Understanding FFT size in OFDM modulation
An FFT does not need to be of length 2^N.  However, FFTs of lengths that are a multiple of only tiny primes run faster.  If you don't mind slower performance (or higher CPU utilization), you can use any length.

Note that multiple modulations can be combined, for instance, modulation of each subcarrier, and modulation of a collection of carriers.  Each can be different, thus differently named.  For instance, each subcarrier in an OFDM system can have a different size of QAM constellation.