08.03.2007 12:00 AM
FCC: Prototypes Fail 'White Space' Tests
The FCC's Office of Engineering and Technology has issued two reports on initial measurement studies performed on low power RF devices that could be operated within unoccupied television broadcast spectrum known as "TV white spaces."

The reports are titled "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." The former described the testing of two prototype white space transmitting devices, and the second report described the testing of three digital television receivers for susceptibility to interference from the low power transmitting devices.

Test results indicate that the potential for such interference did exist.

In the case of one of the prototype devices, the report stated that circuitry designed to scan for a vacant RF slot consistently failed to perform as expected. On average, it failed to identify a channel occupied by an NTSC signal 19.4 percent of the time and failed to recognize a DTV transmission 85.4 percent of the time. The second device was reported to be able to be somewhat more reliable in identifying television occupied TV channels, but both devices had trouble reliably recognizing signals from wireless mics.

Time for the devices to do a complete scan of the frequency band tested (TV channels 21-51) took between four minutes and 14 minutes.

The second report was based on tests performed using consumer digital cable ready DTV receivers. It recognized 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."

In an executive summary, the report acknowledged that the while the scope of the testing was limited and that results gathered should did not form a complete basis for defining protection criteria, the possibility for such interference did exist.

The summary concluded that, "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."

The White Spaces Coalition, which represents industry players desiring approval of the unlicensed transmitter technology, issued a statement on Aug. 1 in response to the FCC reports. It acknowledged the tests and said that, "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 statement also said that the Coalition would work with the commission to resolve outstanding issues with regard to expanding access to the Internet.

Microsoft, one of the members of the White Spaces Coalition declined any comment beyond that given in the Coalition statement.

The complete reports are available on the commission's Web site.

Check the August 22 issue of TV Technology for more details.

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