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Wireless Spectrum Sharing

We are finally coming to the end of days in which spectrum frequencies are parcelled out to allocated bandwidths for different services. This has been impractical for some time now, as older services have had larger frequency ranges, leaving newer services squeezed together into increasingly smaller spaces.

This crowding is the problem. With large swathes of space allocated for older technologies such as AM radio and TV, there simply is not enough frequency spectrum left to squeeze everything else in. Worse, oft times allocated frequency sits idle and unused in one area, whilst heavily utilised in another. It is allocated however, so nothing else can legally use it.

Sometimes time is also a factor, as vast swaths of spectrum can sit idle for minutes or hours at a time, even in the most crowded, desperate city areas where frequency is like gold dust.

Of course, a dynamic wireless network of transmitters/receivers, capable of switching all services to whatever space is available on a constantly altering network, has been impossible. Its essential for a true sensor web, but has been impossible to create.

That has finally started to change. Dina Katabi, an associate professor of electrical engineering and computer science at MIT, is busy teaching wireless technologies how to share what spectrum is left.

Regulatory bodies like the Federal Communications Commission are unlikely to grant additional technologies access to previously allocated spectrum anytime soon. But spectrum sharing could have immediate implications for the so-called white spaces - the frequency bands vacated when television moved from analog to digital. In the United States, the FCC has agreed to leave those bands unlicensed, at least for now, and a coalition of technology companies that includes Google, Microsoft, and Intel hopes to use them for high-speed data connections for portable devices - wherever they are.

Technologies that want to use the white spaces, however, will have to show that they won't interfere with each other, or with devices already authorised to use the same spectrum. One advantage of the MIT researchers' work is that it takes such a general approach to the problem of spectrum sharing that it should work with most existing wireless data devices - and others yet unimagined.

The questions that the work hopes to address, are as follows:

  1. How, without snooping on transmissions, do you determine which sections of the spectrum from your current location to the next network hop, are currently unutilised?

  2. Which services should get priority in utilising unutilised space, and how long should that priority hold for?

Different transmitters might be able to use the same frequency, for instance, if their intended receivers are far enough apart. "The opposite is also not true," Katabi says. "The fact that certain frequencies do not have power does not mean that you can use them, because if you use them, you could potentially leak power to nearby frequencies." A radio transmitter uses filters to concentrate power into specific frequency bands, but the filters under current technology, never work perfectly.

So Katabi and her colleagues propose that, instead of looking at the amount of power in a frequency band, wireless devices look at the changing power profiles of other devices sharing the same spectrum. Most wireless devices will cut their transmission rates if they encounter congestion. By tracking power over time, the MIT system determines whether a particular choice of frequency is forcing other devices to slow down.

"It's very important; it's good stuff," says Anant Sahai, an assistant professor of electrical engineering and computer sciences at the University of California, Berkeley, who specialises in spectrum sharing. "I can see how this kind of thinking is going to be important in the white spaces." Sahai added, however, that Katabi and Rahul's work is at the "protocol level" - the level of the transmission scheme - and that implementing it in the white spaces will require similar innovation in hardware. Nonetheless, he says, "what's very encouraging about their work is that they've actually put together an implementation to test it out."


Sharing the air

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