Better measurement systems

Among the salient issues that became evident during NPR's early mobile signal surveys were intervening variables imposed by unique RF environments and limitations in the measurement platform. So when the Corporation for Public Broadcasting (CPB) released an RFP in early 2006 to research and document predicted digital coverage for around 850 public radio stations, it was already understood that developing a functional model would require new technology and a comprehensive approach. NPR Labs won that grant, and an enormous technical undertaking began.

One unknown was the performance characteristics of the HD Radio receivers themselves. While the available choice in digital receivers was somewhat limited in 2004, an increasing number of products were coming to market. Characterizing an average receiver required the creation of a fixed test bed that could repeatedly manipulate complex variables with the unit under test. The NPR Labs team responded with the construction of a system that could vary desired signal FS, independent levels of co-channel and adjacent channel interference, and simulate Rayleigh (multipath) fading. With such a spread of variables, automated testing and data collection were essential to the process.

Changes in the mobile measurement platform were also needed. A standardized monopole antenna with integral ground plane was fabricated for attachment to test vehicles, greatly reducing pattern/gain variations presented by the mag-mount antenna used in earlier studies. Multiple field strength monitors were added to allow simultaneous measurement of the desired carrier, as well as those on two adjacent channels. With these improvements, detailed measurement data sets needed to verify modeling performance became available.

Once the new technology and methodologies became operational, the research team was left with the massive task of data collection and analysis that would provide the basis for predictive modeling of hybrid IBOC digital coverage.

After testing and characterizing 15 different HD Radio receivers, the data suggested that much of the variability in IBOC digital service coverage can be accounted for by a couple key variables. While field strength of digital carriers is an obvious factor, it was determined that first adjacent channel interference plays a major role in determining whether a desired digital signal will be successfully captured and decoded. Most notably, it was observed that while FM IBOC digital signals can survive a considerable amount of interference from either an upper or lower first-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.

The test sequence also established parameters for the interplay between desired signal levels, co-channel interference and fading/multipath variables.

Based on the extensive data set generated by receiver tests, NPR succeeded in developing a predictive computer model for IBOC digital coverage. In order to test the programs utility, a new set of measurements was taken for WJFK in Manassas, VA, using the improved mobile test platform. A comparison of the predictive versus measured data showed a very strong correlation of 94 percent between data sets, affirming the model's ability to anticipate coverage based on signal and terrain data for both the station under study and first-adjacent channel stations. Since that time 10 more stations have been used as points of comparison between measured and modeled coverage, with correlations ranging between 85 and 95 percent.

The fruition of NPR Lab's effort to build an FM IBOC digital coverage model had its debut at an IEEE presentation in Washington, DC, last October, as John Kean presented a paper detailing his group's work. A complete portfolio of predicted coverage for approximately 850 public radio stations, as well as publication of receiver test data, are forthcoming.

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Krieger, Radio magazine's technical consultant on digital radio, is the director/general manager of WJCU-FM, and a contract engineer in Cleveland.

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An in-depth examination of the work done by NPR Labs on the HD Radio coverage project in online: http://www.nprlabs.org/publications/reports/20071129_MeasurementandModelingofHDRadioCoverage_JCK.pdf.

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