Researchers use active UHF band in the US for 'super Wi-Fi'
14 July 2015
Rice University engineers have demonstrated the first system that allows wireless data transmissions over UHF channels during active TV broadcasts.
If the technology were incorporated into next-generation TVs or smart remotes, it could significantly expand the reach of so-called 'super Wi-Fi' networks in urban areas.
"Due to the popularity of cable, satellite and Internet TV, the UHF spectrum is one of the most under-utilized portions of the wireless spectrum in the United States," says lead researcher Edward Knightly. "That's a bitter irony because the demand for mobile data services is expected to grow tenfold in the next five years, and the UHF band is perfectly suited for wireless data."
The UHF spectrum, which ranges from 400 to 700MHz is often called the 'beachfront property' of the wireless spectrum. Unlike the higher frequency signals used for existing Wi-Fi hotspots, UHF signals carry for miles and are not blocked by walls or trees. Because of these advantages, wireless data hotspots that use UHF are often referred to as "super Wi-Fi."
In the US, TV broadcasters have been given preferential access to the UHF spectrum for more than 50 years. If no TV broadcaster has laid claim to a UHF channel, the Federal Communications Commission allows secondary users to transmit wireless data on that channel, provided that the transmissions do not interfere with TV broadcasts in any part of the UHF spectrum. The rules governing this secondary access are often referred to as 'TV white space' rules in reference to the industry term for used or blank portions of the TV spectrum.
"Unfortunately, in the most densely populated areas of the country, where the need for additional wireless data services is the greatest, the amount of available white space is extremely limited," Knightly says. "In our most recent tests in Houston, one channel is open in parts of the city and none are available in others. This is fairly typical of a large US urban area."
Though most of the UHF band is already taken in US cities, it is largely under-utilized. According to a 2014 report by the TV rating company Nielsen, fewer than 10 percent of US households rely on over-the-air broadcasts for TV programming.
To demonstrate that wireless service providers could make use of the UHF spectrum without interfering with TV broadcasters, Knightly and Rice graduate student Xu Zhang developed a technology called 'Wi-Fi in Active TV Channels' (WATCH), and received FCC approval to test it at the Rice campus in 2014.
WATCH requires no coordination with or changes to legacy TV transmitters. Instead, TV signals are broadcast as normal and the WATCH system actively monitors whenever a nearby TV is tuned to a channel to avoid interfering with reception. The technology to allow this comes in two parts. One aspect of WATCH monitors TV broadcasts on a channel and uses sophisticated signal-cancelling techniques to insert wireless data transmissions into the same channel; that eliminates TV broadcasts from interfering with the super Wi-Fi data signals being sent to computer users.
The other aspect of WATCH is dedicated to making certain that data transmissions do not interfere with TV reception; this part of the technology would require TVs to report when they are being tuned to a UHF channel. In practice, this could be accomplished with either smart TV remotes or next-generation TV sets. In the tests at Rice, Zhang constructed a 'smart-remote' app that reported whenever a test television in the lab was tuned to a UHF channel. When that happened, the WATCH system automatically shifted its data transmissions to another part of the UHF spectrum that wasn't being used.
"Our tests showed that WATCH could provide at least six times more wireless data compared with situations where we were limited only to the traditionally available white-space spectrum," says Knightly. With WATCH in use, it took a fraction of a second longer than normal to tune in a UHF TV broadcast on the test television. While the increment could be measured - it was less than a 5 percent increase - it was almost imperceptible to a person switching channels.
"Allowing the UHF spectrum to be inefficiently used makes little sense today and will make even less sense in the future," says Knightly. "There are already more people in the United States who require mobile data services than there are people using broadcast-only TV. By showing that these two communities can coexist, we hope to spur innovation and a public debate about how this valuable resource could be used."