My question comes from the high spectrum efficiency claim of OFDM. This seems not correct IMHO, at least theoretically.
For me, we lose energy in using cyclic prefix, hence OFDM should be a waste of theoretically achievable rate. That said, OFDM has advantage in implementation and it is easier to get a high throughput.
Long story short, my question is: if complexity is not my concern, can (any) modulation (without channel coding) combined with a Maximum Likelihood (or Maximum a Posteriori) decoder achieve asymptotically the Shannon capacity ?