Double wireless capacity with ‘full-duplex’

RICE U. (US) — A new technology that allows wireless devices to “talk” and “listen” on the same frequency could pave the way for 4.5G and 5G networks.

The “full-duplex” technology developed by engineers at Rice University also could allow wireless phone companies to double throughput on their networks without adding a single cell tower.

“Our solution requires minimal new hardware, both for mobile devices and for networks, which is why we’ve attracted the attention of just about every wireless company in the world,” says Ashutosh Sabharwal, professor of electrical and computer engineering. “The bigger change will be developing new wireless standards for full-duplex. I expect people may start seeing this when carriers upgrade to 4.5G or 5G networks in just a few years.”


Rice University graduate student Melissa Duarte with a “full-duplex” test device. (Credit: Jeff Fitlow, Rice University)

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In 2010, Sabharwal and Rice colleagues Melissa Duarte and Chris Dick published the first paper showing that full-duplex was possible . That set off a worldwide race to demonstrate that the technology could actually be used in a real network.

This summer, Sabharwal and Rice’s Achaleshwar Sahai and Gaurav Patel set new performance records with a real-time demo of the technology that produced signal quality at least 10 times better than any previously published result.

“We showed that our approach could support higher throughput and better link reliability than anything else that’s been demonstrated, which is a plus for wireless carriers,” Sabharwal says. “On the device side, we’ve shown that we can add full duplex as an additional mode on existing hardware.

“Device makers love this because real estate inside mobile devices is at a premium, and it means they don’t have to add new hardware that only supports full duplex.”

Can you hear me now?

To explain why full-duplex wireless was long thought impossible for wireless networks, Sabharwal uses the analogy of two people standing far apart inside an otherwise empty arena. If each shouts to the other at the same time, neither can hear what the other is saying.

The easy solution is to have only one person speak at a time, and that’s what happens on two-way radios where only one person may speak at a given time. Cell phones achieve two-way communications by using two different frequencies to send and listen.

Rice’s team overcame the full-duplex hurdle by employing an extra antenna and some computing tricks. In the shouting analogy, the result is that the shouter cannot hear himself, and therefore hears the only other sound in the arena—the person shouting from far away.

“We send two signals such that they cancel each other at the receiving antenna—the device ears,” Sabharwal says. “The canceling effect is purely local, so the other node can still hear what we’re sending.”

He says the cancellation idea is relatively simple in theory and had been proposed some time ago. But no one had figured a way to implement the idea at low cost and without requiring complex new radio hardware.

“We repurposed antenna technology called MIMO, which are common in today’s devices,” Sabharwal says. “MIMO stands for ‘multiple-input multiple-output’ and it uses several antennas to improve overall performance. We took advantage of the multiple antennas for our full-duplex scheme, which is the main reason why all wireless carriers are very comfortable with our technology.”

Open access

Sabharwal says Rice is planning to roll its full-duplex innovations into its “wireless open-access research platform,” or WARP . WARP is a collection of programmable processors, transmitters and other gadgets that make it possible for wireless researchers to test new ideas without building new hardware for each test.

Adding full-duplex to WARP, says Sabharwal, will allow other researchers to start innovating on top of Rice’s breakthrough.

“There are groups that are already using WARP and our open-source software to compete with us,” he says. “This is great because our vision for the WARP project is to enable never-before-possible research and to allow anyone to innovate freely with minimal startup effort.”

Sabharwal’s team has gone one step further and achieved asynchronous full-duplex too—that is one wireless node can start receiving a signal while it’s in the midst of transmitting. Asynchronous transmission is import for carriers wishing to maximize traffic on their networks, and Rice’s team is the first to demonstrate the technology.

“We’ve also developed a preliminary theory that explains why our system is working the way that it is,” Sabharwal says. “That’s also important for carriers and device makers, because engineers aren’t likely to implement something like this without a clear understanding of fundamental trade-offs.”

Rice’s research has been funded by the National Science Foundation, the Roberto Rocca Education Program, and Xilinx Incorporated.

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