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Since the U.S. government began assigning broadcast rights in the 1920s, the process has worked on a simple principle: A licensee received the exclusive right to the use of a slice of wireless spectrum, usually limited to a specific geographic area. Today, we are facing a big problem. All of the usable spectrum has been allocated, but the explosive growth of smartphones and other wireless devices is creating rapid growth in the demand for bandwidth. Technology that allows existing spectrum assignments to be used more efficiently has helped considerably. The government has also released some existing bandwidth for wireless data use, most recently a big chunk of 700 MHz spectrum that was freed by the transition to digital television.

But neither spectral efficiency nor the slow process of freeing and reassigning spectrum seem adequate to meet soaring demand. A recent report by the President's Council of Advisors on Science & Technology (PCAST) concluded that technology that allows spectrum to be shared among multiple radio systems will be needed: "Spectrum should be managed not by fragmenting it into ever more finely divided exclusive frequency assignments, but by specifying large frequency bands that can accommodate a wide variety of compatible uses."

The Federal Communications Commission, which shares responsibility for spectrum assignments with the Commerce Dept.'s National Telecommunications & Information Agency, is responding. At a recent forum, FCC Chairman Julius Genachowski said: "We are still operating on the same assumptions and parameters as when we began. But the world has changed."

The FCC is currently working on strategic "band plans" for two chunks of spectrum that are going to become available. One is for bandwidth that will be cleared as TV broadcasters agree to auction off spectrum they hold in the 600 MHz band. This will take a few years, because the process mandated by Congress is complex. But it is prime spectrum for wireless data and new technologies will increase the efficiency of its use.

Another band being considered, at 3.5 GHz, poses different challenges. In general, the higher the frequency, the shorter the range of radios using it, and current cellular-type radios operate at between 800 MHz and 2.1 GHz. One idea is to make a virtue of necessity and use the higher frequency spectrum to build large numbers of very small cells, greatly increasing the reuse of channels in any given area. (Wi-Fi and Bluetooth work in much this way.) But there is an additional challenge in the 3.5 GHz band. Certain types of military radar are currently using the band. So the radios will have to avoid interference with these vital installations by respecting "exclusion zones" that bar the use of specific frequencies in specific locations.

To do that, one possibility being debated is the use of "smart," "agile," or "cognitive" radios that are able to operate on a wide variety of frequencies, using software to sense unused and available channels. One recent twist on cognitive radios is to have them interact with a secure database that will warn them away from protected radio systems, such as military radar.  Still, many issues remain to be addressed. For example, one concern expressed by participants in the FCC forum was whether these new, more complex radios could come close to matching the battery life we have come to expect from mobile devices.

These are early experiments in what is likely to become a much broader move toward spectrum sharing as a way to use bandwidth more efficiently. And many critics have doubts about the reliance on unproven technologies. Writes Richard Bennett of the Information Technology and Innovation Foundation: "Advances in technology are often messy and disruptive, so it's much more sensible to continue pursuing a multi-faceted strategy that allows technologies to compete on the basis of the value they offer consumers than to tilt the scales in favor of one and only one system."

Wireless carriers, too, would rather the government just surrender some of its spectrum for wireless data use. Christopher Guttman-McCabe, vice president for regulatory affairs of CTIA: The Wireless Association says that while "it is sensible to investigate creative approaches for making federal government spectrum commercially available, including the development of certain sharing capabilities," the CTIA believes "the gold standard for deployment of ubiquitous mobile broadband networks remains cleared spectrum."

The problem is that it is growing increasingly hard to find spectrum that can be easily cleared. The time and effort needed to clear bandwidth means we will need major technological advances in efficiency to close the gap, including potentially the development of more ubiquitous frequency-sharing technologies.

By Steve Wildstrom

The contents or opinions in this feature are independent and do not necessarily represent the views of Cisco. They are offered in an effort to encourage continuing conversations on a broad range of innovative technology subjects. We welcome your comments and engagement.

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