The FCC has proposed an innovation that might vastly increase the ability of multiple users to share the same band.

The FCC has proposed a set of rules that look innocuous enough, and would apply only to a single, underused band. But they may herald a new way of managing spectrum – a suite of techniques having the potential to vastly increase the number of users that can share a given range of frequencies.

All of the radio spectrum is occupied – at least, all of the most useful parts, below about 60 gigahertz. But the demand for spectrum continues to increase. What the FCC needs most is a way to squeeze new users into spectrum that is already in use, without causing interference to either the incumbents or the newcomers.

Current spectrum management relies on “allocating” each band of frequencies to (usually) several categories of users. Those categories, in turn, come in three different priorities. Those designated as “secondary” may not cause harmful interference to, and must accept all interference from, those called “primary.” Unlicensed users, permitted in most bands, must protect all other users (except each other) from interference, and must accept all interference that comes their way. A few bands, like that used for GPS reception, have only one active user category; a few have as many as seven or eight. Three to five is about typical.

Yet even supposedly occupied spectrum is quiet in most places, for much of the time. Some services, like those using two-way radios, occupy their frequencies only sporadically; others, like some types of satellite earth stations, operate only at wide separated locations. All such users, however, vigorously resist letting others into their bands. A police officer at the scene of an accident, picking up his microphone to request an ambulance, hopes to find an empty channel to make the call. The satellite operator may want the option of installing earth stations at new locations, without interference from other kinds of transmitters nearby.

As we explain below, the FCC thinks it can fully protect all such users while still letting new entrants share the same frequencies.

It proposes a scheme that prioritizes some classes of users over others, and – here’s the hard part – implements those priorities in real time, automatically, on the fly. A good trick, if the technology can be made sufficiently reliable.

This is not the FCC’s first foray into technical rules meant to protect users from each other. Unlicensed transmitters in the 5.25-5.35 and 5.47-5.725 GHz bands are required to sniff the air for certain radars, including those that detect adverse weather conditions at airports, and must avoid transmitting on radar frequencies in use. (In practice, though, sometimes even compliant equipment causes interference.) Similarly, the unlicensed “white space” devices that operate on vacant TV frequencies must avoid interfering with TV reception, some wireless microphones, and many other services. The rules for white space devices are complex, requiring self-geolocation, use of a central database to identify locally non-interfering frequencies, and spectrum sensing. Large-scale deployment having just begun, the jury is still out on how well the techniques will work in practice

Now the FCC proposes an ambitious expansion of these ideas. Its test bed is the 3550-3650 MHz band, possibly combined with an adjacent band at 3650-3700 MHz. Current users of 3550-3650 MHz are high-powered ground and airborne military radars, and non-federal earth stations that receive signals from satellites. Both of these applications are ordinarily ill-suited to sharing. Typically a radar combines a high-powered transmitter with a highly sensitive receiver, and is thus prone to both cause and receive interference. Downlink earth stations are also sensitive, as they must receive useable signals from tens of thousands of miles away. The 3650-3700 MHz segment is widely used for delivery of commercial broadband service. Both bands are part of a larger swath that the FCC has long kept closed to most unlicensed devices, out of interference concerns.

In short, this is not the first part of the spectrum one might look to for additional sharing.

Not content with crossing that tightrope, the FCC wants to juggle at the same time. It proposes to add in new users having urgent communications needs: hospitals, utilities, state and local governments – not usually good candidates for already-occupied spectrum. These will implement “small cell” applications; think of Wi-Fi service, but with better coverage at specific locations. Other new users are expected to offer additional small cell services over areas such as homes, office suites, stadiums, shopping malls, hospitals, and parks. These might be set up by consumers at home, by the management of a mall, say, or by a service provider trying to improve coverage in hard-to-reach places.

All such users will be “licensed by rule,” a legal fiction that entails no actual applications or licenses. This means the FCC will have no record of where the devices actually are. If one of them does cause interference, there will be no way of tracking it down.

The scheme rests on three levels of priority, comparable in some ways to the classic primary, secondary, and unlicensed partitions.

The highest priority, called “Incumbent Access,” consists of the federal radars and satellite downlink earth stations already in the band. Other users would be excluded from regions where they could cause interference to these facilities. Additional precautions, described below, would apply outside those regions.

The second level, called “Priority Access,” includes qualified users needing certain levels of service quality, like the hospitals and utilities mentioned above. Each of these users would be permitted to operate only in a specified geographic area: a utility, for example, might be restricted to regions where it provides service. The FCC expects Priority Access usage to be primarily indoors. These users might have access to half the band, leaving the other half for lower priority users. But no operation would be permitted in areas where the devices might cause interference to, or receive interference from, the Incumbent Access users.

The lowest priority, “General Authorized Access,” would comprise use by the general public. These operations would have to avoid causing interference to the other two categories and accept any interference received.

Keeping track of these levels and locations would be a “spectrum access system,” an ever-changing database that all Priority Access and General Authorized Access devices must consult on when, where, on what frequencies, and at what power they can operate. Devices would use geolocation (presumably via GPS) to report their whereabouts to the database. In some respects this system would be modeled after the database used to control white space devices; in practice, it might be an extension of that same database.

The FCC is considering whether to include the adjacent 3650-3700 MHz, which has been the topic of some controversy. Among other users, that band houses self-coordinated fixed links at more than 25,000 sites. But there is no proposal to explicitly grandfather these; instead, current users in the band might have to transition to operate under the new rules.

The FCC requests public input on all of the above, and on many possible variations. The Notice of Proposed Rulemaking is complex. Those interested should give it careful study. Comments are due on February 20, 2013, and reply comments on March 22.