O rthogonal FDM ? OFDM eases equalization. OFDM prohibits constant envelope modulation like GMSK.
OFDM vs. CPM Simple equalization Few antennas Linear amplifiers Iterative equalization Many antennas Cheap amplifiers MIMO... is just an add on... is the enabling tool.
My Personal Vision of MIMO Dozens (hundreds) of closely spaced antennas CPM modulation High diversity order due to parasitic elements Iterative processing of ISI, MUI, and FEC by belief propagation.