There are a few attempts today to combine polarization with QAM to effectively double data rates again without impacting SNR. You can have two polarizations of radio waves at any time (horizontal + vertical or left-handed + right-handed), and each of these polarizations can carry separate QAM-modulated data. Supposedly this all also works with MIMO as long as you have the appropriate antennas.
Spatial division works regardless of whether the antennas self interfere; it works better when they don't. Overlapping two polarity-isolated qam signals is a good way to explain rudimentary spatial multiplexing but the current state of the art is way way beyond that, I'm pleased to say.
