08.03.2007 12:00 AM
FCC White Space Device Study Reveals Problems in Detecting DTV Signals
Two reports from the FCC Office of Engineering and Technology showed spectrum sensing alone is not sufficient to protect off-air TV reception and interference is likely to digital cable reception from nearby while space devices at power levels well below those being proposed.

The results of FCC laboratory testing of prototype devices designed to operation on unused TV channels (TV "white space") are described in the OET report Initial Evaluation of the Performance of Prototype TV-Band White Space Devices (FCC/OET 07-TR-1006). Field tests of one prototype showed its spectrum sensing scanners will not protect TV viewers from interference, especially interference to ATSC signals. The two units supplied for testing were identified as "Prototype A" and "Prototype B."

In the field tests, Prototype unit A reported channels were available for use even when an analog or digital signal on that channel was viewable on TV sets at the sites. For ATSC signals, at the site where Prototype A performed best, it reported a viewable ATSC channel as available during 40 percent of the scans. At the worst site, it was wrong in 75 percent of scans. The detection rate was better for analog than digital, with channels incorrectly reported as free during zero to 27.8 percent of the scans, depending on the site.

Prototype B, which lab tests showed was more sensitive than Prototype A, wasn't field tested because the FCC report states, "The manufacturer formally requested that this unit not be used in field tests since the device cannot tolerate much jostling."

Bench testing of prototype A found it was generally not able to detect DTV signals on any of the tested channels at the -116 dBm/6-MHz power density detection threshold the FCC requested comment on or even at the -114 dBm level suggested by the device's manufacturer. Reliable reception was possible at levels of -95 dBm or higher. Testing showed Prototype A was "generally unable to sense wireless microphones."

Prototype B was more sensitive. It was able to sense wireless microphone signals located in the center of a TV channel at signal levels as low as -120 dBm. For DTV signals, the prototype was able to reliably detect signals at -115 dBm in single channel tests and at -114 dBm in the two-channel tests, but sensing performance declined rapidly as signal levels were reduced.

Both prototypes were susceptible to false positives in the presence of strong signals. In one case, with a wireless microphone signal at -36.6 dBm, Prototype B incorrectly sensed wireless microphone signals on six additional channels!

Interference to cable TV reception from white space devices is another concern and another FCC report, Direct-Pickup Interference Tests of Three Consumer Digital Cable Television Receivers Available in 2005 (FCC/OET 07-TR-1005) indicates it is likely to be a problem.

The report states, "The test show that an OFDM source operating at an effective isotropic radiated power (EIRP) as low as 6.3 dBm can cause interference to cable DTV reception at a distance of 2 meters and that an EIRP as low as 15.3 dBm can cause interference at a distance of 10 meters." The report cautions that due to the limited scope of the tests, "the results are not intended to constitute a complete basis for defining criteria necessary to protect cable TV viewers from interference by devices operating in the TV white spaces. Nevertheless, the tests provide an empirical demonstration of the potential for such interference at relatively low power levels and, as such, a useful input to the decision process."

For information on how to file comments on the OET reports, see Public Notice DA 07-3457. Comments are due Aug. 15. The deadline for reply comments is Aug. 27.


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