Higher Power IBOC

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Increasing IBOC power is no trivial matter; now is the time to plan.

On Nov. 5, 2009, NPR and Ibiquity Digital jointly submitted to the FCC a proposal for an IBOC power increase. As of this writing, the Commission has not yet acted on the proposal; however, it does appear that an IBOC power increase is imminent. The heart of the proposal is a blanket 6dB increase in IBOC power levels ? but there are a few more details worth looking at. In a nutshell, NPR and Ibiquity are proposing the following changes to the Commission's rules with respect to IBOC:

  • A 6dB blanket power increase (-14dBc) for all stations except super-powered, grandfathered Class B stations. These stations would be limited to the greater of two levels: the current -20dBc level, or at least 10dB below the Class B equivalent power for their height.

  • The potential for a power increase to as much as -10dBc, depending upon conditions that limit harmful interference. Stations wishing for more than the 6dB blanket power increase would have to apply to the Commission for permission to do so. NPR has posted an IBOC power calculator online.

The filing also specifies a means by which harmful interference complaints can be resolved by the Commission.

If you currently have a Class-B super-grandfathered station, then this proposed rule change might not affect you at all. Say for example you have a super-power grandfathered Class-B with an ERP of 80kW, while the Class-B equivalent at that same site and antenna height is 8kW. Your current IBOC power would stay the same because 20dB below 80kW is the same as 10dB below 8kW.

Other options

One means by which some stations may be able to exceed the -14dBc limit is by transmitting the IBOC sideband groups in an asymmetrical fashion (currently those sideband groups — one group above the center frequency, and one below — are transmitted with the same amount of power). If a station wishing to increase its overall IBOC power has a first-adjacent channel station below it in frequency that is physically closer than the next station above it in frequency (as illustrated in Figure 1), then it is clear that for a given undesired-to-desired ratio (based strictly upon the distance between the stations) more power could be transmitted in the upper sideband group.

Figure 1. Physical spacing may allow a station to transmit an asymmetrical hybrid signal to achieve the power increase.

Figure 1. Physical spacing may allow a station to transmit an asymmetrical hybrid signal to achieve the power increase.

A means by which harmful interference complaints can be resolved is also proposed. If, for example, station Y is transmitting IBOC in excess of -20dBc and causes interference inside of station X's protected service contour, then station X may file a complaint against station Y with the Commission by completing the following steps:

  1. Station X must include in its complaint at least three interference complaints and evidence of on-going rather than transitory harmful interference inside its own protected contour;

  2. Station X must document any test measures used to identify the IBOC-related interference, and;

  3. Station X shall document the extent of the interference.

The FCC will have 90 days after station X's filing to resolve the complaint. If the FCC does not do so, then station Y will be compelled to lower its IBOC power, in 3dB steps, to as low as the original -20dBc level (but not lower).

-- continued on page 2

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