Quadrature amplitude modulation
QAM is an extension of PSK in that some phasors are changed in amplitude as well as phase to provide increased bit representation. For instance, 16-QAM encoding increases the bit width of the modulation to 4 as shown in Figure 8.6. Twelve different carrier phasors are used, four of which have two amplitudes to provide further 4-bit combinations. Figure 8.7 depicts all possible carrier phase angles and amplitudes; it is known as a constel- lation map.
A higher order of digital modulation may be employed, for example, in cable, namely 64-QAM encoding, in which each carrier phase/amplitude represents one of 64 possible 6-bit combinations. The constellation map for 64-QAM is shown in Figure 8.8.
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