FCC Investigates LTE Interference to DTV

The FCC’s Public Notice DA 14-98 (containing an Appendix with a "Methodology for Predicting Potential Interference Between Television and Wireless Mobile Broadband Services") has been the object of much criticism. One of these is that it wasn't based on tests with actual receivers. (See my article Broadcasters Flag Problems With FCC Wireless Interference Plan for a summary of the comments filed on the Public Notice.)

Following up on this, the FCC's Office of Engineering and Technology released a Public Notice this week seeking comment on its measurements of LTE interference to TV receivers. I'm sure broadcasters will find much to criticize in this study as it only involved four receivers. In general, the three newer receivers handled the LTE interference at desired-to-undesired (D/U) ratios specified in the earlier Public Notice. The one older device, a 2007 set-top converter box, failed with anything more than a 1 MHz overlap at the D/U ratios specified in the January Public Notice at very weak signal levels.

This is likely to be a problem, as I suspect many of these older converter boxes are still in use and the sample of new receivers tested is too small to determine how many of them will have similar issues.

Interference at weak desired signal levels is a recognized problem, as the interfering base stations and handheld units are likely to be at the edge of a station's coverage area. The tables in the two Public Notices can't be compared directly without adding a factor to account for the received signal level. I calculated that factor using the "very weak" signal level in the June 20, 2014 Public Notice and compared it with the tables in the Public Notice released earlier this year. Based on this, it would seem that the 2007 converter box would receive interference based on the OET measurements.

The results would indicate LTE-into-TV interference may be a problem if LTE spectrum overlaps spectrum used by full-power TV stations. The OET engineers conducting the study recognized the limitations of their measurements and noted this early in their report, saying that measurements were only intended "to identify any gross differences between the interference potential of LTE and DTV signals on DTV reception."

They added that the number of receivers used in the tests and such limited test conditions would be "too small" for any meaningful statistical analysis and warned that there should be no specific conclusions drawn from this set of measurements, adding: "These observations may, however, be considered in context with other measured data."

My take is that the measurements show that, at least in some cases, LTE interference to DTV receivers is going to be a problem if the methodology published earlier this year is used to determine where LTE base stations and consumer units can be located. More testing with a wider variety of receivers at different price points is needed. Along with TV sets, newer devices combining TV tuners with IPTV also need to be tested.

I'm concerned about the impact on broadcasters, but I think the major problem is going to come when the wireless companies that buy this shared 600 MHz spectrum discover that under certain weather conditions—not all that uncommon in areas near water and possible in other parts of the country—their systems will stop working. This problem will involve interference from TV stations located hundreds of miles away.

A station on Channel 48 in Houston that I helped build back in the late 80's was once received in Atlanta, Ga. Reception in other towns in Texas was not uncommon either. I picked the Houston station up inside a hotel room in San Antonio on a portable TV and a salesperson from the station said they were often able to watch it in their car while traveling to Dallas. Reports of interference between TV stations up and down the east coast are common during certain weather events.

To learn more, see Word document Measurements of LTE into DTV Interference: Tests on four DTV Receivers of OFDM 64-QAM Co- and Adjacent-channel Interference and the earlier Appendix: Methodology for Predicting Potential Interference Between Television and Wireless Mobile Broadband Services. See my 2002 TV Technology article Enhanced Propagation of TV Signals, Part 1 and Part II for more on how weather conditions impact TV reception and interference.

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.