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Sep 2nd, 2009 at 1:52pm  

I work for BSkyB in the UK and for some reason for DVB-S2 we only use QPSK.
No one seems to know the reason of why we don't use 8PSK since everyone acknowledges that it is more expensive.
Do you know if there are any other reasons? (Astra doesn't provide enough power, our dishes are too small....)

Also, do you know of any webs where they show a full real link analysis of a satlellite system?

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Eric Johnston
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Reply #1 - Sep 2nd, 2009 at 3:11pm  
DVB-S2 allows several modulation methods (QPSK, 8PSK etc) and many FEC code rates to be used.  These may even vary on the same carrier, so that a carrier may multiplex several programs, each with different modulation and coding, to suit the receive dish sizes of the intended customers.

The idea is to get much more capacity (information bit/s) out of the satellite transponder and thus reduce satellite cost (e.g. half the price price for 16-QAM modulation).

In two-way VSAT systems, using received signal quality feedback from the remote sites, it is even possible to increase the outlink modulation depth and decrease the FEC coding in clear sky conditions and reverse the process during rain so that the capacity of the system is greatly increased in clear sky conditions while falling back to the nominal information rate during rain.  During extreme heavy rain service can be maintained by reducing the information rate further.

The higher order DVB-S2 modulation methods and reduced FEC need higher carrier to noise ratio and so need more power from the satellite and/or larger customer dishes.  New design receivers are needed that will deal with the more complex modulation and FEC coding.

In a broadcast system, such as Astra, a dominant cost in the network is all the existing customer antennas and satellite receivers.  New satellites with extra power are easier to introduce than to upgrade all customer antennas, but there is a risk that installers will exploit, and waste, the higher power by using smaller antenna size and poor pointing.   Upgrading customer receivers can only be done gradually over many years.  Perhaps once a significant proportion of customers have DVB-S2 receivers then more satellite carriers can be switched to the DVB-S2 mode and then progressively have some DVB-S2 features activated.

New services on new satellites can go straight to DVB-S2 since all the customers will be buying new dishes and receivers also.

Here are a couple of documents worth downloading and studying: (367k pdf file) (607k pdf file)

Best regards, Eric.
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