Digital Versus Analog
Digital transmissions require less power to be received clearly than analog ones. The exact figure for various modes depends on how robust the signal is made to begin with, such as modulation, guard interval, and forward error correction. In each of these three factors, the caveat is that a higher data signaling rate means a tradeoff with reduced broadcast range. The hierarchical modulation used on DVB is a unique case, which reduces the range of the full-definition signal, in exchange for an increase in the usable range of the lower-definition part of the video.
Digital stations in North America usually are operated by the same groups as the analog side, and thus operate their own independent facilities. Because of this, the FCC requires U.S. TV stations to replicate their analog coverage with their digital signal as well. However, ATSC digital TV only requires about one-fifth the amount of power to reach the same area on the same channel as analog does. For HD Radio, the figure is only one percent of the station's analog wattage, in part because it is an in-band on-channel method, which uses sidebands that must prevent interference to adjacent channels, especially for older or cheaper receivers which have insufficient sensitivity and/or selectivity.
Read more about this topic: Broadcast Range