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Asymmetrical Sidebands for Maximum Coverage
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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