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- Transmission of information /message from one point to another
Communication enters our lives in many ways:
- Telephone – makes us talk to any person anywhere.
- Radio and television – entertain and educate.
- Communication signals as navigational aids – ships, aircrafts and
satellites .
- Weather forecasting – conditions measured by a multitude of sensors
are communicated to forecasters.
- Videophones, voicemail and satellite conferencing – enable seeing live
images instantly and communicate directly with people located far away.
- Digital data transmission and retrieval – has made realization of e-mail,
FAX and internet possible.
We communicate through speech. In modern communication systems,
the information is first converted into electrical signals and then sent
Two persons talking to each other constitute the simplest communication system.
The person who speaks is the source, the person listening is the receiver and the
intervening air is the communication link between them.
A communication system consists of three basic components:
- Transmitter (source)
- Communication channel (link – medium)
- Receiver
Nature/details of these components depend on:
1. Nature of the signal/ message to be communicated.
2. Distance which separates the source and the receiver.
- Direct talking is possible over short distances – sound waves attenuate
- Long distance communication requires the signal/message to be
converted into an electrical signal/a set of signals /electromagnetic waves.
- Long distance communication requires a link between the source and the
Out put
Transmits the message/signal over the communication channel.
Quite often the original signal is not suitable for transmission over the
communication channel to the receiver. It requires to be modified to a
form suitable for transmission.
Communication Channel:
Provides a link between the transmitter and the receiver. It can
be a transmission line (telephone and telegraphy), an optical fibre
(optical communication) or free space in which the signal is radiated in
the form of electromagnetic waves.
Reconstructs the original message/signal after propagation
through the communication channel.
- We communicate through a message or a signal.
- A transmitter transmits the message over the communication channel to
the receiver.
- As a rule, the message produced by the source is not suitable for
transmission over the communication channel. Accordingly, a suitable
transducer converts it into a time varying electrical signal called the
message signal.
A transmitter, in its simplest form, is a setup which boosts the power of
message signal and feeds it into the communication channel
Loud speaker
Basic constituents of a transmitter are:
1. Message signal
2. Modulation
3. Antenna
Message signal:
A single valued function of time that conveys the information.
Discrete or digital
Analog Signal
Is a continuous function of time, with the amplitude (instantaneous
value of the signal) being continuous.
In its simplest form, the transmitter has following problems:
1. Size of the antenna or aerial
For transmitting a signal we need an antenna. It should have a size comparable
to the wavelength of the electromagnetic wave representing the signal ( at least
/4) so that the time variation of the signal is properly sensed by the antenna.
For an electromagnetic wave of frequency 20 kHz, the wavelength  is 15 km.
Obviously such a long antenna is not possible. Therefore, direct transmission of
such a signal is not possible. If the frequency of the signal is 1MHz, the
corresponding wavelength is 300m and transmission of such a signal is
possible. Therefore, there is a need of translating the information contained in
the original low frequency signal into high or radio-frequencies before
2. Effective power radiated by an antenna
The power radiated from a linear antenna  l /  2
For a good transmission we need high power hence there is need for high
frequency transmission.
3. Mixing up of signals from different transmitters
Direct transmission of baseband signal leads to interference from
multiple transmitters. Thus multiple user friendly communication is
not possible. A possible solution is provided by employing
communication at high frequencies and then allotting a band of
frequencies to each user.
The above arguments suggest that there is a need for translating the
original signal ( low frequency) into a high frequency wave before
transmission such that the translated signal continues to possess the
information contained in the original signal. The high frequency wave
carrying the information is called the carrier wave. The process of
transformation is called Modulation.
Transformation of the signal into a form suitable for transmission
through a given communication channel
Simplest form of an analog signal is a sinusoidal signal having a single
g(t) = A sin t
Signals generated by different sources have their own characteristics
- amplitude, frequency or nature.
Nature – Simple single frequency or a complex superposition of several
frequency components
The signals associated with music or speech are complex; can be
considered as superposition of several sinusoidal signals of varying
amplitudes and frequencies.
The range over which the frequencies in a signal vary is called the
bandwidth (B) (the frequency range between the lowest and highest
frequency components). Bandwidth for audio signals is 20 Hz to 20 kHz.
Discrete Signals
Discrete signals are discontinuous in time; they are defined only at
discrete times.
In case of discrete signals the independent variable (time) takes only
discrete values which are usually uniformly spaced. Consequently,
discrete-time signals are described as sequences of samples whose
amplitudes may take a continuum of values.
When each sample of a discrete-time signal is quantized I.e. its
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)
and then coded, the resulting signal is referred to as a digital signal.
• Modulation
We have seen:
- Direct transmission of audio frequency signals is not possible –
associated wavelengths are large.
- Quite often, the message signals require a carrier and a
communication channel for transmission from source to the receiver.
- An efficient utilization of the communication channel requires a shift
of the range of base band frequencies to another frequency range
suitable for transmission.
A shift of the range of frequencies in a signal is accomplished by
employing a process called modulation. In this process the signal is
superimposed over the carrier wave. The frequency, fc, of the carrier
wave must be much higher than the highest frequency component of the
message signal.
The form of modulation depends on the specific characteristics of the
carrier wave, nature of the message signal/data and the communication
channel. The carrier wave may be (I) continuous or (ii) pulse. The signal
may be analog or digital.
Forms of Modulation
Amplitude Modulation
Analog Signals
Angle/Frequency Modulation
Pulse Modulation
Amplitude shift keying (ASK)
Digital Signals
Frequency shift keying (FSK)
Phase shift keying
Amplitude & Frequency Modulation
A sinusoidal wave conveys no information. To transmit information by the
usual sinusoidal waveform, the characteristics of the wave must be
varied in some manner. A sinusoidal carrier wave C(t) is defined by
C(t) = Ac cos ( c t + o )
The modulation of the carrier wave can be accomplished in two ways:
(1) The amplitude of the carrier wave is varied about a mean value,
linearly with the baseband signal m(t), the angular frequency c
remaining constant. This mode of modulation is termed as amplitude
(2) The phase angle  of the carrier wave is varied according to the
baseband signal, the amplitude of the carrier wave being kept
constant. This mode of modulation is termed as angle modulation.
There are two variations of angle modulation - phase modulation
(PM) and frequency modulation (FM).
Amplitude Modulation:
- Employed for commercial broadcasting of voice signals. Carrier
frequencies – 0.5 to 20 MHz.
- Broadcast noisy – noise signals created by atmospheric static or man
made electric discharges also get amplitude modulated.
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 Signal/Noise ratio, quality of broadcast very good.
FM Radio – 88 to 108 MHz
VHF TV – 47 to 230 MHz
UHF TV – 470 to 960 MHz
• A carrier wave of peak voltage 12V is used to transmit
a message signal. What should be the peak voltage of
the modulating signal in order to have a modulation
index of 75%?
• For an amplitude modulated wave , the maximum
amplitude is found to be 10V while the minimum
amplitude is found to be 2V. Determine the modulation
• Bandwidth refers the frequency range over which the portion of
the spectrum occupied by the signal.
• In a communication system, the message signal can be voice,
music, picture or computer data.
• Each of these has different ranges of frequencies. The type of
communication system needed for a given signal depends on the
band of frequencies which is considered essential for the
communication process.
• For speech signals, frequency range 300Hz to 3100Hz is
considered adequate. Therefore speech signal requires a
bandwidth of 2800Hz(3100Hz-300Hz) for commercial
telephonic communication.
• For speech signals, frequency range 300Hz to 3100Hz
is considered adequate. Therefore speech signal
requires a bandwidth of 2800Hz(3100Hz-300Hz) for
commercial telephonic communication.
• To transmit music an approximate bandwidth of
20khz is required.
• Video signals for transmission of pictures require
about 4.2MHz of bandwidth.
• A T V signal contains both voice and picture and is
allocated 6 MHz of bandwidth for transmission.
* Digital signals are in the form of rectangular waves
as shown in fig.
* This rectangular wave can be assumed to be the
superposition of sinusoidal waves of frequencies
nνo. This implies an infinite band width.
* For practical purposes, the contribution from
higher and lower frequencies can be neglected thus
limiting the bandwidth.
Bandwidth of Transmission medium
• Different types of media offer different bandwidths.
• Commonly used transmission media are wire, free space and
optic cable.
• Coaxial cable is widely used wire medium which offers a
bandwidth of 750MHz.
• An optical fibre can offer a transmission bandwidth in excess of
• Spectrum allocations are arrived at by an international
agreement. The international Telecommunication Union(ITU)
administers the present system of frequency allocations.
Communication through free space
Propagation of EM waves
• Ground wave propagation:
• A radio wave that can travel directly from one point to
another following the surface of the earth is called a
ground wave.
• Ground wave propagation is possible only when the
transmitting and receiving antenna are close to the
surface of the earth.
• When the em wave glides over the earth surface, its
electric field component induces charges hence
current on the earth’s surface.
• Ground wave gets attenuated due to diffraction effect.
• Ground wave propagation is possible at low frequencies, 500 kHz
– 1500 kHz. This region of the AM band is called medium wave
• Maximum range of ground wave propagation depends on two
factors: a) the freq. of the transmitted wave and b) the power of
the transmitter.
• ADVANTAGES: i) Given enough transmitting power, ground
waves can be used to communicate between any two locations on
the earth. (ii) Ground waves are relatively unaffected by changing
atmospheric conditions.
• Disadvantages:
• i) Ground waves require a relatively high transmitting power.
ii) Ground losses vary with surface material and composition.
iii) Large antennas are required for transmission.
Space wave propagation
• If a radio wave transmitted from antenna,
travelling in a straight line, directly reaches
the receiving antenna, it is called a space
• Its also called tropospheric or line of sight
• The range of space wave propagation is
limited by the line of sight distance and the
curvature of the earth.
Range of a transmitting antenna
• The Range of coverage of transmission from an antenna is
d = √2hR, where R= radius of the earth and h = height of the
• The range of space wave propagation between two towers on
earth’s surface is dM = dT + dR.
• A radio wave directed towards the sky and reflected by
the atmosphere towards the desired location of the earth
is called a sky wave.
• Radio waves freq. 2MHz – 20MHz can be reflected by
the ionosphere.
• This region of the AM wave band is called short wave
• The radio waves of different frequencies sent from earth
will be reflected back the appropriate layers of the
ionosphere. The critical frequency for reflection is, therefore,
given by
1/ 2
f c  9( N max )
The TV signals have a bandwidth of 4.7MHz. What is the number of
channels that can be accommodated in a bandwidth of 4700GHz.
(106 channels)
1) A transmitting antenna at the top of a tower has a height 32m and that of the
receiving antenna is 100m. What is the maximum distance between them
for satisfactory communication in LOS mode? Radius of earth 6400km.
2) The TV tower at a particular station has a height of 160m.
a) What is its coverage range?
b) How much population is covered by the transmission. If the average
density around the tower is 1200 /km2.
c) By how much should the height be increased to double its coverage
range? Given radius of the earth = 6400km.
( 45km, 76.37lakhs, 480m)