Communication is the transfer of information or message from point

to another.

Two persons talking to each other constitute Two persons talking to each other constitute the simplest communication system. The the simplest communication system. The

person who speaks is the person who speaks is the sourcesource, the person , the person listening is the listening is the receiverreceiver and the intervening and the intervening

air is the air is the communication linkcommunication link between them. between them.

COMMUNICATION SYSTEMCOMMUNICATION SYSTEM

A communication system consists of three A communication system consists of three basic components:basic components:

-- Transmitter (source)Transmitter (source)- Communication channel- Communication channel

(link – medium)(link – medium)- Receiver- Receiver

Nature/details of these components Nature/details of these components depend on:depend on:

1. Nature of the signal/ message to be communicated.1. Nature of the signal/ message to be communicated.

2. Distance which separates the source and the receiver.2. Distance which separates the source and the receiver.

- Direct talking is possible over short distances – - Direct talking is possible over short distances – sound waves attenuate fast.sound waves attenuate fast.

- Long distance communication requires the - Long distance communication requires the signal/message to be converted into an electrical signal/a signal/message to be converted into an electrical signal/a set of signals /electromagnetic waves.set of signals /electromagnetic waves.

- Long distance communication requires a link between the - Long distance communication requires a link between the source and the receiversource and the receiver

Message signal Transmitter

Communication

channel Receiver

Out put

signal

CUMMUNICATION SYSTEM

Transmitter:Transmitter: TransmitsTransmits the message/signal over the the message/signal over the communication channel. Quite often the original signal is communication channel. Quite often the original signal is not suitable for transmission over the communication not suitable for transmission over the communication channel to the receiver. It requires to be modified to a form channel to the receiver. It requires to be modified to a form suitable for transmission.suitable for transmission.

Communication Channel:Communication Channel:

Provides a link between the transmitter and the Provides a link between the transmitter and the receiver. It can be a transmission line (telephone and receiver. It can be a transmission line (telephone and telegraphy), an optical fibre (optical communication) or free telegraphy), an optical fibre (optical communication) or free space in which the signal is radiated in the form of space in which the signal is radiated in the form of electromagnetic waves.electromagnetic waves.

Receiver:Receiver:

Reconstructs the original message/signal after Reconstructs the original message/signal after propagation through the communication channel.propagation through the communication channel.

TRANSMITTERTRANSMITTER- We communicate through a message or a - We communicate through a message or a signal.signal.- A transmitter transmits the message over the - A transmitter transmits the message over the communication channel tocommunication channel to the receiver. the receiver. As a rule, the message produced by the source is As a rule, the message produced by the source is not suitable fornot suitable for transmission over the communication channel. transmission over the communication channel. Accordingly, a suitable Accordingly, a suitable transducer converts it into a time varying electrical transducer converts it into a time varying electrical signal called thesignal called the message signalmessage signal. .

AA transmitter, in its simplest form, is a setup which boosts transmitter, in its simplest form, is a setup which boosts the power of message signal and feeds it into the the power of message signal and feeds it into the communication channelcommunication channel

Microphone

Transmitter

Amplifier

Receiver

Antenna Antenna

Amplifier Loud speaker

Modulator Amplifier

Transmitter Receiver

Tunable

Amplifier

Demodu-lator

Audio

AmplifierSignal

To

Speaker

Antenna Antenna

Basic constituents of a transmitter are:Basic constituents of a transmitter are: 1. Message signal1. Message signal 2. Modulation 2. Modulation 3. Antenna 3. Antenna

Message signal:Message signal:

A single valued function of time that conveys the information. A single valued function of time that conveys the information.

Analog Analog Signals Signals Discrete or digital Discrete or digital

Analog SignalAnalog Signal Is a continuous function of time, with the amplitude (instantaneous Is a continuous function of time, with the amplitude (instantaneous value of the signal) being continuous. value of the signal) being continuous.

Elementary idea of Analog and Digital Communication

There are two different modes of communication

(1)Analog communication and

(2) digital communication.

Time period;Time period;amplitude I 4amplitude I 4

1) ANALOG COMMUNICATION

Analog signal is a Analog signal is a continuous continuous variation variation of current of current or which is or which is the representative the representative of the of the signal or signal or message.message.

An alternating voltage which varies sinusoidal is an analog signal.

During the transmission of analog signal, there is a chance for

interference and distortion and analog communication

is less reliable.

Discrete SignalsDiscrete Signals

Discrete signals are discontinuous in time; they are defined only at Discrete signals are discontinuous in time; they are defined only at discrete times. discrete times.

In case of discrete signals the independent variable (time) takes only In case of discrete signals the independent variable (time) takes only discrete values which are usually uniformly spaced. Consequently, discrete values which are usually uniformly spaced. Consequently, discrete-time signals are described as sequences of samples whose discrete-time signals are described as sequences of samples whose amplitudes may take a continuum of values. amplitudes may take a continuum of values. When each sample of a discrete-time signal is When each sample of a discrete-time signal is quantizedquantized I.e. its I.e. its amplitude is only allowed to take on a finite set of values (e.g. in a amplitude is only allowed to take on a finite set of values (e.g. in a binary representation low and high signals are designated as 0 and 1) binary representation low and high signals are designated as 0 and 1) and then and then codedcoded, the resulting signal is referred to as a , the resulting signal is referred to as a digital signaldigital signal..

signal which has two levels of current orPulsevoltage. These two levels are high or low level.

values of voltage or current representedby binary digits zero and one.

The binary signals are easy to generate andprocess with a digital circuit. Digital communication is more reliable.

level 0level 0

The low frequency electric signal (audio frequency range] transmitted to very large distances. This is because the energy as s

a low frequency signal will be very small (energy <* frequency).

1 audio frequency electric signal to transmits a large distances, a high frequency electric signal is used. Hence we superimpose the audio frequency (modulating signal) on a high frequency carrier wave. This process is modulation.

Modulation

1) The energy associated with the audio signal is very small (E = hv). Hence the audio frequency signal cannot be transmit such over long distances.

Need for modulation:

3) If the low frequency signals are transmitted from different stations get mixed up. So the different signals cannot be distinguished.

2) For the maximum efficiency of the transmitting and receiving a height of the antenna must be of the order of X/4.

In its simplest form, the transmitter has following problems:In its simplest form, the transmitter has following problems:1. Size of the antenna or aerial1. Size of the antenna or aerial For transmitting a signal we need an antenna. It should have For transmitting a signal we need an antenna. It should have a size comparablea size comparable to the wavelength of the electromagnetic wave representing to the wavelength of the electromagnetic wave representing the signalthe signal ( at least ( at least /4)/4) so that the time variation of the signal is properly sensed so that the time variation of the signal is properly sensed by the antenna. by the antenna. For an electromagnetic wave of frequency 20 kHz, the For an electromagnetic wave of frequency 20 kHz, the wavelength wavelength is 15 km. is 15 km. Obviously such a long antenna is not possible. Therefore, Obviously such a long antenna is not possible. Therefore, direct transmission ofdirect transmission of such a signal is not possible. If the frequency of the signal is such a signal is not possible. If the frequency of the signal is 1MHz, the 1MHz, the corresponding wavelength is 300m and transmission of such corresponding wavelength is 300m and transmission of such a signal is a signal is possible. Therefore, there is a need of possible. Therefore, there is a need of translating the translating the information contained in information contained in the original low frequency signal into high or radio- the original low frequency signal into high or radio-frequencies beforefrequencies before transmission. transmission.2. Effective power radiated by an antenna2. Effective power radiated by an antenna The power radiated from a linear antenna The power radiated from a linear antenna For a good transmission we need high power hence there is For a good transmission we need high power hence there is need for high need for high frequency transmission. frequency transmission.

2/l

3. Mixing up of signals from different transmitters3. Mixing up of signals from different transmitters

Direct transmission of baseband signal leads to interference from Direct transmission of baseband signal leads to interference from

multiple transmitters. Thus multiple user friendly communication is multiple transmitters. Thus multiple user friendly communication is

not possible. A possible solution is provided by employing not possible. A possible solution is provided by employing

communication at high frequencies and then allotting a band of communication at high frequencies and then allotting a band of

frequencies to each user. frequencies to each user.

The above arguments suggest that there is a The above arguments suggest that there is a need for translating theneed for translating the

original signal ( low frequency) into a high frequency wave before original signal ( low frequency) into a high frequency wave before

transmission such that the translated signal continues to possess the transmission such that the translated signal continues to possess the

information contained in the original signal. The high frequency wave information contained in the original signal. The high frequency wave

carrying the information is called the carrying the information is called the carrier wave. The process ofcarrier wave. The process of

transformation is called transformation is called ModulationModulation..

ModulationModulation

Transformation of the signal into a form suitable for transmission Transformation of the signal into a form suitable for transmission

through a given communication channel through a given communication channel

Types of Modulation

The process of changing the amplitude or frequency or phase the carrier wave in accordance with the intensity of the signal is called modulation.

1)Amplitude modulation (AM)

2) Frequency modulation (FM)

3) Phase Modulation (PM)

Accordingly the different types of modulations are

Amplitude modulationAmplitude modulation

The The process of changing the process of changing the amplitude of high frequency amplitude of high frequency carrier wave in accordance with carrier wave in accordance with the intensity of the signal is the intensity of the signal is called the amplitude called the amplitude modulation.modulation.

Amplitude change of the carrier = 2A - A = AAmplitude change of the carrier = 2A - A = A A.M. WaveA.M. Wave m™ *u j i *• r *. amplitude change of carrier A . i«™/ m™ *u j i *• r *. amplitude change of carrier A . i«™/

Now the modulation factor, m = ——Now the modulation factor, m = ——--———————*—————:— =— = 1 ———————*—————:— =— = 1 or 100%or 100%

amplitude of normal earner Aamplitude of normal earner A For effective modulation the degree of modulation should never exceed For effective modulation the degree of modulation should never exceed

100%.100%. ur j i «.• j j Signal wave amplitude Em /T, ,.. . ^f ur j i «.• j j Signal wave amplitude Em /T, ,.. . ^f

Modulation index Modulation index = —2-———————= —2-———————*~*~——— = —sl (Em= amplitude ——— = —sl (Em= amplitude ofof

carrier wave amplitude Ec modulating wave , Ec= amplitude of carrier carrier wave amplitude Ec modulating wave , Ec= amplitude of carrier wave)wave)

Modulation index Modulation index =Emfl*"Emio, where Emax and E^ are the=Emfl*"Emio, where Emax and E^ are the Emax + EminEmax + Emin and minimum amplitudes of the modulated wave,and minimum amplitudes of the modulated wave, »» width of modulation width of modulation = 2fm, where fm is the modulating signal = 2fm, where fm is the modulating signal

frequencyfrequency : In amplitude modulation, the base band signal amplitude is half : In amplitude modulation, the base band signal amplitude is half

the er signal amplitude. Find the modulation index.the er signal amplitude. Find the modulation index.

The process of changing the frequency of high frequency carrier

wave in accordance with the intensity of the signal is called

frequency modulation.

Frequency ModulationFrequency Modulation

Frequency Modulation:- TV broadcast , VHF, UHF, SHF and EHF

broadcasts.- Requires higher carrier wave

frequencies.- Noise generated by atmospheric or man

made electric discharges does no harm to intelligence.

- Higher S/N ratio, quality of broadcast very good.

FM Radio – 88 to 108 MHz VHF TV – 47 to 230 MHz UHF TV – 470 to 960 MHz

Comparison of Amplitude Comparison of Amplitude and Frequency modulationand Frequency modulation

AMAM FMFM1) Noisy 1) Noisy Less noiseLess noise2) Less band width 2) Less band width More band widthMore band width3) low cost3) low cost costlycostly4) Low efficiency4) Low efficiency High efficiencyHigh efficiency5) Transmission and reception easier5) Transmission and reception easier

Transmission and Transmission and reception comreception com

Phase Phase ModulationModulation

The process of changing the The process of changing the phase of the carrier wave phase of the carrier wave accordance with the intensity of accordance with the intensity of the signal is called phase the signal is called phase modulationmodulation

The wave form of the phase The wave form of the phase modulated wave is similar to that modulated wave is similar to that wave. Phase modulation uses a wave. Phase modulation uses a smaller bandwidth than smaller bandwidth than FM. FM. Hence information can be sent in Hence information can be sent in a given band width.a given band width.

Pulse ModulationPulse Modulation

Modulation of a carrier wave may be accomplished by short pulses. Modulation of a carrier wave may be accomplished by short pulses. Conventional telegraphy is the simplest example of this mode of modulation.Conventional telegraphy is the simplest example of this mode of modulation.Pulse systems are based on sampling of the information signal at periodic Pulse systems are based on sampling of the information signal at periodic intervals, usually twice the maximum frequency present (2B). They transmit a intervals, usually twice the maximum frequency present (2B). They transmit a very short pulse of radio-frequency carrier for each sample, with pulse very short pulse of radio-frequency carrier for each sample, with pulse characteristics varied in some manner proportional to the amplitude at the characteristics varied in some manner proportional to the amplitude at the sampling instant. A general name given to these modes of modulation is the sampling instant. A general name given to these modes of modulation is the pulse modulation.pulse modulation.

TheThe commoncommon pulse systems employed in pulse modulation of analog signals pulse systems employed in pulse modulation of analog signals are:are: (i)(i) Pulse – amplitude modulation (PAM)Pulse – amplitude modulation (PAM) (ii) Pulse – position modulation (PPM) (ii) Pulse – position modulation (PPM)(iii) Pulse – duration/width modulation (PDM/PWM)(iii) Pulse – duration/width modulation (PDM/PWM)(iv) Pulse – code modulation (PCM(iv) Pulse – code modulation (PCM

DEMODULATIONDEMODULATION

process process of recovering audio of recovering audio frequency signal from the frequency signal from the modulated carrier wave is modulated carrier wave is known as demodulation or known as demodulation or detection.detection.

In demodulation, the two main In demodulation, the two main operations involved are operations involved are

(1) Rectification of the modulated (1) Rectification of the modulated wavewave

(2) Elimination of radio frequency (2) Elimination of radio frequency components from the rectified components from the rectified wave.wave.

PULSE PULSE MODULATIONMODULATION

Pulse modulation is used in digital Pulse modulation is used in digital communication. In the pulse communication. In the pulse

modulation modulation the carrier is in the the carrier is in the form of pulsesform of pulses

The different types of pulse The different types of pulse modulation aremodulation are

1) Pulse amplitude modulation (PAM)1) Pulse amplitude modulation (PAM)

2) Pulse width modulation (PWM)2) Pulse width modulation (PWM)

3) Pulse position modulation (PPM)3) Pulse position modulation (PPM)

The process of changing the The process of changing the amplitude of the pulse in amplitude of the pulse in

accordance with the accordance with the modulating signal is called modulating signal is called

pulse amplitudepulse amplitudemodulation.modulation.

Pulse Amplitude Modulation (PAM)Pulse Amplitude Modulation (PAM)

Pulse Width Modulation (PWM) Pulse Width Modulation (PWM) or Pulse Duration Modulation or Pulse Duration Modulation

(PDM)(PDM)

The process of changing the The process of changing the width of the pulse in width of the pulse in

accordance with the intensity accordance with the intensity of the modulating signal is of the modulating signal is

called pulse width modulation.called pulse width modulation.

Pulse Position Modulation Pulse Position Modulation (PPM)(PPM)

The process of changing the The process of changing the position of pulse with the position of pulse with the modulating signal is called modulating signal is called pulse position modulationpulse position modulation

Pulse Code Modulation Pulse Code Modulation (PCM)(PCM)

The process of modulating The process of modulating the carrier pulse in the carrier pulse in accordance with the accordance with the

digitized modulating signal digitized modulating signal is called pulse code is called pulse code

modulation.modulation.

The commonThe common modulation modulation techniques used for digital techniques used for digital

data aredata are

1)1) Amplitude shift keying (ASK)Amplitude shift keying (ASK)

2) Frequency shift Keying (FSK)2) Frequency shift Keying (FSK)

3) Phase shift keying (PSK) 3) Phase shift keying (PSK)

DATA TRANSMISSION AND DATA TRANSMISSION AND RETRIEVALRETRIEVAL

Before transmission the message Before transmission the message signal is modulated with a carrier signal is modulated with a carrier wave using a modulator. wave using a modulator.

Modulated wave is amplified using Modulated wave is amplified using a high frequency power amplifier a high frequency power amplifier and it is supplied to the and it is supplied to the transmitting antenna. transmitting antenna.

The modulating signal is received The modulating signal is received using antenna at the receiving using antenna at the receiving station. The signal demodulated station. The signal demodulated and the message signal is and the message signal is retrieved.retrieved.

ModeModemmModem is the abbreviation for Modem is the abbreviation for

modulator and demodulator. It’s used modulator and demodulator. It’s used both at the transmission and the both at the transmission and the receiving ends.receiving ends.

At the transmission modem accepts the At the transmission modem accepts the digital data and converts it into analog digital data and converts it into analog signal modulation with a carrier signal modulation with a carrier signal.signal.

At the receiving end, the carrier wave At the receiving end, the carrier wave

demodulated to get the analog signal and demodulated to get the analog signal and the equivalent digital signal produced.the equivalent digital signal produced.

Fax (Facsimile)Fax (Facsimile)

A fax is an electronic device for A fax is an electronic device for transmitting graphical information transmitting graphical information through wire or wireless. through wire or wireless.

The printed material is scanned The printed material is scanned and converted into an electronic and converted into an electronic signal by fax machine.signal by fax machine.

There are two types of communicationThere are two types of communication

(1) communication through space (1) communication through space called, called,

space communicationspace communication

(2) communication through wire (2) communication through wire called called

line communication.line communication.

The communication utilizing The communication utilizing the physical space the physical space

surrounding the earth is called surrounding the earth is called space communication.space communication.

SpaceSpace

CommunicationCommunication

There are three modes of There are three modes of space communicationspace communication

(1)(1) Ground wave propagationGround wave propagation

(2) (2) Space wave or troposphere Space wave or troposphere wave propagationwave propagation

(3)(3) Sky wave propagation or Sky wave propagation or ionosphere propagation. ionosphere propagation.

In In ground wave ground wave propagation the propagation the

transmitted wave transmitted wave propagate along propagate along the the surface of the earth.surface of the earth.

1) Ground Wave Propagation1) Ground Wave Propagation

2) Space Wave Propagation 2) Space Wave Propagation or Troposphere Wave or Troposphere Wave

PropagationPropagationIn space wave propagation the In space wave propagation the

radio waves from the radio waves from the transmitting antenna reaches transmitting antenna reaches the receiving antenna either the receiving antenna either

directly or after reflecting the directly or after reflecting the troposphere or ground.troposphere or ground.

3) 3) Sky wave Propagation or Sky wave Propagation or Ionosphere PropagationIonosphere Propagation

The atmosphere surrounding the earth at a height The atmosphere surrounding the earth at a height of about 80km to 300km is called of about 80km to 300km is called ionosphere. ionosphere. This region is full of ions produced by ultraviolet This region is full of ions produced by ultraviolet and high energy radiations coming from the sun. and high energy radiations coming from the sun. This region has the property of reflecting e.m This region has the property of reflecting e.m waves coming from earth.waves coming from earth.

By using this reflecting property of ionosphere By using this reflecting property of ionosphere electromagnetic waves coming from transmitter electromagnetic waves coming from transmitter can be sent to far away points as shown in fig. can be sent to far away points as shown in fig. 10.1110.11

IONOSPHERE

60

100

200

300

Height

(km)

C Layer

D Layer

n ~108 (m3)

n ~ 109 (m3)

n ~ 1011 (m3)

n ~ 5 1011 (m3)

n ~ 8 1011 (m3)

E Layer

F1 Layer

F2 Layer

- Surface wave propagation – used for medium wave band and TV - Surface wave propagation – used for medium wave band and TV broadcasting which is done in the frequency rang 100 – 200 MHz. In this broadcasting which is done in the frequency rang 100 – 200 MHz. In this transmission the reception is possible only when the receiver antenna transmission the reception is possible only when the receiver antenna directly intercepts the signal. Thus, if the broadcast is made from a directly intercepts the signal. Thus, if the broadcast is made from a tower of height tower of height hh above the ground, due to the curvature of earth no above the ground, due to the curvature of earth no reception is possible beyond certain points. reception is possible beyond certain points.

- To get larger coverage TV broadcast are made from tall antennas. - To get larger coverage TV broadcast are made from tall antennas. Further, the power transmitted also decreases nearly as the inverse Further, the power transmitted also decreases nearly as the inverse square of the distance hence the signal becomes weak as the distance square of the distance hence the signal becomes weak as the distance increases, which limits the range of transmission by this mode. increases, which limits the range of transmission by this mode. - The ground wave attenuation increases with frequency, so the - The ground wave attenuation increases with frequency, so the transmission via this mode is in practice possible only for frequencies transmission via this mode is in practice possible only for frequencies up to about 1500 kHz or wavelengths greater than 200m. up to about 1500 kHz or wavelengths greater than 200m.- Below 200m wavelength, the communication in AM band is via sky- Below 200m wavelength, the communication in AM band is via sky wave. wave.- The diffraction of electromagnetic waves also affects their - The diffraction of electromagnetic waves also affects their propagation.The frequencies of the waves employed for radio and propagation.The frequencies of the waves employed for radio and television broadcast lie in the range 5 – 1000 MHz and the television broadcast lie in the range 5 – 1000 MHz and the corresponding wavelengths are in the range of 30 cm – 200 m. At these corresponding wavelengths are in the range of 30 cm – 200 m. At these wavelengths the diffraction effects are considerable and therefore these wavelengths the diffraction effects are considerable and therefore these waves lose their directional properties. waves lose their directional properties.

Rhd 2

•AntennaAntenna-- An antenna is a vital component of any communication system. It is An antenna is a vital component of any communication system. It is

employed both at the transmitting end as well as at the receiving end. employed both at the transmitting end as well as at the receiving end.- An antenna is a length of conductor, its length is such that it acts as a - An antenna is a length of conductor, its length is such that it acts as a resonant circuit at the frequency of operation. resonant circuit at the frequency of operation. ll = = /2./2.- It acts as a conversion device. The first conversion takes place at the - It acts as a conversion device. The first conversion takes place at the transmitter where electrical energy is converted into electromagnetic transmitter where electrical energy is converted into electromagnetic waves. The second conversion occurs at the receiving end where the waves. The second conversion occurs at the receiving end where the electromagnetic waves are transformed into electrical signal that is electromagnetic waves are transformed into electrical signal that is applied to the input of the receiver. applied to the input of the receiver.

Two types of antenna:Two types of antenna: 1. Dipole antenna – Length of dipole = 1. Dipole antenna – Length of dipole = /2 ; Omni directional./2 ; Omni directional.

2. Dish antenna – A spherical or parabolic dish is employed as a reflector 2. Dish antenna – A spherical or parabolic dish is employed as a reflector or collector. The resonant element is placed at the or collector. The resonant element is placed at the focus. It is highly directional. focus. It is highly directional.

• Communication ChannelCommunication Channel

- In a communication system, the communication channel or the - In a communication system, the communication channel or the transmission medium transmission medium is the physical path between the transmitter and is the physical path between the transmitter and receiver.receiver.- Transmission media- Transmission media can be classified into two broad categories:can be classified into two broad categories:

(a) Guided (a) Guided - Point – to – point communication- Point – to – point communication (i) Twisted pair(i) Twisted pair (ii) Coaxial cable (ii) Coaxial cable (iii) Optical fibre (iii) Optical fibre

(b) Unguided – Free space (b) Unguided – Free space

- Characteristics and quality of transmission are determined both by the - Characteristics and quality of transmission are determined both by the nature of the signal as well as the medium. nature of the signal as well as the medium.- In guided media, the nature of the medium is more important; in - In guided media, the nature of the medium is more important; in unguided media, the spectrum or the frequency band of the signal unguided media, the spectrum or the frequency band of the signal transmitted by the transmitter is more important. transmitted by the transmitter is more important. Characteristics of a Communication Channel : Characteristics of a Communication Channel : Band width, Band width, Modulation and Data rate. Modulation and Data rate.

•ReceiverReceiver

- Reconstructs the original message or data after its propagation through- Reconstructs the original message or data after its propagation through

the communication channel the communication channel

- The process consisting of decoupling of the carrier wave and the - The process consisting of decoupling of the carrier wave and the

modulating signal is broadly termed as modulating signal is broadly termed as demodulationdemodulation..

- The design of the receiver depends on the modulation process - The design of the receiver depends on the modulation process

employed in the transmitter. employed in the transmitter.

- The antenna receives the modulated wave transmitted from the - The antenna receives the modulated wave transmitted from the

transmitter, which is then amplified by a suitable amplifier and fed to the transmitter, which is then amplified by a suitable amplifier and fed to the

demodulator or decoder. demodulator or decoder.

- The demodulator or decoder extracts the original signal. The process of - The demodulator or decoder extracts the original signal. The process of

demodulation provides a means of recovering the original signal from demodulation provides a means of recovering the original signal from

the modulated wave. In effect, demodulation is reverse of modulation: the modulated wave. In effect, demodulation is reverse of modulation:

therefore, it depends on the modulation process used. therefore, it depends on the modulation process used.

A communication satellite is an electronic device. It has a receiver and transmitter which are

together called transponders.

The signal transmitted from the earth is up linked receiver in the satellite.

The receiver amplifies the signal and download the ground station by its transmitter. The uplink

frequency and the download frequency are different in order to avoid confusion.

The satellite communication is possible by using a geostationary satellite. Geostationary satellite appears to be stationary relative to the earth. This is possible if the satellite is at a height of

about 36000 km from the earth.

- The satellite communication first started in 1962 with the satellite - The satellite communication first started in 1962 with the satellite

Telstar. The first commercially operated satellite was launched in 1965. Telstar. The first commercially operated satellite was launched in 1965.

Since then numerous communication satellites have been launched for Since then numerous communication satellites have been launched for

the services of point-to-point telecommunication circuits, vide area TV the services of point-to-point telecommunication circuits, vide area TV

coverage, direct broadcasting by satellite, navigational communications coverage, direct broadcasting by satellite, navigational communications

to ships and aircrafts. to ships and aircrafts.

- Most of the satellites orbit at heights greater than 600 km to minimize - Most of the satellites orbit at heights greater than 600 km to minimize

atmospheric drag. atmospheric drag.

- The choice of orbit is of fundamental importance, as it determines the - The choice of orbit is of fundamental importance, as it determines the

transmission path loss and delay time, the earth coverage area and time transmission path loss and delay time, the earth coverage area and time

period the satellite is visible from a given area. period the satellite is visible from a given area.

- The orbits of communication satellites are conventionally classified as - The orbits of communication satellites are conventionally classified as

inclined elliptical, polar circular and geo-stationary. inclined elliptical, polar circular and geo-stationary.

- The geo-stationary orbit is the most widely used orbit for - The geo-stationary orbit is the most widely used orbit for

communication satellites. communication satellites.

Polar SatellitePolar Satellite

Polar satellite revolves round the Polar satellite revolves round the earth at a height of about 1000km earth at a height of about 1000km

from earth. Their orbits are from earth. Their orbits are around the earth over the poles in around the earth over the poles in

geographical meridian with a geographical meridian with a speed such that it reaches over a speed such that it reaches over a

place when it is just under the place when it is just under the sun. Hence they are called the sun. Hence they are called the polar satellites. Because of the polar satellites. Because of the

earth's spinning the polar satellite earth's spinning the polar satellite is able to scan the entire surface is able to scan the entire surface

of the earth, of the earth,

Remote sensing and Remote sensing and spy work.spy work.

USE OF POLAR SATTELLITES

Remote sensingRemote sensing

Remote sensing is one of the Remote sensing is one of the applications of the satellite applications of the satellite

communication.communication.

A satellite can take photograph of an A satellite can take photograph of an object or some area the earth or object or some area the earth or collect any other information and collect any other information and

transmit it back to an earth station. transmit it back to an earth station. This is known as This is known as remote sensing. remote sensing.

The process of taking photograph The process of taking photograph from satellites is generally called from satellites is generally called

satellite imagery. satellite imagery.

It is mainly used resourceIt is mainly used resource

survey, town and country planning survey, town and country planning and for archeology etc.and for archeology etc.

Applications of remote Applications of remote sensingsensing

(1) meteorology (1) meteorology (2) climate(2) climate(3)(3) oceanography oceanography (4) archeology (4) archeology (5) geological surveys (5) geological surveys (6) agriculture(6) agriculture(7) forestry (7) forestry (8) predicting natural disaster etc. (8) predicting natural disaster etc.

LINE COMMUNICATIONLINE COMMUNICATION

Line communication is a Line communication is a mode of communication mode of communication

from one point to another from one point to another through a wire or cable. through a wire or cable.

Etc…Etc…

There are three different types of There are three different types of line communication. line communication.

1)1)Two wire transmission lineTwo wire transmission line

2) 2) Co-axial cableCo-axial cable

3) Optical Fibre3) Optical Fibre

In two wire transmission line In two wire transmission line and co-axial cable audio and co-axial cable audio frequency ultra high frequency ultra high frequency (UHF) are used. frequency (UHF) are used.

But in optical fibres, optical But in optical fibres, optical frequencies are used.frequencies are used.

Two wire Transmission Two wire Transmission LinesLines

The best example for two wire The best example for two wire transmission line is a telephone: transmission line is a telephone: figure shows the transmission figure shows the transmission

through two wires.through two wires.

The signal flowing through the wires The signal flowing through the wires produces electric field E and magnetic produces electric field E and magnetic field B as given in the fig. (b). field B as given in the fig. (b).

Each portion of the transmission can be Each portion of the transmission can be considered as a small considered as a small inductor, resistor inductor, resistor and capacitor a. and capacitor a. fig. (c). fig. (c).

Hence each length of transmission line will Hence each length of transmission line will have a ch impedance. Because of this, for have a ch impedance. Because of this, for maximum power transfer, impede maximum power transfer, impede receiving unit should have an impedance receiving unit should have an impedance equal to the ch; impedance. This is also equal to the ch; impedance. This is also called called load matching.load matching.

Twisted pair of wiresTwisted pair of wires

A twisted pair" of wires consists of A twisted pair" of wires consists of two insulated wires twisted two insulated wires twisted around each other to form a around each other to form a

spiral shape as shown in figure.spiral shape as shown in figure.

The electromagnetic interference The electromagnetic interference from other sources can be from other sources can be reduced by the use of twisted reduced by the use of twisted wires. The twisted wires are wires. The twisted wires are widely used in telephone and widely used in telephone and computer networking computer networking

Co-axial cableCo-axial cableMesh conductorMesh conductor

A co-axial cable consists of a single inner A co-axial cable consists of a single inner conductor made of copper, surrounded by solid conductor made of copper, surrounded by solid conductor or a braded mesh of fine wires. An conductor or a braded mesh of fine wires. An

ms medium like teflon or poly ethylene ms medium like teflon or poly ethylene separates the two cables as shown in figure. separates the two cables as shown in figure. The outer conductor is normally connected to The outer conductor is normally connected to

the ground hence it provides an electrical shield the ground hence it provides an electrical shield to the signal carried by the conductor, reducing to the signal carried by the conductor, reducing

radiation loss. The outer conductor is further radiation loss. The outer conductor is further covered with a polymer for protection. covered with a polymer for protection.

A co-axial cable has least A co-axial cable has least interference from other signals.interference from other signals.

Inner conductor Inner conductor Outer coverOuter cover

Telephone links can be established by using two line wire waves, sky waves,

microwaves, co-axial cables and optical fiber individual telephone sets are

connected to local telephone exchange twisted pair of wires. The satellite linking

with telephones are don’t figure. The message from the telephone goes to the earth satellite through local exchange,

main exchange, earth satellite station and transmitter. The message from the geostationary satellite is n another

microwave receiver kept far away. The message from this receiver finally goes to

another telephone set through local exchange in figure.

Satellite In GeostationaryCoaxial Cable

Telephone I/inks

Optical communicationOptical communication is made by using optical carrier

high frequency ranging from 1O12 Hz to 1016 Hz. Hence very large bands and- large number of channels

are possible. Optical communication possible in the past due to the

following reasons.1. Non availability of carrier optical signal.

2. Non availability of a cable carrying optical signal.The optical communication technology is developing fast invention of optical fibers carrying optical signals, semiconductor like LED, diode laser etc arid detectors

like photocjiode. The main advantages

Transmitter

Optical source

Input signal

Modulation

Optical fibre cable

Receiver

Optical detector

Demodulation

Output signal

Optical Communication

Optical FibreOptical Fibre

- An optical fibre is a thin fibre of glass. Its diameter is about the same as- An optical fibre is a thin fibre of glass. Its diameter is about the same as that of a human hair ~ 10 to 100 that of a human hair ~ 10 to 100 m.m.

- Light can be guided in such a fibre by launching it at one end, using an - Light can be guided in such a fibre by launching it at one end, using an intense and focused light source, and allowing it bounce down to the intense and focused light source, and allowing it bounce down to the other end by a series of reflections ( other end by a series of reflections (total internal reflectionstotal internal reflections) from the) from the sides. sides. - An optical fibre essentially consists of an inner cylinder of glass known - An optical fibre essentially consists of an inner cylinder of glass known as the as the core, core, having a refractive index having a refractive index nn11, and an outer cylinder of a, and an outer cylinder of a different glass, called the different glass, called the cladding cladding havinghaving a a refractive index refractive index nn22, , nn = = nn11 nn22 ~ 10 ~ 10 33 . .

- For use in a telecommunications system, many fibres are usually - For use in a telecommunications system, many fibres are usually incorporated into a cable structure for pulling into underground ducts. incorporated into a cable structure for pulling into underground ducts.

The main The main advantages of optical advantages of optical communication arecommunication are

Wide bandwidth and a large number of Wide bandwidth and a large number of channels.channels.

Low transmission losses.Low transmission losses.

Cladding with refractive index,Cladding with refractive index,

Medium of refractive index, nMedium of refractive index, n

Core wiCore wi

refractive index, nirefractive index, ni Buffer coatingBuffer coating

OPTICAL FIBREOPTICAL FIBRE

Point to point communication of light signal Point to point communication of light signal can be realized by using optical fibers. For this can be realized by using optical fibers. For this a transparent medium like glass, polymer or a a transparent medium like glass, polymer or a dielectric material is required. dielectric material is required.

The principle of on optical fiber is based The principle of on optical fiber is based on . the phenomenon of multiple total on . the phenomenon of multiple total internal reflections. internal reflections. The optical fiber can be The optical fiber can be considered as a light pipe like a water pipe.considered as a light pipe like a water pipe.

The important parts of an optical fibre are The important parts of an optical fibre are 1) 1) core, core, 2) 2) cladding cladding

3)3)buffer.buffer.

1)1) CoreCore: : The core is the cylindrical central The core is the cylindrical central part of the optical fibre, made up of glass part of the optical fibre, made up of glass or polymer of refractive index, m. The or polymer of refractive index, m. The diameter of the core is 10-100diameter of the core is 10-100

2) Cladding2) Cladding: : The cladding is a covering on The cladding is a covering on the core, made of glass or plastic, the core, made of glass or plastic, refractive index, n2 where n2 refractive index, n2 where n2 < n\. < n\. The The difference of refractive indices m, very difference of refractive indices m, very small of the order of 0.001.small of the order of 0.001.

3) Buffer3) Buffer: : Buffer is a plastic coating made Buffer is a plastic coating made on the cladding for providing si and on the cladding for providing si and strength.strength.

Principle of an Principle of an optical fibreoptical fibre

The principle of an optical fibre is based on The principle of an optical fibre is based on the phenomenon of multiple total internal the phenomenon of multiple total internal reflections. The total internal reflection reflections. The total internal reflection at core-clad< boundary will take place at core-clad< boundary will take place when the light travels from a denser to a when the light travels from a denser to a rarer medium at an angle of incidence rarer medium at an angle of incidence greater than the critical angle. The dimci of greater than the critical angle. The dimci of the core is so small that the light entering the core is so small that the light entering into the core will have an incident angle into the core will have an incident angle always greater than the critical angle. The always greater than the critical angle. The multiple reflections of Ugt1 opposite multiple reflections of Ugt1 opposite boundaries will continue and the light passes boundaries will continue and the light passes through the core light pipe.through the core light pipe.

There are two types of optical fibres.There are two types of optical fibres.

1.1. Step index fibre:Step index fibre: In step index fibre, there is In step index fibre, there is

an abrupt cl refractive index at an abrupt cl refractive index at core-cladding interface.core-cladding interface.

2. 2. Graded index fibre:Graded index fibre: In Graded index fibre, the In Graded index fibre, the

refractive inde fibre decreases refractive inde fibre decreases radially outwards. There is no radially outwards. There is no abrupt change in n index.abrupt change in n index.

Advantages of fibre Advantages of fibre communicationcommunication

1. High bandwidth. Hence more 1. High bandwidth. Hence more information can be sentinformation can be sent

2. High signal security2. High signal security3. Size and weight are smaller 3. Size and weight are smaller

compared to metal conductorscompared to metal conductors4. Noise free4. Noise free5. Negligibly small transmission 5. Negligibly small transmission

lossloss

LASERLASER

Laser Laser is the abbreviated form foris the abbreviated form for light light amplification by stimulated emission amplification by stimulated emission of radiation.of radiation. Laser is a highly coherent Laser is a highly coherent light.light.

Principle Principle of production of of production of LaserLaserE3E3Short lived stateShort lived stateThe atoms in the ground energy state are The atoms in the ground energy state are

irradiated to higher energy state.irradiated to higher energy state.

Ground stateGround state This is achieved by This is achieved by optical optical pumping. pumping. The The

life spanlife span

the excited state is in the order of 10-8 s. the excited state is in the order of 10-8 s. Therefore the atoms jump down the Therefore the atoms jump down the metastable state (where life span is 10-3 metastable state (where life span is 10-3 s). s).

population inversion population inversion place place when when the number of atoms in the number of atoms in the metastable state is larger the the metastable state is larger the atoms in the ground state.atoms in the ground state.

When a photon of energy hv, equal to When a photon of energy hv, equal to the difference in the energy of the difference in the energy of metastable state and the ground state metastable state and the ground state is incident, all the atoms in is incident, all the atoms in metastable state come simultaneously metastable state come simultaneously to the ground state emitting photons. to the ground state emitting photons. This is called stimulated This is called stimulated emission. emission. All All the emitted photons are in phase with the emitted photons are in phase with the incident photon and the emitted the incident photon and the emitted beam of light is called laser.beam of light is called laser.

Different types of lasers are Ruby Different types of lasers are Ruby laser (694 nm), CO2 laser (10600 laser (694 nm), CO2 laser (10600 mnjft He - Ne laser (5430/633 mn) and mnjft He - Ne laser (5430/633 mn) and Argon laser (488/514 nm)Argon laser (488/514 nm)

ELEMENTARY PRINCIPLE OF LIGHT ELEMENTARY PRINCIPLE OF LIGHT MODULATIONMODULATION Light can be modulated with the information signal. Light can be modulated with the information signal.

This is achieved‘ by changing the intensity, frequency This is achieved‘ by changing the intensity, frequency or phase of the light-signal in accordant with the or phase of the light-signal in accordant with the intensity of the information signal.intensity of the information signal.

Light modulation can be done in two ways. Light modulation can be done in two ways. 1) Direct modulation 2) Indirect or External modulation ;1) Direct modulation 2) Indirect or External modulation ; The direct modulation can be achieved by modulating The direct modulation can be achieved by modulating

the direct curiw supplying the light source. This is the direct curiw supplying the light source. This is obtained by changing the laser biasing current above obtained by changing the laser biasing current above and below the threshold value to make the laser on and below the threshold value to make the laser on and^on. This causes a change in frequency. Thus and^on. This causes a change in frequency. Thus Direct modulation can be used to modulate the Direct modulation can be used to modulate the frequency of light wave.frequency of light wave.

An indirect modulator consists of a material like lithium An indirect modulator consists of a material like lithium (UNbO3) crystal whose refractive index can be controlled with (UNbO3) crystal whose refractive index can be controlled with the help applied Electric field. This control on the refractive the help applied Electric field. This control on the refractive index brings the con phase of the signal. When light passes index brings the con phase of the signal. When light passes through the applied electric suffers cumulative phase change. through the applied electric suffers cumulative phase change. The desired modulation can be ach| nrnnprlv hv switrhinor the The desired modulation can be ach| nrnnprlv hv switrhinor the electric field on and off. The externalelectric field on and off. The external