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Interference Unlocks Faster Mobile Data

Intentionally combining radio signals from different transmitters could allow mobile devices to download at much higher speeds.
August 3, 2011

While demand for wireless data grows exponentially, the supply of available radio spectrum remains stubbornly fixed. A new technology could get more from that precious resource by turning a conventional piece of engineering wisdom on its head.

“Every other wireless system up to now has avoided interference,” says Steve Perlman, founder of technology incubator Rearden Labs, based in San Francisco. “This embraces it.”

Perlman’s team is testing a new kind of wireless network that he says can fit thousands of times more data into the same amount of radio spectrum as a conventional one. The approach is known as DIDO, for distributed input distributed output, and is currently being tested around Palo Alto, California, and in rural Texas.

Today, wireless data is most constrained on cell-phone networks, which are struggling under demand from growing numbers of increasingly capable mobile devices.

All wireless systems have access to a fixed portion of radio spectrum, and hence a fixed capacity for transmitting data, known as bandwidth. Today’s wireless networks, like those that serve data to cell phones, share that bandwidth among the gadgets connected to the network. The more devices that connect, the smaller the slice for any individual user, and the slower the download speeds. By contrast, a DIDO system, says Perlman, “can offer the full bandwidth available to the network to every user.”

DIDO involves intentionally combining signals from multiple transmitters, exploiting interference to create a bubble of crystal-clear reception around every user. Each signal that leaves an individual transmitter is incomprehensible until it encounters, and interferes with, other DIDO signals near a device connected to the network.

This approach removes the need to share bandwidth, says Perlman, because each bubble covers a small area and can occupy all the spectrum available to the network. The size and shape of the bubbles varies depending on the number of antennas broadcasting to a device, says Perlman.

Designing radio signals that will interfere with one another in just the right way takes complex mathematics and careful coordination among the different DIDO transmitters. “The computational requirements are very large, but we solved that by using a cloud server,” says Perlman.

When a device wants to connect to a DIDO network, it contacts the nearest tower and sends information on the local radio conditions back to the server. The server combines that information with data from other nearby devices to design signals that will combine in the right way. Perlman says tests involving up to 10 devices downloading at the same time have been successful. Individual devices can’t easily take advantage of DIDO’s design when uploading, however, so sending data in that direction would be slower.

Mobile carriers are building new, higher-bandwidth networks in many areas, but something like DIDO could add even more capacity, says Perlman. “Urban areas are the worst,” he says. “New York is absolutely dying. You just can’t get any 3G data because there’s not enough bandwidth to divide up.”

If a DIDO network was rolled out to supplement today’s cellular ones, it would use many small towers rather than the large ones typically used now. “You would rely on lots of little towers scattered about that will work together to target you with your own signal,” says Perlman. “They could be on light poles, on top of buildings, in businesses.” Those small base stations would be under the control of DIDO servers constantly calculating how to make signals that interfere in just the right way. Those signals could be altered to deal with changing radio conditions and transmitter availability as gadgets moved, even when users were driving.

The DIDO system that Perlman is testing in Texas sends signals more than 30 miles. Perlman hopes this will catch the eye of telecom companies required by federal law to provide broadband Internet to isolated communities.

“DIDO looks very promising,” says Bhaskar Krishnamachari, a professor at the University of Southern California, who develops techniques to make wireless networks more efficient. “I’ve seen enough similar suggestions in the academic world to believe that this can certainly work,” he says. “Their demonstration sounds significantly beyond anything like this before.”

However, relying so heavily on cloud processing to keep a network functioning is an untested idea, notes Krishnamachari, and it may prove challenging to deploy DIDO in areas with many users.

Furthermore, even as DIDO exploits interference, it is still limited by it, notes Krishnamachari: “If many users are close together, then it is more likely that the channels for the multiple signals will look similar.” This could pose a problem in urban situations (for example, in a busy coffee shop), although Perlman says that with enough base stations, it should be possible to target devices in close proximity.

Jay Jayasimha, chief technology officer at Dialogic, which offers technology that speeds up the wired connections that link cell towers to the Internet, says Perlman’s idea makes sense and will certainly be taken seriously by telecom networks. “They need new ideas, because the problem of wireless data demand is only going to get worse,” he says. “This idea is trying to solve the right part of the problem.”

Jayasimha notes that DIDO will need to be compatible with existing technology if it is to be viable. “There needs to be a migration path,” he says.

Perlman says DIDO could initially be rolled out parallel to existing networks in areas troubled by overwhelming demand. Cell phones would require an additional radio chip to tap into such a network, though.

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