Principle of Communication

The principles of communication are essential concepts that facilitate effective information exchange among individuals, groups, and systems. In our interconnected world, grasping these principles is vital for everyday interactions and specialized fields such as telecommunications and information technology. Understanding types of signals (analog and digital), propagation methods (line-of-sight, ground wave, and sky wave), and modulation techniques (AM, FM, and PM) is crucial for effective information transmission in technical contexts.

1.0Definition Communication System

• It is the act of processing, sending and receiving information of signals from one place to another by some means.
• It is the transmission of information from one place to another.
• Examples of communication system, cable communication, optic fibre communication, ground wave communication, sky wave communication, satellite communication
• Some important forms of communication are Telephony, Fax, E-mail, Mobile

2.0Elements of Communication System

For long distance communication the following elements are essential:

1. Transmitter: The information in the form of speech has to be converted into a corresponding electrical signal. Any device which converts information into electrical signals and vice-versa is known as transducer. The electrical signals corresponding to the information say speech are processed, modulated, amplified and fed to the link (communication channel) or radiated through antenna.
2. Transmission Channel: The transmitter sends the signal to the receiver through the communication channel, it can be transmission lines, wires, cables, optic fibers, or air.
3. Receiver: The receiver extracts the original message or information and sends it to the user or destination. A receiver amplifies, demodulates and filters the received signals. For proper understanding of the message or information, it is necessary that the transmitter and receiver use common language.

3.0Modes of Communication 

1. Point to Point Communication: There is one transmitter and one receiver. The information is transmitted by a transmitter through a medium or link and received by another person through a receiver. Example-Telephone
2. Broadcast Communication: There is one transmitter and many receivers, messages transmitted by a transmitter are received by many receivers. Example-Radio Broadcasting and Television Telecast

4.0Types of Signal

1. Analog Signal: It is an electrical Waveform continuously changing in magnitude and time. Example-Sine wave
2. Digital signal: It is a discontinuous and discrete signal having only binary variations 0 and 1 with time, where 1 represents the closed circuit voltage and 0 represents the open circuit voltage.

Note: 0 and 1 are called Bit, Bits are used to form codes called Binary Codes

5.0Bandwidth of Signals

The wideness of the frequency spectrum of a signal is known as bandwidth(BW).

$BW=v_{max}-v_{min}$

Bandwidth of  Transmission Medium

1. coaxial cable=750 MHz
2. Optic Fibre= $10^{11}$Hz
3. Free Space=580 kHz to 6.5 GHz
4. Voice Frequency Signals=3000 Hz
5. Music Signals=20 kHz

6.0Propagation Of Electromagnetic Waves

Electromagnetic waves are used in radio, television and other communication systems; these waves can be sent from one part of earth to the other part with the help of a transmitter and receiver. Depending upon the frequency, radio waves and microwaves may travel from transmitting to receiving antenna through the following ways:

1. Ground Wave Propagation

• In ground wave propagation ,radio waves travel along the surface of earth. In this type of propagation a large portion of wave energy is in the space just above the surface of earth but the propagation of wave is guided along the earth’s surface and follows the curvature of earth.
• Energy of radio waves decreases as they travel over the surface of earth due to the conductivity and permittivity of earth’s surface.
• The decrease in the value of energy is called attenuation, attenuation increases with increase in the frequency of radio waves.
• Ground wave is limited to 1.5MHz. It is used for transmitting signals through small distances.

2. Sky Wave Propagation

• The radio waves which are reflected back to the earth by ionosphere are called sky waves. In this mode radio waves transmitted by a transmitting antenna and directed towards the ionosphere, these radio waves reflected back to the earth by ionosphere. Preferable range is 3MHz to 40MHz
• Radio waves having frequency greater than 40MHz penetrate the ionosphere and are not reflected back by the ionosphere, these waves are not propagated through this mode.
• Sky wave propagation is useful for very long distance radio communications.
• Critical Frequency: It is the peak frequency that is reverted to the earth by the evaluated layer of the ionosphere after having been sent straight to it.$f_c=9(N_{max}{)}^{\frac{1}{2}}$, where $N_{max}$ is the maximum electron density of the ionosphere, above this frequency wave will penetrate the ionosphere and is not reflected by it.

3. Space Wave Propagation(Line of Sight (LOS) Propagation)

• High frequency waves above 40 MHz called space waves can be transmitted from transmitting to receiving antenna and the mode of traveling of these waves through space.
• High frequency waves can be transmitted if there is no obstruction like curvature of earth or mountains or huge buildings in between transmitting and receiving antenna.
• Propagation of very high frequency(VHF) and ultra high frequency (UHF) is not possible through ground and sky waves .
• Transmission of space wave is not ordinarily possible beyond the radio horizon. Radio Horizon d for space wave is determined by the height of antenna $d=\sqrt{2Rh}$ , R = radius of Earth 

7.0Modulation and Its Types

Modulation:

• The process of mounting a very low frequency signal over a high frequency signal.
• Low frequency signal require some carrier for their transmission, the carrier used to carry low frequency signal are called carrier wave or high frequency signal, carrier wave is represented as, $e_c=E_{c}Sin({\omega }_{c}t+\Phi )$
• When an audio wave or high frequency signal is superimposed then either amplitude, frequency and phase of the carrier wave changes.

Types of Modulation

1. Amplitude Modulation:

• The process of changing the amplitude of a carrier wave in conformance with the amplitude of a very low frequency signal.
• When an audio frequency signal or modulating wave is mounted over the carrier wave,we get amplitude modulated waves.
• Representation of amplitude modulated wave

$e=\left(E_c+e_m\right)\mathrm{Sin}{\omega }_{c}t=\left(E_c+E_m\mathrm{Sin}{\omega }_{m}t\right)\mathrm{Sin}{\omega }_{c}t$

$e=\left(E_c+e_m\right)\mathrm{Sin}{\omega }_{c}t=E_c\left(1+\frac{E_m}{E_c}\mathrm{Sin}{\omega }_{m}t\right)\mathrm{Sin}{\omega }_{c}t$

$\frac{E_m}{E_c}=m_{a^{\prime }},\text{ modulation index }$

$e=E_c\left(1+m_a\mathrm{Sin}{\omega }_{m}t\right)\mathrm{Sin}{\omega }_{c}t$

$e=E_c\mathrm{Sin}{\omega }_{c}t+m_{a}E{}_c\mathrm{Sin}{\omega }_{m}t\mathrm{Sin}{\omega }_{c}t$

$e=E_c\mathrm{Sin}{\omega }_{c}t+\frac{m_a{E}_c}{2}\mathrm{Cos}\left({\omega }_c-{\omega }_m\right)t-\frac{m_a{E}_c}{2}\mathrm{Cos}\left({\omega }_c+{\omega }_m\right)t$

Amplitude Modulation Bandwidth- The bandwidth of an amplitude modulated signal is two times the frequency of the modulating signal.

$\text{ Bandwidth }=\left(v_c+v_m\right)-\left(v_c-v_m\right)=2v_m$

2. Frequency Modulation

• When the frequency of the carrier wave is altered in accordance with the intensity of the modulating signal.

$ModulationFactor=\frac{max.frequencydeviation}{ModulatingFrequency}=\frac{\Delta f}{f_m}$

8.0Sample Questions on Principle of Communication

Q-1. What is the maximum distance at which TV transmissions can be received from a tower that is 300 meters tall?

Solution:

$d=\sqrt{2Rh}=\sqrt{2\times 6400\times 1000\times 300}=62\mathrm{km}$

Q-2. If the combined height of the transmitting and receiving antennas in a line-of-sight communication system is fixed at h, demonstrate that the range is maximized when both antennas are set at a height of $\frac{h}{2}$ each.

Solution:

$d_1=\sqrt{2R{h}_1}\text{ and }{d}_2=\sqrt{2R{h}_2}$

For maximum range,

$d_m=\sqrt{2R{h}_1}+\sqrt{2R{h}_2}$

$d_m=d_1+d_2=d$

$h_1+h_2=x$

$h_1=xand{h}_2=h-x$

$d_m=\sqrt{2Rx}+\sqrt{2R(h-x)}$

$\frac{d}{dx}\left(d_m\right)=\sqrt{\frac{R}{2x}}-\sqrt{\frac{R}{2(h-x)}}=0$

$\frac{1}{2x}=\frac{1}{2(h-x)}\Rightarrow x=\frac{h}{2}$

$\Rightarrow h_1=h_2=\frac{h}{2}$