http://www.32apsk.org/wordpress/?p=301
In the good ol’ DVB days we used the Eb/No as a measure of signal quality. This is because this indication came directly from the information theory and the associated mathematics (remember the Eb/No versus BER curves). The Eb/No is the energy per bit over the noise density (the noise power present in 1 Hz). Now suddenly, in DVB-S2, everyone talks about Es/No… why is that? Well, Es/No is the energy per symbol over the noise density. And to know why this is rather used, we have to trace back how this value is determined.
A demodulator can determine the Es/No rather easy (I believe by looking at the eye-pattern and taking into account the number of corrections that need to be performed; correct me if I’m wrong). From that the Eb/No is simply calculated. Simply? Yes. Because the relationship between bits and symbols (bauds) is nothing more than the spectral efficiency. So Es/No = Eb/No + 10.log(n) with n = spectral efficiency or the number of bits/baud.
Good. In DVB-S there is always a nice 1-to-1 relationship between the used modcod of a carrier and the resulting bitrate. However, DVB-S2 provides mechanisms such as VCM (Variable Coding and Modulation) where a number of S2 streams in a single carrier can be modulated with different modocds. And on top of that there’s ACM with modcods per stream that can vary over time depending on receive conditions. So the simple relationship between carrier bandwidth (or symbolrate) and achievable throughput (bitrate) is no longer there… it is rather complex and can even change over time.
Hence DVB-S2 carriers always use Es/No to quantify the carrier. Of course since we are creatures of habit, in cases of CCM (Constant Coding and Modulation) such as the simple transport of fixed rate video programs, the Eb/No can still be used (that’s why many IRDs or STBs will still display Eb/No) because there is only one modcod that doesn’t change over time.
In fact (and this is my humble opinion) we should have worked with Es/No even back in the days of DVB-S. Because what you see on your spectrum analyser isn’t bits, it’s symbols. So if you measure C/N with your spectrum analyser, you directly measure the Es/No, just as simple as that. Not 100% correct in fact, what you seen on your spectrum analyser is C+N/N which you then have to convert to C/N = Es/No. But maybe more about that later.
Cheers, Dave
