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Asymmetrical Sidebands for Maximum Coverage
The FCC's blessing of up to -10dBc digital injection ratios is a great step forward for HD Radio coverage, and many broadcasters are considering taking advantage of the elevated digital signals. Unfortunately, as many as 4,000 U.S. stations may not be able to take full advantage of the higher digital signal levels because of potential adjacent-channel interference. For these stations, the digital power may be limited to only -14dB. Fortunately, by implementing asymmetrical sidebands, many of these broadcasters can maximize their IBOC signals to achieve greater digital listenership. Figure 1 provides a breakdown.
|Symmetrical IBOC Power Distribution||Asymmetrical IBOC Power Distribution|
|Figure 1. More than 4,000 radio stations could use asymmetrical sideband technology to achieve maximum digital listenership. |
Source: NPR Labs
Increasing HD Radio coverage
Let's take a look at the effects of increased IBOC carrier power levels. On-air tests with elevated carrier power levels reveal substantial increases in the HD Radio coverage area. Tests at Nautel confirmed a -20dBc to -10dBc carrier power increase produces a 30 percent increase in radial coverage of the HD Radio signal. Correspondingly, this translates into a 70 percent increase in coverage area with, potentially, an equivalent increase in HD Radio listenership matching or even exceeding a station's FM coverage.
Due to first-adjacent channel interference concerns, 44 percent (as calculated by NPR Labs) of all FM stations in the United States will not be able to take full advantage of the 70 percent coverage increase as they are limited to operating HD Radio carriers at -14dBc or below. With asymmetric sidebands, 88 percent of all stations can raise at least one sideband to the -10dBc level. Because HD Radio receivers decode both sidebands independently and can even operate on a single sideband entirely, asymmetric sideband operation is an effective way of reclaiming most of the 70 percent listenership increase. For example, bit error ratio (BER) tests at Nautel have shown a -10dBc/-14dBc sideband combination provides a coverage area increase comparable to running both sidebands at -12dBc. Figure 2 shows the spectrum display of a station running asymmetrical sidebands.
For stations wanting to implement asymmetrical sidebands, Nautel has developed HD PowerBoost, which is a suite of tools developed to aid HD Radio users in several ways. First demonstrated as a real product at the 2010 NAB Show, HD PowerBoost is an embbedded implementation of Nautel's unique method of peak-to-average power ratio (PAPR) reduction. This technology permits about 30 percent more hybrid power from a given Nautel transmitter, while achieving up to 7 percent greater operating efficiency. This eases the transition to elevated IBOC carrier levels for many broadcasters. To realize a 70 percent gain in listenership, many stations may require up to 77 percent of additional transmitter power rating. With HD PowerBoost, only 42 percent additional transmitter hardware is required to accomplish the same, effectively reducing the cost of HD Radio listenership.
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