P300H
P300 Series Modem Installation and Operating Handbook
Page 162
Sol
`Inn
Info
ormation Bit
ate 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
E
Dec
RS
It is
inally, after the RS Decoder comes Deframing and baseband processing such as Drop/Insert.
defr
E
b
Bas
8.12
tical 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, 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
b
/ N
o
, but it will of little practical value
if the bandwidth it requires occupies a whole transponder.
When you look at BER performance figures
at a
specified
E
b
/ N
o
you get the same BER
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
b
/ N
o
(as it
does not compensate for framing) and therefore degrade the BER. However when brought back
to the same E
b
/ N
o
you do of course recover the same BER. Framing therefore does not degrade
the BER for a stated E
b
/ N
o
, but it does require more power (and bandwidth) to achieve the same
E
b
/ N
o
.
Following the FEC Decoder (the `inner FEC`, ie Turbo, Viterbi, Sequential or TCM) comes the Reed-
omon `Outer FEC` Decoder (if active). This `Outer FEC` RS Decoder operates similarly to the
er FEC` Decoder, reducing the bit rate as it corrects errors and finally generating the Composite
rmation Bit Rate at its output. The Carrier / Noise expressed at this Composite Inf
R
respectively). Again because the data rate reduces through the RS Decoder E
b
/ N
o
is higher than
/ N
o
as the same power is referenced to a lower bit rate. Allowing for the change in bit rate in the
Decoder gives:
))
n
k
E
E
(
=
Rate
Code
(RS
10
-
N
=
N
10
0
Dec
0
b
log
this
E
b
/ N
o
that is typically displayed on the front panel of Modem equipment.
F
IBS/SMS or IDR deframing does decrease the bandwidth, but the bandwidth reduction effects of
aming are ignored as INTELSAT chose to define the mandatory modem performance in terms of
/ N
o
, specifying that this relates to the
Composite
Information Bit Rate (ie it includes framing).
eband processing has no effect on bandwidth and is totally ignored.
.3 Prac
