COFDM and OFDM are very similar to each other except for the fact that COFDM has a sort of Forward Error Correction(FEC) coding before OFDM.
This is done to make the OFDM system insensitive to Frequency selective fading. i.e. usually some carriers in a OFDM system undergo deep fade, if this information is available at the Tx it can code it in such a way so as to increase the throughput.