Window for Wireless Broadband/TV Interference Issues Comment Filing Closes

The deadline for filing comments about interference issues between wireless broadband and television services passed on March 17, 2014. However, as of midnight, March 18, only three comments had been filed and made it onto the FCC's Electronic Comment Filing System Proceeding 14-14 website.

Proponents of the FCC's plan for determining interference between wireless services and TV broadcasters in a post-auction environment got a glimpse of the flaws in the plan and the complications of attempting to determine inter-service interference to and from broadcast TV stations using the Longley-Rice terrain sensitive propagation model.

The most interesting comments came from the Society of Broadcast Engineers and Ericsson, a major supplier of LTE systems around the world. While they approached the FCC's proposal from a different direction, both agreed that the plan, as presented, has problems that must be resolved before it can be adopted.

SBE supports the use of separation distance to avoid interference between services, stating: "While the Commission desires to avoid using separation distances, SBE urges that appropriate separation distances be retained as the interference avoidance mechanism for all four interference scenarios, precisely because they are conservative, and create the best opportunity to avoid interference ex ante. Interference in any of the four scenarios cannot be remedied post hoc, and the Commission hasn’t the enforcement resources to address such interference on a case-by case basis. The fact is that distance separations are easily implemented, and interference avoidance is predictable. The proper calculation of distance separation to avoid the worst-case interference potential is the key to maximizing spectrum efficiency and reuse in this process."

Making the case against use of OET's Methodology, SBE said: "The Commission’s OET Methodology would, by contrast, over-complicate what otherwise is, has been and should remain a simple and effective process. Furthermore, the separation distance method would allow for future changes in DTV and Wireless modulation systems and techniques, without changing the evaluation of desired-to-undesired (D/U) signal ratios that would be necessitated by the OET Methodology."

Ericsson was more open to the idea of using a terrain-sensitive propagation model for determining interference, but suggested a combination of Longley-Rice (ITM) for long distances and the extended-range Hata model for modeling area coverage and clutter over short distances. It questioned whether ITM would work, noting, "It would be difficult to tune uplink power control to match field measurements if the ITM model were adopted for the cellular link."

The company’s filing continued: "Ericsson notes that while terrestrial long-range links have successfully been modeled using ITM, that model has not typically been used in predicting wireless coverage in cellular networks, where the distances are much shorter than those in television broadcasting. The extended Hata model, which is designed for distances typical of cellular transmissions, is typically used for cellular propagation estimates, including in 3GPP working groups. The extended Hata model has generally been validated for cellular environments when properly tuned for the urban, suburban or rural case."

SBE found several issues with the OET's Methodology that must be resolved if the FCC decides to adopt an ITM model for predicting inter-service interference and sees potential problems for a transition to ATSC 3.0:

"First of all, a prerequisite to the use of D/U ratios for interference avoidance in the four cases described in the Notice is the conducting of extensive laboratory testing on DTV receivers using LTE signals, as well as testing LTE interference to DTV Receivers. Furthermore, this is not a once-and-done process. As broadcasters move toward implementation of ATSC 3.0, the actual interference D/U ratios will likely change, as new modulation methods are likely to be utilized for ATSC 3.0 transmission, such as DVB-T2 or some other system."

In its closing comments, Ericsson indicated that changes to the FCC OET"s methodology will likely be needed. "Ericsson urges the Commission to carefully consider the implications and difficulties either of two approaches: (1) using the ITM model alone; or (2) using the ITM model for DTV in combination with the extended Hata model for cellular links favorable for modeling area coverage and clutter over short distances. It is also important in the methodology to account for each market’s population density when simulating interference performance. To the extent the extended Hata model is adopted for cellular links, the Commission should consider incorporating it directly into the TVStudy software package. In addition Ericsson suggests that the Commission also consider the use of a finer resolution to identify where within an EA interference from DTV is likely to occur, given that the DTV interference contour is not uniformly distributed across an impacted EA."

The SBE noted: "The Notice proposes the use of a new "de minimus" interference threshold of five percent for interference from DTV to wireless services, thus to define ''Impairment.' The proposed five percent threshold is excessive. It would include very significant number of people in large urban environments. A one or two percent threshold is more appropriate. The two percent threshold is proposed in the modified version of OET 69. By creating an overly-lax interference threshold, the Commission risks selling potentially unusable spectrum, which is in no one’s interest. Perhaps a more flexible option that deals with future population changes would be to base the evaluation on geographic area impairment, or a combination of areas and populations impacted, rather than on population alone."

The SBE raised several other concerns with the approach, including the impact on MVPD receivers and translators, whose antennas may be located near the edge of or even outside the station coverage area, the use of F(50,50) rather than F(50,10) statistics for interference into wireless base stations, the potential for ducting along the coasts, and concern about insufficient resources at the FCC to deal with case-by-case interference issues. The organization suggested that the FCC set up a database, similar to CDBS, which would register every base station site operating on or adjacent to channels used for TV broadcasting with technical information about transmission and reception from that site.

The SBE concluded: "It is clear that the most objective, predictable and reliable method of avoiding interaction between and among DTV and wireless facilities, either co-channel or adjacent channel, should be based on geographic distance separation. The dialog should be about the proper distance to address each of the four interaction scenarios, and use the worst-case to determine the proper separation. However, should the Commission nevertheless proceed to adopt the OET Methodology, the foregoing issues should be addressed and the final parameters should be based on actual laboratory testing."

It added: "Therefore, for the reasons discussed herein, SBE respectfully requests that the Commission continue to utilize the distance separation method of avoiding DTV and wireless interaction in the UHF television bands, post-auction. Alternatively, the OET Methodology should not be adopted unless and until each and all of the serious issues raised herein are addressed after appropriate real-world testing." The Comments of the Society of Broadcast Engineers and the Comments of Ericsson are available in the FCC ECFS.

I’ll have a summary of the remaining filings in this proceeding in next week’s RF Report.

Doug Lung

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.