Quantum and Evolution Series Installation and Operating Handbook
8-30
through the FEC Decoder E
Dec
/ N
o
is higher than E
t
/ N
o
as the same power is referenced
to a lower bit rate. Allowing for the change in bit rate in the FEC decoder gives:
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
b
/ N
o
(or
E
bi
/ N
o
or
E
i
/ N
o
with `I` and `b`referring to
Information
and
Bit
respectively). Again because the
data rate reduces through the RS Decoder E
b
/ N
o
is higher than E
Dec
/ N
o
as the same
power is referenced to a lower bit rate. Allowing for the change in bit rate in the RS
Decoder gives:
It is this
E
b
/ N
o
that is typically displayed on the front panel of Modem equipment.
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
b
/ N
o
, specifying that this relates to the
Composite
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
b
/N
o
whatever the settings of
Modulation Inner FEC, and Outer (RS) FEC, as the E
b
/N
o
display compensates for these
parameters. The bandwidth will vary and so will the Carrier/Noise at the input of the
demod, but the displayed E
b
/N
o
will be steady as it compensates for these parameter
changes.
When comparing BER performance for different Modulation/FEC schemes against E
b
/N
o
,
the plotted E
b
/N
o
already accounts for the changes in Carrier/Noise caused by the
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
b
/N
o
, but it will of
little practical value if the bandwidth it requires occupies a whole transponder.
Rate)
Code
(FEC
10
-
N
E
=
N
E
10
0
t
0
Dec
log
))
n
k
(
=
Rate
Code
(RS
10
-
N
E
=
N
E
10
0
Dec
0
b
log
