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CDM-Qx Satellite Modem
CnCPerformance Characterization
The link parameters and results are summarized in the table:
The link asymmetry has increased the CnC ratio at Site A to +5.3 dB. Yet this results in a
degradation of only 0.1 dB. This is a C-Band link so no additional change in signal level
is expected due to rain fade. The CnC ratio at Site B is -5.3 dB so no degradation is
expected.
What is done if the CnC ratio is 10 dB or more? In a C-Band link it is possible to tolerate
the additional impairment, but then the modem is operating with less margin. One
possibility to reduce the CnC ratio is to increase the amount of power transmitted from
the remote site with the smaller antenna. This is feasible in some instances where there
are higher power satellite transponders. If the installation is a new one, a larger, though
more expensive, antenna is possible at the remote site. This simultaneously decreases the
CnC ratio at the hub while increasing it at the remote site.
Another alternative is to reduce the modulation order and/or error correction code rate on
the receive side of the remote site. This decreases the power transmitted by the hub and
reduces its CnC ratio. The hub's CnC ratio will decrease further if it is possible to
increase the modulation order or code rate at the hub. In the above example changing
from QPSK 3/4 to QPSK 1/2 helps reduce the power but requires additional 1.5 times
more bandwidth. The 1.2 dB Eb/No difference between rate 3/4 and 1/2 reduces the CnC
ratio at the hub.
Parameters
Satellite EIRP (dBW)
Satellite BOo (dB)
Satellite BOi (dB)
2
Satellite SFD (dbW/m
)
Satellite G/T (dB/K)
E/S Antenna (meters)
Data Rate (kbps)
Carrier Spacing Factor
Conventional Link
Occupied BW1 for 2 Carriers (kHz)
% of Transponder Power
CnC Link
Occupied BW2 for 2 Carriers (kHz)
% of Transponder Power
CnC Ratio (dB)
Expected Eb/No Degradation (dB)
Site A
37
6
10
-78
0
4.5
192
1.3
8PSK 2/3 TCM
274.6
0.55
QPSK 3/4 Turbo
166.4
0.37
+5.3
-0.1
D–12
Revision 6
MN/CDMQx.IOM
Site B
37
6
10
-78
0
2.4
192
1.3
8PSK 2/3 TCM
274.6
0.16
QPSK 3/4 Turbo
166.4
0.11
-5.3
0.0
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