Channel encoding:Quadrature amplitude modulation

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.