Noise, in communication systems, is any unwanted intervention of energy that tends to disrupt the proper reception and reproduction of transmitted signals. It may come from external sources, material imperfections and other miscellaneous factors that are always present. If left unchecked, noise can cause much damage, not only to the transmitted data but also to the equipment that are used in the communication systems. This is the reason why noise is definitely a basic subject any budding electronics engineer should understand.

There are different types of noise, some can be reduced with good design or fair material compromises while others can only be compensated with a good level of transmit power. However, these types can be generally classified as either one of the following: external, internal, miscellaneous or system-dependent.

External noise is any noise that happens to be outside the scope of the equipment used to transmit the signal. Atmospheric noise, which is actually caused by the unwanted energy surge from a lightning is one example of this type. Extraterrestrial noise, which consists of radiation from solar activities such as solar flares and collective radiation from distant stars, is another type of atmospheric noise. Industrial noise contributes as well.

Miscellaneous noise appears only during very special cases and is inherent in the design of the circuit itself. Burst noise, a low-frequency noise appearing in bipolar junction transistors and avalanche noise, large noise spikes in devices that utilize the avalanche zone, are types under miscellaneous noise.

System-dependent noise are inherent to the type of communication system implemented. For example, microwave systems experience intermodulation distortion, which is created whenever the complex modulating signal passes through any kind of non-linearity of phase or amplitude in the transmission facility.