Wireless Hotspot: Unused TV Spectrum

Wednesday, September 10, 2014 @ 04:09 PM gHale


There is now a way to make the most of the unused UHF TV spectrum by serving up streams of data over wireless hotspots that could stretch for miles.

A multiuser, multiantenna transmission scheme for UHF, a portion of the radio spectrum traditionally reserved for television broadcasts, is now going through the demonstration phase, said researchers at Rice University’s Wireless Network Group.

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“The holy grail of wireless communications is to go both fast and far,” said lead researcher Edward Knightly, professor and chair of Rice’s Department of Electrical and Computer Engineering. “Usually, you can have one or the other but not both. Wireless local area networks today can serve data very fast, but one brick wall and they’re done. UHF can travel far, but it hasn’t had the high capacity of WiFi.

“This provides the best of both worlds,” Knightly said of the new technology.

Rice’s technology combines several proven technologies already widely used in wireless data transmission.

One of these is “multiple-input, multiple-output” (MIMO), a scheme that employs multiple antennae to boost data rates without the need for additional channels or transmitter power. In effect, MIMO allows for a larger wireless “pipeline,” and the technology is standard in the latest generation of wireless routers and networking equipment.

Parts of the UHF spectrum opened after the switch to digital television, which has a smaller broadcast footprint than analog TV. UHF signals travel for miles, and one popular idea for the liberated portion of the spectrum is for “open” wireless access points like those used for today’s WiFi hotspots. Using UHF for broadband Internet is appealing for rural areas where wired brandband is unavailable.

“When comparing UHF and WiFi, there’s usually a tradeoff of capacity for range or vice versa,” said Rice graduate student Narendra Anand, the lead author of the new study. “Imagine that the WiFi access point in your home or office sends data down a 100-lane highway, but it’s only one mile long. For UHF, the highway is 100 miles long but only three or four lanes wide. And you cannot add any lanes.

“To be able to leverage the best characteristics of the UHF band, we need to be able to efficiently use the lanes that we have,” Anand said. “One way to do that is with multiuser MIMO, a multiantenna transmission technique that serves multiple users over the same channel simultaneously.”

Knightly, Anand and Rice graduate student Ryan Guerra designed the first open-source UHF multiuser MIMO test system. Based on Rice’s “wireless open-access research platform,” or WARP, the system allowed the team to perform a side-by-side comparison of multiuser MIMO for UHF and for both 2.4 gigahertz and 5.8 gigahertz WiFi.

“Based on over-the-air experiments in a range of indoor and outdoor operating environments, we found that UHF-band multiuser MIMO compared favorably and produced high spectral efficiency as well as low-overhead wireless access,” Knightly said.



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