Communication System Basics

Introduction to
Communication System
Mr. A. B. Shinde
Assistant Professor,
Electronics and Computer Science Engineering,
P. V. P. Institute of Technology, Sangli

A. B. Shinde
Contents…
• Block schematic of communication
system,
• Simplex and duplex systems
• Modes of communication:
– Broadcast and
– point to point
• Necessity of modulation,
• Classification of modulation,
• Sampling theorem and
• Pulse analog modulation,
• Multiplexing: TDM and FDM
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A. B. Shinde
Communication ?
 Transfer of information from one place to another.
 What is Information ?
 What is Data ?
 Is there any difference between data and Information ?
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A. B. Shinde
Intro to Communication System
• Communication system has five blocks.
• Information source, Transmitter, Channel, Receiver and destination.
• The communication in electrical form takes place mainly in Transmitter,
Channel, Receiver blocks.
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Black diagram of a communication system

A. B. Shinde
 Information Source:
• Information is a very generic word signifying as anything intended
for communication (thought, news, feeling, visual scene & so on).
• The information source converts this information into a physical
quantity ( e.g. thoughts to speech signal)
• We need to convert the message signal to the electrical form,
which is achieved using a suitable transducer.
• Transducer is a device which converts energy in one form to the
other.
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A. B. Shinde
 Transmitter:
• The objective of the transmitter is to collect the incoming message
signal and modify it in a suitable form (if needed), such that, it can
be transmitted via the chosen channel to the receiving point.
• Functionality of the transmitter is mainly decided by the type or
nature of the channel chosen for communication.
• Transmitter block involves several operations like amplification,
generation of high-frequency carrier signal and modulation etc.
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A. B. Shinde
 Channel:
• Channel is the physical medium which connects the transmitter
and receiver.
• The physical medium includes copper wire, coaxial cable, fiber
optic cable, wave guide and free space or atmosphere etc.
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 Receiver:
• The receiver receives the incoming modified version of the message
signal from the channel and processes it to recreate the original
(non electrical) form of the message signal.
• Receiver includes processing steps like reception, amplification,
mixing, demodulation and recreation of message signal.
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 Destination:
• The destination is the final block in the communication system
which receives the message signal and processes it to comprehend
the information present in it.
• Usually, humans will be the destinations.
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Types of Communication
 There are 3 types of Communication Systems.
 Simplex
 Duplex
 Half – Dulex
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 Simplex:
 The most basic type of service is known as simplex.
 This service provides one-way communication.
 It is a unidirectional communication
 Examples of this type of service are TV distribution.
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A. B. Shinde
 Duplex:
 Most networks transfer data in two directions and are known as
duplex communications links.
 It is two-way directional communication simultaneously.
 Some times they are called as Full Duplex
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A. B. Shinde
 Half-duplex:
 A half-duplex network, is one with a full duplex physical layer, but
that on allows the network to be used on one direction at any one
time.
 It is two-way directional communication but one at a time.
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 Comparison:
16
Parameters Simplex Half Duplex Full Duplex
The direction of
communication
Simplex mode is a uni-
directional
communication.
Half Duplex mode is a
two-way directional
communication but
one at a time.
Full Duplex mode is a
two-way directional
communication
simultaneously.
Sender and Receiver
In simplex mode,
Sender can send the
data but that sender
can’t receive the data.
In Half Duplex mode,
Sender can send the
data and also can
receive the data but
one at a time.
In Full Duplex mode,
Sender can send the
data and also can
receive the data
simultaneously
Channel usage
Usage of one channel
for the transmission of
data.
Usage of one channel
data.
Usage of two channels
data.

A. B. Shinde
Types of Communication 17
Parameters Simplex Half Duplex Full Duplex
Performance
The simplex mode
provides less
performance than half
duplex and full duplex.
The Half Duplex mode
provides less
performance than full
duplex.
Full Duplex provides
better performance
than simplex and half
duplex mode
Bandwidth Utilization
Simplex utilizes the
maximum of a single
bandwidth.
The Half-Duplex
involves lesser
utilization of single
bandwidth at the time
of transmission.
The Full-Duplex
doubles the utilization
of transmission
bandwidth
Suitable for
when there is
requirement of full
bandwidth for
delivering data.
when there is
requirement of sending
data in both directions,
but not at the same
time.
when there is
requirement of sending
and receiving data
simultaneously in both
directions
Examples Keyboard and monitor. Walkie-Talkies. Telephone

A. B. Shinde
Modes of Communication
 Modes of Communication
 Broadcast Communication
 Point to Point Communication
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 Broadcast Communication:
 In a broadcast system there is one transmitter and more than one
receivers. A transmitter processes the incoming signal and makes it
suitable for transmission through a channel.
 A receiver receives the signal and extracts the message or contents
from the signal at signal output.
 Considering the broadcast communication system, the radio and
television are examples of broadcasting systems.
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A. B. Shinde
 Broadcast Communication:
 Considering the broadcast communication system, the radio and
television are examples of broadcasting systems.
 For different broadcasting systems we have different frequency bands.
 For standard AM broadcast the frequency band is 540 − 1600kHz.
 For FM broadcast the frequency range is from 88 − 108MHz.
 For television broadcasting systems we have different ranges of frequencies
depending on the type over 50MHz to 900MHz.
 Television broadcast and satellite communication systems use space wave mode
of propagation
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 Point-to-point Communication:
 Point-to-point is a form of communication providing a direct route from
one fixed point to another.
 Here, there exists a dedicated link between two nodes. There is one
transmitter and one receiver
 In this type of system, the smallest distance has most importance to
reach the receiver terminal.
 Here, the communication gives security and privacy to the transmitted
signal because the channel of communication is not shared.
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 Point-to-point Communication:
 An example is a telephone call, in which one telephone is connected to
another.
 The Point to Point Protocol (PPP) is used to create direct communication
between two nodes in the network. It authenticates the connections,
compresses them and transmits them after encryption, thus giving
privacy.
 PPP is designed primarily to link two networks and connections are able
to have bidirectional functions simultaneously
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A. B. Shinde
Modulation
 The term modulate means to regulate.
 The process of regulating is modulation
 For regulation we need one physical quantity which is to be regulated
and another physical quantity which dictates regulation.
• The signal to be regulated is termed as carrier.
• The signal which dictates regulation is termed as modulating signal.
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A. B. Shinde
Modulation
 Need for Modulation:
• Length of Antenna:
• For the effective transmission of a signal, the height h of the antenna
should be λ / 4 in length.
• The low-frequency message signal has a very high value of λ which will
require a very high antenna (practically not possible).
• For example:
• If we have to transmit a signal of 20 kHz then λ = C / f and height of the
antenna h ≈ λ where C is the wave velocity, here C = 3 × 108 m/s.
• h ≈ λ = (3 × 108) / (20 × 103) = 15 km.
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A. B. Shinde
Modulation
• Narrow Banding of Signal
• An audio signal usually has a frequency range (20 Hz to 20 kHz), if it is
directly transmitted then the ratio of highest to the lowest frequency
becomes (20 kHz / 20 Hz) = 1000.
• But if this audio signal is modulated over a carrier signal of
frequency 1000 kHz then the ratio of highest to the lowest frequency
becomes: (1000 kHz + 20 kHz) / (1000 kHz + 20 Hz) ≅ 1.2
• Hence, we need modulation to convert a wideband signal into a narrow
band signal.
•
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A. B. Shinde
Modulation
• Frequency Multiplexing
• All sound is concentrated within the range from 20 Hz to 20 kHz, so that
all signals from the different sources would be inseparably mixed up.
• It is practically not possible to distinguish between the different audio
signals when transmitted simultaneously.
• Hence, each of these signals is translated to a low-frequency range
before transmission which makes it quite easier to recover them and
distinguish each of them from one another at the receiver’s end.
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A. B. Shinde
Modulation
• Effective Power Radiated By Antenna
• Power radiated by an antenna ∝ (l/λ)2
where l is the length of the antenna and
λ is the wavelength of the signal which is to be transferred.
• This relation clearly shows that when signals having a low frequency and
high wavelength is transmitted directly the power radiated by the
antenna is very low and the signal will vanish after traveling some
distance.
• Hence, to transmit such signals over long distances, we superimpose
these low-frequency signals over the high frequency carrier signal so that
the power radiated by the antenna of the same length will be very large.
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A. B. Shinde
Modulation
 Classification of Modulation:
 Analog Modulation
 Amplitude Modulation
 Angular Modulation
Frequency Modulation
Phase Modulation
 Digital Modulation
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A. B. Shinde
Modulation
• Amplitude Modulation (AM):
• In AM, the amplitude of the carrier wave is varied in proportion to the
message signal, and the other factors like frequency and phase remain
constant.
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A. B. Shinde
Modulation
 Frequency modulation (FM):
 Frequency modulation (FM) varies the frequency of the carrier in
proportion to the message or data signal while maintaining other
parameters constant.
 The advantage of FM over AM is the greater suppression of noise.
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A. B. Shinde
Modulation
 Phase Modulation (PM):
 In phase modulation, the carrier phase is varied in accordance with the
data signal.
 In this type of modulation, when the phase is changed it also affects the
frequency, so this modulation also comes under frequency modulation.
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A. B. Shinde
Sampling Theorem
 Analog signals are continuous in time and difference in voltage levels for
different periods of the signal.
 The amplitude keeps changing along with the period of the signal.
 An analog signal can be converted to digital form using the sampling
technique.
 The output of this technique represents the discrete version of its analog
signal.
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A. B. Shinde
Sampling Theorem
• Definition
• The sampling theorem is defined as the conversion of an analog signal
into a discrete form by taking the sampling frequency as twice the input
analog signal frequency.
Fm = Input signal frequency
Fs = Sampling signal frequency.
• The output sample signal is represented by the samples.
• These samples are maintained with a gap, these gaps are termed as
sample period or sampling interval (Ts).
• And the reciprocal of the sampling period is known as “sampling
frequency” or “sampling rate”.
Sampling frequency Fs=1/Ts
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A. B. Shinde
Sampling Theorem 38

A. B. Shinde
Sampling Theorem
• Nyquist Criteria:
• If the sampling frequency (Fs) equals twice the input signal frequency
(Fm), then such a condition is called the Nyquist Criteria for sampling.
• When sampling frequency equals twice the input signal frequency is
known as “Nyquist rate”.
Fs=2Fm
• If the sampling frequency (Fs) is less than twice the input signal
frequency, such criteria called an Aliasing effect.
Fs<2Fm
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A. B. Shinde
Sampling Theorem 40

A. B. Shinde
Sampling Theorem
 Aliasing Effect:
 If, an information bearing signal is not strictly band-limited, some aliasing
is produced by the sampling process.
 Aliasing refers to the phenomenon of a high frequency component in the
spectrum of the signal seemingly taking on the identity of a lower
frequency in the spectrum of its sampled version.
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A. B. Shinde
Sampling Theorem
 Corrective Measures for Aliasing:
 Prior to sampling, a low-pass anti-aliasing filter is used to attenuate those
high frequency components of the signal that are not essential to the
information being conveyed by the signal.
 The filtered signal is sampled at a rate slightly higher than the Nyquist
rate.
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A. B. Shinde
Pulse Analog Modulation
 In analog modulation systems, some parameter of a sinusoidal carrier is
varied according to the instantaneous value of the modulating signal.
 In Pulse modulation methods, the carrier is no longer a continuous signal
but consists of a pulse train.
 Some parameter of which is varied according to the instantaneous value
of the modulating signal.
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A. B. Shinde
Pulse Analog Modulation
 Types of Pulse Modulation
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A. B. Shinde
Pulse Amplitude Modulation
 The amplitude of the pulses of the carrier pulse train is varied in
accordance with the modulating signal, that is amplitude of the pulses
depends on the value of m(t) during the time of pulse.
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A. B. Shinde
 In fact the pulses in a PAM signal may of Flat-top type or natural type or
ideal type.
 The Flat-top PAM is most popular and is widely used. The reason for
using Flat-top PAM is that during the transmission, the noise interferes
with the top of the transmitted pulses and this noise can be easily
removed if the PAM pulse as Flat-top.
 In natural samples PAM signal, the pulse has varying top in accordance
with the signal variation. Such type of pulse is received at the receiver, it
is always contaminated by noise. Then it becomes quite difficult to
determine the shape of the top of the pulse and thus amplitude
detection of the pulse is not exact
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A. B. Shinde
 Transmission Bandwidth of PAM:
 In PAM signal the pulse duration τ is assumed to be very small compared
to time period Ts between the two samples
τ < Ts
 If the maximum frequency in the modulating signal x(t) is fm then
sampling frequency fs is given by
fs>=2fm or Ts <= 1/2fm
 Therefore, transmission bandwidth >= fmax
But fmax = 1/2 τ
B.W >= 1/2 τ
B.W >= 1/2 τ >> fm
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A. B. Shinde
 Drawbacks of PAM signal:
 The bandwidth required for the transmission of a PAM signal is very large
in comparison with modulating signal frequency.
 Since the amplitude of the PAM pulses varies in accordance with the
modulating signal therefore the interference of noise is maximum in a
PAM signal. This noise cannot be removed easily.
 Since the amplitude of the PAM pulses varies, therefore, this also varies
the peak power required by the transmitter with modulating signal.
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A. B. Shinde
Pulse Width Modulation
 In PWM, Width of the pulses of the carrier pulse train is varied in
accordance with the modulating signal but the amplitude of the signal
remains constant.
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A. B. Shinde
Pulse Width Modulation
 Advantages of PWM:
 Noise is less, since in PWM, amplitude is held constant.
 Signal and noise separation is very easy
 PWM communication does not required synchronization between
transmitter and receiver.
 Disadvantages of PWM:
 In PWM, pulses are varying in width and therefore their power contents
are variable this requires that the transmitter must be able to handle the
power content of the pulse having maximum pulse width.
 Large bandwidth is required for the PWM as compared to PAM
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A. B. Shinde
Pulse Position Modulation
 Pulse Position Modulation (PPM) is an analog modulating scheme in
which the amplitude and width of the pulses are kept constant, while the
position of each pulse varies according to the instantaneous sampled
value of the message signal.
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A. B. Shinde
 Advantages of PPM:
 Like PWM, in PPM, amplitude is held constant thus less noise
interference.
 Signal and noise separation is very easy
 Because of constant pulse widths and amplitudes, transmission power
for each pulse is same
 Disadvantages of PWM:
 Synchronization between transmitter and receiver is required.
 Large bandwidth is required for the PPM as compared to PAM
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A. B. Shinde
Comparison
PAM PWM PPM
Amplitude is varied Width is varied Position is varied
Bandwidth depends on
the width of the pulse
the rise time of the pulse
the rise time of the pulse
Instantaneous transmitter
power varies with the
amplitude of the pulses
power varies with the
amplitude and width of
the pulses
power remains constant
with the width of the
pulses
System complexity is high System complexity is low System complexity is low
Noise interference is high Noise interference is low Noise interference is low
It is similar to amplitude
modulation
It is similar to frequency
modulation
It is similar to phase
modulation
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A. B. Shinde
 Multiplexing is a technique of combining more than one signal over a
shared medium channel.
 Types:
 TDM
 FDM
 CDM
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A. B. Shinde
Time Division Multiplexing
 This happens when the data transmission rate of media is greater than
that of the source, and each signal is allotted a definite amount of time.
 These slots are so small that all transmissions appear to be parallel.
 In time-division multiplexing, all the signals operate with the same
frequency at different times.
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A. B. Shinde
 TDMA Transmission
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A. B. Shinde
 Types :
• Synchronous TDM:
• The time slots are pre-assigned and fixed. This slot is even given if the
source is not ready with data at this time. In this case, the slot is
transmitted empty.
• Asynchronous (or statistical) TDM:
• The slots are allocated dynamically depending on the speed of the source
or their ready state. It dynamically allocates the time slots according to
different input channels’ needs, thus saving the channel capacity.
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A. B. Shinde
Frequency Division Multiplexing
 Here, number of signals are transmitted at the same time, and each
source transfers its signals in the allotted frequency range.
 There is a suitable frequency gap between the two adjacent signals to
avoid over-lapping.
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A. B. Shinde
Frequency Division Multiplexing
 Since the signals are transmitted in the allotted frequencies so this
decreases the probability of collision.
 The frequency spectrum is divided into several logical channels, in which
every user feels that they possess a particular bandwidth.
 A number of signals are sent simultaneously at the same time allocating
separate frequency bands or channels to each signal.
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This presentation is published only for educational purpose
abshinde.eln@gmail.com

Communication System Basics

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