d UMTS. of one bit per modulation signal. The phase of a carrier signal takes two possible values (separated by ) depending on the transmitted bit. 4-Quadrature 3.4.4 Hierarchical modulation amplitude modulation (4-QAM), also referred to as Quadrature phase shift keying (QPSK or 4-PSK), trans- Hierarchical modulation [19] is a digital modulation mits two bits on each carrier. Thus, it achieves twice the technique that enables transmission of two independent data rate of BPSK. In Figure 3.4-13, one possible con- information bit streams with unequal priority on a single stellation realization is presented, where the carrier channel. As part of the digital terrestrial television phases are /4, 3/4, 5/4, and 7/4. In 16-QAM and standard DVB-T [18], it offers new possibilities in 64-QAM, because there are 4 X 4 and 8 X 8 different organizing scarce radio frequency bandwidth. In this constellation points, respectively, four and six bits, section, we first outline the main concepts underlying respectively, can be sent per modulation signal. The hierarchical modulation and compare it to standard assignment of the bit streams to the digital states is digital nonhierarchical modulation techniques; then, we usually determined using Gray-code mapping SO that the give examples of possible applications. assignments of the closest constellation points differ in Figure 3.4-13 shows constellations of four basic linear one bit. The data rate is increased compared to 4-QAM digital modulation techniques [38,39]. Each possible at the expense of a lower noise tolerance (due to smaller digital state (constellation point) in the phase diagram distances between neighboring states in the phase dia- (represented by a dot in Figure 3.4-13) uniquely gram). For example, compared to 4-QAM with the same determines one phase of the carrier signal. Each trans- code rate, the minimum tolerated SNR is approximately mitted bit stream is assigned to one constellation point. 6 dB and 12 dB higher with 16-QAM and 64-QAM, The performance of a digital mod