Coded OFDM

Hello. Can anybody put me through on how to compute the noise variance in coded OFDM? My implementation is as follows: Ebno=1:10; snr=Ebno+10*log10(Nst/N) + 10*log10(N/(Ncp+N)); % converting to symbol to noise ratio sigma=sqrt((10^.(snr)/10)) where Nst=Number of total used sub-carriers, N=FFT size, Ncp= Number of symbol allocated to CP If this is correct,where do i need to consider the code rate and the BitPerSymbol. If not, kindly assist with the proper implementation. Thanks. Adedapo.