Licensing of 6.25 kHz

The communications radio spectrum contains frequencies ranging from 30 kHz to more than 300 GHz. These frequency ranges are often grouped into bands. IDAS operates in two bands - VHF (very high frequency) and UHF (ultra high frequency).

In the US, the spectrum is regulated by the FCC for non-Federal spectrum and by National Telecommunications and Information Administration (NTIA) for Federal use. VHF and UHF operation requires licensing for radios with a power output of greater than 10mW. Within these bands the FCC grants licenses for slices of frequencies called channels. When the FCC licenses a channel, it authorizes a center frequency (or carrier frequency) and a maximum excursion from that frequency – meaning the spectrum used to transmit the voice, or data, can not extend beyond the stated excursion. To avoid interference between systems, the FCC licenses the frequencies so a specific channel is granted to only one user in any given area. To obtain a license, an agency must first confirm availability of specific frequencies from private, FCC-authorized coordinator. The coordinators also may provide assistance when evaluating frequency design issues.

The licensed channel widths have been either 25 kHz, called Wide Band, or 12.5 kHz, called Narrow Band. This means the slice of spectrum used to carry a single voice is 25 kHz or 12.5 kHz, wide, as allowed by limits of the analog technology used to transmit the signal. Now radio users can add 6.25 kHz wide channels, called very narrow band, or sometimes ultra narrow band or skinny band to the list. These use ¼ the spectrum of a 25 kHz channel. This is good news as it will contribute to freeing up the spectrum increasing the availability in congested areas.

To help visualize the 6.25 kHz advantage, imagine a dump truck (a single geographic region) full of bowling ball sized rocks (25 kHz channels). There is a fair amount of air space (unused channels) between each rock. Now imagine a bunch of baseball sized rocks (6.25 kHz channels) poured on top. With a little work, such as the truck driving down the road (frequency coordinators), the smaller rocks would begin to find their way in between the larger ones. Efficiency is the result – more rocks in the same volume truck.

Unfortunately, the licensing schemes are not as easy to visualize. That's why there are experts, frequency coordinators, who assist in the process most of the time. You'd be wise to use them. But for a brief overview...

Very High Frequency

Very High Frequency channels are established as shown in the diagram below. Notice there is significant overlap with both 25 kHz wide band and 12.5 kHz narrow band channels. This is because the steps between center frequencies of each channel in VHF are 15 kHz (for 25 kHz wide band channels) and 7.5 kHz (for 12.5 kHz narrow band channels). The only way to make sure that there is no interference on adjacent channels is to place 'geographic isolation' between transmitters on adjacent channels, which can be as much as 20 miles or more. This effectively reduces the number of channels available in a given geographic area.

With 6.25 kHz, the channel steps are still 7.5 kHz so the 6.25 kHz IDAS channels fit nicely within each step with no overlap to its adjacent 6.25 kHz channel. This allows adjacent channel usage with no need for geographic isolation, allowing much more efficient use of channels and making getting frequencies in heavily congested urban areas much easier.

Ultra High Frequency

In UHF, the situation is a little different. The channel steps are 12.5 kHz for wide band and for narrow band channels (as opposed to 15 kHz for wide band and 7.5 kHz for narrow band in VHF). These steps create overlaps for adjacent narrow band 12.5 kHz channels, there is no overlap and they can operate normally. Similarly, the 6.25 kHz IDAS channels can be operated at full power when adjacent channels are occupied with 6.25 kHz modulated signals.

There are interesting licensing transition strategies addressing the transition from wide band to very narrow band.  LMCC members are experts at understanding these and determining what frequencies will work when making the transition.