I just realized the EVM feature has been there since FW 16.1 and I don’t understand it. Looks like EVM is the same as what I call MSE, and that 450 radios can finally measure the actual vector error from the rcv constellation rather than extrapolating from BER? And you can set a reference EVM for each SM. Can someone explain this to me, or point me to a document? How should I be using the statistics, and what happens if I set a reference EVM? Does this affect reported LQI and more importantly does it affect what modulation is selected?
I found the 16.1 release notes and it describes it pretty well in there. You can find that in the support site under downloads. That should answer your questions.
Basically, yes, EVM is like MSE and the lower it is the smaller the delta betwen symbols in the constellation diagram from where the symbols should arrive in signal space. For the general audience, by lower we mean larger negative number. -20s dB is a pretty good EVM for unlicensed links and -30s dB is a clean channel with low noise.
After an SM has been well aligned you can hit the populate EVM button to set that as the reference. You can also configure the Link Quality Metric in the AP to be based on EVM instead of the rate.