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.)
