Last Tuesday representatives from AT&T met by conference call with several top FCC officials to discuss the TV UHF broadcast band incentive auctions, as revealed in AT&T's ex parte notice. The notice is slightly longer than a page, but it reveals that AT&T discussed potential issues with the 600 MHz band plan options. AT&T emphasized that “its examination of these issues is in the early stages, and that AT&T has not drawn any conclusions to their impact, if any, on services using 600 MHz or other frequencies.”
Interference scenarios discussed included:
• The potential for intermodulation products generated by interactions between uplink transmission originating from 600 MHz mobile devices and television transmissions originating from frequencies within the 600 MHz uplink/downlink duplex gap.
• The potential for third-order harmonics generated by 600 MHz uplink transmissions in the frequencies currently occupied by television Channels 42 through 46.
• The potential for television transmissions to cause blocking in 600 MHz devices using nearby frequencies.
The first concern appears to argue against the FCC's proposal to establish a nationwide wireless downlink allocation using frequencies from TV Channel 36 and down, and a separate uplink allocation using Channels 51 and lower. It isn't clear, however, what plan they would prefer other than keeping the frequencies away from channels used for TV transmission. In that regard, a band starting at Channel 51 down may be better.
The ex parte notice said “AT&T also discussed design and performance tradeoffs (e.g., antenna size, antenna performance, and MIMO) associated with the location of the uplink and downlink frequencies within the 600 MHz frequency band.”
While we don't know the details of the technical discussions between AT&T and the FCC officials, the issues they raised--antenna size, antenna performance and MIMO--are consistent with the problems with lower frequencies/longer wavelengths.
As I've written in the past, as wireless carriers move to more and smaller cell sizes to improve efficiency and handset makers attempt to cram more antennas into ever thinner and smaller devices, frequencies as low as 600 MHz are not going to be as useful for wireless broadband except in rural areas, where finding 700 MHz and higher spectrum isn't likely to be a problem. MIMO depends on receive antenna isolation for performance, and the lower the frequency the harder that is to achieve.
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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