Communication Systems: The process of transmission and reception of information over extended distances.
Elements of a communication system are the set up used in the transmission and reception of information from one place to another.
Every communication system has three essential elements:
- – Transmitter
- – Medium/channel
- – Receiver
In a communication system, the transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter and the channel is the physical medium that connects them.
There are two basic modes of communication
– Point-to-point: In point-to-point communication mode, communication takes place over a link between a single transmitter and a receiver e.g. telephony.
– Broadcast: In the broadcast mode, there are a large number of receivers corresponding to a single transmitter. e.g. radio and television.
The terminology used in the electronic communication system
Transducer: Any device that converts one form of energy into another is known as a transducer.
Signal: Information converted in electrical form and suitable for transmission is known as a signal. Signals can be either analog or digital.
Noise: It refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system.
Transmitter: A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception.
Receiver: A receiver extracts the desired message signals from the received signals at the channel output.
Attenuation: The loss of strength of a signal while propagating through a medium is known as attenuation.
Amplification: It is the process of increasing the strength of a signal using some suitable electronic circuit.
Range: It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.
Bandwidth: It refers to the frequency range over which equipment operates or the portion of the spectrum occupied by the signal.
Modulation: It is the phenomenon of superimposing the low-frequency message signal (called the modulating signal) on a high-frequency wave (called the carrier wave).
Demodulation: The process of retrieval of information from the carrier wave at the receiver is known as demodulation. Demodulation is the reverse process of modulation.
Repeater: A repeater is the combination of a receiver and a transmitter. It increases the strength of the signal to increase the range of communication.
Bandwidth of signals
For speech signals, frequency range is from 300 Hz to 3100 Hz. Thus bandwidth of speech signals is 2800 Hz (3100 Hz – 300 Hz).
The audible range of frequencies is from 20 Hz to 20 kHz. As the frequencies produced by musical instruments are high, therefore, approximate bandwidth for music is 20 kHz.
For transmission of pictures, video signals require a bandwidth of 4.2 MHz.
The bandwidth of transmission medium
The commonly used transmission media are wire, free space, and optical fiber cable.
Coaxial cable is a widely used wire medium. It offers a bandwidth of approximately 750 MHz. These cables are normally operated below 18 GHz.
Communication through free space using radio waves takes place over a very wide range of frequencies ranging from a few hundreds of kHz to a few GHz.
Optical fiber communication is used in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.
The gaseous envelope which surrounds the earth is known as earth’s atmosphere.
The earth’s atmosphere mainly consists of nitrogen 78%, oxygen 21% along with a little portion of argon, carbon dioxide, water vapor, hydrocarbons, sulfur compounds, and dust particles.
The density of the atmospheric air goes on decreasing as we go up.
It extends from 65 km to 400 km. In this region, the temperature rises to some extent with height, hence it is called the thermosphere. The ionosphere which is composed of ionized matter
(i.e., electrons and positive ions) plays an important role in space communication. The ionosphere is subdivided into four main layers as D, E, F1, and F2 as shown in the table.
Propagation of electromagnetic waves in the atmosphere
The radio waves emitted from a transmitter antenna can reach the receiver antenna by any of the following modes of propagation, depending on the factors like, frequency of operation of radio waves, distance between transmitter and receiver antennas etc. These are:
- – Ground wave propagation
- – Skywave propagation
- – Space wave propagation
Ground wave propagation
When the radio waves from the transmitting antenna propagate along the surface of the earth so as to reach the receiving antenna, the wave propagation is known as ground wave or surface wave propagation.
The radio waves which progress along the surface of the earth are known as ground waves or surface waves. These waves are vertically polarised in order to prevent short-circuiting of the electric component. These waves induce currents in the ground as they propagate due to which some energy is lost by absorption.
Apart from it, as a ground wave or surface wave propagates over the surface of the earth, the wavefront of the wave gradually tilts over the surface of the earth as shown in the figure and the tilt of the wavefront of the wave increases as the wave propagates over the earth.
As a result of which, the strength of the wave decreases with the propagation of waves along the surface of the earth. It is due to this tilt that, the propagation of the ground wave is limited.
The ground wave propagation is suitable for low and medium frequency, i.e., up to 2 MHz only, hence it is also known as medium wave propagation. The maximum range of ground or surface wave propagation depends on:
- – Frequency of the radio waves
- – Power of the transmitter
When the radio waves from the transmitting antenna reach the receiving antenna after reflection in the ionosphere, the wave propagation is known as skywave propagation. The skywave propagation is also known as ionosphere propagation.
The sky waves are the radio waves of frequency between 2 MHz to 30 MHz.
These radio waves can propagate through the atmosphere and are reflected back by the ionosphere of the earth’s atmosphere.
With the help of skywave propagation, communication over a very long distance around the globe is possible.
Critical frequency: It is that highest frequency of radio wave, which when sent straight (i.e. normally) towards the layer of ionosphere gets reflected from the ionosphere and returns to the earth. If the frequency of the radio wave is more than critical frequency, it will not be reflected by the ionosphere.
– The critical frequency of a sky wave for reflection from a layer of atmosphere is given by
Space wave propagation
When the space waves from the transmitting antenna reach the receiving antenna either directly or after reflection from the ground in the earth troposphere’s region, the wave propagation is known as space wave propagation. Space wave propagation is also known as tropospheric propagation and line of sight propagation.
Modulation and Its Necessity
Low frequencies cannot be transmitted to long distances as such. Therefore, they are superimposed on a high-frequency carrier signal by a process known as modulation.
Need for modulation
– Size of the antenna
– Effective power radiated by an antenna
– Mixing up of baseband signals from different transmitters
Types of modulation
In modulation, some characteristic of the carrier signal like amplitude, frequency or phase varies in accordance with the modulating or message signal giving rise to following three types of modulation:
– Amplitude modulation
– Frequency modulation
– Phase modulation
Pulse modulation: The carrier wave is in the form of the pulses. The pulse modulation can be classified as:
– Pulse amplitude modulation (PAM)
– Pulse duration modulation (PDM) or pulse width modulation (PWM)
– Pulse position modulation (PPM).
The Internet: It is a system with billions of users worldwide. Its applications include:
(ii) File transfer
(iii) World Wide Web (WWW)
The modem is a modulator and a demodulator.
FAX or facsimile means the exact reproduction of the document like a picture, letter, map, etc. at a distant place.
The concept of mobile telephony was developed first in the 1970’s and it was fully implemented in the following decade. The central concept of this system is to divide the service area into a suitable number of cells centered on an office called MTSO (Mobile Telephone Switching Office). Mobile telephones operate typically in the UHF range of frequencies. (about 800-950 MHz).
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