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Quantum and Evolution Series Installation and Operating Handbook
through the FEC Decoder E
to a lower bit rate. Allowing for the change in bit rate in the FEC decoder gives:
E
N
Following the FEC Decoder (the `inner FEC`, ie TPC, Viterbi or TCM) comes the Reed-
Solomon `Outer FEC` Decoder (if active). This `Outer FEC` RS Decoder operates
similarly to the `Inner FEC` Decoder, reducing the bit rate as it corrects errors and finally
generating the Composite Information Bit Rate at its output. The Carrier / Noise
expressed at this Composite Information Bit Rate is referred to as the E
or E
with `I` and `b`referring to Information and Bit respectively). Again because the
/ N
i
o
data rate reduces through the RS Decoder E
power is referenced to a lower bit rate. Allowing for the change in bit rate in the RS
Decoder gives:
E
b
N
0
It is this E
that is typically displayed on the front panel of Modem equipment.
/ N
b
o
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
Information Bit Rate (ie it includes framing). Baseband processing has no effect on
bandwidth and is totally ignored.
Practical Implications of Displayed Eb/No
Practically this means the following:
For a constant Tx Power, you will get the same displayed E
Modulation Inner FEC, and Outer (RS) FEC, as the E
parameters. The bandwidth will vary and so will the Carrier/Noise at the input of the
demod, but the displayed E
changes.
When comparing BER performance for different Modulation/FEC schemes against E
the plotted E
/N
already accounts for the changes in Carrier/Noise caused by the
b
o
different Modulation and FEC settings. Any BER difference you see with different
schemes is a real difference - it is not necessary 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
little practical value if the bandwidth it requires occupies a whole transponder.
/ N
is higher than E
Dec
o
E
Dec
t
=
-
10
(FEC
log
10
N
0
0
E
Dec
=
-
10
log
(RS
Code
10
N
0
/ N
, specifying that this relates to the Composite
b
o
/N
will be steady as it compensates for these parameter
b
o
8-30
/ N
as the same power is referenced
t
o
Code
Rate)
/ N
is higher than E
b
o
k
Rate
=
(
))
n
/N
whatever the settings of
b
o
/N
display compensates for these
b
o
(or E
/ N
/ N
b
o
bi
o
/ N
as the same
Dec
o
/N
,
b
o
/N
, but it will of
b
o
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