CDM-750 Advanced High-Speed Trunking Modem
Revision 2
Appendix C
MN-CDM750
C–2
C.2
Background
The
CDM
‐
750
system
is
based
on
DVB
‐
S2
standards.
The
DVB
standardized
a
suite
of
28
ModCods
that
work
over
an
Es/No
range
of
‐
2.3
dB
to
16.0
dB
(ideal).
They
have
also
defined
much
of
the
system
for
both
ACM
and
a
related
feature
called
Variable
Coding
and
Modulation
(VCM)
.
DVB
‐
S2
did
not
specify
the
means
of
passing
necessary
information
between
system
components,
so
the
ACM
schemes
adopted
by
different
manufacturers
are
generally
not
interoperable.
During
ACM
operation,
control
parameters
between
modems
are
transferred
over
the
satellite
link
along
with
user
traffic
in
what
is
usually
referred
to
as
Inband
Signaling
.
DVB
‐
S2
also
describes
an
ACM
method
using
“out
‐
of
‐
band”
ACM
controllers
(per
EN
302
307
Annex
D.1)
that
is
not
supported
with
the
CDM
‐
750.
The
CDM
‐
750
supports
duplex
ACM
operation
between
a
pair
of
modems
in
a
point
‐
to
‐
point
link.
While
point
‐
to
‐
multipoint
operation
is
not
currently
supported,
the
modem’s
physical
layer,
software,
and
overall
architecture
are
designed
to
allow
future
expansion.
In
ACM
mode,
the
symbol
rate
remains
constant,
and
the
modulation
and
coding
(ModCod
in
DVB
‐
S2
terms)
changes
to
preserve
the
data
integrity.
Most
links
are
designed
with
enough
Es/No
margin
to
provide
error
free
performance
under
faded
conditions
when
there
is
higher
attenuation
in
the
uplink
or
downlink
path
to/from
the
satellite.
Depending
on
the
geographical
region
and
link
budget
criteria,
faded
conditions
can
occur
less
than
1%
of
the
time.
In
such
as
case
the
operating
Es/No
of
the
link
is
higher
than
needed
for
more
than
99%
of
the
year.
ACM
takes
advantage
of
this
link
margin
by
increasing
the
ModCod
during
unfaded
conditions
allowing
the
link
to
operate
at
a
higher
data
rates
during
these
periods.
A
simplified
example
illustrates
the
point.
shows
the
several
ModCods,
the
Spectral
Efficiency
(SE),
and
Es/No
for
the
CDM
‐
750
assuming
Normal
FEC
Frame
(64,800
bits).
Table C-1. ACM Example for Standard FECFrame (64,800 bits)
ModCod #
ModCod
Spectral
Efficiency
Pilots ON
Guaranteed
Es/No (dB)
Data Rate
(Mbps) at 20
Msps
4
QPSK 1/2
0.9653
1.4
19.31
5
QPSK 3/5
1.1600
3.7
23.20
6
QPSK 2/3
1.2908
3.4
25.82
7
QPSK 3/4
1.4521
4.6
29.04
8
QPSK 4/5
1.5494
5.1
30.99
9
QPSK 5/6
1.6153
5.6
32.31
10
QPSK 8/9
1.7244
6.7
34.49
11
QPSK 9/10
1.7460
6.8
34.49
Example:
If
a
link
is
designed
to
operate
at
an
Es/No
of
3.7
dB
during
a
3.0
dB
fade,
the
3.0
dB
fade
is
commonly
referred
to
as
link
margin
.
When
fade
conditions
are
not
present,
the
link
1
VCM
operation
does
not
provide
any
advantage
in
a
point
to
point
link
and
therefore
is
not
supported
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