James E. O'Neal /
08.22.2007 12:00 AM
Devices Fail FCC White Space Tests
The FCC last month released the results of two different investigations conducted to ascertain whether or not licensed television broadcasters and cable service providers can co-exist with the proposed unlicensed “white space” RF devices.
It looks now as if the concept of sharing or repurposing this broadcast spectrum will have to go back to back to the drawing board.
According to the commission, the units tested consistently failed to detect the presence of operational television transmitters and also failed to acknowledge, or were confused about, wireless microphone signals. “Our tests also found that the transmitter in the prototype device is capable of causing interference to TV broadcasting and wireless microphones,” the commission said.
The two reports, “Initial Evaluation of the Performance of Prototype TV-Band White Space Devices” and “Direct-Pickup Interference Tests of Three Consumer Digital Cable Television Receivers Available in 2005,” were in general agreement that harmful interference could be generated to broadcast television services by White Space transmitters.
Both reports were released on July 31 by the FCC’s Office of Engineering and Technology (OET) and were based upon testing work done at the commission’s laboratory.
The Association for Maximum Service Television Inc. (MSTV), which has strongly lobbied against allowing the use of unlicensed devices in the TV band, was quick to issue its response to the test reports.
“Based on the Report, allowing these devices in the television band will result in massive amounts of interference, harming consumers that have purchased new DTV sets and jeopardizing the government-subsidized digital-to-analog converter box program,” said David Donovan, president of MSTV. “These levels of interference will derail the digital transition. It confirms everything that MSTV and the television industry have said for the past three years. I think that they have significant problems here and that this is a diversion from the digital transition. It’s time to stop this and move forward with the transition itself and get that done.”
When testing the ability of the white space devices to locate “clear” spectrum, one of the units submitted for evaluation failed an occupied digital television channel 84.5 percent of the time on average. The same unit failed to spot an analog TV transmission between 11.1 percent and 27.8 percent of the time. The device evaluation report stated further that when DTV signal strength was sufficient for reception, one of the units tested reported that the (occupied) channel was free for use 58.2 percent of the time on average. Further, when the unlicensed devices scanned the band of frequencies chosen for the study (TV channels 21-51), it sometimes took between slightly more than four minutes for one unit tested, and, in the case of another device, up to 14 minutes.
The report dealing with the evaluation of interfering sources on consumer DTV sets simulated the connection of such receivers directly to a CATV system 256-QAM signal source, without any intervening set-top box. Interfering signals were located on unused television channels that overlapped the frequencies used by cable systems. Data showed that signals typical of such unlicensed devices could cause interference to the DTVs under test at an effective isotropic radiated power (EIRP) as low as 6.3 dBm at a distance of two meters. At a distance of 10 meters, a 15.3 dBm signal was sufficient to disturb DTV reception. These unlicensed transmitter-DTV set distances were chosen because they typified what might be encountered in a real world situation in a townhouse or single family home environment.
The White Spaces Coalition, a group of industry players pushing for adoption of the unlicensed transmitting devices issued a short statement in response to the FCC’s issuance of the lab reports. It acknowledged the reports and stated that the group was pleased that the testing had been completed and a report issued in a timely manner. The group was also optimistic that white space utilization would ultimately be possible.
“Coalition members are encouraged that FCC engineers did not find fault with our operating parameters and remain confident that unlicensed television spectrum can be used without interference,” the coalition said in a statement.
Microsoft, a coalition member, was contacted for reaction, but declined comment beyond what was contained within the coalition statement.
Charlie Rhodes, columnist for TV Technology and a television industry veteran, has been vocal in his opposition to any scheme that threatens to cause significant interference to DTV reception.
In commenting on the FCC’s DTV receiver testing, Rhodes said that the tests reinforced what he has been stating in his column—intermodulation within television tuners is a serious problem.
“They show that I was right,” said Rhodes. “Co-channel interference is being manufactured in the tuner. As the white space devices will probably use TV tuners to look for spaces in which to operate, why wouldn’t this happen to them also?
“Tuners manufactured today have poor selectivity and are easily overloaded. Where are they going to get tuners that are bullet-proof?”
Rhodes said that he imagined that the white space devices would have to fall within a $100 pricing entry point, and that this may limit the quality of TV tuners that could be used for spectrum scanning purposes within the units.
“The receiving part of these white space devices can’t cost more than about $15,” Rhodes said. “This forces manufacturers to build to a price, not to a spec. The people who build tuners don’t seem to worry about this. The FCC knows that this is an important element; but what they’ll do, I don’t know.”
No plans have been announced yet for additional testing.