Noise

Noise is an undesirable electric energy that enters the communication system and interferes with the desired signal. It is always and random and unpredictable in nature.

External Noise:

1)Atmospheric Noise: also called as static noise is mainly due to electric disturbances such as lightening.

Atmospheric noise effects the reception of frequencies less than 30MHz and less effected for the frequencies above 30MHz.

2)Extra-terrestrial Noise: 

a) Solar Noise: Primary source of solar noise is Sun. Sun radiates a wide range of signals in a noise spectrum which includes the frequencies we use for communication.

b) Cosmic Noise: Cosmic noise generated by stars is not great because of the great distance between the stars and earth.

Internal Noise:

Electric components such as resistors, diodes, transistors etc provide noise internally in the circuit known as Internal Noise.

a) Shot Noise: Shot noise occurs in electronic devices because of discrete nature of current flow in the device.

Example:

In Vacuum tubes noise is generated due to random emission of electrons from cathode.

In semiconductor devices noise is generated due to random diffusion of electrons or random combination of electrons with holes.

In photodiode noise is generated due to random emission of photons.

b) Thermal Noise: The random movement of electrons inside the conductor resulting in randomly varying voltage across the conductor is known as Thermal noise also known as Johnson Noise.

In order to avoid noise in communication system we need to use Noise Suppression Filter.

In the above fig we can see that our message signal or information after going through channel and mixing up with up with some random noise is not exactly the same as information that is there is a loss in our information due to the noise. So, we need to use Noise Suppression filter in order to avoid loss in our information.

This Noise filter is added in between channel and receiver section as shown in the fig above.

What is Range of Communication System?

The largest distance over which the signal can be transmitted from source to destination with sufficient signal strength is called Range of Communication.

For example, we can see in the fig above that our signals strength or its range of communication is till the first station only. But our need is to send the signal to the station which is mounted on the mountain shown in picture. So, we need to send the signal without any signal loss to the destination but its range of communication is very less. In order to increase the range of communication 

Repeaters are used. Repeater is a device that receives a signal and retransmits it. It is a combination of both Transmitter, Receiver and Amplifier.