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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)RbBlog2(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.