NPR Labs Weighs in on IBOC Coverage, Interference

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Naturally, interference is only part of the overall coverage picture, and the DRCIA report goes a long way toward quantifying discrepancies between indoor vs. outdoor/mobile reception. Structure penetration creates a special set of issues for digital signals, as general findings of the report indicate that on average, current mobile digital coverage is about 85 percent that of the corresponding analog coverage, while indoor digital reception averages only about 38 percent that of analog. That's a gap that troubles many in the radio industry.

The report also introduces a number of less dramatic but significant points. Even though existing FCC rules covering adjacent-channel spacing and signal overlap were developed for an analog world, the amount of adjacent-channel digital interference that would result from universal FM IBOC hybrid digital operation at present levels would not be substantial, and would reduce coverage to indoor analog receivers by an average of about 6 percent. Only about 5 percent of FM analog translator input receivers would be affected, while mobile receivers would experience a negligible increase in objectionable interference.

But the digital/analog interference changes dramatically when a 10dB across the board increase of digital carrier is applied to a universal FM IBOC hybrid digital operation scenario. In such a case, analog coverage areas could drop by 26 percent on average, with 20 percent of stations losing as much as half of their current analog coverage. In return, digital mobile coverage would jump to about 117 percent of current analog coverage, while indoor coverage would be boosted to 83 percent of current analog coverage. The result would be significantly improved digital coverage, but at a considerable loss of analog listeners. Surprisingly, commercial stations would suffer nearly as much as their NCE counterparts down the dial. And it's a trade off that NPR, regardless of their enormous investment in IBOC technology and infrastructure, appears flatly unwilling to endorse.

The road ahead

As it turns out, the release of the DRCIA final report comes at a pivotal moment for digital radio decision makers at the FCC. While the study casts doubt on a one-size-fits-all approach to digital coverage improvement, such as an unqualified 10dB increase of digital carrier power, it does suggest a number of individually tailored solutions. These include the use of new technologies, such as single-frequency networks, which can be used locally to exclusively boost the field strength of digital carriers and sidebands, providing coverage fill in problem areas, such as downtowns and office parks. Other suggestions include the use of directional antennas on digital signals only, relying on space combining to control digital signal patterns independently from analog. In every case, digital signal improvement would likely involve an individualized approach to coverage studies using a new, more sophisticated set of analytic and measurement tools.

In conclusion, the DRCIA final report has answered a lot of questions about IBOC digital coverage and its relationship with analog FM signals in a hybrid environment. Yet it also poses some new questions for industry and the FCC. Will the NRSC now be asked to take up the complex task of drafting standards and procedures for a digital signal upgrade path? If not, how can the Commission establish new rules that best enhance digital service, while protecting a vast majority of listeners still relying on analog FM service?

NPR Labs and the CPB have made a great contribution to the science of IBOC digital broadcasting with the release of the DRCIA final report. How effectively that knowledge can be put to use is left to our industry, working in close cooperation with an informed and open-minded FCC.

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