Narrowband Radio Modem

In  RF Technology, narrowband describes a RF channel in which the bandwidthof the data do not significantly exceed the channel's coherence bandwidth. Coherence bandwidth is a statistical measurement of the range of frequencies over which the channel can be considered "flat", or in other words the approximate maximum bandwidth or frequency interval over which two frequencies of a signal are likely to experience comparable or correlated amplitude fading.

Narrow band is a small-defined band of frequency to transfer the information signal. In India licensed frequency, bands like VHF 136- 174 MHz & UHF 470- 520 MHz are commonly called as Narrow frequency band.Allowed bandwidths are in the range of KHz (25/12.5/6.25KHz) and so the obvious result is less speed of communication. But Lower frequencies make them useful for Long distance applications without Line of Sight.

Hence in Industrial communication requirements, Narrowband is termed as Non Line Of Sight technology with use of lower frequencies.

The opposite of narrowband is wideband/ Broadband

As per 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.