Modulation and coding selection » History » Version 1

GAY, Adrien, 03/24/2015 11:12 AM

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h1. Modulation and coding selection
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h2. Modulation and Coding:
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As developed in _"Constraints for the physical layer"_ , we have the following relation between the modulation, the coding and the roll-off:
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p=.  $\frac{\log_{2}(M)\rho}{1+\alpha}>\frac{R_{b}}{B}$
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For the sake of simplicity, we will fix the roll off to $\alpha=0.2$, which is a typical value enabling a good tradeoff between the spectral efficiency and the threshold detector performances (used in DVB-S2 for example).
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Then, considering that _M_ is a power of 2 and $\rho<1$, we can estimate from the previous relation the minimal MODCOD required:
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p=. $M = 2^{nextpow2(2^{\frac{(1+\alpha)*R_{b}}{B}})}$
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p=. $\rho = \frac{(1+\alpha)\cdot R_{b}}{B\cdot log_{2}(M)}$
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+Conclusion:+ *After fixing the value of the roll-off (here $\alpha=0.2$), the MODCOD is directly conditioned by the values of _B_ and _Rb_.*