CDM-750 Advanced High-Speed Trunking Modem
Revision 2
Appendix C
MN-CDM750
C–3
margin
is
directly
converted
into
a
higher
link
Es/No:
3.7
dB
+
3.0
dB
margin
=
new
Es/No
of
6.7dB.
This
non
faded
Es/No
of
6.7
dB
allows
for
a
much
higher
ModCod
to
be
used,
offering
a
higher
Spectral
Efficiency.
The
corresponding
Spectral
Efficiencies
in
this
example
range
from
1.1600
to
1.7244.
The
Data
Rate
(DR)
and
Symbol
Rate
are
related
by
the
symbol
rate
as
follows:
DR (Data Rate) = SR (Symbol Rate) x SE (Spectral Efficiency)
For
a
20
Msps
link,
this
corresponds
to
the
data
rates
listed
in
compares
the
ACM
link
with
its
CCM
counterpart.
In
ACM
mode,
the
link
operates
most
of
the
time
with
clear
sky
conditions
at
34.49
Mbps
and
drops
back
to
23.20
Mbps
only
when
the
link
is
faded.
In
CCM
mode
the
link
must
remain
in
its
worst
case
condition
at
23.20
Mbps.
Table C-2. ModCod Comparison: ACM vs. CCM
Condition
Es/No (dB)
ACM
ModCod
ACM
Data Rate
CCM
ModCod
CCM
Data Rate
Clear Sky
6.7
QPSK 8/9
34.49 QPSK
3/5 23.20
Faded 3.7
QPSK
3/5
23.20 QPSK
3/5 23.20
There
are
a
few
important
factors
to
consider
for
ACM
operation:
•
The
digital
communication
system
must
tolerate
a
change
in
data
rate.
This
excludes
synchronous
interfaces
such
as
G.703
T3/E3
interfaces,
which
operate
at
a
fixed
data
rates
(44.768
/
34.368
Mbps).
•
Ethernet
is
a
suitable
data
stream
because
the
packet
nature
of
this
interface
accommodates
changing
data
rates.
•
The
bit
rate
is
not
allowed
to
change
arbitrarily.
Link
conditions
determine
the
operational
ModCod,
as
measured
by
the
far
side
demod
Es/No
reading.
The
ModCod
chosen
must
always
operate
at
or
above
the
minimum
threshold
for
reliable
communications
for
any
given
ModCod.
•
There
are
generally
two
types
of
scenarios
where
ACM
operation
is
run:
o
Minimum
Guaranteed
DR:
A
link
budget
is
run
using
worst
case
fade
conditions
and
a
fixed
availability
(ex.
99.8%).
The
ModCod
selected
in
this
link
budget
is
considered
to
be
the
lowest
acceptable
ModCod
that
meets
the
DR
and
availability
of
the
link
budget.
99.8%
of
the
year
the
data
rate
of
the
link
will
be
at
or
greater
than
this
minimum
data
rate.
For
most
of
the
year
the
DR
will
exceed
the
minimum
guaranteed
figure,
0.2%
of
the
year
the
link
will
be
down
or
at
a
DR
that
does
not
meet
the
minimum
guaranteed
figure.
o
Relaxed
Availability:
A
link
availability
below
typical
thresholds
(ex:
97%)
is
used
to
calculate
a
worst
case
fade
condition
at
a
desired
data
rate.
It
is
understood
that
although
the
availability
is
less
than
desirable,
for
the
majority
of
the
year
the
DR
will
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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