C–1
Appendix C. ADAPTIVE CODING
AND
MODULATION (ACM) OPTION
C.1
Introduction
Adaptive
Coding
and
Modulation
(ACM)
is
a
technique
that
allows
for
automatic
change
in
modulation
and
FEC
Code
Rate
in
response
to
changing
link
conditions.
ACM
may
be
used
in
packet
‐
based
satellite
links
(as
when
using
the
CDM
‐
750
with
Ethernet
Interface)
to
boost
system
throughput.
The
basic
goal
of
ACM
is
to
capture
historically
unused
satellite
system
link
margin
and
convert
this
margin
into
additional
data
throughput.
An
ACM
system
is
set
up
using
a
constant
physical
layer
symbol
rate
–
and
therefore
occupied
bandwidth
–
and
power,
but
with
a
varying
assortment
of
modulation
and
coding
combinations
called
ModCods
.
The
ModCods
are
selected
to
span
a
range
of
Es/No
(Energy
per
Symbol
to
Noise
density
ratio)
so
that
if
a
system
detects
a
change
in
link
margin
–
e.g.,
fading
–
it
can
use
a
different
ModCod
to
preserve
the
link,
albeit
at
a
different
user
throughput
rate.
Therefore,
in
an
ACM
system,
as
Es/No
increases
or
decreases,
so
does
the
user
data
rate.
ACM
operation
is
a
FAST
‐
enabled
option
in
the
CDM
‐
750
Advanced
High
‐
Speed
Trunking
Modem.
When
enabled
with
ACM,
the
CDM
‐
750
offers
significant
operational
enhancements:
•
ACM
turns
fade
margin
into
increased
link
capacity
–
gains
of
100%
or
more
are
possible,
compared
to
traditional
Constant
Coding
and
Modulation
(CCM).
This
is
accomplished
by
automatically
adapting
the
modulation
type
and
FEC
code
rate
to
give
highest
possible
throughput.
•
ACM
maximizes
throughput
regardless
of
link
conditions
(noise,
clear
sky,
rain
fade,
inclined
orbit,
antenna
pointing
error,
or
other
impairments).
Initial
setup
is
easy
and
requires
no
further
user
intervention.
•
With
a
CCM
system,
severe
rain
fading
can
cause
the
total
loss
of
the
link
–
the
result
being
zero
throughput.
ACM
can
keep
the
link
active
during
heavy
fades
with
lower
throughput,
and
can
yield
much
higher
system
availability.
•
As
Comtech
EF
Data’s
DoubleTalk
Carrier
‐
in
‐
Carrier
(CnC)
operates
in
conjunction
with
ACM
in
the
CDM
‐
750,
the
benefits
of
CnC
and
ACM
are
available
simultaneously.
Summary of Contents for CDM-750
Page 76: ...CDM 750 Advanced High Speed Trunking Modem Revision 2 Updating Firmware MN CDM750 4 12 Notes...
Page 224: ...CDM 750 Advanced High Speed Trunking Modem Revision 2 Appendix A MN CDM750 A 4 Notes...
Page 234: ...CDM 750 Advanced High Speed Trunking Modem Revision 2 Appendix B MN CDM750 B 10 Notes...
Page 256: ...CDM 750 Advanced High Speed Trunking Modem Revision 2 Appendix C MN CDM750 C 22 Notes...
Page 280: ...CDM 750 Advanced High Speed Trunking Modem Revision 2 Appendix D MN CDM750 D 24 Notes...
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