[BobbyTables2]

There are a few drawbacks with transmitting QAM with two signals

- one needs 2 signals instead of one (2x total bandwidth) - requires each channel bandwidth to extend to to DC, which had many other challenges

If one modulates the signal to shift it away from DC, the “negative/mirror” frequencies also shift, which means now bandwidth has doubled.

A QAM signal still has double the bandwidth of an equivalent PAM one but pays for it by encoding two PAM signals.

Of course, Discrete Multitone Modulation puts QAM to shame for non-flat channels as it can adapt near-perfectly to such. Not likely to happen for high speed interconnects in our lifetime. I suspect photonics will happen first.