In fact, in 802.11a, it does not use the symmetric conjugate technique,
i.e., the output of IFFT module is not the real signal, but complex
siganl.

Why they did this? Because 802.11a uses the carrier modulation technique,
whos e central carrier frequency is 5 GHz. In application, as I know,
ADSL1 uses the symmetric conjugate input of IFFT module, also in MB-UWB
system case as well.

I understand to obtain a real output from IFFT, its input has to be
conjugate symmetric. In 802.11a, 64-point IFFT is used with 48 data
carriers and 4 pilot carriers.

If all the 48 data carriers are carrying data then where are its
conjugate carriers?