A previous edition of RF Report and stories in TV Technology and other publications covered the failure of white space devices tested at the FCC laboratory to protect DTV reception (off air and cable) and wireless microphones from interference. Microsoft revealed that the devices it submitted to the FCC were “broken,” implying that a working device might have offered more protection.
NAB and MSTV, in comments filed with the FCC Wednesday
, disagreed. The comments include a study by Bruce Franca and Victor Tawil and field measurements from Meintel, Sgrignoli and Wallace showing “even if devices were able to sense at the Further NPRM’s
detection threshold, they would still fail to protect television services from harmful interference.”
NAB and MSTV also raised concerns that if the custom built prototypes were unable to effectively sense wireless microphone and DTV signals in the laboratory, “one can only imagine the consumer outrage if personal/portable devices are allowed to enter the market and result in millions of Americans losing access to DTV signals.”
The study shows several examples of locations with DTV reception located close to other locations where white space devices would not sense a signal above -120 dBm. For example, in a kitchen in a residence, DTV signals were too weak to detect, but in another room in the same residence, DTV reception was possible. Photos and spectrum analyzer photos show off-air reception, the test setup and signal strengths.
The comments conclude, “MSTV and NAB accordingly commend the Commission for testing and reporting on the prototype devices submitted. As the results of these tests confirm, the Commission must not allow personal/portable devices into the broadcast spectrum. To promote broadband deployment in rural and other underserved areas, it should instead move forward with rules to authorize fixed devices with appropriate protections.”
Ignoring the poor performance of the prototypes, their design and fragility do not inspire confidence that mass-produced devices competing on price will even provide the proposed protections MSTV and NAB have shown are inadequate. The Philips device was too fragile to be taken out of the laboratory for field tests, and the Microsoft devices “broke.” The Microsoft device included a high-performance external filter to achieve compliance with FCC out-of-band emission limits. This filter reduced in-band emissions from 200 mW to 7 mW and could only tune to a single DTV channel. MSTV and NAB note, “There does not appear to exist a filter that could be practical for a consumer device (e.g. of a small size and low cost), tune to all channels, and
meet the out-of-band performance observed on DTV Channel 30 in the Microsoft prototype.”
If Congress is worried about a “train wreck” when analog TV is shut off, the situation that will be created by allowing portable unlicensed white space devices on “unused” TV channels could be considered a Category 5 hurricane.
MSTV and NAB studies show spectrum sensing won’t work and these devices, even if they could meet the proposed specifications, will interfere with TV reception and wireless microphone use over a wide area. The FCC laboratory studies showed that switching to digital cable won’t help — nearby white space devices will interfere with digital cable channels.
Using prime UHF TV spectrum for short haul devices doesn't make sense when there are plenty of alternatives, including 802.11a/b/g/n devices and new UWB technologies capable of providing much higher data rates for in-house use. The highest value use of the “white space” is for fixed links that are professionally installed and use updated databases and GPS to avoid causing interference to existing services in the TV band.