# 4.3 Interference and noise

## Sources of interference

#### Most interference encountered in digital communications systems (except for deep space missions!) arises from either other communications systems or machinery. For example, crosstalk in telecommunications lines is classed as interference, as is the ignition noise generated by a car engine. |

| #### In radio systems in particular, a major source of interference is from other users of the radio spectrum. For example, equipment radiating on frequencies close to the wanted channel can pass through the receiver selection filtering, causing what is termed adjacent channel interference. In cellular applications, mobile users in different geographical locations are assigned the same frequency for their calls, and if they are not separated by sufficient distance, co-channel interference occurs. In both radio and television, multipath interference is common, manifest as ghosting on the television screen, caused by signals travelling by many different paths between transmitter and receiver each with slightly different time delay.

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