Basics of Electronic Communication

8/12/2019 Basics of Electronic Communication

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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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Basics of Electronic Communication