The FCC order allowing an FM IBOC power increase outlines how a station can determine whether it can increase its digital power and by how much; it also sets out interference remediation procedures.
Most stations are covered by the blanket increase of 6 dB, to 14 decibels below carrier. (Super-powered FMs are an exception; see below.) To increase FM digital effective radiated power beyond that, a station would need to submit an application to the Media Bureau.
The bureau said its experience with higher-powered digital experimental authorizations suggests that the formula developed by NPR and endorsed by iBiquity in their compromise agreement is “over-predictive of the potential for interference.” However the bureau will use that formula because it believes “the protection this methodology provides to first-adjacent channel stations can be used to establish an expeditious ‘go-no go’ mechanism.”
Before applying for digital power beyond the –14 dBc just approved for most FMs, you must calculate your analog field strength along the protected 60 dBu contour of a potentially affected first-adjacent analog FM. Once you determine the most restrictive analog field strength of your station, use this table to determine your maximum permissible FM digital ERP. Where this method is not applicable, the FCC will evaluate case by case. Stations that want to raise their FM digital power beyond 6 dB must calculate the station’s analog F(50,10) field strength at all points on the protected 60 dBu F(50,50) contour of a potentially affected first-adjacent channel analog FM station. This calculation must be done using the station’s licensed analog facilities and the standard FCC contour prediction methodology. Once the most restrictive analog F(50,10) field strength of the proponent station has been determined, the licensee will use a table, shown, to determine the proponent station’s maximum permissible FM digital ERP.
In situations where this simple method isn’t applicable because of unusual terrain, environmental or technical considerations, or when the method produces anomalous results, the Media Bureau will accept applications for FM digital ERP in excess of –14 dBc but these must include a detailed showing containing an explanation of the prediction methodology used as well as data, maps and sample calculations. The bureau will evaluate these applications on a case-by-case basis.
“Super-powered” FMs are treated separately in the order. The FCC went beyond the compromise recommendation and restricted all superpowers, not just Class Bs, from the blanket hike. It also noted that less than 2 percent of the approximately 9,600 licensed FM stations are super-powered.
FM digital ERP for such stations will be limited to the higher of either the current –20 dBc level, or 10 dB below the maximum analog power that would be authorized for the station’s class, adjusted for antenna height above average terrain.
Super-powered FMs must file an informal request for any increase in digital ERP.
Also in its order, the Media Bureau said it anticipates that widespread FM IBOC power increases “will provide valuable coverage and interference data that will be useful in developing a prediction methodology. We are convinced that it is imperative for us to implement a power increase promptly and that the record establishes that the digital power limits set forth in this order will provide the necessary protection to analog FM stations.”
However, it continued, “out of an abundance of caution” it is adopting interference remediation procedures to address instances of loss of analog service within a full-service FM’s protected contour.
In cases of alleged interference, the FCC will look for sustained and verifiable instances. If a full-service analog FM is receiving verifiable listener complaints of interference within its protected contour from digital FMs operating at higher than –20 dBc, the affected station should contact the interferor. The stations should try to resolve the issue by first verifying interference and trying to eliminate it by lowering the digital power of the interfering station by degrees. Both stations must agree how much to reduce the digital power; then the interferor needs to notify the FCC of the lowered digital ERP.
If the stations cannot agree, the affected licensee may file a complaint with the Media Bureau.
In their compromise agreement, iBiquity Digital and NPR had proposed that at least three interference reports would have to be received before station could seek recourse to the FCC; the commission increased that minimum to six, citing the effects of varying terrain, tall buildings and other FMs.
So in order to be considered by the bureau, a complaint must contain at least six reports of ongoing, rather than transitory, objectionable interference. For each interference report, the affected FM must submit a map showing the location of the reported interference and a detailed description of the nature and extent of the interference being experienced at that location. The FCC also said it needs a complete description of the tests and equipment used to identify the alleged interference as well as the scope of the unsuccessful efforts to resolve the interference.
Interference reports at locations outside a station’s protected analog contour will not be considered.
The Media Bureau will review each complaint and “order appropriate action” by the interferor within 90 days of the date on which a “complete and sufficient complaint” has been filed (this is as recommended by iBiquity and NPR). If the bureau doesn’t act within 90 days, the interfering station must immediately reduce its FM digital power.
In those cases, stations operating with FM digital ERP in excess of –14 dBc must immediately reduce to –14 dBc. If ongoing complaints of objectionable interference persist, the FCC may require subsequent 3 dB reductions to –17 and –20 dBc, respectively, until it acts on the pending complaint.
Stations operating with –14 dBc or less must immediately reduce to –17 dBc. If interference complaints persist, the bureau will order the station to reduce to –20 dBc and require it to remain there until the bureau acts on the complaint.
And while it believes its procedures won’t result in “numerous or non-resolvable interference cases,” the FCC stated that that if increased power results in “a widespread level of interference to existing analog FM stations that we deem unacceptable,” it will revisit the maximum permissible values.