much bandwidth as required
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QAM demodulation
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Coherent modulation is used at the receivers
LPF is used to recover the desired signal
The main drawback is that the oscillator should be tightly
synchronized
If in phase oscillator has a phase delay theta:
After using LPF, the received signal:
Co-channel Interference
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VSB modulation
Signals that do not contain an energy gap near DC cannot be
easily SSB modulated
Vestigial Side Band (VSB) modulation is used to overcome
problem
VSB transmits a single side band, plus a small amount (or
vestige) of the other sideband using a filter
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VSB Modulation and Demodulation
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Removing double frequency components due to LPF, we get
Output filter must satisfies:
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Example 1
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The carrier frequency is 20 kHz. Baseband signal bandwidth is 6 kHz. Hi(f) is
shown if fig (a). Determine H0(f).
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Application VSB: Analog TV
Video signal:
large bandwidth (4.5 MHz)- don't want DSB
significant low-frequency component - no SSB
The demodulation of the tv signal must be simple (cost)
envelope detector; this implies the addition of the carrier tothe VSB modulated wave
Since the transmitted power is large, it would be expensive to
carefully control the shape of the filter sideband.
Instead the VSB filtering is applied at the receiver, where thepower level is lower
The transmitter includes a filter to simply limit the bandwidth
of the signal
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Analog TV
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Notice
Class Test 01: lecture 1 to
todays lecture
Next Monday
On 27/01/2014 at 1 pm2ndfloor of ECE building
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