Noise is an unwanted form of energy or sounds which likely to interfere with the proper reception and reproduction of transmitted signals. There are some examples of noise such as many electrical disturbances produce noise and thus modifying the required signal in an unwanted form.

Types of noise – There are two different types of noise

• External Noise
• Internal Noise

External Noise – It is that type of noise which is created external to a communication system. It cannot be examined quantitatively.  This type of noise cannot be controlled. It is also divided into three types

1. Industrial Noise – Industrial noise is a man-made noise. It is produced by several sources such as electrical motors, automobiles and aircraft ignition, switch gears, and some other heavy electrical equipment.
2. Atmospheric Noise – It is also called as static noise. Atmospheric noise is generated by lightning discharges in thunderstorms and other electrical disturbances occur in the atmosphere.
3. Extraterrestrial Noise – Extraterrestrial noise is also known as space noise. There are two types of extraterrestrial noise
   • Solar Noise
   • Cosmic Noise

Internal Noise – This type of noise is generated internally and also within the communication system or receiver. Internal noise may be treated as quantitatively which can be decreased by proper system design. There are several types of internal noise

1. Shot Noise – This type of noise arises in active devices due to random behavior of charge carriers. Shot noise is generated due to the random emission of electrons from cathodes in electron tubes and in semiconductor devices shot noise is produced due to the random diffusion of minority carries or simply random generation and recombination of electron-hole pairs.
2. Thermal noise – It is also known as White noise or Johnson noise. Thermal noise is the random noise which is produced in a resistor or the resistive component of complex impedance. This is because the random and rapid motion of the atoms, electrons and molecules.
3. Partition noise – This type of noise is generated in a circuit when a current has to divide between two or more paths. This means that random fluctuations in the division are the reason for the generation of noise.
4. Low-frequency or flicker noise – It is also called as flicker noise. Flicker noise is generated or observed in low frequencies below few kHz. When the frequency decreases the power spectral density of this noise increases. Due to this it is also called as low-frequency noise.
5. High-frequency or transit-time noise – This type of noise is generally observed in semiconductor devices when the transit-time of charge crossing a junction is comparable with the time-period of the signal. It is also known as transit-time noise. When the frequency increases, the power spectral density also increases. That’s why it is also called high-frequency noise.