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This paper examines the limits that must be imposed on single and multiple fixed cognitive broadband devices (BDs) transmitting simultaneously on multiple unused TV channels in order to avoid interference with the reception of licensed TV channels. Single and multiple fixed BDs are assumed to radiate 4W peak power relative to an isotropic source and transmit within 10 meters of the nearest digital television (DTV) set antenna. The power and separation limits are shown to depend on whether BDs are licensed, on whether mandated minimum front-end performance specifications for DTV sets are in place, and on the definition of an ldquounused channel.rdquo Short and long-term general conclusions are drawn regarding the maximum allowable directional power and regarding taboo channels for interference-free coexistence with DTV sets. It is shown that unlicensed BDs would result in the least efficient use of the available DTV spectrum because the ldquoworst caserdquo interference limits must be imposed in an unlicensed environment. The most efficient utilization of the DTV spectrum would be achieved if BDs were licensed and then allowed to migrate, together with DTV channels, into two blocks of contiguous channels, one for DTV and one for BDs.