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In radio, narrowband describes a channel in which 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.
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.
Two-way radio narrowband 
Two-Way Radio Narrowbanding refers to a U.S. Federal Communications Commission (FCC) Order issued in December 2004 requiring all CFR 47 Part 90 VHF (150-174 MHz) and UHF (421-470 MHz) PLMR (Private Land Mobile Radio) licensees operating legacy wideband (25 kHz bandwidth) voice or data/SCADA systems to migrate to narrowband (12.5 kHz bandwidth or equivalent) systems by January 1, 2013.