IBOC Update: Data and Asymmetrical Sidebands


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To the sidebands

Let's shift gears now and talk some about asymmetrical IBOC sidebands. Why is this necessary to begin with? To answer that, just a small snippet of the Commission's own reasoning in their public notice (regarding MM 99-325) from Nov. 1, 2011:

A significant number of FM stations currently are precluded from taking advantage of the full 10dB digital power increase [from -20dBc to -10dBc] permitted by the Order [of Jan. 29, 2010] due to the presence of a nearby station on one but not both of the two first-adjacent channels. If asymmetric digital sideband operation is permitted, such stations presumably could increase their digital power on the sideband away from the limiting station.

So now with a reason to go ahead with this new facet of IBOC, two questions are rather obvious. What if anything will this do to plain old FM (analog) reception? And how effective is the transmission of asymmetrical sidebands with respect to the IBOC reception? Studies were undertaken to answer both of these questions.

Nautel engaged NPR Labs to perform laboratory testing on its peak-to-average-power reduction algorithm (PAPR) HD Powerboost. The purpose of the testing was to see how this PAPR affected FM reception of the host station. During the same tests, the effects of asymmetrical sideband transmission were also tested. I should mention also that the tests were performed operating in MP3 (extended hybrid) mode, which adds an additional set of partitions for OFDM carriers closer in to the host frequency. This was done because it represents a tougher set of test conditions than that of MP1 mode. Also, the tests utilized automotive, home stereo and shelf system receivers for analog FM compatibility tests and an analog automotive receiver for RBDS performance tests.

The entire report is available online. Here's a summary of the results:
> In comparison with conventional IBOC operation, Nautel's PAPR had little or no measureable effect on the performance of analog test receivers, even with the highest-tested asymmetrical digital sideband ratios of -10/-20dBc.
> Asymmetrical operation reduced the audio noise level of the home receivers by up to 3dB as either sideband was reduced
> RBDS reception sensitivity with mobile fading was slightly affected by -10dBc symmetrical sidebands, but showed improvements similar to analog FM reception with asymmetrical operation

The report was directed by and submitted by John Kean of NPR labs, and after a detailed analysis of the measurement data (seen near the bottom of page 10) the report states: "In sum, broadcasters are best off maintaining symmetrical sideband levels, but coverage improvements are possible with an increase of only one sideband."

To answer the question regarding the performance of the digital receivers, we look to iBiquity's report published in December 2011, which is also available online.

- continued on page 4



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