Sinclair Warns of Interference in Duplex Gap

WASHINGTON—There will more interference than FCC anticipates in the duplex gap. That’s the upshot of a filing on the Federal Communications Commission’s Incentive Auction docket made by Mark Aitken, vice president of Advanced Technology for Sinclair Broadcast Group.

“Interference prevention is the raison d’être of the FCC,” he wrote. “The laws of physics have not changed and repacking broadcasters in bands reserved for mobile wireless broadband highlights the commission’s challenge.”

The commission is proposing to place orphaned TV stations in the duplex gap—the band of spectrum between wireless up- and downlink frequency blocks. Orphaned TV stations will be those not sold in the auction, and that the FCC can’t find a channel for in the TV band after the auction. The FCC is proposing to put these TV stations into the duplex gap, something both broadcasters and wireless providers oppose. The FCC, however, says it needs the flexibility of assigning stations to the duplex gap in order to reach its spectrum-clearing goals.

The impact of doing so remains disputed.

Aitken submitted an article by TV Technology contributor Charles Rhodes, “Assessing Post-Repack Channel Options.” Rhodes was chief scientist at the Advanced Television Test Center, a non-partisan, independent testing facility set up to develop digital TV technology. He is also a recipient of the David Sarnoff Medal and holds several patents. He now conducts signal-interference tests in his own facility and reports the results in TV Technology.

“Using the commission’s recently adopted ISIX model, Mr. Rhodes cautions that harmful interference will indeed occur and be far greater with the aggregation of ‘super blocks’—10 MHz wide assignments—by wireless carriers,” Aitken writes. “ISIX interference can arise from signals offset in frequency by more than 6 MHz. In fact, Mr. Rhodes notes that two Super Blocks of 10 MHz each may generate third-order distortion products spanning a significant 27 MHz. In short, broadcast and wireless broadband will demonstrably interfere with each other and that will be exacerbated by placing broadcast channels in close proximity to wireless users in the duplex gap.”

Aitken also cites a study by the European Broadcasting Union, “Can LTE Share Spectrum With DTT,” which found that cellular long-term evolution transmissions to be incompatible for sharing with direct terrestrial, or broadcast, TV.

The study concludes that terrestrial TV transmitters would have to be between 19 and 56 miles by land from a single LTE base station, depending on its characteristics, to avoid interference from that LTE operation. Given the greater likelihood of several LTE base stations all operating on the same frequency, a distance of between 124 and 184 miles would be needed.

LTE base stations also are vulnerable to interference from digital TV transmitters and would need between 124 and 372 mile of separation, depending on the transmitter.

The EBU study, “which investigated potential sharing of wireless LTE and broadcast spectrum similarly concludes that such sharing is impractical; LTE cannot share spectrum with digital broadcasting,” Aitken said. “These cautionary predictions should inform the commission as it seeks to repack broadcast channels in the duplex gap. Neither broadcasters nor wireless broadband users will find the sharing acceptable.”

Sinclair’s filing comes during the last days of the comment period on a previous commission Notice of Proposed Rulemaking on preserving one UHF channel in the TV band in each market for wireless microphones and unlicensed white-space devices. Opponents of duplex gap placement question potentially kicking a TV station out of the TV band to accommodate an undefined number of wireless mics and the minimal number of white-space devices. Comments are due on the docket Sept. 30.