CDM-750 Advanced High-Speed Trunking Modem
Revision 2
Appendix C
MN-CDM750
C–8
C.4
CDM-750 ACM Scheme
The
description
of
ACM
operation
in
this
section
serves
as
the
prelude
to
configuring
the
CDM
‐
750
for
ACM
operation.
The
key
items
discussed
are:
•
Target
Es/No
Margin
•
Distortion
–
allowing
for
degradation
by
modulation
type
•
Pilots
–
ON/OFF
•
Standard
and
Short
Frames
•
Monotonic
operation
•
ModCod
selection
C.4.1 Target Es/No Margin
The
Target
Es/No
Margin
adds
a
fixed
amount
of
margin
to
all
of
the
modem’s
specified
Es/No
values.
Links
normally
include
margin
to
meet
performance
criteria.
When
ACM
is
enabled,
a
method
is
needed
to
add
margin
to
the
guaranteed
Es/No
specification
so
the
modem
will
switch
to
the
next
ModCod
in
a
way
that
maintains
a
level
of
performance,
generally
near
the
QEF
point
(PER
≈
10
‐
7);
the
Target
Es/No
Margin
provides
this.
This
Es/No
margin
is
added
to
the
published
guaranteed
Es/No
specification
per
ModCod
for
ACM
operation
–
the
value
of
Target
Es/No
ranges
from
0
to
4.5
dB.
Example:
An
Es/No
Margin
of
1.0
dB
would
mean
that
the
minimum
Es/No
required
to
run
at
ModCod
(5)
QPSK
3/5
would
be
the
guaranteed
Es/No
(2.7
dB)
plus
the
Target
Margin
(1.0
dB)
=
3.7
dB.
The
Target
Es/No
Margin
can
and
should
also
be
used
as
a
way
to
dial
in
a
maximum
fade
slope.
ACM
messages
sent
from
the
far
side
demod
to
the
near
side
modulator
contain
the
Es/No
estimate
that
the
demod
was
seeing
at
that
particular
time;
the
ACM
message
is
sent
four
(4)
times
per
second.
In
the
worst
case
scenario,
due
to
the
distance
traveled
over
satellite
and
the
symbol
rate
of
the
link,
there
can
be
300
‐
400
ms
of
path
delay
AND
a
250
ms
(1/4
second)
messaging
gap
from
the
last
time
the
Es/No
is
measured
by
the
far
side
demod
to
the
time
the
near
side
modulator
can
act
on
this
measurement.
If
the
maximum
slope
of
a
fade
(e.g.,
rain
fade)
is
1.0
dB/s
it
would
desirable
to
minimally
set
the
Target
Es/No
to
a
figure
that
would
be
greater
than
the
maximum
possible
fade.
To
help
choose
the
Es/No
Margin,
depicts
a
graph
of
ACM
messaging
delay
(including
satellite
delay).
Example:
A
10
Msps
link
is
run
and
has
a
maximum
rain
fade
of
1
dB
per
second.
At
10
Msps
there
will
be
a
single
hop
latency
of
300ms
and
an
additional
maximum
ACM
message
gap
of
250
ms.
So,
if
a
1.0
dB
rain
fade
began
just
after
an
ACM
message
was
sent
it
would
be
300+250=550ms
before
an
updated
ACM
message
with
the
reduced
Es/No
is
received
and
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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