Hello Ina,
The 7.5GHz is the carries frequency used to carry the QAM/OFDM modulated signal. Users can set their own carrier frequency.
Typically spectral efficiency of a modulation scheme is calculated based how many bits/s/Hz. However, users need to consider the number of bits/symbol and number of subcarriers, actual bandwidth used by the modulated signal, etc…
A very good reference can be found at
http://etutorials.org/Networking/wimax+technology+broadband+wireless+access/Part+Two+WiMAX+Physical+Layer/Chapter+5+Digital+Modulation+OFDM+and+OFDMA/5.2+OFDM+Transmission/
Regards,
Ahmad