CBRS: A Foray Into Spectrum Sharing

Earlier this spring the FCC announced the creation of what it calls the Citizens Broadband Radio Service (CBRS) — a swath of spectrum between 3.5-3.7 GHz that will be opened to both licensed and unlicensed services. This spectrum has traditionally been reserved for military radar and satellite uplinks; now it may become a sandbox for dynamic use of the public airwaves.
This particular slice of spectrum falls between two established Wi-Fi allocations, so one obvious potential use is for the provision of last-mile (or last-foot) broadband access. Incumbent users (the Navy and satellite ground stations) will remain on the band, but they’re so geographically sparse that for all intents and purposes this spectrum has been fallow in the majority of the United States. Under CBRS, instead of licensing devices to work on a particular channel within a band, they will be effectively permitted to use the entire band. The devices themselves will be programmed to sniff the local airwaves to find and utilize non-congested channels in its immediate area. Google is developing a database of CBRS users and devices that will be updated in real-time based on operating feedback from the devices themselves — the Internet of Things coming to life.
What’s most interesting about the CBRS proposal is that it authorizes both licensed and unlicensed use of the band. Incumbents and licensed users (expected to be telecommunications companies and other Internet service providers) will have “geographically-targeted, short-term priority rights” to the spectrum, as determined by auction. However, the FCC will reserve a healthy segment of this band for unlicensed wireless use, providing a critical space to experiment with small-scale mesh networking and other innovative uses of spectrum in concert with smart devices.
Not surprisingly, telecoms and ISPs aren’t keen on the idea of sharing spectrum. Some are throwing around CB Radio analogies, portending chaos will ensue, wholly ignoring the fact that real-time sensing and mapping of local RF conditions automatically mitigates this risk. Public Knowledge raises the concern that telecom comapnies may try to hoard or squat on this spectrum by leveraging the priority of licensure to interfere with other (unlicensed) users.
There’s a host of technical details yet to be determined, including what information will be required of devices and users for the purposes of maintaining the spectrum-sharing database and just how fine-tuned the sensors on CBRS devices will actually be. That said, the fact that CBRS has gone from concept to policy in just three short years — greased lighting in spectrum-policy terms — would seem to suggest that the FCC is serious about maintaining space for unlicensed experientation. That is, if during further rulemaking wireless incumbents don’t lobby the “citizen” portion of CBRS into marginality.