White Space Rules Show FCC Interference Fears
November 21, 2008
It seems some at the FCC are seriously concerned about the impact White Space devices will have on TV viewers and on other uses of the spectrum such as wireless microphones.
The FCC released the White Space rules Nov. 14, detailing its decisions concerning and the rules for unlicensed operation in the TV broadcast bands.
While unlicensed TV band devices have been referred to as "White Space Devices" or WSDs (not to be confused with WMDs), the Second Report and Order calls them "TV Band Devices" or TVBDs. After working my way through the 130 pages in the Second Report and Order, I focused on the actual rules for the TVBDs. I'm expecting to see an erratum soon, as one of the rules prohibits the use of TVBDs in the top markets, including Los Angeles, New York and Chicago. Section 15.712(f)(2)(a) states: "TVBDs will not be permitted to operate within 134 km of the 13 metropolitan areas listed in Section 90.303(a) of this chapter."
I searched through the rules looking for details on the modulation types allowed for TVBD transmission. Could someone put a 4-watt analog video transmitter on the air as a TVBD? How about a 4-watt 8-VSB translator? While I don't see any restrictions on emission type, the requirement to "perform in-service monitoring of an operating channel a minimum of once every 60 seconds" would preclude it being used for any continuous broadcast service.
One rule indicates the FCC's concern about TVBD interference to TV reception.
Section 15.715(j) states, "The database must have functionality such that upon request from the Commission it can indicate that no channels are available when queried by a specific TVBD or model of TVBDs."
This would allow the FCC to shut down TVBDs that are causing or are expected to cause interference.
My biggest concern with the rules released last Friday is the -114 dBm threshold for sensing- only devices. According to the Second Report and Order, the -114 dBm level was determined by assuming a DTV signal has to be at or above 41 dBµV/m (-84 dBm) within its service area and subtracting 30 dB to allow for the difference in signal level between a TV receive antenna and the TVBD sensing antenna.
In any area with hills or mountains, there will be places where the DTV signal is below -114 dBm, but where viewers on nearby hilltops in view of the TVBD will have reception, perhaps with the use of outdoor antennas and preamplifiers that allow reception at signal levels below -84 dBm. The -114 dBm number does not take into account the dipole factor the FCC used in determining DTV service area. For example, the 39.5 dBµV/m contour defines the service area for a channel 21 DTV station.
The justification for the -114 dBm detection threshold is based on UHF service area only, While a more sensitive receiver would be needed to detect VHF signals, which have service areas defined at 36 dBµV/m for channels 7-13 and 28 dBµV/m for Channels 2-6, only fixed-mode devices, which must access a database to identify available channels, are allowed below Channel 21.
The FCC appears to realize that -114 dBm is useful only as a "first cut" in qualifying sensing-only devices.
Section 15.717(b) states, "Compliance with the detection threshold for spectrum sensing in Section 15.711(c), although required, is not necessarily sufficient for demonstrating reliable interference avoidance."
However, once a device is certified, the FCC will allow additional devices that are identical in electrical characteristics and antenna systems to be certified under the procedures of Part 2, Subpart J of the FCC rules. It is critical that groups interested in protecting free over-the-air TV monitor the device testing and have the ability to comment on the testing before sensing-only devices are approved.
The "white spaces debate" is not over. I expect there will be considerable arguments over the results of the testing of the first sensing-only device. While the rules appear to offer technical solutions to resolving interference problems, it isn't clear how they will be handled. If a TV viewer starts to experience problems in receiving one or more over-the-air TV channels or cable channels, he or she will likely call the TV station or cable company, who will then have to contact the database administrator to find out if devices are using the channel in the area where the interference is being received. At that point, will the database administrator send a command to shut off all the devices using that channel in the area, whether or not they are the ones actually causing the interference? Will a more exhaustive study be needed?
In her statement on Second Report and Order, Commissioner Deborah Taylor Tate summed up the situation nicely:
"I wanted to ensure that our rules specify that, in the event of significant interference caused by an unlicensed device, the party responsible for this device will also be responsible for rectifying the problem and assume the cost," Tate said. "Some companies have assured us that this will be the case; that their business reputation requires it and it is indeed 'good business' for them to correct the situation."
She added that the impact of potentially millions of such devices in use called for a recall process, interference mitigation procedures, along with a clear exposition of fines and penalties for violations.
Tate concluded, "I felt it was imperative to deal with this on the front end, but today's item is not sufficiently clear on these matters. I hope the next Commission will address this before, rather than after, any harm occurs."