Jockeying for TV white space continues

A coalition of high-tech companies and Motorola have filed separate proposals that are similar in concept, but different in approach, concerning how portions of unused “white space” spectrum in the television broadcast bands can be utilized by unlicensed wireless broadband devices.

Both entities believe devices can be built so they don't interfere with existing public-safety communications and TV reception. They also both believe the development of such devices may prove to be a boon to first responders, by providing alternative communications mechanisms and lower-cost broadband equipment.

“We are suggesting that [consumer devices] simply not use any of the channels that public-safety devices are operating on,” said Scott Blake Harris, the attorney for the White Spaces Coalition. “Public-safety devices operate under [TV] channel 21. We suggest the use of channels 21 through 51.”

In addition to gaining exclusive bandwidth in the lower channels, public safety also would be able to utilize consumer devices that have been retuned to the lower frequencies, enabling the sector to leverage economies of scale delivered by standards-based technologies, according to Harris. “This should give them significant cost advantages,“ he said.

Last year, the FCC issued a notice of proposed rulemaking regarding unlicensed operation in the TV broadcast bands. The commission is in the midst of its Final Rulemaking and is expected to make public the results of its testing of prototype devices in July, followed by a Final Report and Order in October.

In its filing, the White Spaces Coalition — consisting of Dell, Google, Hewlett-Packard, Intel, Microsoft and Philips — advocates the use of spectrum-sensing technology to find open white space and believes it is the most reliable and efficient way to avoid harmful interference. “Public-safety devices could use the same technologies and techniques to avoid interference to broadcasters,” Harris said. “These techniques will work the same for channels 4 through 20 as for 21 through 51.”

The coalition supported its filing by developing a prototype — dubbed “Microsoft TV White Spaces Development Platform Version 1” — designed to let the commission explore and evaluate potential hardware and software solutions.

While Motorola also filed comments promoting TV white space usage for both consumer and public-safety applications, it suggested that public safety receive a bigger portion of the spectrum. “We recommended they give public safety priority access on some channels, in particular on channels 14-20, [in addition to] adding two VHF/UHF channels,” to public safety's allocation, said Stu Overby, Motorola's senior director of global spectrum strategy.

Overby thinks that simply opening up channels 14-20 may not be enough for public-safety usage in some cities. Dedicating two more channels from channels 21-25 to public safety would provide much-needed additional spectrum.

In its FCC filing, Motorola also recommended a more conservative technical approach for white space devices. The company would like to see a geo-location database approach in combination with spectrum sensing. A device would reference a database of licensed 700 MHz users and then use the information to avoid transmitting on occupied frequencies. “We believe, at least in the early stages of cognitive radio and TV white space, the database approach is more reliable than the spectrum-sensing approach,” Overby said.

But advocacy groups that long have promoted TV white space use see alternative motives in Motorola's approach. “I think they see [white spaces] for things like their fixed wireless broadband products,” said Michael Calabrese, director of New America Foundation's Wireless Future Program.

Calabrese also said geo-location has a number of drawbacks, including the need to keep the transmitter information up-to-date and the question of who would pay for and maintain a transmitter database. Spectrum sensing is a far easier and more reliable approach to avoiding interference, he said. “You can detect a TV signal that is a thousand times weaker than what a television needs to display a picture. You don't have to rely on the accuracy of external information.”

This is a snapshot of how current frequencies allocated to analog television might be used after the transition to DTV takes place in 2009.