TV White Spaces

TV White Spaces: New Era for Wi-Fi?

Posted by Sam Churchill on November 5th, 2008

Tuesday’s decision to allow “white spaces” for WiFi-like services on unused television frequencies, represents a new era to provide broadband wireless using unlicensed devices.

One potential use that could come from the decision is neighborhood wireless zones. Carriers could provide, for example, rural broadband Internet access without incurring the huge costs of buying spectrum for a limited number of subscribers.

Roger Cochetti, group director of U.S. Public Policy for the Computing Technology Industry Association, said the FCC’s ruling will soon make Internet access “appear as common and ubiquitous as air” and will work to collapse the digital divide. The Wireless Innovation Alliance includes Microsoft, Google, Motorola, HP and many other high tech firms.

In his blog, Larry Page, Google co-founder referred to that as “Wi-Fi on steroids.” He also praised the FCC for adopting the same successful unlicensed model used for Wi-Fi. “Now that the FCC has set the rules, I’m sure that we’ll see similar growth in products to take advantage of this spectrum.

Yesterday FCC Chairman Kevin Martin and fellow commissioners unanimously agreed with proponents of free public access, including Google and Microsoft. The FCC in October released a report (pdf) that found the interference issue to be inconsequential, and noted that, “when factoring in actual operation under nonstatic conditions, the situation only improves.” They also noted that wireless microphones would be protected in several ways. For example, the commission has mandated that devices include the ability to listen to the airwaves to sense wireless microphones as an additional measure of protection for these devices.

Full power analog television broadcasts operate between the 54 MHz and 806 MHz in the United States (Channels 2-69). In February 2009, full power TV stations will be required to switch to digital transmission and operate only between 54-698 MHz. One of the advantages of DTV is that million watt DTV television stations can operate on adjacent channels (ie Ch 41, 42, 43…) because a digital signal can more easily filter out interference. That also frees up lots of unused open channels.

The broadcast television spectrum is even lower than the 700 MHz band. It should transmit further and penetrate better than today’s Wi-Fi technology, so broadband access can be spread using fewer base stations. But power is limited and the bandwidth is only the width of one (unused) television channel - 6 MHz. That’s less than one third the bandwidth of the 20Mhz channels found on the 2.4 GHz band (which is 85MHz wide).

According to Martin’s description, portable devices that use both spectrum-sensing technology and geolocation databases to prevent interference to DTV would be limited to 100 mW of power, or 40 mW when operating on channels adjacent to active DTV channels. If the devices use only spectrum sensing, they would be limited to 50 mW, or 40 mW on adjacent channels.

The Association for Maximum Service Television said that the 40 mW level on adjacent channels would “eviscerate” DTV in some areas, particularly in cities.

A white spaces access point will use the IEEE 802.22 standard ( wikipedia), which utilizes two different strategies to avoid interference. First, it listens for other users, and moves to another channel if there’s the potential to cause interference. But this technique is only partially successful. For example, an access point in the “shadow” of a building may not detect a distant tv station. An ajoining house, with a clear shot to the distant television station, might then pick up both transmitters on the same channel, causing interference.

The second technique — required in “white space” access points — would utilize GPS to determine the radios position and interrogate the FCC’s list of broadcast transmitters. Access points would automatically avoid using nearby television channels based on its location.

source : dailywireless.org