NPR Labs Weighs in on IBOC Coverage, Interference

November 7, 2008

ecent publication and distribution of NPR Lab's Digital Radio Coverage and Interference Analysis Research Project (DRCIA) seems to reiterate a point once made by renown physicist Albert Einstein, when he suggested we ought to “make everything as simple as possible - but not simpler.”

The comprehensive report, commissioned by the Corporation for Public Broadcasing in 2006 to answer question about IBOC DAB coverage, is a meaty piece of work. At almost 280 pages, it requires time to digest, which explains why project manager John Kean and other key NPR personnel have taken time to present their findings over an NPR closed circuit and at a Public Radio Engineering Conference in Austin, TX, just ahead of the NAB Radio Show.

According to NPR, CPB funded the DRCIA project with the primary goal of determining real-world coverage of analog FM and hybrid IBOC digital radio services, as well as interference occurring within and between both families of service. The study was recommended to the CPB in 2005, well before discrepancies in digital signal coverage had emerged as a significant issue in digital radio's rollout.

Laying the foundation

In their early phases of inquiry, it became obvious to NPR engineers that a better understanding of marketplace receiver performance was essential to building a profile of hybrid IBOC DAB coverage and interference issues. After testing and characterizing a sizeable number of both analog and HD Radio receivers, a mobile test bed was used to generate measured analog service coverage maps on a number of NPR stations around the country. Building on this work, researchers constructed similar field measurement sets using actual digital receivers and leveraged that data into the first predictive computer model for IBOC digital coverage.

Eventually, NPR Labs was able to generate both digital and analog coverage maps based on real-world receiver data and measured fields for 850 NPR stations nationwide – a remarkable achievement. In addition, 75 of these stations had their digital coverage analyzed using both the normally licensed value of 1 percent IBOC digital power, versus an elevated level of 10 percent. The latter level corresponds directly to an increase recently proposed to the FCC by the NAB and other interested parties.

Because the studies were detailed and relied on actual receiver data, an unprecedented multi-dimensional model of coverage based on analog/digital signal levels, as well as the effects of co-channel and adjacent channel analog/digital interference, emerged.


Among the points raised during the DRCIA project, one central observation stands out: Digital versus analog coverage ratios for IBOC hybrid stations are both individualized and variable, with interference playing a significant role in many cases.

Just as real-world digital transmissions don't operate in an RF vacuum, neither do digital RF components, particularly low-level sidebands occupying spectrum beyond traditionally defined channel limits. Thus, much of the variability in IBOC digital service coverage is linked to a couple key factors.

While raw field strength of digital carriers is important, the study suggests that first-adjacent channel interference is a significant player in the successful capture and decoding of digital signals. While FM IBOC digital signals can survive a considerable amount of interference from either an upper or lower 1st adjacent channel, simultaneous interference both above and below can degrade receiver performance by as much as an additional 10dB beyond that of a single interferer. This means that stations with short-spaced first-adjacent channels or in crowded urbanized markets may find their digital coverage well below expectation.

Table 1

Table 1
Select table to enlarge

The DRCIA report looks at an IBOC digital signal's impact on adjacent channel analog reception as well. Even at current 1 percent levels, the interference potential of an IBOC digital signal for adjacent analog channels is significantly higher than that of an analog-only signal, while a 10dB increase in digital carrier power has a major impact on reception of adjacent channel analog signals. Tables of desired-to-undesired signal ratios for indoor, outdoor and portable receivers provided in the DRCIA final report make these points in striking fashion. One table is included here as Table 1.

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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