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RF/Microwave??

White spaces marked off as wireless arena

Posted: 16 Oct 2008 ?? ?Print Version ?Bookmark and Share

Keywords:DTV switch? white spaces? RF spectrum?

When the long-awaited DTV switchover in the United States occurs next February, broadcasters will relinquish the analog channels some have held for more than half a century. Considering the prices mobile operators paid for "beachfront" spectrum above 700MHz earlier this year, these unlicensed VHF/UHF channels!known as white spaces and scattered throughout the 54MHz to 698MHz region of the RF spectrum!would easily have pulled in billions had they been auctioned.

But they weren't!and a range war is raging over how they should be used.

The first squatters in the newly unlicensed channels will likely be a handful of tech giants, including Google, Hewlett-Packard, Microsoft and Motorola, which have joined with public-interest groups and media companies to create the Wireless Innovation Alliance (WIA). Several RF companies with links to WIA members are testing prototype products known as white-spaces devices (WSDs) with the U.S. Federal Communications Commission (FCC).

Superior UHF propagation
Transmissions from these WSDs would enjoy UHF propagation characteristics far superior to those of the now overcrowded 2.4GHz band. Applications range from a data network that Google's Larry Page has described as "Wi-Fi on steroids" to all sorts of video, to networks of humble household objects like washing machines, toasters and light bulbs, all communicating with one another.

The brave new wireless world envisioned by the WIA could possibly disrupt the airwaves with interference!and undoubtedly will upset the broadcasters' VHF/UHF monopoly. The broadcasters are livid, describing the plan as unworkable. And although neither side is saying so, it is not difficult to translate "broadband" into video and "unlicensed" into dollars.

The FCC has just concluded its field testing of WSD submitted by Motorola, Philips, Adaptrum, Shared Spectrum, and Singapore's Institute for Infocomm Research. The test results will almost certainly start another round of vituperation between the techs and the broadcasters.

There is quite a bit of white space out there, particularly in rural areas of the U.S., according to the New Frontiers Foundation, a member of WIA. But the contested turf is in large media markets where the broadcasters rake in most of their profits. New Frontiers, which believes the socially proactive use of white spaces can help close the digital divide, has estimated the percentage of unused spectrum varies from 18 percent in New York to more than 60 percent in less densely populated areas, such as Wichita, Kansas.

These figures are contested by the broadcast lobby, led by Maximum Service TV (MSTV), which describes itself as the engineering arm of the National Association of Broadcasters. David L. Donovan, president of MSTV, argues that "you can't just add up the DTV channels" that are not licensed in a geographic area, because high-power TV signals can travel many miles beyond their licensed region. More important, he said, is the presence of low-power devices licensed to operate in the VHF/UHF bands.

Wireless microphones such as those used by sportscasters who interview coaches and players on the sidelines of sporting events can be licensed to operate legally in the VHF/UHF bands (although many more operate unlicensed). Because wireless microphones are mobile and transmit at much lower power than DTV stations, they represent a tougher interference problem for WSDs.

WIA is not to be deterred. Noting that maximum WSD power transmission has been limited to 100mW, WIA consultant Edmund Thomas is confident the interference problem can be solved by deploying the right set of technologies.

At first glance, most RF engineers might think it unduly cautious to fear that low-power (below 100mW) use of part of an unoccupied 6MHz channel could cause problems for DTV, even with the longer range of UHF signals. RF engineers have been solving interference problems for years, notably the Wi-Fi/Bluetooth contention in the 2.4GHz band. But the WIA has set its counterinterference bar high. Two technologies!and possibly a third!would be employed: global positioning systems, cognitive radio and beacons.

To guard against interference with wireless mikes, some WIA prototypes include beacon technology. When a broadcaster sets up at a sporting event, for example, its technical team will deploy a beacon that will broadcast at a power the white-spaces devices can easily sense. Incumbents in the VHF/UHF space, which include broadcasters and wireless-microphone interests led by U.S. manufacturer Shure Inc., have launched a spirited verbal attack on WSDs.

TV white space operation is illustrated in this simplified diagram.

Infection analogy
MSTV and a group of stations asked the FCC why it would consider allowing "millions and millions of these interference-causing devices, like 'germs,' to spread throughout America with the ability to attack the TV receivers in people's homes, apartments, hotel rooms, hospital rooms, dormitories, etc., with no way for the owner of the TV set ('the victim') to determine who was causing the 'illness' to his or her TV set?"

That sort of rhetoric might not impress engineers, but it has induced no fewer than 70 members of Congress to sign a statement expressing deep concern about the use of white spaces for personal wireless devices.

It's not that there isn't cause for concern. The most often noted technology trap is the so-called hidden-node problem. Essentially, it is quite possible for a WSD to be in a position where it cannot sense a DTV signal while a nearby TV!in an adjacent apartment, for example!is receiving the signal. This is most likely in areas dense with large concrete and steel structures.

A wireless-technology deployment with as large a market and potential public impact as WSDs would typically attract a lot of attention from the IEEE. In the political wrangle over white spaces, however, it seems as though nothing is typical.

Excluding mobile
The IEEE does, in fact, have a working group!IEEE P802.22!tasked with developing a detailed radio interface specification for a cognitive radio that would share airwaves with TV transmitters and wireless microphones in the UHF band. The specification includes cognitive-radio techniques to avoid or at least minimize harmful interference.

At the group's very creation, however, its scope was limited to fixed wireless devices, such as those used by wireless Internet service providers. Mobile devices like WSDs were specifically excluded.

P802.22 chairman Carl R. Stevenson has no trouble describing WSDs and the plans to deploy them as ill-conceived. "I believe that what the WIA is proposing to do is a recipe for disaster," said Stevenson, a seasoned RF engineer with a long and distinguished career in both industry and standards groups. "The UHF spectrum actually propagates too well for short-range devices."

Given this perspective, it should be no surprise that Stevenson was instrumental in limiting 802.22's purview to fixed devices. As is typical at IEEE, his views are well aligned with those of the companies sponsoring his chairmanship.

Although most IEEE chairs are employees of companies large enough to handle the expenses equivalent to a salary (Intel is a good example), some consultants support their IEEE work by acquiring sponsors. Stevenson took the latter route. MSTV, Shure and Qualcomm!all opponents of WSDs!sponsor his IEEE activities. This is within IEEE guidelines, and sponsorship disclosures are made to the IEEE.

Stevenson says there is no conflict of interest and that none of his corporate sponsors have "ever pressured me in any way." The broadcasters, in particular, brought a great deal of knowledge and expertise in broadcast technology to the working group, he said.

The outcome of FCC testing is still uncertain, but if the commission allows WSDs, the lack of a standard will probably limit their deployment to a handful of companies. Unless extraordinary measures are taken to level the playing field, the companies testing prototypes will have a very big jump on the rest of the RF community.

- Jack Shandle
EE Times





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