Asymmetrical Sidebands for Maximum Coverage

August 1, 2010


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 DistributionAsymmetrical IBOC Power Distribution
-14dBc3,252-14dBc695
-13dBc305-13dBc111
-12dBc294-12dBc111
-11dBc240-11dBc137
-10dBc5,132-10dBc8,156
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.

Asymmetrical sidebands

Figure 2. The spectrum display of a Nautel NV10 running asymmetrical sidebands. Click to enlarge.

Figure 2. The spectrum display of a Nautel NV10 running asymmetrical sidebands. Click to enlarge.


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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HD PowerBoost also provides the ability to have asymmetric sideband power for those stations unable to increase above -14dBc because of an adjacent conflict on one side. A demonstration at the 2010 NAB Show showed a Nautel NV10 FM transmitter feeding a dummy load. This transmitter could operate at 10kW in analog mode, but at -10dBc in the normal hybrid iBiquity mode, output dropped to about 5.5kW. With HD PowerBoost, the power rose to 7.25kW. Then, still operating at about 7.25kW, the mode could be changed instantly to operate at -14dBc/-10 dBc, or other combination as requested.

HD PowerBoost implements a unique method of PAPR reduction; the iBiquity PAR is turned off via a software switch because the current iBiquity's PAR only processes the digital portion of the system. Nautel wraps the entire envelope into its peak reduction scheme with a processor that yields far greater efficiency than just dealing with the digital portion alone. Nautel offers this today and does not require any future release of iBiquity IBOC code to achieve asymmetrical capabilities.

Implementing asymmetrical sidebands

Implementing asymmetrical sidebands is possible today given two requirements. First, a station needs to request Special Temporary Authority (STA) from the FCC and show that one of the station's adjacent channels is clear of interference. Currently, the IBOC rules do not allow for this differential power, however, much like the earlier days when stations wished to use separate antennas for analog and digital, an STA was required until this became part of the rules. The general consensus among attorneys and consultants is that asymmetrical power will eventually become standard.

Second, the station needs hardware that can support asymmetrical sideband technology. A Nautel transmitter can be purchased with the HD PowerBoost option. Existing Nautel NV series transmitters can add PowerBoost by installing a replacement exciter circuit board and the necessary firmware and software.

If a station cannot achieve -10dBc on both HD Radio sidebands but could accomplish it on one side, it would be feasible using HD PowerBoost to then operate at -14dBc on one side and -10dBc on the other. This would yield an overall digital RMS power of 7 percent of the analog, or -12dBc effective symmetrical power. This would permit this station to maximize its digital coverage yet not create interference beyond what would be permitted. Stations that will operate at symmetrical power levels may also employ HD PowerBoost to gain efficiency or power output, but no FCC action is required in this case.


Schmid is a research engineer at Nautel, Hackett's Cove, NS.


Inside Nautel PowerBoost

Several years of research have been applied to creating Nautel PowerBoost. Because the system takes control of the PAPR reduction, it is able to assimilate the HD Radio carriers on both sides of the analog host and regenerate them at whatever amplitude is required. HD PowerBoost with asymmetrical sidebands is expected to be available in the third quarter of 2010.



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