QAM involves both amplitude and phase variation to map the digital data over the subcarriers. QPSK involves only phase variation while amplitude remains constant. 16QAM for example maps 4 bits of data on one single carrier and QPSK maps 2 bits of data on one single carrier. Hence 16QAM help increase data rate over QPSK but receiver will become complex in order to retrieve the modulated symbols if it is passed through the noisy channel environment.   _{Your Comment}