Filling in the Blanks
With most of the commercial opportunity in the 700 MHz band being claimed in a landmark auction earlier this year, the wireless industry already has begun focusing on the next frontier, the so-called TV white spaces spectrum.
As with 700 MHz, television broadcasters promise to play a key role in determining the development path for these airwaves. But there is a significant difference: unlike the 700 MHz band, television broadcasters have no intention to clear this spectrum; in fact, they are expected to continue to transmit their powerful digital signals using existing channels for the foreseeable future.
“White spaces” refer to the unused spectrum below the 700 MHz band that exists between the licensed 6 MHz-wide TV channels — airwaves that have been the subject of much discussion in wireless circles during the past few years and which are fraught with both opportunities and challenges.
The opportunities are based on the fact that there is a lot of unused spectrum in the TV bands that could host an array of wireless technologies on airwaves with favorable propagation characteristics.
Key challenges revolve around the reality that TV broadcasters will retain their existing channels to continue offering over-the-air programming, and protecting those signals from interference is of paramount importance to Washington, D.C., regulators and lawmakers. Failure to adequately protect TV channels is a potential political firestorm, particularly in the wake of questions surrounding the digital-television transition to clear the 700 MHz band.
Determining how much protection is appropriate is the key question facing the FCC. In short, the greater the protection provided to incumbent users, the less available white-spaces spectrum will be available for other wireless uses, said Phil Weiser, an associate professor in the University of Colorado’s telecommunications program.
“If it’s perfect interference protection/mitigation, then white spaces will never happen,” Weiser said.
Another complication stems from the fact that over-the-air TV channel usage is not uniform across the United States, meaning that white-spaces spectrum is not quickly identifiable on a traditional nationwide spectrum chart. So while there are vast amounts of spectrum that are unused — an undesirable situation for the FCC, which is charged with making the most efficient use of airwaves — these airwaves do not lend themselves easily to the traditional auction model used for licensed technologies.
With this in mind, most industry observers believe the whites spaces will be used for unlicensed applications, following a model similar to the wildly popular 2.4 GHz band but on spectrum with significantly better propagation characteristics.
“We see it as unlicensed, and we think that makes the most sense,” said Steve Sharkey, senior director of regulatory and spectrum policy for Motorola. “We look at what happened with Wi-Fi [at 2.4 GHz]. You had a band that was opened up for unlicensed use. I don’t think anybody knew exactly what applications or uses would come out of it, but we ended up with Wi-Fi as a primary use of the band — the applications and kinds of uses have just exploded beyond what anybody envisioned.”
MAKING THE VISION A REALITY
Such possibilities have become increasingly important in the wake of the 700 MHz auction, which resulted in the bulk of the most valuable spectrum being purchased by AT&T Mobility and Verizon Wireless, the two leaders in the U.S. commercial wireless marketplace. Many view the TV white spaces as a final chance for new entrants to have an impact on the industry and bring new applications and services to the marketplace.
Of course, this will require equipment, and the unique nature of TV white space dictates that gear is able to perform in ways that have not been seen before in a commercial environment. Not only do the devices have to be able to avoid interfering with each other, they must avoid interfering with digital TV signals that operate on different frequencies in each market.
To accomplish this, technologies such as software-defined and cognitive radio need to prove their effectiveness in a frequency-agile environment. By using sensing capabilities, such devices can avoid operating at frequencies where TV channels exist and can find open spectrum on which to transmit packets of data without conflicting with one another.
Initial FCC lab tests of products last year failed to meet this standard, but subsequent tests this year have been much more encouraging, according to industry sources. The hope is that outdoor tests will confirm that devices can operate in the white spaces without compromising the adjacent TV channels.
“There’s no question that this is a technically solvable problem,” said Sharkey, who reiterated Motorola’s belief that combining geolocation and sensing technologies is the best way to protect incumbent users. “The advances in technology, computing power and memory make it very doable in a cost-effective way. It’s more a question of getting the right rules in place to make sure that the users that need to be protected get protected.”
Television broadcasters are not the only incumbent users that need to be protected. In 13 metropolitan areas, public-safety entities are operating narrowband land mobile radio (LMR) voice systems in the Channels 14-20 range. The FCC’s order issued last year prohibits portable white-space devices from operating on these frequencies, but the commission has left open the possibility of allowing fixed devices to operate on these airwaves in areas where the channels are not used by public safety.
Sharkey said Motorola believes it can design devices that would not interfere with the public-safety systems operating in this space.
“I’m reasonably confident that any authorized devices are going to be required to have a number of safeguards that would avoid such interference,” he said, adding that in situations where the interference is knowable, it should not be too difficult to establish effective safeguards.
Sharkey said that the more likely white-spaces challenge would occur in circumstances where the interference is not as easily discernible. For example, problems cropped up when people started using wireless microphones in some bands for which their use wasn’t contemplated, much less authorized. “How to safeguard against interference of that kind is a much more difficult problem,” he said.
But public-safety officials typically have taken a different view, although their attention generally has been focused more on policy issues regarding 700 MHz rules and 800 MHz rebanding, said Robert Gurss, director of legal and government affairs for the Association of Public-Safety Communications Officials.
“We had expressed some concern early on to make sure that, whatever the commission does, it doesn’t create interference in Channels 14-20, where there’s land mobile, and to the upper end of 700 [MHz],” he said.
Indeed, while white-space devices should be able to easily detect the presence of high-power television signals, those devices may not be able to detect lower-power public-safety communications, said John Powell, senior consulting engineer for the National Public Safety Telecommunications Council. In particular, surveillance equipment used by public safety on these frequencies is specifically designed not to be detectable. And geographic restrictions are not adequate safeguards, even for fixed devices, he said.
“I would suggest that the commission just restrict [white-space devices from operating on Channels 14-20],” Powell said. “The difficulty becomes, how do you prevent somebody who’s got one of those devices in San Francisco — where Channel 15 is not used — from taking that device to Los Angeles — where Channel 15 is used — and firing it up down there?”
Meanwhile a key question looms: how should public safety use white-space devices operating outside of Channels 14-20? All sources interviewed for this article agreed on one aspect of the multidimensional answer: unlicensed spectrum in the TV white spaces is not suitable for mission-critical communications.
“I think public safety will want to stay on licensed spectrum, at least for mission-critical systems,” said Charles Werner, fire chief for the city of Charlottesville, Va., and executive committee chair of the Department of Homeland Security’s SAFECOM Program.
Mobile wireless consultant Andrew Seybold noted that potential interference from unterminated cable outlets and leaky transformers threatens to undermine the dependability of white-space devices to an extent that even commercial customers may be disappointed with their performance.
“Let’s put it this way: If I were a communications director for a public-safety system, would I use it?” Seybold said. “No, I would not.”
Jeff Cohen, senior legal counsel for the FCC’s Public Safety and Homeland Security Bureau, echoed this sentiment.
“If there’s a trend in this part of the spectrum, it’s [public safety’s] desire to get more basic land-mobile spectrum allocated for the traditional land-mobile public-safety operations [rather] than … looking for white space,” Cohen said.
While first-responder agencies may not want to use unlicensed TV white spaces for mission-critical transmissions, public safety could leverage such an unlicensed band in other ways, Sharkey said.
“We do see public safety as being able to use the white spaces,” Sharkey said, while noting that Motorola believes mission-critical communications should be transmitted on licensed spectrum. “The characteristics of this band make it very desirable for signals over larger distances, and we think it could facilitate data for public-safety uses.”
Powell agreed, citing administrative-related communications as a logical use of white-space technologies. In addition, the favorable propagation characteristics of the white-space spectrum could make it ideal for the transmission of video from fixed surveillance cameras, freeing spectral bands such as 700 MHz, 2.4 GHz and 4.9 GHz to carry only mobile data and video signals.
Even if these public-safety applications never evolve, Powell said he believes public-safety communicators should monitor closely the developments within the white-space spectrum. For years, public safety officials have debated the potential of next-generation technologies such as frequency-agile and cognitive radios for first responders, but most have acknowledged the need to see them operate in a live environment to determine their usefulness. Powell believes the TV white spaces could provide such an environment.
“It would be a great opportunity as a proving ground — to take some of the stuff that’s being developed in the test beds, put it in the real world and see what it does. It’s an opportunity to work out all the kinks in the various technologies,” he said. “Test beds are great as a technology proving ground, but it’s not real world. This kind of stuff would be real world.”
Weiser agreed, noting that technological advancements achieved in the TV white spaces could be adapted to licensed spectrum.
“I do think one opportunity for public safety is how these multimode radios develop, providing legacy backward compatibility as well as new technologies,” Weiser said. “I could see white spaces, as a phenomenon, helping to further that development.”
When the FCC will establish rules for operations within the TV white spaces is a matter of considerable conjecture and debate, complicated by the political reality of this year’s presidential election likely resulting in a turnover on the commission, particularly in the chairman position currently held by Kevin Martin.
However, Motorola’s Sharkey expressed optimism that the FCC will release white-spaces rules before any transition, noting that the current commission has devoted considerable resources to the matter in recent years.
“I think they’re all supportive of it and very interested in it, so I think there’s certainly possibilities for them to move forward with this commission on it,” he said. “Frankly, I think it would be a great thing for the chairman and his legacy to move a technology issue like this forward sooner rather than later.”
But Weiser said he believes it will be extremely difficult for the FCC to meet such a timetable, particularly with Martin stating that his three priorities are resolving the 700 MHz D Block issue, universal-service reform and intercarrier-compensation reform.
“Those are each Herculean tasks — to get one of those done in a six-month period would be a pretty big accomplishment,” Weiser said. “To try to do all three of those makes it difficult to do something like white spaces, which is not a trivial issue either. It’s basically been on the agenda for four years and hasn’t happened. It’s hard to predict it would happen now.”
LMR HOME IN THE TV BANDS
Markets in which Channels 14-20 (470 MHz to 512 MHz) can be used for land-mobile radio.
- LOS ANGELES
- NEW YORK
- SAN FRANCISCO
- WASHINGTON, D.C.
* Systems have not been deployed because of Canadian border restrictions.