For a few years now, the FCC and the wireless technology industry have been hashing out what to do with space opened up in our radio waves by switching from analog to digital TV. The so-called “white space” which exists between TV stations so that they don’t interfere with each other has become a bit more delineated due to the ease of correction and strength of digital signals. This opened up more than just a can of worms for wireless technology that was looking to stretch further than low power devices like wireless modems, but doesn’t need to be as strong as radio and TV.
One of the more recent modifications to the FCC’s white space policy happens to be the removal of the spectrum-sensing requirement for certain devices. Spectrum-sensing allows devices to hop between channels while running in order to avoid interfering with other low-power devices running near the same channels by detecting their broadcasts and backing off. However, since a lot of different unlicensed devices already run within this spectrum—like low power wireless microphones—having this requirement would give them precedence over the new technologies; thus, the FCC felt that they needed to discard it.
The remaining restriction is the use of a frequently updated geo-location database to help prevent devices from interfering with each other when they’re broadcasting. Specifically the geo-location service would work well for the larger, more infrastructure-like devices that act as base stations rather that free-floating low power devices.
Ars Technica has an excellent analysis of how this policy change will affect the adoption and adaptation of technologies in these new white spaces,
The elimination of spectrum sensing, however, creates a new problem for the new service. The procedure represented a crucial moment in the original plan in which some of these gadgets reported their interference status to home base. The FCC divides white space devices into two categories—”Mode I” and “Mode II” gadgets. Mode II and fixed location devices communicate with the central database command and include lists of available channels. Mode I devices float more freely, but, under the 2008 plan, used spectrum sensing to communicate their situation to the Mode II or fixed machines.
To compensate for the elimination of sensing, devices in Mode I mode now must receive regular signals from Mode II gadgets that provide an updated list of good-to-go channels, or they must contact the Mode II channel themselves at least once per minute. Meanwhile the Mode II and fixed devices must check their own locations at the same rate, except if in “sleep mode”—that is, the machines aren’t transmitting data, but they aren’t powered down either.
The situation is a bit more sticky, but since most applications for the white space spectrum will cover broad-area radio (for example to connect emergency services) and wireless CCTV regional setups, they will most likely be a bunch of satellite Mode I devices (radios, cameras) that call back into a fixed infrastructure of Mode II machines (the radio switcher, video recording station.) Google—who have been a big part of the FCC talks about white space—have already started their own tests on the newly temporary licensed spectrum for use with a hospital within the two above paradigms.
The final consumers of these devices will probably not notice the outcome of this sort of wrangling, but it will shape the future of their use. This means that it’s unlikely we’ll see very many mesh technologies coming out of white space use in the beginning—until better geo-location spread topology algorithms are written to enable them.
However, with this relaxation in policy, we will probably also see a flourishing of cheaper technologies as they won’t need to carry as much hardware and firmware to deal with spectrum sensing in order to remain licensed.