Signal Power in OFDM

Hello

I have written a OFDM transmission reception chain in MATLAB.
I would like to calculate the signal power and compare it with the
MOS(Minimal operational Sensitivity). I am using the following equation:
P = 10log10((abs(S).^2)/R1000)
S is the FFT result after the channel
R = 50 ohms

This calculated power is getting bigger with the noise power.
Do I need to subtract the power with the channel attenuation in order to
compare with the MOS ?

When I compare with the MOS, should I take the mean power or the maximum
power or something else ?

Thank you in advance