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Boffin breakthrough doubles Wi-Fi speed

Two-way packet pumper

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A team of researchers has developed a technology that has the potential to double the speed of wireless radios – such as those used by Wi-Fi – by allowing signals to be received and transmitted simultaneously.

"Textbooks say you can't do it," said Stanford University assistant professor Philip Levis when announcing the breakthrough. "The new system completely reworks our assumptions about how wireless networks can be designed."

"Designing radios that can receive and transmit at the same time was thought to be generally impossible," says fellow assistant professor and team member Sachin Katti in a video accompanying the announcement.

The Stanford team – Levis, Katti, and three Stanford graduate students – developed a deceptively simple method to support full-duplex radio transmission and reception. Their idea was so straightforward, Levis says, that one researcher told the team it couldn't work because the idea was so obvious that that someone else must have already tried it and failed.

The technique works with radio signals much the same way that noise-cancelling earphones work with sound waves: a dual-antenna system cancels the outgoing signal, thus enabling a precisely placed third antenna to hear a much-weaker incoming signal.

The main reason why traditional transmitters can't send and receive simultaneously, Levis says, is because "When a radio is transmitting, its own transmission is millions, billions of times stronger than anything else it might hear [from another radio]. It's trying to hear a whisper while you yourself are shouting."

As Levis explains, "In the first design [using the technique] the radio has – rather than one transmit antenna – two transmit antennas. And then it has a receive antenna positioned between them. So what happens is the two transmit signals interfere destructively at the receive antenna, and the receive antenna doesn't hear the [transmitted] signal.

"It effectively hears not silence, but a very, very quiet version of the signal because those two conflicting signals cancel each other," he says. "And so you create this null position where the receiver doesn't hear itself, so it can actually hear packets from other radios."

When we asked Levis about the potential for commercializing the technology and what the incremental cost of upgrading Wi-Fi access points to take advantage of it might be, he said: "Like anything, what matters is scale." Referring to the fact that 802.11n MIMO transmitters already have two antennas, he said: "You'll have to put another antenna on there, but that's not that expensive – 30 cents, 40 cents."

The cancellation circuitry would add another expense, but if a custom chip could be designed to handle the necessary computation in hardware rather than through software code, Levis said, "It wouldn't be expensive to make in quantity."

Not that all the challenges have yet been met. "There are some things that are hard," he said. "The things which are hard relate mostly to RF engineering. We've been able to demonstrate it in a lab setting – not a tightly controlled lab setting, but in offices – but the real world is more difficult."

More work needs to be done before the technology becomes a product, and the team – which has a provisional patent on the technology – has plans to get that work done. One of the three students involved in the project, Kannan Srinivasan, has graduated from Stanford and is now doing post-doc research at the University of Texas at Austin; the other two, Jung Il Choi and Mayank Jain, will be graduating from Stanford this spring. At that point, Levis said, "The students and I hope to do a startup to commercialize it" with Katti "at the very least in an advisory or consulting role," perhaps beginning as early as this summer. ®

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