FFC testing details "white space" interference to TV signals


The FCC's Office of Engineering and Technology released the results of study which revealed that portable, unlicensed devices cause interference to television broadcast signals. The FCC report concluded the sample prototype White Space Devices submitted to the Commission for initial evaluation do not consistently sense or detect TV broadcast or wireless microphone signals and that the transmitter in the prototype device is capable of causing interference to TV broadcasting and wireless microphones.

These new devices would operate on frequencies in the broadcast television bands that are unused in each local area. These locally unused frequencies are known as "white spaces." This research is part of the FCC's ongoing proceeding to consider rules for permitting such devices to operate on TV white spaces. As established previously by the Commission, fixed "white space devices" (WSDs) will be allowed into the TV spectrum simultaneous with the completion of the transition from analog to digital television broadcasts on February 17, 2009. The Commission is also considering whether to allow unlicensed "personal/portable" WSDs to operate in the TV spectrum.

One approach under consideration for determining the unused frequencies in local areas is for a WSD to employ a "detect and avoid" or "listen before talk" strategy. This approach would use "spectrum sensing" techniques that listen for the signals of TV stations, wireless microphones and perhaps other incumbent services. The Commission has requested comment on whether to require that the sensing capability of devices using this approach be able to detect signals as low at -116 dBm.

A second issue is the potential for WSDs to interfere with TV reception and wireless microphone operations. To address these issues, the Commission announced that it would conduct testing of WSD spectrum sensing and transmitting capabilities.

The FCC also notes the devices tested represent an initial effort, and do not necessarily represent the full capabilities that might be developed with sufficient time and resources. Accordingly, the report says the Commission is open to the possibility that future prototype devices may exhibit improved performance.
Measurements were limited to TV signals on UHF channels 21-51, the operating range of the prototype devices.
In the cases where the NTSC signal is being broadcast, the scanner reports the channel to be free or available between 11.1% and 27.8% of the time, with the average of 19.4% of the time. Where a DTV signal was being broadcast but was not received on the site's TV set, the scanner reported its channel to be free or available 81.3% to 91.7% of the time, with an average of 85.4% of the time.

Where a DTV signal was strong enough to be received on the TV, the scanner reported its channel to be free or available 40% to 75% of the time with an average of 58.2% of the time. When no signal was expected to be present, the scanner reported the channel to be free or available from 78.1% to 91.7 % of the time, with an average of 85.2 % of the time.

SmartMedia observation: Much of this new technology uses pulses of energy using multiple different frequencies and sometimes spectrums-often just above the noise floor. We're a bit surprised it would interfere with existing stations, as the devices can be programmed to just not use those frequencies in any given market. Needless to say, when the VHF band opens up for mobile applications in 2009, it should change the picture a bit.