Shared use of 3.5 GHz band promises greater efficiency, but much work still needs to be done before operations can begin.

In a move described by the Commissioners as a “significant step forward” (Chairman Wheeler), history-making (Commissioner Rosenworcel), and “excit[ing]” (Commissioner O’Rielly), the FCC has opened up the 3.5 GHz (3550-3700 MHz) band for a wide variety of new uses. The band will now be home to the new Citizens Broadband Radio Service (CBRS). Perhaps most importantly, the new users will share the spectrum with themselves and incumbents through a three-tiered access model that depends on an automated frequency assignment and control database mechanism known as the Spectrum Access System (SAS). A number of details still need to be worked out in a further rulemaking, but the FCC’s action unquestionably reflects an innovative approach to increasing the efficient use of spectrum.

Historically, the 3.5 GHz band has been used by the Department of Defense for radar systems. Radar, of course, is a notoriously difficult technology to share with. A portion of the band has also been used for delivery of commercial broadband service, another sensitive use not ordinarily open to sharing. The Fixed Satellite Service (FSS) is another incumbent which has been pushed around to accommodate the CBRS. Through a system of priorities embodied in the tiering approach, the Commission believes that those incumbent uses can be protected from interference while the band is opened to other users. And those new users will enjoy the flexibility of opting for either priority access when reliability is important (at least in license areas where there is demand), or general access when it is less critical.

The new scheme contemplates that the licensed incumbents in the band will be joined by two new categories of users: those with Priority Access (who would hold Priority Access Licenses (PAL)) and General Authorized Access (GAA), whose use would not be subject to individually-issued licenses. The amount and type of incumbent spectrum use will determine the amount of spectrum available for new entrants in each license area. Spectrum availability for all three tiers of users will be determined continuously by the SAS. While this database system is modeled after the TV White Spaces database system – whose readiness for prime time has recently come under heavy fire – the FCC’s 3.5 GHz plan relies on Google’s promise that its prototype SAS will be capable of even more complex database control than is required of the TV White Spaces database. To maximize database control and incumbent protection, CBRS systems will have to be capable of operating across the entire band and automatically shutting down or changing frequencies at the direction of the SAS. 

The sharing system will work like this.

Incumbent Access (IA) users will enjoy the most protection of all, primarily through the creation of “exclusion zones” with respect to which the SAS will exclude or control access by new entrants. The size of these exclusion zones has been the subject of much wrangling. The FCC has convinced the National Telecommunications and Information Administration (NTIA) – which is generally responsible for spectrum allocated for government use, including radar systems – to reduce the size deemed necessary for such zones. The zones adopted by the FCC are 77% smaller than originally proposed. But concerns remain. In a separate statement Commissioner O’Rielly noted that even the shrunken exclusion zones “capture several of this country’s largest cities, where the shortage of spectrum is most acute. Thus, we must exercise diligence in ensuring that the zones continue to shrink.”

PAL: The FCC will reserve a maximum of 70 MHz (in seven 10 MHz unpaired channels) in each license area for PA licensees. Any unused channels may be accessed by GAA users. As proposed, a PAL area will consist of census tracts (of which there are more than 74,000, ranging in size from less than 1 square mile to 85,000 square miles). PA users will have to protect IA users and each other.

Contrary to its initial proposal last year, the FCC has decided to offer these licenses for three-year license terms (with an optional two consecutive terms the first time around). This will provide for more predictable business planning than the one-year license term originally proposed, although we agree with Commissioner Pai that it’s not clear that even this concession will be sufficient to encourage investment, at least by certain types of potential users. Unlike most FCC licenses, PALs will not be subject to any “renewal expectancy” at the end of a license term. While that may be a good tactic to prevent spectrum warehousing, it could also discourage those with longer range business plans looking to use equipment for, say, 15-20 years.

PA licensees will obtain multiple contiguous channels when available, and they may aggregate up to four licensees (i.e., 40 MHz of spectrum). When demand for the PALs exceeds the supply of available spectrum, the FCC will apply its usual auction process (luckily, one that is much less complex than the incentive auction). However, no auction will be held – and no PALs will be issued – when there is only one applicant in a license area; in such a case, only GAA use will be available in that area. (Commissioner O’Rielly has criticized both the spectrum aggregation limit and the unavailability of PALs when there are no other applicants.)

GAA: The rules provide for opportunistic use of the spectrum by GAA users, which means that such users can operate wherever spectrum is unused by higher-tiered (i.e., IA and PA) users. In license areas where there are no PALs (or incumbents), GAA users may have access to all 150 MHz of the band; in other areas where PALs are in high demand, but there are no IA users, GAA will have access to at least 80 MHz. (Note that this is a change from the percentage-based plan originally proposed, which the Commission concluded would be confusing and uncertain.) GAA is licensed by rule, meaning that GAA users have the status of licensed users (i.e., priority above unlicensed users), but are not required to obtain (or pay for) individual licenses. However, GAA has no interference protection from, and must not cause harmful interference to, higher tier users.

The CBRS technical rules are complex, with different rules for “Category A” devices (designed for lower power, often indoor and/or small cell use) and “Category B” devices (designed for professionally-installed, longer range outdoor backhaul). Devices in each of those categories may be used by both GAA and PAL users. For tech geeks, we recommend a read of the section on received signal strength limits to see how the FCC mandates receiver requirements without actually mandating receiver requirements. Another twist from the norm is that holders of adjacent channel or geographic licenses may consent to alternative reception thresholds by working with the SAS.

An additional note: the FCC has imposed special rules for the upper 50 MHz (3650-3700 MHz) that grandfather existing wireless services and allow GAA access around a “Grandfathered Wireless Protection Zone.” 

What the Future Holds

There is a substantial to-do list before commercial CBRS service can begin: 

The Commission must refine its rules in a number of respects. Among other things, it must: (1) define “use” of spectrum by PAL, a determination that will affect the amount of opportunistic use by GAA; (2) develop certain auction-related rules and procedures to govern PAL auctions; (3) establish secondary market (i.e., spectrum leasing) rules; and (4) craft more specific rules to protect FSS earth stations (one incumbent that was pushed around to make room for CBRS). The FCC has issued a Second Further Notice of Proposed Rulemaking (SFNPRM) seeking comments on these questions.

The Commission must request proposals for and, eventually, approve one or more commercial SAS. Before they can be approved, of course, the FCC will have to review them and confirm that they do the job. Given the complexity of the SAS, this step may require considerable time.

Environmental Sensing Capability (ESC) will have to be developed, authorized and deployed. ESC will involve systems of sensors that detect federal uses in and adjacent to the 3.5 GHz band. The ESC will interface with the SAS so that the SAS in turn can direct commercial users to vacate a channel when their commercial use threatens interference to federal operations. ESCs will be developed by the private sector, but before deployment any ESC will first have to be reviewed, certified and approved by the FCC, much like SAS.

Priority Access Licenses will have to be auctioned. (GAA use can occur on CBRS spectrum prior to licensed use by a PAL).

But still more questions lurk, including whether to permit use of technologies such as LTE-Unlicensed (a/k/a LTE-U)/Licensed Assisted Access (LAA) in the 3.5 GHz band. This controversy has brewed for some time. LTE-U/LAA is a new technology, promoted most vigorously by Qualcomm, that relies on bonded channels in licensed bands. By placing an additional chip in cell phones, wireless carriers using LTE-U/LAA can use unlicensed frequencies in additional to their licensed frequencies. Verizon already has announced its intent to use both 3.5 GHz and 5 GHz unlicensed spectrum for LTE-U by 2016. Broadband proponents, led by Microsoft, have fought against this, arguing that LTE-U/LAA will interfere with Wi-Fi. Further complicating matters is the fact that LTE-U/LAA is not yet standardized worldwide: Europe and Japan require listen-before-talk protocols to prevent interference to Wi-Fi, but other forms do not. Rather than address this conundrum in the SFNPRM, Chairman Wheeler has promised to open a docket specifically devoted to LTE-U/LAA questions in the very near term.

Several Commissioners have categorized the CBRS as a “playground” or “sandbox” for technological development. This may be true. But it remains to be seen what type(s) of business for this spectrum (other than offloading data) can be designed and successfully implemented given the rules the FCC has adopted. The answer will depend on the type and availability of equipment that is developed for the band, and whether investment in that equipment is  sound business decision in view of the varying spectrum access that the Commission has provided. Certainly, a great range of businesses have become successful on other unlicensed bands (902-928 MHz being the granddaddy of them all), but the CBRS scheme has enough new twists, such as database control, that the outcome for the 3.5 GHz band remains to be seen.