The QPSK is the most robust modulation. It can be represented by a constellation:
The radius, R, represents the amplitude.
The angle, ?, represents the phase.
There are 1 amplitude but 4 phases to 4 different states.
2 bits can be coded with 1 QPSK symbol.
The 16-QAM can modulate 4 bits per symbol.
The 64-QAM can modulate 6 bits per symbol.