) or by changing the constellation's a factor, as in Then, the coverage radius (which is determined by the Figure 3.4-15. The a = a/b factor [18] is defined as the noise tolerance) of the second TV program will be ratio between a, the minimum distance separating two roughly the same as in the nonhierarchical case; the constellation points that carry two different HP bit coverage radius of the first program, however, will be streams, and b, the minimum distance separating any two smaller than with nonhierarchical 4-QAM, but can be constellation points. Constellations with a > 1 are called enlarged by increasing the a factor (at the expense of nonuniform constellations. The increase of a makes the HP a smaller coverage radius of the second TV program) or bit stream more robust at the expense of a less robustness by using error protection (at the expense of decreasing of the LP bit stream. (The DVB-Tstandard uses a E {1, 2, the information rate). Thus, one immediate advantage of 4}.) Thus, hierarchical modulation splits the actual hierarchical modulation over a nonhierarchical one is the communication channel in two virtual channels whose savings in transmission channels because two streams characteristics depend on the whole constellation with different data rates and different coverage areas can (64-QAM), a factor, and code rates of the HP and LP bit be transmitted on a single frequency channel. streams. Hierarchical modulation efficiently addresses the Hierarchical modulation was originally proposed to problem of heterogeneity in clients' available bandwidths, enable two different coverage areas for a given receiver resolution capabilities, and channel conditions. transmitter in digital terrestrial TV. It offers great design For example, a single frequency channel can be used to broadcast a video bit stream to mobile (or portable) receivers and fixed receivers. The mobile receivers will decode the HP bit stream, whereas the fixed receivers will be able, in addition, to decode the LP bit stream (due to their large