Elements of Communication System: Communication is the art of transmitting, processing and receiving information by electronic means. For the communication to be successful, the sender and the receiver should understand the common language. Human beings have constantly made endeavours to improve the quality of communication with other human beings. Methods and Languages used in communication have kept evolving from prehistoric to modern times to meet the growing demands in terms of complexity and speed of information.

The message signal can be a voice signal, computer data or picture. The signals have a different range of frequencies. The type of communication system needed for a particular signal depends on the band of frequencies considered essential for the communication process. This article investigates the basic elements of communication system, its types, and the bandwidth of a signal.

Basic Elements of Communication System

Any communication system has three important elements. They are the transmitter, channel, and receiver. The block diagram of a general form of communication system is as shown.

In a communication system, the source of information can be a human being or a machine. A transmitter is located at one place and the receiver at another with the channel as a medium. The signal from the source is called a message signal, and these signals are fed into the transmitter.

The main purpose of the transmitter is to convert the message signal produced by the source of information into a suitable form for transmission through the channel. These signals are called transmitted signals. If the message signal from the source of information is the nonelectrical signal is converted into an electrical signal by a transducer

The channel (wired or wireless) is the physical medium that connects the transmitter and receiver. When the transmitted signal propagates, it may be distorted due to its imperfection, and some noise is added to the transmitted signal.

Detailed Study of Basic Components of Communication System

Information: The idea or message that is to be conveyed is information. The message may be single or a set of messages. The message can be a symbol, code, group of words, or any pre-decided unit.

Transmitter: The transmitter consists of a transducer, modulator, amplifier, and transmitting antenna in radio transmission.

Note:

1. A transducer converts sound signals into electric signals.
2. The modulator performs the mixing of the electric audio signal with the high-frequency radio wave.
3. The amplifier boosts the power of the modulated signal.
4. The signal is radiated in space with the aid of an antenna.

Communication channel: The duty of the communication channel is to carry the modulated signal from transmitter to receiver. The communication channel is also called a transmission link or medium.

The term channel refers to the frequency range allocated to a particular transmission or service.

Table: Different channels

Type of communication	Channels or links
Radio communication (The basic elements of radio communication system are a transmitter and a receiver)	space
Telephony and Telegraphy communication	Transmission line
Optical communication	Optical fibre

Receiver: The receiver consists of:
Pickup antenna: To pick the signal
Demodulator: To separate the audio signal from the modulated signal
Amplifier: To boost up the weak audio signal
Transducer: converts back audio signal in the form of electrical pulses into sound waves.

Types of Communication System

Communication systems are classified according to the nature of information, mode of transmission, or types of the transmission channel.

According to the nature of the information source, there are three types of communication systems-
(i) Speech transmission
(ii) Picture transmission
(iii) Facsimile transmission (FAX): This involves the exact reproduction of a document or picture which are static.

According to the mode of transmission of information, there are two types of communication systems-
(i) Analogue communication: The communication system, which uses analogue signals, is called an analogue communication system.

Table: Few analogue communication systems

System	Specification
Telegraph	Code messages are sent.
Television broadcast	Both sounds, as well as pictures, are sent.
Telephony	It sends a voice signal from one place to another utilizing wire.
Radar	It is an acronym for ‘radio detection and ranging’. It is used for determining the distance and direction of objects using a microwave.
Tele-printing	Message can be typed and telegraphed to distant receivers

(ii) Digital communication: In this system, digital signals are used.

Table: Few digital communication systems

System	Specification
Facsimile transmission (FAX)	It involves the exact reproduction of a document or picture which are static.
Mobile phone	Such telephones are also called cellular phones because they operate within a network of radio cells.
E-mail	The message sent via a computer network is called e-mail.
Teleconferencing	It is a system in which persons are sitting at coloured television screens. They see and talk to each other via a computer communication network.
Communication satellite	It is used to relay radio and television programmers.
Global positioning system (GPS)	It is a navigation system based on a network of earth-orbiting satellites.

According to the transmission channel, there are two types of communication systems-
(i) Line communication          (ii) Space communication

According to the type of modulation, there are six types of communication systems-
(i) Amplitude modulation (AM)
(ii) Frequency modulation (FM)
(iii) Phase modulation (PM)
(iv) Pulse amplitude modulation (PAM)
(v) Pulse time modulation (PTM)
(vi) Pulse code modulation (PCM)

Analogue and Digital Signals

In the communication system, a signal means a time-varying electrical signal containing information.

Analogue signals: It is a continuous waveform that changes smoothly and continuously over time.
(i) Analogue signals can be easily generated from the information source by using a suitable transducer, e.g. pressure variations of sound waves can be converted into corresponding current or voltage pulses by a microphone(mic).
(ii) An analogue signal is represented by a sine wave.

(iii) The frequency of analogue signals(speech or music) varies in audible range i.e. \(20\;{\rm{Hz}}\) to \(20\;{\rm{kHz}}\).
(iv) Bandwidth is the range over which the frequencies of a signal vary.

(v) The term baseband designates the band of frequencies representing the signal supplied by the source of information.
(vi) A signal consisting of two or more waves of different frequencies is a complex analogue signal.

Digital signals: It is a discontinuous function of time. It has only two voltage levels, i.e. either low \((0)\) or high \((1)\).

Either \(0\) and \(1\) is known as bit. A group of bits is called a byte.

A byte comprising \(2\) bits can give on the four code combinations, i.e. \(00, 01, 10\) and \(11\).

The number of code combinations increases with the number of bits in a byte by \(N = {2^x}\), where \(x=\) the number of bits.

Bits or the number of binary digits per second is called its bit rate. It describes a digital signal and is expressed in bits per second (bps).

Modulation

Digital and analogue signals are of low frequency and hence cannot be transmitted to long distances. These signals require some carrier. These carriers are known as carrier waves or high-frequency signals. The placement of a low frequency (LF) signal over the high frequency (HF) signal is known as modulation.

Need for modulation: The sound wave (\(20\;{\rm{Hz}}\) to \(20\;{\rm{kHz}}\)) cannot be transmitted directly from one place to another because-

(i) Height of antenna: For efficient radiation and reception, the height of transmitting and receiving antennas should be comparable to a quarter of the wavelength of the frequency used. For \(15\;{\rm{kHz}}\), it is \(5000\;{\rm{m}}\) (too large), and for \(1\;{\rm{MHz}}\), it is \(75\;{\rm{m}}\).
The energy radiated from an antenna is practically zero when the signal frequency to be transmitted is below \(15\;{\rm{Hz}}\).

(ii) Detecting signals: All audible signals are in the range of \(20\;{\rm{Hz}}\) to \(20\;{\rm{kHz}}\), so the signals from all sources remain heavily mixed up in the air. It won’t be easy to differentiate or detect the broadcast signal at the receiving station.
Thus modulation is necessary for a low-frequency signal when it is sent to a distant place so that the information may not die out in the way itself and for the proper identification of a signal and keeping the height of the antenna small.

Amplitude Modulation (AM)

Amplitude modulation (AM) is the process in which the amplitude of the modulated wave is varied in accordance with the amplitude of modulating wave. In AM, the frequency of the carrier wave is unchanged.

Modulation index: It isthe ratio of change of amplitude of carrier wave to the amplitude of original carrier wave. It is also called the modulation factor or degree of modulation\(\left( {{m_a}} \right)\).
\({m_a} = \frac{{{\rm{ Changen\, amplitudef\, carrier\, wae }}}}{{{\rm{ Amplitudef\, originatarrier\, wae }}}} = \frac{{k{E_m}}}{{{E_c}}}\)

Where \(k=\) is a factor determining the maximum change in the amplitude for a given amplitude \({E_m}\) of the modulating signal. If \(k=1\) then \({m_a} = \frac{{{E_m}}}{{{E_c}}} = \frac{{{E_{\max }} – {E_{\min }}}}{{{E_{\max }} + {E_{\min }}}}\)

If several sine waves modulate a carrier wave, the total modulated index \({m_t}\) is given by \({m_t} = \sqrt {m_1^2 + m_2^2 + m_3^2 + \ldots \ldots } \)

Voltage equation for AM wave: Let us consider the voltage equations for carrier wave and modulating wave is- 

\({e_c} = {E_c}\cos {\omega _c}t\) and \({e_m} = {E_m}\sin {\omega _m}t = m{E_c}\sin {\omega _m}t\)

where \({e_c} = \) Instantaneous voltage of carrier wave, \({E_c} = \) Amplitude of carrier wave, \({\omega _C} = 2\pi {f_c} = \) Angular velocity at the carrier frequency \({f_c},{e_m} = \) Instantaneous voltage of modulating, \({E_m} = \) Amplitude of modulating wave, \({\omega _m} = 2\pi {f_m} = \) Angular velocity of modulating frequency \({f_m}\)

The voltage equation for AM wave is

\(e = E\sin {\omega _c}t = \left( {{E_c} + {e_m}} \right)\sin {\omega _c}t = \left( {{E_c} + {e_m}\sin {\omega _m}t} \right)\sin {\omega _c}t\)

\( = {E_c}\sin {\omega _c}t + \frac{{{m_a}{E_c}}}{2}\cos \left( {{\omega _c} – {\omega _m}} \right)t – \frac{{{m_a}{E_c}}}{2}\cos \left( {{\omega _c} + {\omega _m}} \right)t\)

The above AM wave indicates that the AM wave is equivalent to the summation of three sinusoidal waves, one having amplitude \({E_c}\) and the other two having amplitude \(\frac{{{m_a}{E_c}}}{2}\).

Sideband Frequencies and Bandwidth in AM Wave

Side-band frequencies: The AM wave contains three frequencies \({f_c},\left( {{f_c} + {f_m}} \right)\) and \(\left( {{f_c} – {f_m}} \right)\).

Here \({f_c}\)  is called carrier frequency, \(\left( {{f_c} + {f_m}} \right)\) and \(\left( {{f_c} – {f_m}} \right)\) are called sideband frequencies.

 \(\left( {{f_c} + {f_m}} \right)\): Upper sideband (USB) frequency

 \(\left( {{f_c} – {f_m}} \right)\): Lower sideband (LSB) frequency

Side-band frequencies are generally close to the carrier frequency.

Bandwidth: The two sidebands lie on either side of the carrier frequency at equal frequency intervals \({f_m}\).

So, bandwidth \( = \left( {{f_c} + {f_m}} \right) – \left( {{f_c} – {f_m}} \right) = 2{f_m}\)

Power in AM waves: Power dissipated in any circuit \(P = \frac{{V_{rms}^2}}{R}\). Hence

(i) Carrier power \({P_c} = \frac{{{{\left( {\frac{{{E_c}}}{{\sqrt 2 }}} \right)}^2}}}{R} = \frac{{E_c^2}}{{2R}}\)

(ii) Total power of sidebands \({P_{sb}} = \frac{{{{\left( {\frac{{{m_a}{E_c}}}{{2\sqrt 2 }}} \right)}^2}}}{R} + \frac{{\left( {\frac{{{m_a}{E_c}}}{{2\sqrt 2 }}} \right)}}{R} = \frac{{m_a^2E_c^2}}{{4R}}\)

(iii) Total power of AM wave \({P_{{\rm{Total }}}} = {P_C} + {P_{sb}} = \frac{{E_c^2}}{{2R}}\left( {1 + \frac{{m_a^2}}{2}} \right)\)

(iv) \(\frac{{{P_t}}}{{{P_c}}} = \left( {1 + \frac{{m_a^2}}{2}} \right)\) and \(\frac{{{P_{sb}}}}{{{P_t}}} = \frac{{m_a^2/2}}{{\left( {1 + \frac{{m_a^2}}{2}} \right)}}\)

(v) Maximum power in the AM (without distortion) will occur when \({m_a} = 1\) i.e. \({P_t} = 1.5P = 3{P_{sb}}\)

(vi) If \({I_{c = }}\) Unmodulated current and \({I_{t = }}\) total or modulated current \( \Rightarrow \frac{{{P_t}}}{{{P_c}}} = \frac{{I_t^2}}{{I_c^2}} \Rightarrow \frac{{{I_t}}}{{{I_c}}} = \sqrt {\left( {1 + \frac{{m_a^2}}{2}} \right)} \).

Limitation of Amplitude Modulation

(i) Noisy reception
(ii) Low efficiency
(iii) Small operating range
(iv) Poor audio quality

Frequency Modulation (FM)

It is the process of changing the frequency of a carrier wave in accordance with the audio frequency signal. It is known as frequency modulation.

The audio quality of AM transmission is poor. Amplitude-sensitive noise has to be eliminated by removing amplitude variation. (I.e. a need to keep the amplitude of the carrier constant).  In FM, the overall amplitude of the FM wave remains constant at all times. In FM, the total transmitted power remains constant.

Frequency deviation: Frequency deviation isthe maximum change in frequency from the mean value \({f_c}\). It is also the change or shift either above or below the frequency \({f_c}\) and called frequency deviation.

\(\therefore \quad \delta = \left( {{f_{\max }} – {f_c}} \right) = {f_c} – {f_{\min }} = {k_f} \cdot \frac{{{E_m}}}{{2\pi }}\)

\({k_f} = \) Constant of proportionality. It determines the maximum variation in the frequency of the modulated wave for a given modulating signal.

Carrier swing (CS): Carrier swing is the total variation in frequency from the lowest to the highest. i.e.

\(CS = 2 \times \Delta f\)

Frequency modulation index \(\left( {{m_f}} \right)\): The ratio of maximum frequency deviation to the modulating frequency is called modulation index.

\({m_f} = \frac{\delta }{{{f_m}}} = \frac{{{f_{\max }} – {f_c}}}{{{f_m}}} = \frac{{{f_c} – {f_{\min }}}}{{{f_m}}} = \frac{{{k_f}{E_m}}}{{{f_m}}}\)

Frequency spectrum: FM sideband modulated signal consist of an infinite number of sidebands whose frequencies are

\(\left( {{f_c} \pm {f_m}} \right),\left( {{f_c} \pm 2{f_m}} \right),\left( {{f_c} \pm 3{f_m}} \right) \ldots \ldots \)

The number of sidebands depends on the modulation index \({m_f}\).

In the FM signal, the information (audio signal) is contained in the sidebands. Since the sidebands are separated from each other by the frequency of modulating signal \({f_m}\) so
Bandwidth \( = 2n \times {f_m}\); where \(n=\) number of significant sideband pairs

Deviation ratio: The ratio of the maximum permitted frequency deviation to the maximum permitted audio frequency is the deviation ratio. Thus, the deviation ratio \( = \frac{{{{(\Delta f)}{\max }}}}{{{{\left( {{f_m}} \right)}{\max }}}}\).

Percentage modulation: It is the ratio of actual frequency deviation to the maximum allowed frequency deviation. Thus, percent modulation, \(m = \frac{{{{(\Delta f)}{{\rm{actual }}}}}}{{{{(\Delta f)}{\max }}}}\)

Table: Range of frequency allotted for FM radio/TV broadcast

Type of broadcast	Frequency band
FM radio	\(88\) to \(108\,{\rm{MHz}}\)
UHF TV	\(47\) to \(230\,{\rm{MHz}}\)
UHF TV	\(470\) to \(960\,{\rm{MHz}}\)

Pulse Modulation

Pulse Modulation(PM) is a process in which the carrier wave is in the form of pulses.

(1) Pulse amplitude modulation (PAM): The pulse amplitude varies in accordance to the modulating signal(message signal).

(2) Pulse width modulation (PWM): The pulse duration varies in accordance with the modulating signal.

(3) Pulse position modulation (PPM): In PPM, the position of the pulses of the carrier wave train varies according to the instantaneous value of the modulating signal.

Demodulation

Demodulation is the process of extracting the audio signal(message signal) from the modulated wave(modulated signal).

The wireless signals consist of radiofrequency (high frequency) carrier waves modulated by audio baseband frequency(low frequency). The diaphragm membrane of a telephone receiver or a loudspeaker cannot vibrate with high frequency. Therefore, it is necessary to separate the audio frequencies from the radio frequency carrier wave.

Simple demodulator circuit: A diode can detect or demodulate an amplitude modulated (AM) wave. A diode acts as a rectifier, i.e. it reduces the modulated carrier wave into a positive envelope only.

The AM wave input is shown in the figure. It appears at the output of the diode across PQ as a rectified wave (since a diode conducts only in the positive half cycle). After passing through the RC network, this rectified wave does not contain the radio frequency carrier component. Instead, it has only the envelope of the modulated wave.

In the actual circuit, the value of \(RC\) is chosen such that \(\frac{1}{{{f_c}}} < RC\); where \({f_{c = }}\) carrier signal frequency.

Summary

1. The modulation factor determines the strength and quality of the transmitted signal.
2. A straight conductor of the length \(l = \frac{\lambda }{2}\) of radio signals transmitted or received is called a Hertz antenna. The Marconi antenna is a straight conductor of length \(l = \frac{\lambda }{4}\).
3. In a digital signal, information is carried by the pattern of pulses and not by the shape of pulses.
4. The three important elements in a communication system are Transmitter, Channel, and Receiver.
5. The transmitter converts the message signal produced by the source of information into a suitable form for transmission.

FAQs on Basic Elements of Communication System

Q.1. What are the essential elements of a communication system?
Ans: The basic components of a communication system are information source, input transducer, transmitter, communication channel, receiver, and destination.

Q.2. What are the parts of a communication system?
Ans: Primarily communication system consists of three main parts:
(i) transmitter,
(ii) communication channel
(iii) receiver
The transmitter and the receiver are erected at different places. At the same time, the channel is the physical medium that connects them.

Q.3. What is the function of a repeater used in a communication system?
Ans: The function of a repeater in a communication system is to extend the range of communication.

Q.4. What is the function of a transducer used in a communication system?
Ans: Any device that converts one form of energy into another is called a transducer.

Q.5. How does the effective power radiate from a linear antenna depend on the signal’s wavelength?
Ans: Effective power radiated decreases with an increase in wavelength, i.e. \(P \propto \frac{1}{{{\lambda ^2}}}\).

Q.6. Give one example each for point-to-point communication mode and broadcast mode of communication.
Ans: Telephone is an example for point-to-point communication, and television is an example for the broadcast mode of communication.