Doug Lung /
11.07.2008 12:00AM
FCC Approves White Space Devices, but it Could Have Been Worse

This week, as I'm sure most broadcasters know by now, the FCC approved rules allowing unlicensed devices to use “vacant” TV channels.

As of 1 a.m. Friday morning (EST) the FCC had not released the text of the Second Report and Order establishing the rules, but commissioner comments suggest they have endorsed power levels that are well above those shown to cause interference in FCC testing.

Listening to the commissioners' statements during the long delayed open Commission meeting, I heard them state they were concerned about interference to broadcast and over-the-air TV reception but that they had been assured by the scientists and engineers at the FCC's Office of Engineering and Technology and the FCC Laboratory that it would not be a problem.

“I have placed much faith in the Chief of OET, Julie Knapp, and his entire staff of engineers,” said Deborah Taylor Tate. “Their recommendations, while always useful in any rulemaking, played an especially persuasive role in my final decision to support this item. I have been assured by Chief Knapp and our top engineers that the risks of interference have been appropriately considered.”

Tate expressed doubts, however, about the lack of specific details on how the FCC will deal with interference complaints, stating:

The Commission, through our Enforcement Bureau and in consultation with the Office of Engineering and Technology, will investigate complaints of interference and take appropriate action, as we do with all cases of interference. I regret that my colleagues were unwilling to set forth in this item a more specific and swift process to deal with complaints of interference. I remain concerned that the item is too vague and does not provide necessary protections after the interference has occurred.  I also would like to acknowledge the legitimate concerns of the many industries that already provide valuable services using the core TV bands. Regarding the concerns of my friends in the broadcasting industry, I want to recognize the important service they provide, especially all they currently are doing to make the DTV transition as smooth as possible. ... In addition, broadcasters play a key role in providing emergency alerts to the public. It is, therefore, imperative that TV broadcasting continue to be protected from interference.

After expressing these concerns, she said, “The power limits and other technical rules, proposed by OET and adopted here are designed to do just that.”

I'm concerned by her solution to WSD interference to cable TV reception, an issue I raised in last week's RF Report. Tate said:

With regard to cable systems in the home, OET has attempted to establish power limits that will lower the risk of interference between devices in the home, though this risk is not, in my mind, fully mitigated. However, the Commission does not generally focus on interference that users cause to themselves. Just as we have all learned to move devices away from each other in the office and we no longer place our cell phones next to the computer, so might consumers need to reduce interference inside their home by moving devices, at least until the next generation of cable equipment becomes more widely disseminated.

As with the potential for interference to broadcast reception and wireless microphones, Commissioner Tate doesn't seem to be completely comfortable with the rules.

Unfortunately, the solution she mentions about moving the device away from the cable box or TV set won't work in a condominium or apartment house, where the device may be located in a home office far away from the user's TV set but only one wall away from the living room or bedroom cable box of their neighbor! What will that neighbor do when his or her favorite channel disappears from the cable box? How will they know it's the person in the next unit using a white space device that's wiping out their reception? It is unlikely “the next generation of cable equipment” will solve the problem, since digital cable channels were affected as much as analog channels in the FCC testing. Replacing the wiring in the condo or apartment with laboratory quality cable, as was done in the FCC testing, might solve the problem but that's unlikely to happen in older apartment buildings.

I'm not picking on Commissioner Tate. Of all the Commissioners, she had the best appreciation of the impact white space devices have on over-the-air and cable TV reception. I quote her because listening to her presentation and reading her comments, it's clear she isn't happy with the protections offered TV viewers in the Second Report and Order.

In the headline I note, “It could have been worse.” The good news is that at this time only devices with geolocation capability are allowed. They will have to access a database of available channels before transmitting and while I feel the allowed power levels are too high, the requirement for adaptive power control should reduce the actual power of devices communicating with other devices inside a dwelling. However, it may not help much if the devices are communicating with distant fixed transmitters.

Devices will require an ID, which will probably be something similar to the MAC address on all Wi-Fi and Ethernet devices. Through the geolocation database, it may be possible to shut off devices that are identified as causing interference. If transmitter power can be controlled through the geolocation database, it may even be possible to force units causing interference to cable reception to reduce power below the maximum the rules allow. Conceivably, if this turns out to be as big a fiasco as some fear, it may even be possible to turn off all the devices or limit them to a smaller number of channels that cable operators could avoid using for popular programming.

The rules will allow for sensing only devices to be sold once they pass a rigorous series of tests. There may be a lot of pressure to lower the threshold to get sensing-only devices on the market. Broadcasters and wireless microphone operators will need to carefully watch the testing to make sure devices are sensitive enough to detect a DTV or wireless microphone signal.

Sensing must work even in situations where the sensing receiver is blocked from the DTV transmitter or microphone if it could cause interference to receivers at locations where the channel is receivable. I'm not sure that's physically possible, but detecting weak DTV signals from a “hidden” location is the only way sensing can be used without wrecking havoc on the reception of DTV signals over a wide area.

I discuss the MSTV and NAB comments in another article in this week's RF Report. Meanwhile, take some time to review the comments of Commissioner Deborah Taylor Tate and the other commissioners at the FCC Web site. I'll cover the technical details after the text of the Report and Order is released.

 



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