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FCC Defends White Space Tests

After it revealed the results of its “white space” tests at the end of July, the FCC tried to clear up some of its signals, inviting some 50 industry heavies to its suburban Maryland lab and placing its top technology staff on the hot seat to discuss how the prototype white space devices (WSD) caused interference to DTV, cable and licensed wireless audio devices.

With a decision on mobile WSDs expected from the commission in October, broadcasters remain fearful that unleashing millions of untrackable RF-spewing consumer devices on the unused channels will derail the DTV transition, and the briefing at the lab only deepened their concerns.

The mid-Au-gust briefing seemed to be an attempt by the FCC Office of Engineering and Technology to demystify its testing process and push back on any suggestion it was not being sufficiently transparent in its testing.

The meeting offered few conclusions or predictions and gave little comfort to either side. OET Chief Julius “Julie” Knapp indicated more tests could come, although he wouldn’t specify if that would include re-testing one of the boxes, which was submitted by Microsoft; and he and his staff repeatedly noted that the tests performed so far are extremely limited in their scope.

The OET ran the two devices—from Microsoft (Prototype A) and Philips Electronics (Prototype B)—through tests of interference on DTV receivers, cable receivers, and wireless audio devices. (The devices are not actual products, but merely boxes containing the technology that could eventually form the basis of mobile consumer devices.)

(click thumbnail)This prototype from Microsoft failed the FCC’s white space interference test.Microsoft’s device failed to detect signals below –114 dBm, the threshold touted as an appropriate spec by the White Spaces Coalition, a group of eight major high-tech companies. The Philips device reliably detected signals as low as –115 dBm in bench tests (the manufacturer said it was not ready for field tests).

But the minimum performance threshold itself is in debate, with Knapp indicating the OET did not know whether it would be enough to protect over-the-air DTV reception. The IEEE 802.22 standard for fixed white space calls for a detection down to –116 dBm.


OET’s tests included co-channel and adjacent channel interference analysis of the prototypes. OET’s Steve Jones, who led those tests, said very little previous testing like this had been done before, so engineers had to invent some of their own methodology, with guidance in some areas from IEEE 802.22.

To bench test the prototypes’ spectrum sensing powers, OET staff sent an ATSC signal of –60 dBm over 6 MHz (using a Rohde and Schwarz SFU test system) through an attenuator to lower the power until the prototype failed to detect it. (The approach reflects the “detect and avoid” or “listen before talk” technique of the devices to avoid creating interference.)

Field tests of DTV detection were performed (on the Microsoft box only) at four sites (suburban homes) in the Washington-Baltimore area, and at different locations within each site, where the prototype “did not provide consistently accurate determinations on an overall basis or with respect to any of the subcategories in the field tests,” the OET reported.

Microsoft, which uses a proprietary algorithm in the box, rejiggered the box and told the FCC it once again was able to detect DTV at its claimed spec of –114 dBm.

The devices also took their time scanning: The Microsoft box took 27 seconds to scan a single channel, or 14 minutes for all 30; the Philips box took eight seconds for each channel, or four minutes for all 30.

NAB and DTV manufacturers leaped on the shaky results, and the Microsoft crash, as further evidence that the DTV transition cannot afford the risk of such devices.

“If the custom-built ‘prototype’ device Microsoft created specifically for testing could not hold up, how can the Commission expect spectrum scanners in actual personal/portable devices to work for years after they are sold to consumers?” NAB and the Association for Maximum Service Television (MSTV) asked. “If the FCC... could not detect or identify the degradation in the scanner’s performance, then surely customers will be unable to know when their unlicensed devices malfunction.”

Philips, meanwhile, found “remarkably good news” in the tests. “WSDs have been proven to detect consistently and very robustly, and therefore, effectively protect digital television and wireless microphone signals,” the company told the FCC.

Philips conceded that Prototype B’s test results raised questions, but said adjusting the algorithms would be the solution in most instances. The company also claimed that even where test results were less than perfect, there was no risk for harmful interference.

NAB dismisses that conclusion, calling the results “incomplete at best” and noting that Philips was not subjected to any field trials. Nor did the Philips box detect signals as low as –116 dBm.

Ed Thomas, spokesman for the White Spaces Coalition, said the lab event offered no new information. He said the FCC’s methodology seems fine; it’s just that the Microsoft device it tested was defective. News reports have overlooked that the Philips device performed exactly as expected, he said.


For the FCC, the tests are uncharted territory and there’s disagreement about how low a DTV signal the WSDs should have to detect. NAB and MSTV have argued that even detection of –116 dBm may be inadequate, and MSTV President David Donovan noted that at the discussion at the FCC lab, staff agreed that –114 dBm detection may not be sufficient.

“If you’re not sure whether –114 dBm is going to protect DTV, then why are you testing at –114 dBm?” Donovan asked.

Others disagree. “No one has empirically established that this level of sensitivity is necessary to protect viewers from harmful interference,” wrote a coalition including the New America Foundation and the Media Access Project.

Donovan also said field tests should be done in more real-world situations, in places like New York (with its post-9/11 weak DTV signals and wireless users from law enforcement to Broadway), and the hills of rural Pennsylvania. “And I didn’t hear a commitment to that,” he said.

OET staff repeatedly stressed the time-consuming nature of the tests and noted that they had to carefully pick the most representative tests possible to maximize their useful data.


OET tests also found direct-pickup interference in digital cable TV tuners when even a weak 4.8 MHz wide orthogonal signal was placed in a neighboring townhouse or elsewhere nearby. Cable packs several channels into the spectrum of a single DTV channel and has no white space between them, so interference could wipe out several cable offerings at once.

“Cable has got to be scared stiff,” said Donovan.

The National Cable and Telecommunications is in fact concerned, not just about interference in the home but also at the cable headends, many of which receive broadcast signals beyond the Grade B Contour of a station but still are subject to must-carry. NCTA has urged the FCC to not only restrict operation of portable devices within the Grade B Contour but to require spectrum coordination before any unlicensed devices, fixed or portable, can operate outside the Grade B Contour.

At the lab meeting, Knapp noted a chicken-and-egg relationship: FCC rulemakings sometime proceed simultaneously with the development of the devices those rules will govern. So the regulators don’t know, for certain, the capabilities of the devices, while the manufacturers don’t know the rules the devices will have to work under.

Both sides in the debate can expect more tests, and the FCC continued to receive comments on the matter into late summer, but it remains to be seen if white space rules will be developed by mid-October as FCC Chairman Kevin Martin has promised Congress. For broadcasters, as fast as they can derail mobile WSDs is not soon enough.

“This is a no-brainer,” said Donovan. “This should not be going on.”