Hi Mohammad Ben,
acoording to you, the frequency spacing, then the subcarrier frequency will be as mention before
frequency spacing (dalta F)= Bit rate/(log2(M)*Nsc) where the M is the M-array of the modulation, and Nsc is the number of subcarriers
for bit rate of 10 gbps and no. of subcarriers = 4, and M = 4, we will get delta F = 10ghz /[(log2{4})*4] = 1.25 ghz;
You really make me understand, it may also be another explanation.
Thank you.