H/B= log2 (1+ Eb/No * H/B)
where:
H/B=> bit rate/band width(spectral effieciency)
Eb/No=> energy per bit/noise densiny.
SO from obtained Eb/No you can find the required theoretical Rx for QAM.& by adding the extra noise yuo will get the real.
# bit per symbol is constant= Nb=log2(M) = 4bit.