NTIA TEST RESULTS RAISE QUESTIONS ABOUT UWB IMPACT ON GPS

Study looked at maximum allowable UWB power levels that could be tolerated by GPS receivers when used in different operational applications without causing degradation to GPS systems. Earlier this year, NTIA reported mixed test results for UWB in non-GPS band, citing potential to operate technology in 3-6 GHz range without interference but pointing to challenges with interference below 3 GHz, where airport surveillance radar and some federal systems operate. GPS industry has been closely following UWB proceeding at FCC on such issues as impact on navigation systems for maritime and aviation systems and pending Enhanced 911 requirements for wireless phones that will rely on GPS applications. NTIA tests examined 32 UWB signal permutations when evaluating impact on GPS receivers, NTIA official said Fri.

NTIA report evaluated tolerance of one type of GPS receiver to low-duty cycle pulsed interference. Report essentially said that if UWB device operated below 100 kilopulses per sec., GPS interference wasn’t concern, regardless of number of UWB devices involved, said Robert Fontana, pres. of Multispectral Solutions, which is developing UWB for non-GPS bands. At levels below 100 kilopulses per sec., however, it becomes difficult for UWB operates to achieve high data rates that are competitive with rival broadband wireless systems such as Bluetooth, Fontana said. High speeds are critical for markets such as wireless local area networks, he said. NTIA report appeared to raise fewer concerns about low data rate applications such as ground-penetrating radar, Fontana said.

NTIA report said that if UWB operations at 100 kHz were limited to “applications such as ground-penetrating radar and through-the-wall imaging” there would be fewer concerns with UWB interference. Under certain conditions, maximum allowable power levels of 4.3 db below existing Part 15 level for outdoor UWB operations “would satisfy the restrictions imposed by the aviation en route navigation operational scenario,” report said. Report said that pulse rate frequencies of 1, 5 and 20 MHz showed that maximum power level “necessary to satisfy the measured GPS performance criteria must be less than the current Part 15 level for most of the operational scenarios considered.” Even if certain regulatory restrictions were put in place, there still would be cases in which power levels would have to be reduced to 27 db below Part 15 levels to circumvent GPS interference concerns, report said.

But data that point to possibility of UWB operations for ground-penetrating radar in GPS bands still are expected to raise concerns in GPS community, particularly as aviation officials seek to increase use of that technology for navigation, several sources said. David Stempler, pres. of Air Travelers’ Assn., said NTIA report showed that UWB had “the potential to affect or degrade the performance of GPS. That’s a very significant impact for us.” He said several UWB developers, such as Fantasma, had decided to operate only in non-GPS bands, and not GPs. “If we lose the usability and viability of this GPS system we don’t have any fallback,” Stempler said. “GPS is going to be essential for improving the safety of aviation around the world.”

NTIA report, while outlining multiple scenarios and impact of wide range of variables, didn’t make policy recommendations to FCC. NTIA is evaluating 4 types of GPS receivers, with first 2 covered in report that was released Fri. and data on 2nd pair expected to be released in June or July. One source said that although some tests still needed to be completed, report given to FCC last week contains major data that NTIA had been collecting and still set stage for Commission to move ahead with decision on UWB operations.

Johns Hopkins U. Applied Physics Lab also submitted to FCC Fri. its analysis of UWB and GPS interaction. That study was funded by Time Domain, which said it gave university “complete editorial control” over evaluation. Study analyzed data gathered by Applied Research Labs at U. of Tex. Time Domain lauded Hopkins’s analysis, saying it showed that UWB had no more impact on GPS receivers than devices that complied with FCC’s Part 15 rules. Time Domain’s Ross said analysis also showed that impact of emissions from multiple UWB devices was same as effect of emissions from multiple Part 15 devices. “The signals add as random white noise,” he said.

Ross told us Fri. that Time Domain was “encouraged” to see few areas of agreement between Hopkins and NTIA results. For example, both sets of tests concluded that if UWB operator were “dithering” pulses, emissions would appear to be noise-like. “If you dither your pulses in the time domain, it is effective in spreading spectrum lines in the frequency domain, which is what we have been saying all along, which is that we are like Part 15 devices,” he said. Hopkins results also showed that effect of emissions from multiple UWB devices was identical to the effect of emissions from multiple digital devices that complied with Part 15 rules, he said. Hopkins said UWB devices that had average power level that complied with FCC Part 15 regulations showed that GPS receivers exhibited “severe degradation” when separation between GPS receiver and UWB devices was less than about 3 m. Part 15 device that was analyzed for similar distances had similar impact on degrading GPS receivers, Hopkins said. At separations greater than 3 m, “GPS receiver performance converges to nominal levels,” report said. “The minimum separation at which degradations are acceptable depends on individual user scenarios including performance thresholds, GPS receiver and UWB devices.”