Field Guide · term

Also known as: occupied bandwidth, OBW, 99% bandwidth

Occupied bandwidth (OBW) is the width of the frequency band that contains a specified fraction of a signal’s total transmitted power — by long-standing ITU convention, 99%, with 0.5% left over above and 0.5% below.1 Unlike the vague “bandwidth” of a signal, OBW is a precise, measurable number, which is what makes it useful for channel planning, guard-band sizing, and the emission designators regulators assign to every licensed transmission.

0.5% 0.5% occupied bandwidth (99%) 99% of P
Occupied bandwidth is the span holding 99% of the total power, with 0.5% of the power excluded on each edge of the spectrum.

How it works

To find OBW you integrate the signal’s power spectral density across frequency to get the total power, then find the two frequencies below which 0.5% and above which 0.5% of that total lies. The distance between them is the occupied bandwidth — the band that, by definition, carries the central 99%. Because the measure is defined on the actual power distribution, it captures the real spectral footprint including modulation sidebands and pulse-shaping skirts, not just an idealized main lobe.

A closely related regulatory term is necessary bandwidth: the minimum width sufficient to carry the information at the required rate and quality for a given emission class. OBW is what a transmitter actually occupies; necessary bandwidth is what the class of emission theoretically needs. Both feed the ITU emission designator — a code such as 16K0F3E (16.0 kHz, F = frequency modulation, 3 = analog telephony, E = telephony) whose leading field is the bandwidth. That designator is how a license precisely specifies the width and nature of an emission.

In practice

Occupied bandwidth is the number that sets channel spacing. A regulator chooses spacing so that each emission’s OBW fits inside its channel with a guard band to spare, and a spectrum mask bounds how far the skirts may extend beyond OBW before they count as spurious emissions. Tightening OBW — through sharper filtering, a smaller roll-off factor, or more efficient modulation — is what lets a band be re-planned at narrower spacing (the VHF/UHF move from 25 kHz to 12.5 kHz “narrowbanding” is exactly this), improving spectral efficiency.

Relevance to SDR

For an SDR, a signal’s occupied bandwidth dictates how much bandwidth the receive chain must pass and how wide the channel filter should be: too narrow clips the modulation sidebands and degrades the demodulator; too wide admits extra noise and adjacent-channel energy. Measuring OBW off a spectrogram or PSD estimate is a quick way to identify an unknown emission and to set the right decimation and filter width before decoding.

GopherTrunk sizes its per-channel filters and decimation from each protocol’s known channel rate and occupied bandwidth — for example, the ~12.5 kHz footprint of a P25 C4FM channel — so that the channelizer passes the full modulation without dragging in neighbours. The concept is therefore directly baked into how GopherTrunk carves channels out of a wideband capture.

Sources

  1. Bandwidth (signal processing) — Wikipedia, the 99%-power occupied-bandwidth convention. ↩

See also