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E
b
N
0
E
Dec
N
0
10log
10
(RS Code Rate (
k
n
))
P300H P300 Series Modem Installation and Operating Handbook Page 160
It is this E / N that is typically displayed on the front panel of Modem equipment.
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Finally, after the RS Decoder comes Deframing and baseband processing such as Drop/Insert. IBS/SMS
or IDR deframing does decrease the bandwidth, but the bandwidth reduction effects of deframing are
ignored as INTELSAT chose to define the mandatory modem performance in terms of E / N , specifying
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that this relates to the Composite Information Bit Rate (ie it includes framing). Baseband processing has
no effect on bandwidth and is totally ignored.
8.12.3 Practical Implications of Displayed Eb/No
Practically this means the following:
For a constant Tx Power, you will get the same displayed E / N whatever the settings of
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Modulation Inner FEC, and Outer (RS) FEC, as the E / N display compensates for these
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parameters. The bandwidth will vary and so will the Carrier / Noise at the input of the demod, but the
displayed E / N will be steady as it compensates for these parameter changes.
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When comparing BER performance for different Modulation / FEC schemes against E / N , the
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plotted E / N already accounts for the changes in Carrier / Noise caused by the different Modulation
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and FEC settings. Any BER difference you see with different schemes is a real difference, you
don’t have to further compensate for the changes in Carrier / Noise this change induces.
You do however have to be aware of Bandwidth considerations. You might find a Modulation /
FEC scheme which gives an exceptional BER for a given E / N , but it will of little practical value if the
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bandwidth it requires occupies a whole transponder.
When you look at BER performance figures at a specified E / N you get the same BER
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performance whether the data is framed or not. Adding framing to a Closed Network link would
increase the bandwidth, decrease the Carrier / Noise, decrease the displayed E / N (as it does not
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compensate for framing) and therefore degrade the BER. However when brought back to the same
E / N you do of course recover the same BER. Framing therefore does not degrade the BER for a
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stated E / N , but it does require more power (and bandwidth) to achieve the same E / N .
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