Comparing the BER curves for PSK and QAM, the intuitive advantage of
QAM over PSK drawn from the constellation diagram is borne out, with an approximately
3.5 dB gain in noise immunity for 16-QAM compared with 16-PSK. This improvement comes at
the expense of a somewhat more complicated modem design, needing to handle both amplitude
and phase information, and to combat both amplitude and phase errors on the channel. In
practice, the performance benefits usually outweigh the complexity such that QAM is
frequently used in preference to PSK modulation.
If the channel amplitude distortion is particularly severe, such as to dominate errors due
to noise, then PSK could prove superior under these conditions.
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