Roberson Testing Proves LTE/GPS Coexistence, Ligado Says
Signals from Ligado Networks' proposed LTE network at the power and out-of-band emission (OOBE) levels that the company has worked out with GPS companies don't appear to interfere with GPS navigation devices, according to Roberson and Associates testing commissioned by Ligado. The coexistence plan that Ligado has proposed has "sufficient limits in those adjacent band signals to ensure GPS receiver performance," Roberson Chief Technology Officer Ken Zdunek said Thursday on a news conference call discussing those test results and in a related filing posted in FCC docket 12-340.
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The test results in the filing strictly focused on locational accuracy based on the reduced operating parameters agreed upon in recent weeks by Ligado (formerly LightSquared) and GPS companies Deere, Garmin and Trimble (see 1602040015 and 1512180020). While Roberson testing also looked at other performance factors, such as signal acquisition, it's still analyzing those results, Zdunek said: "We have literally a mountain of results we are going through and analyzing and graphing.”
The filing was in response to a question at a January meeting (see 1601140037) during which one federal agency representative asked whether the settlement agreement protects GPS devices, Ligado counsel Gerry Waldron of Covington and Burling said. Beyond the filing, which focuses on a handful of navigation devices, "We'll give the data the relevant policy makers think is helpful," Waldron said, though he wouldn't commit to making all the data for testing of 28 different devices publicly available. "We've been as transparent as any person in this saga over the years," he said.
For the testing, Garmin established a device performance baseline by letting each device sit for two hours without any LTE or adjacent band signals, then adding LTE signals across the four frequency bands to see how they affected simulated motion, and then comparing that baseline performance with performance under the influence of LTE signals, Zdunek said. The motion-testing procedure involved creating a simulated driving route that was input to a GPS generator that created GPS signals to the devices, as if the device were in motion, he said. "We didn't see any differences between the GPS baseline without LTE and when we added LTE," Zdunek said.
The filing focuses on a select few devices: the Garmin handheld eTrex GPS navigation device, a Motorola workstation commonly used in public safety settings such as police cars, and Samsung S5 and S6 smartphones. According to Ligado, the Roberson testing shows the S6 performed better than the S5, though both were within acceptable limits. Waldron said preliminary results for other GPS navigation devices "indicate the 'no impact' analysis for these devices looks to be consistent across the other devices." Waldron added, "I want to emphasize, that's preliminary." Garmin didn't comment Thursday.
When asked about test results for the devices under the old power and OOBE levels Ligado had proposed before the GPS company agreements, Waldron said they were "not very interesting." The new limits "are our reality," he said. "This is what we're limited to.”
Ligado also said the testing shows that the Transportation Department-proposed yardstick for adjacent band coexistence testing -- a 1 dB (decibel) rise in carrier-to-noise ratio -- "is fatally flawed." Even minus any LTE signal, the GPS receivers tested sometimes showed signal to noise variations that exceeded 1 dB, Zdunek said: "It's not a good predictor -- it's a terrible predictor, really.”
Ligado and others have asked the FCC to issue a public notice seeking comments on a proposed auction of the 1675-1680 MHz band and of Ligado's related license modification request (see 1602120052 and 1602040015). "By incorporating limits on Ligado’s operations that were agreed to by major GPS manufacturers, this proposal appears to satisfy the concerns of various stakeholders about adjacent band interference issues," Technology Policy Institute President Thomas Lenard said in a similar urging filed last week in the docket. Waldron said he had no sense of when the FCC might issue such a public notice. The FCC didn't comment.