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MSTV Opposes Qualcomm Use of OET-69 for Interference Studies

NAB, MSTV and other broadcasters have filed comments with the FCC opposing Qualcomm's request to use the commission's data to determine whether Qualcomm's MediaFLO 700 MHz transmitters will interfere with broadcast signals. The data is derived from the FCC Office of Engineering and Technology Bulletin 60 (OET-69) and would be used for Section 27.60 interference studies; broadcasters take issue with Qualcomm's assertion that its transmitters would be allowed to cause up to two percent de-minimis interference to broadcast stations.

I reviewed both the Qualcomm and NAB/MSTV filings. Qualcomm is asking that its transmitters on the out-of-core TV channels it secured in the FCC auctions be treated the same as if they were DTV broadcast stations on the same channel. In other words, OET-69 interference ratios and calculation methodology would be used to determine the amount of interference allowed to TV stations and the interference to any individual station could decrease its population coverage by up to 2 percent, provided the total interference, including interference from other stations, and did not exceed 10 percent.

MSTV and NAB argue that these parameters were set at the start of the DTV transition and are not applicable to new services sharing channels with broadcast stations. If you have been following Charlie Rhodes columns in TV Technology, you know the OET-69 desired to undesired interference ratios do not apply to strong signals. MSTV also notes that the FCC has dropped the allowable amount of interference to a station from a new or modified DTV facility to 0.1 percent, not 2 percent. They also questioned the validity of using OET-69 for doing the interference analysis, when the FCC acknowledged in its digital LPTV proceeding that it did not accurately reflect the broader elevation patterns of the lower gain antennas often used for LPTV broadcasting and which Qualcomm is likely to use at its stations.

In my Nov. 10, 2004 RF Technology column, I pointed out that while using the FCC OET-69 default antenna, patterns could result in significant errors in the predicted signal levels on the ground when the antenna is located high above the community it serves, an error in the way the FCC OET-69 implementation of Longley-Rice calculates depression angle can largely offset it.

In reply comments, Qualcomm submitted engineering studies showing that in two cases, OET-69 studies gave essentially the same results whether the default pattern based on a 24 gain UHF antenna was used or whether the actual pattern of the 12 gain antenna Qualcomm is planning was in calculating field strength. If the OET-69 depression angle error was included in these calculations or if the antenna was low enough that most of the population fell within the peak in the OET-69 UHF elevation pattern at 0.75 degrees below the horizon and above, these results are not unexpected.

The real interference problem, as I see it, occurs if the Qualcomm transmitters are not co-located with the TV broadcast facilities, but instead are positioned on cell phone towers. In this scenario, a broadcaster on an adjacent channel to the Qualcomm transmitter would likely find that people around the Qualcomm transmitter are unable to receive the broadcast station. Many people in the TV station's community could lose the ability to receive the station. Since the OET-69 interference parameters are based on weaker signals, studies based on OET-69 won't pick up the interference. The net result is many people in the most densely populated areas of a TV station's community of license could lose free OTA TV reception if the Qualcomm petition is granted.

As long as broadcast operations continue on adjacent channels, Qualcomm is likely to experience interference problems as well. If Qualcomm's 50 kW station is co-located with a 1,000 kW adjacent channel broadcast station, its customers' receivers may have problems with interference from the broadcast station unless they are designed to handle the strong DTV signals without creating intermodulation or desensitization. Using multiple transmitters closer to the customers, as Qualcomm seems to be proposing, solves the problem for them but now creates areas of interference to the broadcast stations.

Qualcomm argues that broadcast use of channel 52 and above is temporary and any interference will go away once the DTV transition is complete. That may be the case, but for DTV stations now using channels above 52, interference to reception of their signals, especially at cable head ends, could slow the DTV transition.

What is the solution? My opinion is the 2 percent threshold will be much more of a problem for broadcasters if the interference is in the core of the community they are serving rather than near the edges of their coverage area. Since the interference ratios in OET-69 are not applicable to strong signal areas, different ratios must be used. For low elevation sites, this could be achieved even in existing OET-69 software by increasing the power of the new, close by, station before doing the study to compensate for the change in D/U ratio. While this won't overcome the 2 percent interference problem, it will give results closer to reality for viewers near the Qualcomm transmitter sites. The FCC is going to encounter similar problems with strong signal interference when dealing with distributed transmitters for DTV broadcasting, so it may be worthwhile to defer deciding how the interference from the Qualcomm transmitters should be treated until rules are in place for broadcast distributed transmission systems.

For more details on Qualcomm's petition and the comments in response to it, visit the FCC's Electronic Comment Filing System Search for Filed Comments. Enter "05-7" for the proceeding. This should take you to the page with all the comments in the proceeding as well as the technical exhibits. When I last checked there were 27 comments available.