08.02.2007 02:37 PM
FCC OET report paints ugly picture for white space devices

The FCC Office of Engineering & Technology July 31 issued a report finding that prototypes of white space devices (WSD) submitted for testing fail to detect TV broadcast and wireless mic signals on a consistent basis and that the prototype evaluated with a transmitter can interfere with TV and wireless mic transmissions.

The report appears to be a serious setback for companies wishing to use unoccupied slices of TV spectrum in various locales for a variety of consumer electronics, including wireless broadband connections. While noting that certain envisioned technologies, like dynamic power control and adjustment of power levels based on signal levels in adjacent bands, were not included in the prototypes evaluated, the report found that given the results of OET’s testing, “further testing of these devices was not deemed appropriate at this time.”

OET’s research was part of an FCC proceeding considering rules to allow fixed devices to operate in TV white spaces following the completion of the digital television transition in February 2009. The agency also is deciding whether or not to allow personal/portable WSDs to use unoccupied slices of TV spectrum. Among the issues the commission wants answered is whether or not WSDs using detect-and-avoid technology to prevent interference should be required to find signals as low as -116dBm.

WSD proponents have contended that technology to detect the presence of a TV or wireless microphone transmission and avoid interference by looking for a free channel can prevent WSDs from creating harmful interference. The OET tests of prototype WSDs from two sources appear to contradict that assertion. The report identifies the tested devices as only “Prototype A” and “Prototype B.”

The FCC Laboratory tests looked at several issues, including the ability of the devices to sense TV broadcast signals and their ability to sense wireless mic signals. Because only Prototype A included a transmitter it was the sole unit tested for interference to DTV reception. Interference to Part 74 wireless mic systems relied on simulations and Prototype A.

For TV signal detection, the report found Prototype A “often reports a channel to be available, or vacant, when the broadcast signal is expected to be present.” For instance, where a DTV signal was strong enough to be viewed on a digital TV, Prototype A’s scanner said the channel was available 40 percent to 75 percent of the time with an average of 52.8 percent of the time. When no signal was expected to be present, it said the channel was available 78.1 percent to 91.7 percent of the time, with an average of 85.2 percent.

Additionally, bench testing of Prototype A showed it was “generally not able to detect DTV signals on any of the tested channels at the -116dBM/6Hz level detection threshold” for digital TV about which the commission sought comment, nor could it do so at the -114dBm level that device manufacturers had suggested.

Testing of Prototype B showed it to be “generally able to reliably” sense DTV signals at -115dBm in single channel tests and -114dBm in two-channel tests. However, its sensing performance declined “very rapidly as the signal levels are reduced.”

The results of the tests to determine the prototypes’ ability to sense the presence of signals from Part 74 wireless mic systems found Prototype A to be unable in general to do so, while the performance of Prototype B was “mixed.”

The lab conducted “limited” field tests of Prototype A for interference to DTV reception. A DTV receiver was placed in the test area with an indoor antenna pointed towards a DTV transmitter on channel 29. The lab placed the WSD tuned to the same channel in the main beam of the receive antenna. The WSD was activated at various distances from the antenna and the interference effects on the DTV were noted. First and second adjacent channel testing were also conducted with and without the use of an external transmit filter.

According to the report, co-channel interference occurred out to 285ft from the DTV. First adjacent channel interference without the filter occurred to between 154ft and 164ft. Second adjacent channel interference was found to distances between 36ft and 46ft. With the transmit filter used, interference was observed at a maximum of 6.5ft. However, the report noted that with the filter attached the transmit power is attenuated by 14dB. “In practice, the distance at which adjacent channel interference occurs would be expected to be greater if the device were operating at the proposed output power level of 100mW EIRP,” the report said.

The report acknowledged that the devices tested represented “an initial effort” and don’t embody “the full capabilities that might be developed with sufficient time and resources.” Thus, the report said OET is “open to the possibility” that future prototypes “may exhibit improved performance.”

To read the entire report, visit: hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-275666A1.pdf.



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