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FCC Unleashes Digital Modulation

Fri, 06/01/2001 - 5:05am
By Mitchell Lazarus

Forget those pesky spread spectrum rules! Starting soon, the FCC won't even ask about processing gain. Frequency hoppers can skip over crowded parts of the band. Most striking of all, the FCC will soon permit unlicensed radios using any digital modulation at all, running at tens of megabits/second, possibly at a full watt of output power. In short, the FCC is about to allow almost unlimited design flexibility and much higher throughput in the spread spectrum bands. At somewhat lower powers, the new radios are even lawful today.

A Brief History
Twenty years ago, when the FCC first proposed authorizing spread spectrum, unlicensed radios could use only tens of microwatts — enough power for garage door openers and short-range cordless phones, but not much else. Even then, spread spectrum had a long history at military frequencies. But the idea of extending it to civilian uses ran into a lot of opposition, based mostly on uncertainties about the effects of spread spectrum on existing applications (much like the dispute over ultra-wideband today). It took a contentious, four-year-long proceeding before a 1985 order allowed spread spectrum at an unprecedented full watt. The FCC responded to interference concerns in part by limiting spread spectrum to three ISM bands — 902 - 928, 2400 - 2483.5, and 5725 - 5850 MHz. But even those had multiple non-ISM users, including amateurs, police and public safety, military and weather radar, and (later) a commercial vehicle location service. To protect those, the FCC approved only two modulations — frequency hopping and direct sequence — and put strict limits on power spectral density.

Almost no companies requested product certifications. Several, though, did request changes to the rules. In 1990 the FCC allowed 6 dBi of antenna gain, adjusted the frequency hopping provisions, and required direct sequence systems to show processing gain. A 1995 public notice allowed applicants to measure processing gain with a CW jamming margin test. In 1997 the FCC allowed much higher antenna gains, and reduced the required number of hops at 902 - 928 MHz. An off-the-record proceeding in 1998 found CCK modulation complies with the rules, greatly increasing throughput. Then, last year, after a lengthy and disputatious proceeding, the FCC allowed "HomeRF" wide-band frequency hopping systems. All these rule changes, and the parallel advances in technology, had a dramatic effect. The number of spread spectrum devices certified each year went from just a dozen or so in 1990 to well over 300 last year.

But the more the FCC changes the rules, the farther behind it gets. Nowadays a rulemaking typically requires two or three years. By the time it's over, and the new rules have taken effect, technical advances have made them obsolete.

The Battle for Bandwidth
The FCC did not originally think of spread spectrum as a broadband technology. The original modulations first authorized in 1985 were capable of 1 - 2 Mbps at most. That proved adequate for dozens of useful applications, including retail, airline baggage handling, hospital records, and more. But manufacturers had their eye on a bigger prize. Computers in the workplace were still a relative rarity in 1985, when spread spectrum first appeared, but were almost universal just ten years later. And most of those computers were interconnected through expensive and inflexible in-the-wall wiring. Wireless LAN networks using spread spectrum could offer a cost-competitive alternative, were it not for the 1 - 2 Mbps speed limit, far inferior to 10 Mbps wire-in-the-wall performance. When the 1998 approval of CCK suddenly made possible direct sequence systems at 11 - 12 Mpbs, the market took off overnight. This year alone, spread spectrum wireless LAN manufacturers expect to sell equipment worth $2 billion.

But while the FCC was catching up to wired network speeds, user needs changed again. The late 1990s saw growing demand for spread spectrum systems capable of speeds in the multiple tens of megabits. The products were technically feasible. Several manufacturers even went through the design process, built prototypes, and submitted applications to the FCC. But there they hit a brick wall.

The FCC rules say the radio bandwidth of a spread spectrum system must indeed be spread —" deliberately widened" over that otherwise needed to transmit the information. But as the data rate goes up, and the available spectrum stays the same, the spreading ratio necessarily goes down. In the popular 2400 MHz band, the FCC staff drew the line at 11 Mbps. Any data speed higher than CCK, the staff said, does not spread enough to qualify.

Industry Pushes Back
One such rejectee, the Canadian company Wi-LAN, Inc., retorted that its product in fact "deliberately widens" the occupied bandwidth, and asked the FCC staff to reconsider its denial. (Wi-LAN had other problems, too — the FCC ruled its multiple-carrier modulation would not qualify as spread spectrum at any speed.) After being turned down a second time, Wi-LAN invoked a little-used procedure that asks the full Commission to review a denied equipment certification. (Necessary disclosure: I represented Wi-LAN through all of these proceedings.)

In the meantime, other sectors of the spread spectrum community saw growing unrest. First, a 1999 FCC proposal suggested measuring processing gain with a jamming test that uses a Gaussian noise source. That alarmed CCK manufacturers, who feared the test could be abused, and set off a fervent debate over what, if anything, processing gain is supposed to measure. At around the same time, several companies asked for a further change to the frequency-hopping rules. Noting that the new HomeRF rules permit fewer, but wider, hops, several companies asked for a further change to allow fewer hops at the old, narrow bandwidth, allowing 2400 MHz systems to use only part of the band and bypass occupied frequencies. Finally, while the FCC staff was mulling over Wi-LAN's application for the third time, other companies appeared at industry forums to promote their own high-speed direct sequence systems, despite the near certainty that the FCC would treat them just as it had Wi-LAN.

The FCC Acts — Again
In mid-May of this year, the FCC proposed yet another set of changes to the rules. One official disclosed privately that the agency's goal was a set of rules the industry could live with for at least a few years. "We want to work on something else for a change." The proposals have three components . . . "something for everybody," according to the same official.
•Processing gain. Rather than continue debating the relative merits of different processing gain tests, the FCC proposes to drop the processing gain requirement altogether. Europe already has a similar rule.
•Frequency hopping. As requested, the FCC proposes to reduces the minimum number of hops in the 2400 MHz band from 75 to 15, with two constraints: power in this mode is limited to 125 mW (the same as HomeRF); and the device must avoid hopping on occupied channels.
•"Digital modulation." In its most far-reaching proposal, the FCC offered to allow any digital modulation - not just spread spectrum - at a full watt of power. The only applicable limits would be occupied bandwidth, power spectral density, and out-of-band emissions. Formerly disqualifying characteristics, such as adequate spreading and multiple carriers, simply would not count any more.

On the Wi-LAN matter, the full Commission solemnly affirmed the staff denial of the original application. But, at the same time, it proposed digital modulation rules that suit Wi-LAN's technology. And the FCC granted Wi-LAN a waiver to market its product immediately, even while the rulemaking is still underway. Any other company can also apply for certification under the same waiver, so long as its power level is no higher than Wi-LAN's, at 100 mW.

What Next?
It seems inevitable that industry engineers' best and newest ideas will fall outside the FCC rules. Right now is a good time to get any such ideas in front of the FCC, while it's still rethinking a more flexible regulatory structure. If the staff has its way, this may be the last chance for a while.

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