Deborah D. McAdams /
03.07.2007 12:00 AM
Race for White Space Intensifies
The first prototype device for use in taboo TV channels should be in the hands of the FCC by now. Thus begins the testing phase of the commission's proceeding to loose consumer transmitters into TV spectrum traditionally left fallow to prevent co-channel interference—roughly 6 MHz swaths between licensed broadcast channels, aka “white spaces.”
The long-awaited mystery gizmo should be able to sniff out TV signals, then shut down or jump to a vacant channel if any are detected. Dave Donovan, head of the Association for Maximum Service Television, is itching to get a look-see.
“Is it representative of what's going into the market? Is this a real device,” he said, “or an experimental thing for the purposes of testing?”
Donovan is particularly solicitous about what goes into the TV bands because of the potential for interference. He's also aware that the technical complexities of the DTV transition are often lost in lawmaking.
His lobby spent significant energy ensuring stations had a final DTV channel designation before Congress pulled the plug on analog television. With those issues settled, political pressure shifted to opening broadcast spectrum for personal devices, which may not have to be licensed by the FCC.
Two U.S. senators have introduced bills to open the spectrum before Feb. 17, 2009, when analog transmitters are shut down and TV signals become exclusively digital. Either bill would override the current FCC effort to establish technical specifications for the devices, whatever they are.
Until just a few weeks ago, only vague descriptions were floated: The devices would be low powered—no more than 100 mW. They would facilitate wireless broadband—the raison d'être for opening the spectrum—or they might just move files around ye olde McMansion.
Little else was known, which vexed Donovan, who wanted something for lab tests. Would these things really extend broadband to underserved areas, or would they be glorified Clappers that turned off the lights through the wall? He remains skeptical.
“The great fallacy in this debate is that this is for rural broadband,” he said. “It's for personal portable devices and in-home wireless networking. Those are the things that are going to cause most of the problems.”
Not so, said Scott Harris, managing partner of Harris, Wiltshire and Grannis, the Washington law firm representing the prototype's creators—Microsoft, Intel, Dell, Hewlett-Packard, Philips and Google—in the FCC proceeding.
“We believe broadcasters need to be protected,” Harris said, “and we believe we can do it. It's up to us to prove to the commission that we can protect broadcasters.”
The coalition proposes using spectrum-sensing technology to steer clear of broadcast signals. Harris noted that such technology is already used to protect military radar in the 5 GHz band; military radar being far more stealthy than TV signals.
However, the Institute of Electrical and Electronics Engineers in its FCC filing noted that protecting radar is actually easier than protecting TV signals because radar transmitters and receivers are co-located. TV signals emanate from a transmitter to multiple receivers scattered willy-nilly.
Radar signals are also pretty darn strong compared to a TV signal on the edge of its coverage area. Even relatively close to a transmitter, the strength of a TV signal can vary with respect to the surroundings, bouncing between buildings or off the ground. Technology robust enough to sense DTV signals and immediately jump to other frequencies would have to be highly sophisticated, but Harris said it has indeed arrived.
“Anything that a TV receiver picks up, this technology can sense,” he said.
The filing authored by Harris and his colleagues for Microsoft, et al, provides some general characteristics of the proposed white-space devices. They will not rely on global positioning systems as proffered by the FCC, but rather the spectrum-sensing technology previously mentioned. Their signal-detection threshold will exceed the established dynamic range of DTV receivers and they'll have fixed, gainless antennas that can't be pried off and replaced by the user.
The coalition favors spectrum sensing because GPS would be too clunky with its reliance on satellite links and locational data for every TV transmitter. With regard to dynamic receiver range, the group proposes a –114 dBm threshold for establishing channel vacancy—about 30 dBm below that of a DTV receiver.
The devices would have affixed receive and transmit antennas with no capacity to intensify signals (0 dBi), compared to 6 dBi suggested by the FCC. A transmit power control range of 20 dB is proposed, rather than the 6 dB used by personal devices in the 5 GHz band. (A 20 dB power control range means the power level of a device in the TV band would drop by 10 dB when a TV signal is detected.)
Finally, the coalition opposed the 10-second interval put forth by the FCC to recheck for TV signals.
“In light of the 'always on' nature of incumbent licensees, a recheck requirement on the order of one minute is more appropriate,” the group's filing said.
It went on to say that the described power parameters would overcome landscape-induced anomalies in DTV signals, as well as protect other professional gear sometimes operated in TV bands, such as wireless mics. (The IEEE said a 2-second recheck interval was necessary for wireless mics, which operate intermittently.) It also indicated that the –114 dBm sensing threshold combined with the 20 dB power control range would allow devices to operate in spectrum immediately adjacent to TV channels.
Many broadcast engineers, including TV Technology contributor Charlie Rhodes, are dubious about adjacent-channel operation because signal power tends to “leak” out of assigned channels. This power leakage can also combine in such a way to produce interference on channels far above or below where the offending transmissions are taking place. It's this phenomenon that concerns Byron St. Clair, president of the National Translator Association.
The nearly 5,000 translators in the United States convey broadcast signals beyond stations' established coverage areas to regions that otherwise could not receive free TV. St. Clair said the far-flung nature of the translator system makes it more vulnerable to interference.
“Say you have a station on Channel 22 and Channel 26,” he said. “If you overload some circuit someplace, it will produce unwanted distortion products four channels above 26 and four up and down from 22.”
Potential interference problems will only be worse if the proposed devices are unlicensed, St. Clair said.
“If someone can buy a device and use it without having to register with the FCC,” he said, “they have nothing to compel them to follow the rules.”
When the FCC initially opened the white space proceeding in December 2002, it was for the express purpose of unlicensed devices. Only in a Further Notice of Proposed Rulemaking issued last October did the commission seek feedback about licensing. Dorothy Robyn of the Brattle Group, a Washington law firm representing Qualcomm in the proceeding, said licensing white spaces will spur innovation because the resulting devices will have greater protection from the FCC. Harris contends the opposite is true, because spectrum license auctions would limit the number of participating companies.
“If you give the spectrum to one company that decides what service will be used and what devices,” he said, “will that lead to more innovation than if you have spectrum available to anyone to use it for anything and any type of device they want?”
So far, however, the number of parties angling for the space, licensed or otherwise, is relatively limited—to the coalition.
“After four years of people saying there's demand,” Donovan said, “there's one filing.”