For weeks we've reported that the wireless industry has been pressing the Federal Communications Commission to figure out ways to get the mobile services more spectrum licenses. Experts are darkly warning of a looming spectrum crisis, as consumers rush to buy bandwidth-gobbling smart phones. CTIA, the Wireless Association, says its member companies urgently need 800MHz of license space below the 3GHz mark.

That, of course, means looking at broadcast television territory, aka the VHF and UHF bands in the 52 through 698 MHz zone. But the question is, how should the industry tap into this TV spectrum in, shall we say, a sharing and collegial manner. CTIA and the Computer Electronics Association (CEA) now say they've got a plan to do just that. We can't predict how the television broadcasters will react to it, but here's the proposal.

Smaller footprints

There are about 1,800 high-power licensed television stations in the United States; the gist of the scheme is to move broadcast TV from a system based on single, high power transmitters towards "distributed networks"—closely spaced arrays of lower-power towers. The latter deployment would create "a much smaller interference footprint" around TV transmitters, CTIA/CEA say, "and therefore should permit closer spacing between co-channel and adjacent channel television operations." This would free up between 100 to 180 MHz of spectrum for wireless, the two trade groups estimate, perhaps worth as much as $70 billion, when put to its new and improved uses.

"This substantial amount of spectrum is clearly worth pursuing, even if it does require significant adjustments to the broadcast network," their filing to the FCC insists.

The problem, as CTIA/CEA sees it, is that high-powered, high-altitude television transmitters cover big geographic areas, but they also occupy large amounts of spectrum in any region. A high-powered transmission via channel 25 by one broadcaster sometimes makes that channel unusable by any other licensee for up to 150 miles. The system also circumscribes adjacent channel use, CTIA/CEA complain. And although these powerful broadcasts are licensed to use 6MHz of spectrum (eg, KQED-TV in San Francisco, California: 566 - 572 MHz), they sometimes occupy far more—up to 18MHz, three times that much.

Single-frequency networks

This high-power/high-altitude transmitter system was devised back in the early TV days when wireless service wasn't so omnipresent. Now critics charge that the TV bands are seriously under-utilized. So what's the solution? Replace the status quo with "single-frequency network" (SFN) architecture, CTIA/CEA recommends. Scotch those big transmitters and use bunches of shorter and lower-powered ones in their stead.

It's not like SFN is something these guys just pulled out of their hat. The FCC has authorized the practice in the recent past. As zero hour for the DTV transition approached, the agency gave the green light for TV stations to use SFN Distributed Transmission Systems (DTS) to get at hilly areas that agency engineers feared would be left in the dark by the switchover. Plus: "Multiple DTS transmitters generally operate at a lower power than a single transmitter to achieve the same coverage and thereby reduce the likelihood of causing interference to neighboring licensees," the Commission noted in its Order.

But CTIA/CEA acknowledge that SFN has risks, most notably a potential "ghosting" effect—transmission echoes that can interfere with TV reception if the arrays of simultaneous signals aren't carefully synchronized. Some work on DTS synchronization standards already exists, but more will likely be needed, their filing concedes, "to confirm that a wide scale deployment of DTS for broadcast television is achievable and would allow for the significant gains in spectrum efficiency expected."

Once that is done, however, increased reliance on DTS would be good for broadcasters, CTIA/CEA insist, "opening up new segments of the market and giving broadcasters access to consumers who may have previously relied upon other video distribution systems for their television programming," especially those in mountainous areas. And these clumps of low-power transmitters could offer better reception quality, being closer to the ground, therefore nearer to homes.

Repacking risks

The ultimate point of this plan, of course, is to free of lots of spectrum for the wireless industry by squishing the nation's full-power TV stations into a more compact area of the spectrum band. This "could represent a major step towards satisfying the pressing need for substantial amounts of additional mobile broadband spectrum that is imminent for our country," CTIA/CEA concludes.

No word as to what the National Association of Broadcasters (NAB) and its engineering arm, the Association of Maximum Service Television (MSTV), think of this proposal, which was submitted to the FCC just before the holidays. But strong hints of their stance can be found in another FCC proceeding. In early December the agency issued a Notice of Inquiry on how efficiently the TV bands are being used. The Notice pointedly asked for an assessment of the "impact to the U.S. economy and public welfare if the coverage of free over-the-air broadcast television was diminished to accommodate a repacking of stations to recover spectrum."

That would be very bad indeed, NAB and MSTV quickly responded. "Repacking DTV broadcast stations could cause serious disruptions to the public’s broadcast television service, and many viewers could lose substantial or all local television service," they warned. "These disruptions would be orders of magnitude worse than those that the public experienced during the transition to digital television, when a limited number of broadcasters instituted necessary facility changes."

Still, give CTIA/CEA credit for moving the discussion past dire warnings of spectrum droughts. Now we've got a plan to consider, which can be answered with criticisms and concerns, followed by discussion, and ultimately compromise. At least that's the Holiday Season version of the outcome.