Wi-Fi Bulks Up

New technical rules for unlicensed 5 GHz will yield better device performance.

We hear a lot about the shortage of spectrum that wireless carriers need for delivering silly cat videos to our smartphones and tablets. Also in short supply, although it gets less attention, is spectrum used by “unlicensed” services like Wi-Fi and Bluetooth. Access to this spectrum is free: no multi-billion-dollar auctions. The chips that use it are inexpensive, despite sometimes being housed in pricey tablets. There are no monthly charges. These frequency bands carry far more data every day than do carrier-provided 3G and 4G data services.

Older forms of Wi-Fi used only a band at 5.8 GHz band or, much more commonly, a band at 2.4 GHz. Some newer Wi-Fi protocols can use either or both, or other sub-bands in the 5 GHz range – whatever gives the best performance at a particular time and place. These technologies are amazingly good at working around interference, but still, can tolerate only so much congestion. A mathematical theorem sets the limit. As more of our devices send and receive more data, everybody’s performance gets worse.

A recent FCC order will help.

The oddly-named Unlicensed National Information Infrastructure (U-NII) section of the FCC rulebook governs four separate sub-bands, subject to differing technical rules, between 5.15 and (now) 5.85 GHz. The sub-bands are not fully contiguous, consisting of two contiguous pairs with a gap in between. A Notice of Proposed Rulemaking back in February 2013 suggested making the rules more uniform, filling in the missing gap, adding another sub-band at the top end, and reconciling inconsistencies with another rule covering some of the same frequencies. It also proposed to address problems of interference from some U-NII devices to airport weather radars.

The new order takes on some of these tasks, while leaving the addition of new bands for another day.

Most significant is an overhaul of the 5.15-5.25 GHz segment. Although this 100 MHz stretch is bigger than the main Wi-Fi band, operation within it has long been the 97-pound weakling of U-NII, limited to a paltry 50 milliwatts of transmitter power and restricted to indoor use. These constraints served to protect satellites receiving uplink signals on the same frequencies. But the FCC decided it could adequately protect the satellites even if it dropped the indoor-only requirement, raised the transmitter power limit 20-fold to a full one watt, and allowed antennas that can focus the power to an effective four watts. (More precisely, four watts is the maximum “effective isotropic radiated power,” or EIRP – a product of both the transmitter power and the antenna focus.) The new rules protect the satellites overhead by keeping signals close to the ground: outdoor, full-power operation is limited to fixed U-NII devices at angles below 30 degrees above the horizontal. At elevation angles above 30 degrees, where the signal might more plausibly be aimed toward a satellite, the EIRP cannot exceed 125 milliwatts.

The satellite licensee in the band remained concerned that large deployments of U-NII devices could still disrupt its operations. Interference from unlicensed devices, although rare, can be difficult to track down. Once the FCC certifies an unlicensed device as eligible for sale, it has no clue how many units are in operation or where they are. For this band, however, the FCC adopted an unusual rule: before deploying an aggregate of 1,000 access points, the company responsible must file a letter with the FCC acknowledging that it has to take corrective action if its devices cause interference. The names and address of companies filing these letters will at least give the FCC a starting point for investigation if interference does arise.

The uppermost U-NII sub-band, at 5.725-5.825 GHz, is where the big boys operate, allowed a full one watt of transmitter power and, with narrowly focused antennas, up to 200 watts EIRP. A different FCC rule regulated unlicensed “digital modulation” devices on almost the same frequencies – just a little wider, at 5.725-5.85 GHz. At one time the predecessor digital modulation rules and U-NII rules had significant differences, giving engineers good reasons for designing equipment to comply with one or the other. But over the years the two have gradually converged, to the point where almost-identical rules for almost-identical bands no longer make any sense. The FCC has now taken 5.725-5.85 GHz out of the digital modulation rules (two other bands remain), expanded the U-NII rules to the full 5.725-5.85 GHz, and tweaked the technical details to incorporate the best of both prior rule sections.

The most debated element of the consolidation concerned a digital modulation rule that allowed one watt of power into any antenna, no matter how narrowly focused, with no limit on the resulting EIRP. This allowed a well-designed system to cover several tens of miles in one hop: Wireless Internet service providers (WISPs) could bring the Internet to far-flung rural subscribers; oil companies could communicate with distant offshore oil platforms; wireless telephone companies could set up easy connections between network facilities and cell towers. All of these parties, and more, protested the FCC’s proposal to retain the U-NII limit of 200 watts EIRP. The FCC must have heard them, for the final package still has the former digital modulation rule permitting unlimited EIRP from a one-watt transmitter.

The biggest single problem in the U-NII band overall has been interference into the radars used at many airports to alert pilots to dangerous wind conditions. These Terminal Doppler Weather Radars (TDWRs) operate at 5.6-5.65 GHz, which is also part of a U-NII sub-band limited to 250 milliwatts transmitter power and one watt EIRP. A device in this sub-band (and one other) must “listen” for radar signals, and if hears them, switch to a frequency without those signals – a technology called “dynamic frequency selection” (DFS). After some fiddling with the details, DFS now works pretty well at protecting the radars.

Then why is there interference?

Many other countries besides the United States have U-NII-like services, but the precise frequency bands, power limits, etc. vary around the world. Rather than make different hardware separately designed to meet each country’s separate requirements, manufacturers prefer to make a single generic hardware radio capable of covering all bands globally, with software controls to maintain compliance with each separate country’s standards. Thus, a radio sold in the United States comes with software installed that keeps operation within U.S. specifications, activates DFS in bands that require it, and so on. But unscrupulous vendors sometimes modify the software to change the frequency, increase the power, or disable the DFS. The result is a noncompliant radio that might cover greater distances, but can cause interference to the radars. The FCC has uncovered several such instances, most recently this one.

The new rules require manufacturers to “take steps to prevent unauthorized software changes” that could take the radio out of compliance. (“Prevent” may be too strong a word; no system can be completely hack-proof.) Manufacturers can implement security any way they want – the FCC suggests a few methods, but does not prescribe any – and have to explain their approach in the certification application. The FCC promises guidance on what types of security measures work effectively, and the level of detail the FCC needs to evaluate the application.

It will be another 14 or 15 months before new devices must comply with the new rules, and perhaps another year or two after that before they become commonplace. But eventually your laptop, tablet, and smartphone Wi-Fi are all going to work better, even in places where many other devices are operating simultaneously. Enjoy those cat videos.

(FHH represents clients in this proceeding.)

5 GHz Reply Date Extended

Reply comments in the proceeding on 5 GHz unlicensed operation are now due July 24.

In February we reported on an FCC proposal that would not only add new 5 GHz frequencies but also overhaul – maybe even simplify – a confusing stretch of the rules. One possible upshot would be the opening up of 195 MHz of spectrum for Wi-Fi-type operation.

Comments were filed on May 28. The FCC has extended the date for reply comments, originally June 24, to July 24.

Update: Comment Deadlines Set in U-NII 5 GHz Rulemaking

In February we reported on an FCC proposal that would not only add new 5 GHz frequencies but also overhaul – maybe even simplify – a particularly confusing stretch of the rules. One possible upshot would be the opening up of 195 MHz of spectrum for Wi-Fi-type operation.

The Notice of Proposed Rulemaking (NPRM) has now been published in the Federal Register which, as our regular readers know, establishes the relevant filing deadlines. Comments in response to the NPRM are due to be filed by May 28, 2013, and replies by June 24.

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.

Sometimes The FCC Says Please

FCC seeks voluntary cooperation in protecting airport weather radars from interference.

Having occasionally criticized the FCC for what might be perceived as over-enforcement – here, for instance, or here, or here – we welcome the chance to balance the record by crediting the Commission when it takes a kinder, gentler approach.

Not everyone gets Internet service through the phone company or cable company. Some subscribe to a wireless Internet service provider, or WISP, that installs a radio link to the user’s home or business. Many WISPs favor three particular frequency bands that do not require an FCC license, yet allow relatively high power operation that can reliably cover distances of several miles.

Seeking to add capacity, the FCC began a proceeding in 2003 to enlarge one of these bands, at 5 GHz. The frequencies it sought to add were occupied, as is most usable spectrum – in this case by federal government radars.

With the help of federal spectrum experts, the FCC wrote rules that allow WISPs and others to use the band, so long as their radios are capable of detecting and avoiding the radar signals. That took some time. Another delay ensued while the FCC, the government users, and the manufacturers worked together on test procedures to evaluate whether candidate devices in fact met the detect-and-avoid requirements. But eventually everything was in place.  The FCC began issuing equipment certifications, and WISPs began offering service in the band.

But a problem arose.

Some of the radars began registering interference. And some of those, called Terminal Doppler Weather Radar (TDWR) systems, are used to monitor weather conditions near airports. This was worrisome, as good weather information is an important component of aviation safety.

The FCC and the FAA investigated. Sure enough, the interference turned out to be caused by WISP transmitters, particularly those operating high off the ground, in direct line of sight with the TDWR units.

Some of the WISPs were using non-certified transmitters. That is a plain violation of the rules, and the FCC has taken enforcement action. But other affected WISP transmitters are certified. Having met the technical requirements for coexistence with TDWR, they cause interference nonetheless. That is another violation of the rules, which prohibit an unlicensed transmitter, such as the WISPs’ units, from interfering with any other kind of radio equipment. The offending transmitter must cease operation until the problem is fixed. Failure to do so can draw fines from the FCC.

For once, though, rather than gear up for all-out enforcement, the FCC showed restraint. In a letter to WISPs and their manufacturers, the FCC provided a list of the TDWRs that use the band, including their latitude and longitude, frequency, elevation, and tower height. It worked with an association of WISPs to disseminate that information. It encouraged the association to set up a database of WISP locations to help track down interference. And it asked the WISPs voluntarily to avoid TDWR frequencies when operating within 35 km or line-of-sight with a TDWR site. In short, it treated the WISPs as professional equals, and sought their help in a common cause – a welcome trend.

In the meantime, the FCC has suspended new certifications for outdoor transmitters in this band, and is working with the FAA and the manufacturers on new technical standards to better protect the TDWR systems.

There is, we must note, a less pleasant alternative available to the Commission, an alternative which the letter hints at, but just barely. Two people signed the FCC letter. One is Julius Knapp, Chief of the FCC’s Office of Engineering and Technology, co-architect of the 5 GHz rules, and a likely proponent of the voluntary approach. But the other is P. Michele Ellison, Chief of the Enforcement Bureau. Her presence on the letter provides a sobering reminder: If the interference does not stop through cooperative efforts, the WISPs might expect something in the mail on blue FCC letterhead.

Thanks to David Case at Cisco for bringing this item to our attention.