WASHINGTON—Airwave regulators are exploring how smartphone and broadcast signals will get along in the same neighborhood. The Federal Communications Commission’s Office of Engineering and Technology issued a Public Notice asking for comments on interference between cellular and TV signals under a variable band plan.
A variable band plan would allow the commission to auction off a different amount of spectrum in discrete geographic areas. It favors this approach because it would bring more spectrum to, and therefore more revenue from, the auction. Both broadcasters and wireless providers oppose a variable band plan because their signals would overlap on the same channel in different markets and wreak havoc with one another.
The OET’s Jan. 29 Public Notice indicates the agency is sticking to it’s guns. The 26-page document seeks a second round of comments on TV-cellular interference, assuming a variable band plan. Given that the first round treated a variable band plan theoretically, the resulting comments didn’t provide the type of details the commission sought.
General consensus in the first round was to establish a “separation distance” between wireless and TV operations. The commission said proposed distances ranged from 100 to 500 kilometers with “limited technical analyses in support of these proposals.” It also characterized the approach as “spectrally inefficient and overly conservative” because it treats all interference scenarios identically and without regard for variations in terrain, transmitter power, antenna height and so forth.
“Such an approach also fails to account for technologies and techniques that wireless licensees might employ to mitigate potential interference, such as antenna characteristics and resource block provisioning,” the Notice said.
Rather than a fixed separation distance, the commission is proposing using predictive analysis based on the Longley-Rice model now used for estimating potential interference between TV signals in OET Bulletin No. 69. (Longley-Rice signal propagation image at right.)
“The methodology described… may be used to predict whether interference is expected to occur at a location and thereby establish the necessary separation distances in a deterministic way,” the Notice said.
OET-69 has never been applied to wireless services nor in a mixed-use context, but the OET said that it could work in three of four possible interference scenarios: DTV transmitter to wireless uplink; DTV transmitter to wireless downlink; and vice versa for both.
Use of the ATSC standard for TV stations and LTE for wireless providers is assumed. Signal overlap virtually is guaranteed in a variable band plan because wireless providers will be licensed in 5 MHz chunks while broadcasters transmit on 6 MHz channels. The amount of overlap will vary. To account for it, the OET proposes to define co-channel TV/wireless pairs as having 1 to 5 MHz of overlap, and adjacent-channel pairs as being within 5 MHz of one another.
First-round comments focused on co-channel rather than adjacent-channel interference because of the presumed use of guard bands. With a variable band plan, “guard bands could be slightly different in each market, creating the potential for interference between DTV and wireless services that otherwise would not exist,” the Notice said. The OET therefore asks for feedback on “the impact of adjacent-channel interference constraints in the context of market variation.”
The OET is soliciting comments on “whether this methodology can provide greater accuracy than a generic separation distance in predicting potential harmful interference between services.” It also asks if analog TV transmissions should be considered, since some low-power U.S. TV stations and translators still use the format, as well as broadcasters in Mexico and a handful in Canada.
The commission will hold a workshop and webinar on its predictive interference proposal on Feb. 21. Comments are due on Dockets No 14-14 and 12-268 (the incentive auction Notice of Proposed Rulemaking) by Feb. 28, 2014.
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