## M-ary Amplitude and Phase Keying (M-ary APK)

#### In some applications, the constraint of symmetry imposed by QAM systems may not best suit the characteristics of the channel or the detection process. None the less, the designer wishes to have the freedom to place the symbol points anywhere in the constellation diagram, implying both amplitude and phase modulation.

#### The constellation shown here is that used in the V29.bis telephone modem standard, and correctly belongs to the more general Amplitude and Phase Keying (M-ary APK) class rather than the QAM subset class. The reason for this particular symbol layout is to maximize the phase difference between symbols of the same energy to 90o rather than only 37o for 16-QAM at the expense of increased amplitude levels. This is on the basis that there is predominantly phase distortion in a telephone line. |

| #### Interestingly, the later V32.bis modem standard uses square 16-QAM modulation, but also uses more sophisticated equalization and coding. Circular QAM constellations are sometimes used in order to alleviate the problems of phase ambiguity in carrier recovery systems and to facilitate differential detection by reducing the number of amplitude levels in the APK format.