Using MIMO to tap unused UHF for long-range wireless

Researchers have found a way to make the most of unused UHF TV spectrum by serving up fat streams of data over wireless hotspots.

Researchers from Rice University’s Wireless Network Group have unveiled a multiuser, multiantenna transmission scheme for UHF.

“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 Wi-Fi.

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

Rice’s technology combines several proven technologies that are already widely used in wireless data transmission. One of these is multiple-input, multiple-output (MIMO), a scheme that employs multiple antennas to boost data rates without the need for additional channels or transmitter power.

Essentially, MIMO allows for a larger wireless pipeline, and the technology is standard in the latest generation of wireless routers and networking equipment.

Researchers used the Rice Wireless Network Group's WARP platform to build the world's first open-source MIMO test system capable of serving multiple users over UHF.

Parts of the UHF spectrum were opened after the recent switch to digital television, which has a smaller broadcast footprint than analog TV. One popular idea for the liberated portion of the spectrum is for ‘open’ wireless access points like those used for today’s Wi-Fi hotspots.

UHF for broadband internet is particularly appealing for rural areas where wired broadband is unavailable.

Rice graduate student Narendra Anand compares the trade-off of capacity for range to lanes on a road.

“Imagine that the Wi-Fi 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.

Rice University Wireless Network Group's Edward Knightly, Narendra Anand and Ryan Guerra.

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 enabled the team to perform a side-by-side comparison of multiuser MIMO for UHF and for both 2.4 and 5.8 GHz Wi-Fi.

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

The research was supported by the US National Science Foundation and Cisco Systems.