Unlicensed Wireless Devices Face Tall Task in Operating Reliably, CEA Says

To find out how critical a factor it is, CEA commissioned field tests at 10 homes in the Washington area to probe building attenuation and other key issues raised in the FCC’s rulemaking that proposes to allow the operation of unlicensed wireless devices within the frequency bands now used for TV broadcasting. The survey found that higher building attenuation can reduce the chance that an unlicensed device in one home will interfere with TVs or other devices in a neighbor’s home, CEA said. However, the tests found that higher attenuation also “makes it more difficult for an unlicensed device inside a home to detect vacant channels,” CEA said.

The tests, conducted by the Meintel, Sgrignoli & Wallace consulting firm of Waldorf, Md., also studied the signal- strength differences between a rooftop antenna that would typically be used for TV reception and an unlicensed device trying to detect that same signal inside a home, CEA said. The survey found that about 10% of the samples tested had a signal-strength difference in the 39-43-dB range, CEA said. “Autonomous sensing” functions in such unlicensed devices “will need to overcome this difference to reliably detect vacant channels,” CEA said.

CEA conceded the study covered “a relatively small sample” of the actual universe, considering the large installed base of TVs and the diversity of geography and home construction. However, it said it hopes that “experts working on techniques to avoid interference from unlicensed device operation in the TV bands” can make effective use of the study. CEA said it will make the raw data from the study available for download within 2 weeks.

In a summary of the test methodology and results, Meintel, Sgrignoli & Wallace said field strength data was gathered on adjacent RF channels 39 and 40 using a vector sector analyzer. “This allows a scenario where a strong adjacent channel exists, but does not preclude reception of the weaker channel 40 signal using an outdoor antenna,” the summary said: “Sensing the presence of the weaker channel 40 signal is critical and difficult in this scenario due to the strong channel 39 signal.”

For an unlicensed wireless device to detect the presence of an existing broadcast signal, “it must receive the DTV signal by whatever means it has available to it,” the summary says. It must detect that signal wherever in the house the user has placed it, “with whatever self-contained small omni- directional antenna it has, and in whatever interference condition that exists at that location,” it says. That the unlicensed wireless device transmits its own signal makes the task of detecting an existing over-the-air signal all the more challenging, it said.

Making matters worse, successful DTV reception of a weak ATSC signal using an outdoor antenna at 30 ft. above-ground level in the same or a nearby house may be possible, “but sensing the presence of a DTV signal inside the house on the first floor with an omni-directional indoor antenna may not be feasible,” the summary says. “In this case, the unlicensed device would then incorrectly select this particular channel on which to transmit its data and possibly interfere with DTV receivers.” A DTV receiver connected to the outdoor antenna “will have a much better chance of getting an adequate signal for DTV reception” than a set not so connected, it said. That’s because the unlicensed wireless device will typically have a low-gain, omni- directional antenna while an outdoor antenna -- especially one situated farther away from the transmitter site -- will have a high-gain, directional antenna, and perhaps even a low-noise preamplifier at its output, the summary said.