Two months ago, the President’s Council of Advisors on Science and Technology (PCAST) unveiled a bold plan to share 1,000MHz of federal spectrum with cellular providers. It wasn’t exactly what carriers were looking for. They’d prefer exclusive licenses to use spectrum whenever and wherever they need it.

But the Federal Communications Commission has decided to adopt the plan, or at least its first steps. By the end of this year, the FCC announced this week, it will "initiate formal steps to implement the key recommendations of the PCAST report." The first target is freeing up 100MHz of spectrum in the 3.5GHz band for small cell use.

"Today’s iPhone announcement and last week’s release of the new Kindle Fire, Windows 8/Nokia Phone, and Droid RAZR by Google/Motorola offer the latest evidence that, over the past few years, the US has regained global leadership in key areas of communications technology," FCC Chairman Julius Genachowski said Wednesday. "These high-performance devices, and the demands they place on our broadband networks, underscore a critical challenge."

The brief announcement actually doesn’t mention the word "sharing." Perhaps the FCC is trying to avoid controversy. But we confirmed with an FCC spokesperson yesterday that the commission plans "to take spectrum in 3550-3650 (currently used for radar) and share it with wireless carriers," as recommended by PCAST.

A small first step

This is one step toward setting 1,000MHz of spectrum between 2700MHz and 3700MHz aside for sharing between federal agencies and multiple carriers. This isn’t what cellular carriers like Verizon, T-Mobile, AT&T, and Sprint would consider beachfront spectrum (LTE frequency bands are anywhere between 700MHz and 2,600MHz). With the use of higher frequencies that don’t travel as far, cell providers will have to build lots of "small cells" instead of traditional towers. Because spectrum will be shared instead of exclusively licensed, a potentially complicated system to determine who gets to use spectrum when and where will have to be implemented.

PCAST is chaired by Mark Gorenberg of Hummer Winblad Venture Partners and includes people like Microsoft Chief Research Officer Craig Mundie and Google Executive Chairman Eric Schmidt. The group's report also supports work on tightening GPS receiver standards, in response to the failure of LightSquared to build a network that wouldn't disrupt GPS devices on adjacent airwaves. LightSquared's 1525-1559 spectrum isn't part of the 1,000MHz targeted by PCAST, but PCAST said the controversy illustrated the need for better receiver management.

In a response to the July PCAST report, the CTIA Wireless Association issued some muted criticism, saying "the gold standard for deployment of ubiquitous mobile broadband networks remains cleared spectrum." We contacted the CTIA yesterday about the FCC proposal in the 3550-3650 band, and haven’t heard back.

PCAST doesn't suggest ending the practice of clearing and reallocation of spectrum, but it does think that approach won't be enough to handle growing wireless needs. The FCC is not abandoning exclusive licenses either, as it's moving ahead with plans to auction TV airwaves to mobile carriers.

The fact that the FCC jumped on the small cells for 3550-3650 proposal isn’t surprising, as even before the PCAST report Genachowski had supported the dedication of that band to small cells. But implementing the entire PCAST plan will be difficult.

Exclusivity is problematic, and hard to get rid of

Exclusive licenses to spectrum give government agencies and private businesses the right to use that spectrum at all times and in all US geographic regions, even if it’s only needed at certain times and in certain places.

For something like air-to-air combat training missions, this makes sense. They aren’t being flown 24/7 and they aren’t conducted everywhere, so those same airwaves could be reused in many times and places. But do carriers want them? Even when they have exclusive access, cell phone companies like Verizon don't always build out the infrastructure needed to use the airwaves. In the case of Verizon, it bought spectrum in an auction, never used it, and then bought a few billion dollars worth of more spectrum from cable companies Comcast, Bright House Networks, and Time Warner Cable.

The process of clearing federal users off entire swaths of spectrum so it can be reallocated for commercial use is costly. The National Telecommunications & Information Administration (NTIA) has estimated it would take $18 billion and 10 years to clear 95MHz of prime spectrum from 1755-1850.

Michael Marcus, who worked at the FCC for 25 years and is now a consultant on wireless technology and spectrum policy, points out that the auction process is burdensome for federal agencies, because the money they get in exchange for giving up spectrum isn’t received until well after they need it. Another problem: the NTIA and its Interdepartment Radio Advisory Committee act more like "chums" than "regulators" when it comes to spectrum management, in Marcus’s view. PCAST proposes creation of a senior committee in the White House to oversee NTIA, a good step toward bringing "adult supervision" to the process, he said in an interview before this week's announcement.

Continuing an exclusionary approach won’t meet everyone’s needs anymore—the airwaves are too congested, PCAST argues. But PCAST’s plan of sharing will have to overcome technical hurdles and a lack of information—we simply don’t know what all the spectrum controlled by government agencies is used for.

"I think we're going to be operating in a highly constrained environment for a long time to come," said Peter Rysavy of Rysavy Research earlier this summer. He says difficulties in freeing up spectrum for commercial use may well lead to increased network congestion and higher prices for consumers.

‘The sharing concepts are going to take longer than people anticipate, and I think it means we're not going to offer as much capacity in our mobile broadband networks as we would like, and I think its going to be harmful to the economy and to many businesses," Rysavy told Ars. Rysavy’s full analysis is available on his website.

How White Spaces databases fit in

PCAST’s proposal expands upon the well-known "White Spaces" concept, in which empty TV channels are repurposed and used for WiFi networks. So far, White Spaces has been used for machine-to-machine applications, things like video surveillance, and Internet access in rural areas where the "last mile" from the network to the homes aren’t wired, says CTO Peter Stanforth of Spectrum Bridge, which operates the first TV White Spaces database and could play a role in the broader sharing initiative proposed by PCAST.

The database is used to prevent users from interfering with each other. "We see ourselves like the air traffic controller," Stanforth said. It’s shared space, just like the skies, and the "air traffic controller makes sure everyone gets where they’re going and no one runs into each other."

White Spaces has two sets of users. There are the incumbents, the TV broadcasters, who get first priority. Everyone else can use the spectrum, if they make a reservation for a particular time and place. "Everything else is on a first-come, first-served basis for secondary access," he said.

PCAST’s proposal to share 1,000MHz of federal spectrum would work in much the same way. While PCAST would greatly expand the use of spectrum sharing, "technologically it’s not a big difference," Stanforth said.

In the current implementation of White Spaces, end-user devices are passive. Instead of making decisions on whether to use spectrum or not, devices take their orders from above.

"The original TV White Spaces proposal, to build a cognitive radio that turns on and each unit senses what spectrum is in use and makes its own decision, is hugely controversial and is not in commercial use anywhere in the world," Marcus said. "The TV White Space system we have is not that. It looks at a map to see where it is, it doesn't do any sensing of its own."

If the concept were applied to PCAST-style sharing, cell phones would only transmit when they see a compatible base station, just as they do today. But if a band was needed for, say, a military exercise, the base stations in the area would turn the band off.

"That is not rocket science," Marcus said, expressing optimism that sharing spectrum is at least technically feasible, even if bureaucratic problems make it hard to implement.

Although current White Spaces deployments rely on passive end-user devices, the PCAST report banks on advances in cognitive radio technology that will put the intelligence needed to enable sharing on both the receiving and transmitting ends.

"Both the Department of Defense and commercial parties have been developing cognitive radio technology that allows secondary users to operate if they sense that the primary user is not active," PCAST wrote. "For example, the DOD and NTIA collaborated with industry to develop the Dynamic Frequency Selection (DFS) technology that enables 5.8GHz Wi­Fi devices to detect and avoid military radars."

But not everyone agrees that the technological leap from White Spaces to PCAST-recommended spectrum sharing is so small.

TV White Spaces involves fixed or nomadic use cases, Rysavy notes. "More complex problems involving mobile networks have never been attempted,’ he said. End-user devices may have to be reconfigured to use different frequencies and work within mechanisms for querying databases to find out whether spectrum is available to them. "It's all plausible, but I think we have to be realistic about the time frame involved," he said.

Bureaucracy, as always, stands in the way

Bureaucratic problems are also, well, problematic. For one thing, just figuring out what each chunk of spectrum is used for is difficult, said Michael Calabrese, who contributed to the PCAST report and is director of the Wireless Future Project at the New America Foundation.

"Congress needs to mandate a granular spectrum inventory," he said. "Right now, what's operating on federal spectrum is incredibly opaque. It’s like pulling teeth to get these guys to say how much you’re actually using, what kinds of systems, and where does it operate."

The FCC manages 2 million active frequency licenses and the NTIA manages about 450,000 frequency assignments for government uses, Matthew Hussey, telecommunications aide to US Sen. Olympia Snowe (R-ME), noted before this week's announcement. To illustrate the complexity, he pointed to the NTIA report from March that said, "Over 20 agencies utilize more than 3,100 individual frequency assignments" in just 95Mhz of spectrum from 1755-1850.

Spectrum freed up through incentive auctions could end up being just a fraction of what the industry says it needs, but cellular companies are still "going to have to be convinced" of the benefits of sharing, Hussey said.

PCAST targets higher, perhaps less attractive spectrum than that mentioned in the NTIA report, but Calabrese thinks the strategy of using higher frequencies and building small cells across the country fits in well with a strategy carriers are already deploying, to offload cellular network traffic onto WiFi hotspots.

Targeting smaller geographic areas also makes sense in a sharing system, in which interference between conflicted uses must be avoided.

"We're simply running out of bands that can be repurposed for exclusive licensing," Calabrese said. The "spectrum crunch" or "crisis" described by carriers means only that "we're running out of spectrum that can be cleared for exclusive use over wide areas," Calabrese said. "What's abundant is spectrum capacity."