Full-duplex radio breakthrough can double Wi-Fi capacity

Stanford University researchers have found a way to double the capacity of wireless networks, while at the same time making them more reliable and efficient.

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Their discovery supports full-duplex connections - allowing data to be transmitted between devices in both directions at the same time rather than in just one direction as has been the case until now, the researchers say. Both Wi-Fi and cellular networks could benefit.

The speed in either direction remains the same, however. So if a wireless network has a speed of 15Mbps, that doesn't change with the new technology, it just opens up the channel so both ends can send and receive at 15Mbps simultaneously. With current technology, one side sends then waits while the transmitter at the other end takes its turn.

"This technology breaks a fundamental assumption about the way wireless networks work," says Philip Levis, an assistant professor of computer science and electrical engineering at Stanford, one of the leaders of the research.

Equipment initially used to demonstrate this new capability has been refined so it requires two rather than three antennas, and details of this upgrade will be made public in a paper next month. This summer, the researchers will look into making commercial products based on their research, he says.

Using a cancellation technique, the researchers make it possible both parties in a wireless conversation can talk at the same time without drowning each other out. This can help reduce the number of failed connections on Wi-Fi networks that are dense and busy, researchers say.

Until now, the volume of a transmitted radio message overpowered the relatively weak incoming signal from the radio at the other end. Or as the researchers put it, "when a node transmits, its own signal is millions of times stronger than other signals it might hear: the node is trying to hear a whisper while shouting."

Technology developed at Stanford cancels out the signal created by sending machines so they can pick up the relatively quiet transmissions coming at them.

That assumption was that if both ends of a radio connection spoke at once, the signals being received would be obscured. The traditional way around the problem is to have radios communicating on the same frequency take turns transmitting so the outgoing signal doesn't interfere with the incoming. That's why people carrying on a conversation over wireless say, "Over," when they are done talking so there is no overlap.

Most of the time this isn't a problem in Wi-Fi networks because conversations take place between access points and individual end-user devices with little likelihood of simultaneous transmissions, Levis says. But in crowded environments, such simultaneous transmissions do occur and none succeeds.

With full-duplex transmissions enabled, when an access point starts to receive from an end device, it can immediately send a message back saying it is busy so other end devices in the area know not to transmit, Levis says.

In cellular networks where carriers use repeaters to extend the range of base stations, this technology could be used to make transmitting boosted signals from cell phones to base stations and base stations to cell phones more efficient.

The researchers' design uses two transmitting antennas, one receiving antenna at each node. Both transmitting antennas send the same data, but the receiving antenna is placed so it receives signals from the transmitting antennas that are out of phase so they cancel out. The result is there is no signal - or at least a very reduced signal - for the receiving antenna to receive.

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Tags wirelessNetworkingWi-FiStanford UniversityWLANs / Wi-Fi

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Tim Greene

Network World
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