Modulation and coding selection¶
As developed in "Constraints for the physical layer" , we have the following relation between the modulation, the coding and the roll-off:
log2(M)ρ1+α>RbB
For the sake of simplicity, we will fix the roll off to α=0.2, which is a typical value enabling a good tradeoff between the spectral efficiency and the threshold detector performances (used in DVB-S2 for example).
Then, considering that M is a power of 2 and ρ<1, we can estimate from the previous relation the minimal MODCOD required:
M=2nextpow2(2(1+α)∗RbB)
ρ=(1+α)⋅RbB⋅log2(M)
Conclusion: After fixing the value of the roll-off (here α=0.2), the MODCOD is directly conditioned by the values of B and Rb.