## Sources of noise

#### Unlike interference, noise originates predominantly from within the communications link itself and is usually totally random in nature, making it very difficult to deal with. There is a variety of mechanisms by which noise is generated, the most commonly referenced forms being thermal noise, shot noise, flicker noise and atmospheric noise. |

#### Thermal noise often dominates in communicationssystems and originates from the free movement of electrons within a conductor. The name arises because the energy and hence degree of movement of electrons increases proportionally with the temperature of the conductor. The current and hence voltage generated by this movement has a waveform that is entirely random in nature and which will, over time, have an average power spectrum that is flat over all frequencies. This property of thermal noise to contain all frequencies has resulted in it being called white noise to mirror the property of white light to contain all colours. A good text on the subject of noise in digital communications systems is Schwartz (1990).

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