Basic Electronics - Unit 3

Fundamentals of Communication systems

Communication is the basic process of exchange of information or transmission of
information from one point to another. Basically, the information is generated from the
thought process in the human mind and it is sent to the receiver in the form of speech, sign or
in any other understandable form.

Communication enters our daily life in many different ways – such as telephone, radio, TV,
computers…..Communication provides directions for ships, aircrafts, rockets and satellites in
space. A number of modern communication systems include mobile communication, point to
point communication, radio telemetry and so on. Communication need not directly involve
human beings always. For example communication between two or more computers, the
human decision/intervention is required only while giving commands or to monitor the
results.

Basic Blocks of Communication Systems:

Irrespective of the applications, basic blocks of electronic communication system is as shown
in figure 1. This involves transmitter, channel and receiver. The transmitter is located in one
point and the receiver is located somewhere else and the channel is the medium that connects
them.

Receiver
Information Transmitter Channel Destination
source

Noise

Figure 1: Block diagram of communication system

The elements of communication system are as follows:

 Information
 Transmitter
 Communication channel or medium
 Noise
 Receiver
Information: Information or message to be communicated are generated from information
sources. Examples for main sources of information are human brain or changes in the
physical environment. Information may be the speech signal, image, data music, video etc.
The amount of information is measured in bits.

Transmitter: The information generated from a source is not in a suitable form to be

transmitted directly through the channel. The transmitter is an electronic circuit designed to
convert the information into a signal suitable to be transmitted over a given communication
channel. Signal in its original form will be non-electrical in nature, such signal cannot be
transmitted through the channel. Using suitable transducers the messages are converted to the
electrical form and then they are processed and coded before the transmission. The signal
processing techniques are also applied at this stage to make the communication more
effective.

Transmitter includes – transducer, modulator, filters, encoder and also amplifiers to ensure
faithful signal transmission and reception.

Communication channel:

The communication channel is the medium through which the electronic signal is transmitted.
The channel connects the transmitter and receiver either through wire or without wire. In
general the communication channel can be classified as:

 Wired channel or line communication

 Wireless channel or radio channel

Wired channel: The examples for a wired medium are: copper wire, coaxial cable, fibre
optic cable. For applications like telephony two physical wires or conductors are connected
between the transmitter and the receiver. Latest applications use optical fibre cables, which
are well known for their high capacity and immunity to noise. In optical fibres the
information will be transmitted in the form of light wave. Coaxial cables are preferred over
pair of wires as they have higher bandwidth and lower losses. Optical fibres are logical
extension of coaxial cables which can be operated at higher frequencies and greater
bandwidths. Optical fibres are also cheaper and are more immune to interference and noise.

Wireless channel: most commonly known as radio channel or electromagnetic medium. This
type of channel connects the transmitter and receiver wirelessly. No physical wire is
necessary to carry the information, signal is sent through air or free space. Radio
communication requires two antennas for the transmission and reception. The signal received
at the receiving antenna will be attenuated and hence its amplitude will be smaller. With
proper amplification, the signal will be processed further to get back the original information.
Radio communication allows the signal to be transmitted to any longer distance – thousands
of kilometres, perhaps even more.

Noise: Noise is basically any unwanted signal that disturbs the communication. Noise
enters the communication system normally in the medium or channel. Noise may also be
generated at the transmitter or receiver. Means noise can be internally generated in a system
or externally added to the signal.

Noise can also be classified as natural noise or man-made noise. Natural noise includes –
lightening during rainy season, radiations from the sun and cosmic radiations. Man-made
noise is the noise generated by the electric ignition systems, industrial noise, fluorescent
lights etc.

Noise imposes serious problem on electronic communication systems, if not controlled.

Noise cannot be completely eliminated, but its effect can be reduced using different methods.

Receiver: The receiver is a collection of electronic circuits designed to convert the signal
back to its original form. The process includes amplification, mixing, demodulation and
decoding etc. The receiver performs the task of operating on the received signal to generate
the estimation of the original signal. If the estimated signal is same as the original transmitted
signal, then we say that the reception is proper.

Modulation:

The transmission of an information- bearing signal over a communication channel requires a

shift in the range of frequencies from the original to another frequency range suitable for
transmission. This is accomplished through the process of modulation. Modulation is defined
as the process by which some characteristic of a carrier signal is varied in accordance with a
modulating signal. The message signal is referred to as modulating signal and the result of
modulation is referred to as modulated signal. The carrier signal is usually a high frequency
signal than the message signal. The message signal modifies the amplitude, frequency or
phase of the carrier signal in the process of modulation. Hence, the types of modulation are:
  Amplitude modulation – amplitude of the carrier is varied in accordance with the
modulating signal, keeping the frequency and phase of the carrier constant
 Frequency modulation – the frequency of the carrier signal is varied according to the
modulating signal
 Phase modulation – phase of the carrier is varied in accordance with the modulating
signal

Need for modulation:

The message signal cannot be transmitted in its direct form, over the communication channel.
It should be modulated suitably. The advantages of the modulation are:

 Reduces the antenna height

 The range(distance) of communication can be increased
 Signals can be multiplexed
 Bandwidth can be used efficiently
 Quality of reception can be improved
 Avoids mixing of the signals

Height of the antenna: Minimum height of the antenna for proper transmission and reception
is λ/4, where λ is the wavelength. We know that, λ =c/f, where c is velocity of light and f is
the frequency. At lower frequencies wavelength is high and antenna height is also high.
Hence by modulating the message signal using a high frequency carrier, the height of the
antenna can be decreased.

Range of communication: Since message signal frequency range is low (20 Hz to 20 kHz),
the possibility of signal attenuation is more. Modulation increases the frequency and hence
the range of communication may be increased.

Multiplexing: modulation allows the multiplexing of signals. Multiplexing means two or

more message signals are transmitted simultaneously over the same channel. Eg: broadband
data services, TV channels operating simultaneously, radio stations in MW and SW band
simultaneously.
Bandwidth utilization: Bandwidth of the modulated signal can be made smaller or larger than
the original signal.

Quality of reception: by using specific types of modulation schemes such as FM, noise
performance of the receiver can be improved.

Avoids mixing: The message signal frequencies are located within the range from 20 Hz to
20 kHz. Simultaneous transmission of multiple message signals may lead to mixing of the
signals, which may cause interference in the reception. Instead, if each message signal is
modulated separately using different carrier frequencies, then each signal will be located at
different portions in the spectrum. The receiver tuned to particular carrier will be able to
receive the signal. Therefore modulation avoids mixing of signals.