Question regarding the IFFT of a OFDM system

Greetings,

we are currently developing a DSRC 5.9 GHz system that uses OFDM modulation. My group has been assigned the PHY layer, and its block diagram (just the transmitter side) is the picture i've attached.

The part that is raising some questions is the transition between the QAM mapping and the IFFT. Since the modulation is adaptive, this means that we can either have BPSK, QPSK, 16-QAM or 64-QAM. The IFFT we are using is a parametrizable IP-core from Altera, and the minimum number of bits of the input (both for I and Q) is 8.

It happens one needs, at most, 4 bit to quantize each coordinate of any constellation point. Thus, there are 4 unused bits at the IFFT's input (either for I and Q).

Our main doubt is if there is any defined usage for those unused bits, like error detection and correction. We could come up with solutions of our own, but we wanted to know if there's anything that's been tested before.

Thank you.