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V. Chandra Sekar Oxford University Press 2013
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Communication Basics
Communication deals with the principle of transferring
information from one place to another.
It involves transmission and reception, and processing of
information between these two locations. The source could be in continuous form as in the case of
analog communication and as discrete signals as in the case of
digital communication.
Short distance transmission of information is called basebandtransmission.
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Communication Basics
For long distance transmission, information has to be
impressed upon an high frequency component to beable to reach the reception end of communication.
The high frequency component is termed as a carrier
and the entire process is called modulation.
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Need For Modulation
To translate the frequency of a low-pass signal to a higher
band so that the spectrum of the transmitted bandpass signal
matches the bandpass characteristics of the channel.
For efficient transmission, it has been found that the antennadimension has to be of the same order of magnitude as the
wavelength of the signal being transmitted.
Since C=f for a typical low-frequency signal of 2 kHz, the
wavelength works out to be 150 km. Even assuming the
height of the Antenna half the wavelength, the height works
out to be 75 km, which is impracticable.
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Need For Modulation
To enable transmission of a signal from several
message sources simultaneously through a singlechannel employing frequency division multiplexing.
To improve noise and interference immunity in
transmission over a noise channel by expanding the
bandwidth of the transmitted signal.
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Frequency Translation
The modulation process shifts the modulatingfrequency to a higher frequency, which in turndepends on the carrier frequency, thus producing
upper and lower sidebands. Hence, signals are upconverted from low frequencies
to high frequencies and downconverted from highfrequencies to low frequencies in the receiver.
The process of converting a frequency or a band offrequencies to another location in the frequencyspectrum is calledfrequency translation.
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Types Of Modulation
Depending on whether the amplitude, frequency, or phase ofthe carrier is varied in accordance with the modulation signal,we classify the modulation as
Amplitude modulation
Frequency modulation
Phase modulation.
The method of converting information into pulse form andthen transmitting it over a long distance is calledpulsemodulation.
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Transmitter
The message as it arrives may not be suitable for direct
transmission. It may be voice signal, music, picture, or data. The
signals, which are not of electrical nature, have to be converted
into electrical signals. This is the role of transmitter. Typical block
diagram is illustrated below.
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Receiver
A receiver is meant to receive the electromagnetic signal which
carries the information. It is tuned to receive the required
information at a predetermined frequency. The output of the
receiver is usually fed into a transducer which converts the
information into understandable signal.
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Multiplexing
When it is required to transmit more signals on the same channel,
baseband transmission fails, as in the case of audio signals being
broadcast from different stations on the same channel.
To encounter this problem either frequency division multiplexing
or time division multiplexing is employed.
This method of transmitting several channels simultaneously is
known asfrequency division multiplexing (FDM).
In Time Division Multiplexing (TDM)several signals are
transmitted over a time interval. Each signal is allotted a time slot
and it gets repeated cyclically. The only difference compared to
FDM is that the signals are to be sampled before sending.
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V. Chandra Sekar Oxford University Press 2013
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Signals An Introduction
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Signals:
Any function that carries information.
Shows how a parameter varies with anotherparameter.
Will be dealing with signals with time or frequency as
an independent variable
Signals
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Signals are classified as:
Continuous and discrete.
Causal and Non causal.
Even and Odd.
Deterministic and Random
Real and complex
Energy and power type
Signals
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Discrete Signals
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Continuous Signals
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Causal Signals
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Even & Odd Signals
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( )sinsin ( ) ,
tc t
t
=
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Special Signals
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Sgn(t) = 1, t > 0
= -1, t < 0
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Signum Signals
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Impulse or Delta signal
( ) 1
( ) ( ) ( )
t
and v t t dt v t
=
=
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Classification Of Systems
Discrete time and Continuous Time systems.
Time Invariant and Time varying systems
Causal and Non Causal system
Instantaneous and Dynamic systems
Stable and Unstable systems
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Fourier Series & Transform
1. Fourier series:
- Any periodic of function of time x(t) having a fundamental
period T and frequency 1/T can be represented as an infinite
series of sinusoidal waveforms of fundamental and itsharmonic frequencies.
2. If a function is x(t), its Fourier series is given by:
0
1 1
( 0 cos(2 ) sin(2 )n nn n
x t a a fnt b fnt
= =
= + +
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Where:
2
0
2
2
2
1( )
2 2( ) cos
2( ) sin(2 )
T
T
T
n
T
n
a x t dt
T
nta x t dt
T T
b x t nt dt T
=
=
= =
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Fourier Transform
To represent aperiodic function Fourier transform is used
Unlike Fourier series, this representation will be continuous in
frequency domain
It is given by:
Also x(t) can be obtained from X(f) as:
x(t) =
2( ) ( ) j ftX f x t e dt
=
dfefX ftj 2)(
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Laplace Transform
1. It converts time domain signal into frequency domain a plane called s plane having
as the real part and as the imaginary part.
2. Laplace transform is given by the expression:
3. The inverse Laplace transform is given by:
( )
( ) ( )
. . ( ) ( )
st
jw t
x x t e dt
i e X x t e dt
+
=
=
1( ) ( )
2
stx t X s e ds
j
=
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Z Transform
Z transform is a polar representation compared to rectangular
representation in Laplace transform
It is for discrete time function
Z transform of a function x(t) is given by:
Inverse Z transform is given by:
In Z transforms a term ROC is defined as region of convergence
where the Z transform of a function has finite value.
[ ] [ ] nX z x n z= 1
[ ] [ ]x n x zZ
=
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Amplitude Modulation
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Amplitude of the carrier is changed in proportion to the
instantaneous amplitude of a message signal
Carrier frequency must be relatively higher than the
message frequency
Modulation index m is the ratio of Em/Ec
Percentage of modulation = m x 100%
Amplitude Modulation
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AM Envelope
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Frequency Spectrum Of AM Wave
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Power Spectrum Of AM
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Suppressed Carrier Systems
Double side band (DSB) system
Single side band system(SSB)
SSB with pilot carrier
Independent side band (ISB) system
Vestigial side band (VSB) system
Other AM Systems
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AM Waveforms For AM, DSB & SSB
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Single Sideband
Advantages:
Lesser power consumption.
Conservation of bandwidth.
Noise reduction. Less fading.
Disadvantages:
Requires complex receiver.
At the receiver, coherent carrier has to be generated.
In case of pilot carrier, at the receiver end it has to be boosted
properly.
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Square law Modulators
Switching Modulators
Transistor ModulatorsLow level
Medium level
High level
AM Modulators
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Balanced Modulators
1. Balanced ring Modulator
2. Balanced bridge Modulator
3. Transistor balanced Modulator4. FET balanced Modulator
SSB Generation
1. The filter method
2. The phase shift method
3. The Third method
Types Of Modulators
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D d l t Di t ti &
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AM Demodulators
1. Rectifier detector
2. Envelope detector
Detector Distortions1. Diagonal peak clipping
2. Negative peak clipping
SSB Reception
1. Coherent detection2. Reception with pilot carrier
Demodulators, Distortions &
Reception
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AM Transmitters
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AM Transmitters
Low Level AM DSBFC Transmitter
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High Level DSBFC Transmitter
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SSB Transmitter
SSB suppressed carrier Transmitter: BPF is used to remove the other
sideband
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Phase Shift Method
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SSB T itt With Pil t
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SSB Transmitter With Pilot
Carrier
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AM Receiver
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Super Heterodyne Receiver
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SSB Pilot Receiver
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Communication Receiver
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Communication Receiver
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Selectivity
Sensitivity
Dynamic range
Fidelity
Bandwidth
Noise temperature and equivalent noise
temperature
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Receiver Parameters
Costas Loop
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Costas Loop
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V. Chandra Sekar
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Angle Modulation
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Angle modulation includes both frequency and phase
modulations.
In Frequency Modulation(FM), the frequency of the
carrier is changed with respect to amplitude of themessage signal
In phase modulation(PM), the phase of the carrier is
changed with respect to amplitude of the message signal
Unlike AM, both FM and PM are nonlinear, hence much
more difficult to implement and analyze.
Introduction
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M d l i I d & D i i
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1. Modulation index for FM wave is given by:
Where f is the frequency deviation and fm
is the modulating frequency
2. The expression for an FM wave is:
3. Modulation index for PM wave is given by:
where, is the phase deviation given by:
4. The expression for an PM wave is:
m
f
f
=
( ) cos[2 sin{2 ( )}]FM c mf t A f t f t = +
p mm k E=
pK
p
m
kE
=
( ) cos[2 cos{2 ( )}]PM c m
f t A f t f t = +
Modulation Index & Deviation
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Frequency & Phase Modulator
Phase modulator can be used to generate FM wave and FM modulator can be used to
generate PM wave as shown:
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FM & PM Waves
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FM & PM Waves
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Narrowband FM
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FM with
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FM with > 10 is called wideband FM
Expression for wideband FM:
f(t) = Jo() cos ct J1(){ cos(c m) t cos(c m) t}+ J2 () { cos
(c- 2m) t + cos (c+ 2m) t} J3 () { cos (c- 3m) t cos (c- 3mt) }
+ -------
The function Jn() is called the Bessel function.
The spectrum is composed of a carrier with an amplitude Jo () and a set
of side bands spaced symmetrically on either side of the carrier at
frequency separation of m, 2m, 3m--- and so on. Unlike AM, FM has an infinite number of side bands along with carrier.
These side bands are separated from the carrier by fm, 2fm, 3fm---------.
Wideband FM
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B l F ti A A F ti Of
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Bessel Function As A Function Of
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Bandwidth Requirements For Angle
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Carsons formula for bandwidth of FM system
Band width = 2(f + fm) HZ
For low modulation index, in case of narrow band FM since 2f > fm,
equation reduces to Band width = 2f.
Average power in sinusoidal wideband FM:
PT= Vc2Jo
2 () /R + 2Vc2/R { J1
2() + J22() + J3
2() + ---------- }
= Vc2/R [ J0
2() + 2 { J12() + J2
2() + J32() + -------------- }]
= Pc[ Jo2
() + 2 { J12
() + J22
() + J32
() + ------------------- }]
where Pcis the unmodulated power Vc2/R.
Bandwidth Requirements For Angle
Modulated Waves
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The expression for sinusoidal FM is:
Kpem(t) = KpEmsin mt = sin mt
where = KpEm, is defined as Peak phase
deviation and is directly proportional to the peak
modulating signal.
Sinusoidal Phase Modulation
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Ph R t ti
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Phasor Representation
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FM & PM Generation
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FM generation
Varactor diode modulators
Reactance modulators
Modulators using linear integrated circuits
Indirect methods for narrow band and wideband
PM generation:
Varactor diode in direct PM modulators
Direct method with transistor
FM & PM Generation
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FM Detectors
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Slope detector
Balance slope detector
Foster Seeley discriminator
Ratio detector
Demodulator using PLL
Quadrature detector
Zero crossing detector
FM Detectors
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FM Transmitter
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Crosby Direct FM Transmitter:
FM Transmitter
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Indirect FM Transmitter
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Indirect FM Transmitter
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FM Receivers
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Super heterodyne Receiver
FM Receivers
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Double Superheterodyne Receiver
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Double Superheterodyne Receiver
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Ph d L k L
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Phased Lock Loop
It is a feedback system that generates a signal that has a
fixed relation to the phase of a reference signal .
A phase locked loop circuit responds to both the
frequency and phase of the input signals, by changing thefrequency of the voltage controlled oscillator until it
matches to the reference input in both frequency and
phase. Hence it is a negative feedback system except that
the feedback error signal is a phase rather than a current
or voltage signal as usually the case in conventional
feedback system.
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PLL Block Diagram (Analog)
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PLL Block Diagram (Analog)
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PLL Block Diagram (Digital)
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PLL Applications
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Data and Tape Synchronization
Modems
FSK Modulation FM Demodulation
Frequency Synthesizer
Tone Decoding
Frequency Multiplication and Division
PLL Applications
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Direct Digital Synthesis
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Is a powerful technique to generate RF signals.
A direct digital synthesizer operates by storing the
points of a waveform in digital format, and then
recalling them to generate the waveform. The rate at which the synthesizer completes one
waveform then determines the frequency.
Direct Digital Synthesis
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Direct Digital Synthesis
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Block Diagram :
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V. Chandra Sekar
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Pulse Modulation
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Pulse Modulation
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Pulse Modulation
In analog pulse modulation, the carrier is a periodic
pulse train
The amplitude, position and width of the carrier pulse
train are varied in a continuous manner in accordancewith the corresponding sample value of message
signal.
Thus in Pulse modulation, information is transmitted
basically in analog form, but the transmission takes
place at discrete times. Oxford University Press 2013
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In the case of digital pulse modulation the message signalis represented in a form that is discrete in both time andamplitude
The data is transmitted as a sequence of coded pulse.
This type of modulation is also called pulse codemodulation (PCM).
PCM is the most widely used form in the field ofTelecommunication.
Digital Data transmission provides a higher level of noise
immunity, more flexibility in the band width Power tradeoff possibility of providing more security to
data and ease of implementation using large scaleintegrated circuits.
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Predominant Methods Of Pulse
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Pulse width modulation (PWM)
Pulse position modulation (PPM)
Pulse amplitude modulation (PAM)
Pulse code modulation (PCM)
Modulation
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Pulse Width Modulation
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Pulse Width Modulation
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Pulse Amplitude Modulation
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Pulse Amplitude Modulation
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Pulse Amplitude Modulation
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Pulse Amplitude Modulation
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Pulse Code Modulation (PCM)
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PCM offers a method of over coming some of the disadvantages
of other type of pulse modulation.
In PCM the instantaneous amplitude of the sample is
represented by a binary code resulting in a series of ones and
zeros or mark and space.
All pulses have the same height and same shape
Since only ones and zeros are sent. The receiver has only to
detect the presence or absence of a pulse.
A distorted pulse does not degrade the signal as long as the
pulse can still be recognized. Hence PCM is less sensitive to noise
than wither PAM or PWM
Pulse Code Modulation (PCM)
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PCM Transmitter & Receiver
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PCM Transmitter & Receiver
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Multiplexing
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When more than one application or connection share thecapacity of one link it is called multiplexing.
This results in better utilization of resources.
A typical example is, many conversations over telephone
line, trunk line, wireless channel, etc.
A few examples of multiplexing are:
TDM- Time division multiplexing
FDM- Frequency division multiplexing
WDM- Wavelength division multiplexing
CDMA- Code division multiple access
Multiplexing
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FDM Transmitter
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FDM Transmitter
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FDM Receiver
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FDM Receiver
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Synchronous TDM Transmitter
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Synchronous TDM Receiver
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Synchronous TDM Receiver
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Analog Carrier System Using FDM
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Analog Carrier System Using FDM
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Digital Carrier System Using TDM
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