DOT's Draft GPS/LTE Interference Study Sees Support, Criticism

The DOT test plan is "a commendable first step in advancing the goal of ensuring ... harmonious coexistence" between GPS and other uses in adjacent bands, the GPS Innovation Alliance (GPSIA) said in its filing. The group made technical modification suggestions for the testing, including that the test bed should be constructed so the devices sit at least two feet apart so as to not interfere with one another, and that global navigation satellite system signals to be simulated in the testing include GPS L2 and GLONASS G2.

Greenwood Telecommunications Consultants, while backing the test plan broadly, also criticized the methodology of the DOT plan, which involves an array of GPS devices laid out next to each other and simultaneously exposed to an interfering signal, while each device is monitored simultaneously and independently. "Elaborate chamber testing buys little or nothing in terms of replicating the real world and can slow down progress in terms of receivers tested," Greenwood said in its submission. It recommended instead "parallel conducted and radiated test paths" since it allows for cross-checking results. In its filing, Greenwood also pushed for acquisition performance testing, saying that testing is currently set to be tested in later phases, but it should be a more immediate priority as "likely ... the most sensitive compatibility factor." And Greenwood also said the baseline test power levels should be modified to better simulate real-world metro environments where high adjacent band signals are likely.

Given the telecommunications industry's reliance on fixed GPS receivers for such applications as precision timing, which then is used in triangulation, the DOT testing should also include testing of such precision timing receivers and their timing signals instead of just contrast-to-noise measurements, the Alliance for Telecommunications Industry Solutions said in its filing. "A problem with a precision timing GPS receiver located at a wireless base station could impact all wireless handset users that use that base station to connect the handsets to the fixed part of the wireless carrier’s network," ATIS said. The group also recommended simulator test files and receiver log files be made public for outside testing and verification.

LightSquared's criticisms are legion, including that the testing should be done by the FCC and Commerce Department and that DOT's test plan includes "significant omissions that leave the scope and schedule of the proposed testing uncertain (and) fails to respond to or even consider concerns ... on critical issues such as device selection, transparency, independence of the testing process, testing procedures, the repeatability and verifiability of the testing regime, and the evaluation of test results," it said in its filing. The biggest shortcoming, LightSquared said, is that the DOT plan "refuses to measure what counts: the impact of any interference on performance of the device. The test won't actually add to the dialog that is needed to ensure that this vital spectrum can be used by both GPS and next-generation broadband services, and the long delay caused by the DOT testing will be for naught."

Much of the debate between LightSquared and critics such as the GPSIA and Garmin involves what to test for, with LightSquared contracting with Roberson and Associates for a test looking at device functionality from an end-user perspective and the DOT testing to look for a 1 dB change in the noise floor (see 1510160022). “Choosing to measure a 1 dB change may have been justified in the last century, when testing equipment was not as advanced as it is today and so interference analysis was based on changes in the noise floor -- not because it was necessarily a useful metric but because that was all that could be measured," LightSquared said in its filing. But such an "old and flawed metric" is off track today because the technology can -- and the test should -- "measure what consumers care about: whether interference causes the GPS device to give an end user faulty position or timing information," LightSquared said.

While DOT also has proposed recording other performance metrics like signal lock or degradation of pseudo-range or position accuracy, GPSIA called those "inappropriate metrics for interference assessment" since for some applications some GPS receivers may focus not on position but on other parameters such as acceleration, time or velocity. "Degradation of accuracy or otherwise attempting to determine effects on the 'user experience' are not practicable interference metrics," GPSIA said. Meanwhile, given the variety of GPS applications beyond navigation, how one would determine "material degradation" -- or what that even would mean -- is unclear, GPSIA said. And defining harmful interference by looking at degradation of performance among existing GPS devices and applications "fails to account for and support future GPS-based innovation," GPSIA said. "Use of a defined change in the noise floor (1 dB) provides a readily identifiable and predicable metric that all interested parties can take into account now and in the future."

GPSIA also raised one objection to DOT plans to ask GPS manufacturers before testing for details regarding everything from antenna characteristics to front-end design, as such information is sensitive and proprietary. Such information should be necessary only if the testing "produces anomalous results," GPSIA said.