FCC Outlines Rules for Predicting and Limiting Interference in Repacking

The FCC this week released a Second Report and Order and Further Notice of Proposed Rulemaking (2nd R&O and FNPRM) rejecting proposals for a cap on any new interference between television stations as a result of the repacking and establishing a methodology and associated values to be used in predicting inter-service interference between television and wireless services for use during the incentive auction (ISIX Methodology). The FNPRM proposes a post-auction interference methodology and input values as well as protection standards for any television stations and new nearby 600 MHz Band wireless services on co- or adjacent-channel frequencies.

While the FCC rejected broadcasters' proposals for a one-percent cap on the total or aggregate interference allowed to any station during the repacking, the R&O and FNPRM provides more protection to broadcast TV stations from wireless services than wireless commenters proposed.

In rejecting the request from the NAB for the one-percent cap, the commission said that such fears are “exaggerated” and that most stations “are unlikely to experience aggregate new interference of more than one percent.” It also said it was taking steps to alleviate such concerns in cases where the one percent interference was exceeded. It also said such a cap was not “realistic.”

“Even if the broadcasters had identified a means of implementing it (they have not), an aggregate interference cap would deprive the reverse auction bidding process of its speed and, therefore, compromise the success of the incentive auction,” the commission said. The FCC also rejected NAB's contention that the results of the staff analysis showing the total amount of interference stations would receive under different scenarios was skewed by the spectrum recovery estimates and thus understate the potential for new aggregate interference.

The FCC adopted two measures to address “exceptional” cases where a station is predicted to receive aggregate new interference in excess of one percent.

The first measure is the use of optimization techniques that seek to avoid final channel assignments that would result in aggregate new interference above one percent. These would be employed after the incentive auction bidding closes and the stations that will remain the air is known. The 2nd R&O notes, “Although our current rules do not provide broadcasters with complete protection from aggregate interference caused by other broadcast stations, we choose a one percent threshold in light of broadcasters’ stated concerns about aggregate interference exceeding this amount.” The FCC acknowledged that this final optimization procedure could still leave some stations with aggregate interference above one percent.

The second measure the FCC proposes would give a station predicted to receive new interference above one percent on its final channel an opportunity to file an application proposing an alternate channel of expanded facilities in a priority filing window, along with a limited number of other stations that have been assigned the same priority. This opportunity will be available not only to stations repacked to a new channel, but to stations remaining in their current channel that are predicted to receive new aggregate interference above one percent as a result of the repacking.

The ISIX Methodology and input values in an appendix to the 2nd R&O will be used during the auction to estimate the extent to which 600 MHz wireless license areas may be “impaired” due to predicted interference to or from broadcast TV stations assigned to the 600 MHz band as a result of market variation, where frequencies assigned for wireless operations may overlap frequencies used for broadcasting. Impaired areas may include “infringed” areas, where wireless operations predicted to receive harmful interference from a TV station, and/or “restricted” areas where wireless operations would be predicted to cause harmful interference to a TV station placed in the 600 MHz band, depending on how the wireless operations are deployed.

The 2nd R&O rejected calls for use of a distance-based approach, concluding that while the ISIX Methodology may be more complex, its “ability to account for different inter-service interference scenarios, local terrain obstacles and other factors make it significantly more spectrally efficient than a distance-based approach, and these benefits outweigh the costs of greater complexity. Also, its granularity is better suited to the requirements of conducting the incentive auction than a distance-based approach.”

The FCC also rejected calls to use propagation models other than Longley-Rice 1.22 to predict inter-service interference. The Longley-Rice “Error Code 3” will be ignored and the value returned will be used to determine if interference exists. This treatment is different from that employed in the incentive auction, where cells returning Error Code 3 are assumed to have service and no interference even if Longley-Rice predicts interference will result. The FCC decided not to include clutter loss in the interference calculations, noting that “application of a single clutter value in a four-square kilometer area would not improve the accuracy of the ISIX Methodology.”

In its comments to the inter-service interference Public Notice, NAB disagreed with the values of off-frequency rejection where the spectral overlap equals 1 MHz. It challenged the values in the Public Notice because they do not take into account the asymmetry in interference rejection that occurs in a DTV receiver based on where the spectral overlap occurs. The FCC also rejected claims that the measurement data in the record is not reliable because it does not consider factors such as multiple LTE interferers, third-order intermodulation (IM3), taboo interference and splatter, noting that its existing rules do not include provisions to protect DTV signals from the effects of multiple DTV interferers, IM3, or splatter. The FCC concluded “that the measurement data in the record is reliable despite the lack of information regarding these factors.”

The 2nd R&O adopted the D/U (desired-to-undesired) ratios proposed in the Public Notice for adjacent channel interference but found that LTE signals create slightly more co-channel interference to DTV reception than other DTV signals. To reflect this, it increased the co-channel D/U ratios by 1 dB from 15 dB to 16 dB. Regarding the one receiver that was predicted to receive interference at the D/U ratios adopted, the 2nd R&O states, “We decline to adjust the D/U ratios we adopt based on the susceptibility to LTE signal interference of obsolete analog-to-digital converter boxes, the vast majority of which will no longer be in service during and after the 39-month Post Auction Transition Period.”

The 2nd R&O rejects claims that the parameter values for wireless base station power and height proposed for Case 3 in the ISIX Public Notice (interference from wireless base stations to DTV reception) are inconsistent with real-world wireless facilities. The FCC notes these typical values were obtained from advisory committees and industry submissions in the record and that it has previously considered typical operating parameters in predicting interferences.

Although the decisions adopted in the 2nd R&O should increase opportunities for broadcast and wireless sharing of 600 MHz spectrum and thus create more potential for inter-service interference, the rules the FCC are proposing in the FNPRM should help mitigate the actual impact this sharing would have on broadcasters. Interference would be calculated using the same D/U and offset frequency ratios adopted in the 2nd R&O as defined in OET Bulletin 74.

The FNPRM proposes a zero percent threshold for harmful interference, stating, “Under this approach, 600 MHz Band wireless licensees would not be permitted to cause harmful interference within the service area of a full power station or the protected contour of a Class A station, to the degree it affects population within that service area or protected contour.”

The FCC outlined its reasons for this decision: “First, we believe that a different, more cautious approach may be warranted than in the context of preventing harmful interference between television stations because we will be applying the proposed methodology for the first time. Second, we do not believe that a zero percent interference threshold would undermine our goals for the incentive auction. Third, we are concerned that there is a potential for significant aggregate new interference from wireless operations to television stations if we set a de minimis threshold.”

A 600 MHz Band wireless licensee would have to perform an interference analysis using the OET-74 methodology prior to deploying a base station that's co-channel or adjacent-channel with TV stations within the set culling distance. Wireless operators could use their own network software to do the analysis or TVStudy. This analysis will have to be based on the actual location, HAAT, ERP, and antenna pattern of each base station. While the 2nd R&O procedures used in the auction will not combine signal levels from multiple hypothetical transmitters when calculating interference to TV reception, under the rules in the FNPRM, a wireless licensee would have to use the root-sum-squared (RSS) method (similar to that used for calculating interference between a distributed transmission system and other TV stations), to combine all signal levels in the cell being studied. This would prevent a situation where no single transmitter provided enough signal to cause interference even though when multiple signals were combined they exceeded the OET-74 allowed D/U ratios. The base station data used in calculating the RSS interference will have to be up-to-date and accurate.

The FNPRM provides another protection for broadcasters stating that “while we propose to use a predictive model to prevent inter-service interference to television stations based on wireless base station deployments, we also propose to require a wireless licensee to eliminate any actual harmful interference to television service in the 600 MHz Band, even if no harmful interference is predicted.” However, a TV station receiving interference would be required to contact the co-channel or adjacent-channel wireless provider causing the interference to resolve the issue. The wireless provider would have to provide the TV station with the results of its OET-74 analysis demonstrating no interference was predicted to occur in the specific geographic area at issue. If the broadcaster is unable to resolve the issue, they can submit a claim of harmful interference to the commission.

As proposed in the FNPRM, the ability of broadcast stations with co-channel or adjacent-channel wireless operations to increase coverage would be limited. After stations submit their initial construction permit application on a new channel, they would not be able to file for expanded facilities on their new channel unless they can demonstrate that the proposed expanded facility will not increase their contour in the direction of a wireless license area.

The FNPRM seeks comment on appropriate modifications to the ISIX Methodology to predict interference to 600 MHz Band wireless operators from LPTV and TV translators. Based on its analysis, the FCC concluded, “we propose to use the same field strength values as full-power television for the interference thresholds of co-channel and adjacent channel emissions for LPTV and TV translators to wireless service in the ISIX Methodology.”

For more information on the ISIX Methodology and OET Bulletin 74, refer to Second Report and Order and Further Notice of Proposed Rulemaking (FCC 14-157).

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.