FCC Proposes to Simplify and Expand Unlicensed 5 GHz Use

Suggested rules would combine and harmonize the rules for various sub-bands, and open another 195 MHz for Wi-Fi-type operation

Blame it all on Congress. The Middle Class Tax Relief and Job Creation Act of 2012, best known for extending the since-expired payroll tax cuts, took just a few lines for that task, but continued on for another hundred pages of unrelated legislation. The statute has been good for CommLawBlog; we reported on incentive auctions, microwave issues, 911 implementation, and lots more. Now the FCC has responded to yet another mandate in the act: to expand unlicensed operations in the 5 GHz band.

But the FCC is doing more: it has issued a Notice of Proposed Rulemaking  (NPRM) that would not only add new 5 GHz frequencies but also overhaul and (we hope) simplify a particularly confusing stretch of the rules. We will touch on that first, and then take up the proposed expansion.

It is hard to overstate the importance of having enough unlicensed spectrum. The vast majority of radio transmitters in use today are unlicensed. We would be hard pressed to get along without Wi-Fi, Bluetooth, cordless phones, nursery monitors, automatic toll payment, automatic braking in cars, and myriad other such consumer conveniences. Industry as well relies on unlicensed communications and, increasingly, on unlicensed radar. Equally important, though less often mentioned, is the importance of unlicensed spectrum as a technology test bed. Licensed frequencies, if auctioned, are usually too expensive to risk on untried technology, while non-auctioned, site-licensed spectrum is governed by technical rules so restrictive as to preclude experimentation. Much innovation benefits from the technical flexibility inherent in the FCC’s unlicensed rules.

Updating Rules on the Present Bands

Unlicensed operations in the frequency range 5.15-5.825 today are governed by four sets of technical rules. Three are collected under the heading of “Unlicensed National Information Infrastructure Devices,” or U-NII, detailed in Section 15.407 in the FCC rulebook. The fourth comes under the “digital modulation” rule, in Section 15.247. Here is the breakdown:

Below 5.725 GHz, the U-NII rules are the only choice. The power limits are relatively low, and the need to avoid certain airport weather radars adds complication. The region above 5.725 GHz is popular because it offers the highest power for Wi-Fi standards a and n (and also the proposed standard ac). The band is a favorite of wireless Internet service providers, or WISPs, which offer Internet service via a roof antenna, mostly in regions not served by either broadband cable or telephone.

Before 2003, most manufacturers working above 5.725 GHz opted for the U-NII standards, which have no express limit on data speed, because Section 15.247 then limited speeds to about 11 megabits/second. But a rule change that year eliminated the limit in Section 15.247, and thus put the two sections on an equal footing for speed. Today Section 15.247 is the favorite because in fixed point-to-point applications it allows more focused antennas with no penalty in transmitter power, offers 125 MHz of bandwidth versus 100 MHz for U-NII, allows more power per megahertz, and has more relaxed limits for out-of-band emissions.

The FCC now proposes to harmonize the two rule sections. In some respects it suggests changing the U-NII provisions to match those in Section 15.247: namely, extending the upper bound on U-NII-3 by 25 MHz, to 5.85 GHz, and allowing the same power-per-megahertz as Section 15.247. In other respects, however, the harmonized rules would follow the U-NII provisions: a power penalty in fixed point-to-point applications for antenna gains above 23 dBi, and the more stringent U-NII limits on unwanted emissions.

A separate proposed harmonization would amend the rules for U-NII-1 at 5.15-5.25 GHz to more closely match those for U-NII-2 at 5.25-5.35 GHz in three respects: raise the power limit from 50 mW to 250 mW; raise the power-per-megahertz to match U-NII-2; and drop the limitation to indoor-only operation. As an alternative, also up for discussion, is raising the U-NII-1 power limits to U-NII-3 levels (1 watt), and again allowing outdoor operation. Neither proposal would require DFS or TPC in the U-NII-1 band.

The NPRM revisits the stubborn problem of U-NII-2 devices causing interference to airport weather radars operating at 5.6-5.65 GHz. Dynamic frequency selection (DFS) capability is required in the U-NII-2A and U-NII-2C bands specifically to protect those radars: the U-NII device must “listen” for the presence of a radar signal and, if found, move to a different frequency. Interference has persisted nonetheless, some from illegally operated U-NII devices that may lack DFS, but also from fully compliant systems.

Some interference to radars comes from users unlawfully changing the frequency of a certified device. This can result in a transmitter possibly overpowered for its band, with no DFS, operating on the same frequency as the radars. The FCC has found that some U-NII transmitters are easily modified in this respect. It proposes to require security safeguards to prevent such reprogramming, and/or the transmission of ID information to help locate offending units. It also asks for comment on these additional measures:

  • built-in geolocation capability in combination with database registration and access, so that units within a certain distance of a radar will automatically avoid its frequencies;
  • tighter limits on unwanted emissions to reduce interference from U-NII transmitters operating close by a radar frequency;
  • improved sensing capability; and
  • changes to certification test procedures to better assess sensing capability.

(Note to manufacturers and test labs: The above is only a rough summary. Please consult the proposed rules and measurement procedures in NPRM at pp. 39-45.)

Expanding into New Bands

As shown in the table above, the FCC is considering an expansion into 195 MHz of new U-NII spectrum. It hopes to find 120 MHz in the tentatively-named U-NII-2B band at 5.37-5.47 GHz, and another 95 MHz in the U-NII-4 band at 5.85-5.925 GHz. This would yield an unbroken sweep of 775 MHz, albeit subject to differing technical rules in the various sub-bands.

The problem, of course, is that both of the proposed new bands are occupied. The 5.37-5.47 GHz U-NII-2B band houses military and other government radars, weather radars (some used by broadcasters), NASA systems, unmanned aircraft, satellite observations of the planet, and border surveillance. At 5.85-5.925 GHz, U-NII-4, are more military and other government radars, automatic communications with and between vehicles, and a secondary amateur band.

U-NII, being an unlicensed service, will be required to protect licensed services, i.e., all of the above. We expect it will be a challenge to accomplish adequate protection to the satisfaction of the federal government (not to mention the amateur radio folks) while leaving enough technical elbow room for U-NII to be useful.

As a starting point for discussion, the FCC proposes:

  • U-NII-2B to operate under the same rules as the adjoining bands, U-NII-2A and U-NII-2C, providing 475 MHz of contiguous and consistently regulated spectrum; and
  • U-NII-4 to operate under the same rules as U-NII-3, with the same rules also applying to the 25 MHz in between. This would provide another 200 MHz that is contiguous and consistently regulated, at somewhat higher power.

The FCC seeks comment on whether DFS and TPC requirements should apply to U-NII-2B and U-NII-4, and if so, what the technical characteristics should be.

Manufacturers of future devices and present users of the proposed expansion bands should pay close attention. Reading between the lines, we have the sense that the FCC is tired of tinkering with these rules. The outcome of the proceeding may set the technical provisions for many years to come.

Comments and reply comments will be due 45 days and 75 days, respectively, after publication in the Federal Register. Watch this space (or subscribe) and we will let you know when that happens.

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Comments (1) Read through and enter the discussion with the form at the end
David Sumner - February 26, 2013 10:24 AM

Anyone with an in-depth interest in this subject would be well advised to review the NTIA study at http://www.ntia.doc.gov/report/2013/evaluation-5350-5470-mhz-and-5850-5925-mhz-bands

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