OFDM transmission under rayleigh fading

Hi all, 

I would like to find theoretical 802.11g BER performances in the case of a
transmission under rayleigh channel.
I found theoretical M-QAM curves, but I don't know how to compute BER
curves for OFDM transmission due to the use of several sub-carriers...

Is the OFDM BER something like that:
BER = (1/N)*SUM(BER_i, i=1..N)
where N is the number of sub-carriers
and BER_i the BER on each sub carrier
(N = 52 for a 802.11g transmision)

In that case:
- for an AWGN transmision, the BER for a OFDM communication and the
classial M-QAM curves would be the same ? why not !!
- How could I write BER for each sub-carries in a rayleigh channel ??

Thanks a lot for your help