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'Narrowband' refers to a situation in radio communications where the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.
The opposite of narrowband is wideband.
In the study of wireless channels, ''narrowband'' implies that the channel under consideration is sufficiently narrow that its frequency response can be considered flat. The message bandwidth will therefore be less than the coherence bandwidth of the channel. This is usually used as an idealizing assumption; no channel has perfectly flat fading, but the analysis of many aspects of wireless systems is greatly simplified if flat fading can be assumed.
Narrowband can also be used with the audio spectrum to describe sounds which occupy a narrow range of frequencies. In telephony narrowband is usually considered to cover frequencies 300–3400 Hz.