DOT's Draft GPS/LTE Interference Study Should Not Go Forward, Roberson Says

The Department of Transportation's plans to test for interference between LightSquared's proposed wireless broadband network and GPS devices is so inherently flawed that there is no "rationale for proceeding," Roberson and Associates President Dennis Roberson said Friday. That day at 11:59 p.m. was DOT's deadline for accepting public comments on its draft test plan for GPS adjacent band compatibility. And at a news conference Friday, Roberson -- in the midst of conducting its own, LightSquared-sponsored testing for possible interference between LightSquared broadband uplink and downlink signals and neighboring spectrum GPS signals -- had numerous critiques of the DOT plan. DOT didn't comment.

The testing's end goal is "to find means of peaceful coexistence" with GPS, Roberson said -- with that peaceful coexistence meaning identifying any GPS devices that do show interference problems and determining what changes the manufacturers need to make. The majority of the roughly 40 devices -- ranging from smartphones to numerous GPS navigation units -- that Roberson is testing haven't shown any functionality degradation, he said.

Roberson's testing revolves around GPS device functionality from an end-user perspective, with a focus on such issues as position accuracy and timing function, he said. "It's a device-by-device thing," Roberson said. By contrast, the DOT test is focused on looking for 1 dB degradations in carrier noise, which doesn't even correlate to less functionality in a device, Roberson said. LightSquared and GPS maker Garmin also have been bickering in FCC filings over use of that 1 dB increase in noise floor as a benchmark for interference (see 1510140052).

While DOT has said it expects to start testing in March, that is a questionable time frame considering the test procedures have yet to be developed and the devices to be tested have not been identified, Roberson said. The DOT-proposed methodology also would let manufacturers each test their own devices and report the data, he said: "You don't have independence, you don't have consistency."

The Roberson and DOT tests differ in other key ways, with DOT looking at potentially 100 devices, while Roberson is testing close to 40. "There's always the next device you can test," Roberson said, adding that his firm's aim is a mix of "the highest-use devices and the most-critical devices." The FCC didn't comment on how it plans to use the DOT testing. The DOT test plan has a variety of allies in the GPS universe, including the GPS Innovation Alliance (GPSIA) (see 1508180019), and the Alliance for Telecommunications Industry Solutions. It last week urged the FCC to hold off on any decisions until after the DOT test results are in (see 1510140052). The original deadline for comment on the DOT draft plan was Oct. 9 until GPSIA asked for a one-week extension.

GPSIA continues to back "the multi-stakeholder process currently underway (at DOT) rather than engaging in a debate with LightSquared consultants," said Executive Director-Legislative Heather Hennessey in a statement Friday. "GPSIA believes that anyone testing the compatibility of terrestrial broadband and GPS should use the 1 db standard as an interference protection criterion. It has a long and well-established history in international and domestic regulatory proceedings as the appropriate interference protection standard; and it is not fraught with the difficulty of other standards that are impractical due to the vast number of applications of GPS and the exceedingly complex ways that interference impacts the accuracy, integrity, availability and continuity requirements of a particular application.”

"A reasonable way to limit interference" would be for the FCC to require any 4G LTE licensees operating in band 24, and any future L band licensees, "to limit noise power in the adjacent L1 band," General Motors said in a DOT filing posted Oct. 9 regarding the draft test plan. It also suggested DOT use a 4G LTE simulator in lab tests, and that if all the antennas tested are global navigation satellite system (GNSS) patch antennas, testing 100 GPS/GNSS receiver antennas "at once could create a significant potential for receiver-on-receiver interference," GM said.