Comtech EF Data CDM-625, Installation And Operation Manual

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CDM-625 Advanced Satellite Modem Revision 1
DoubleTalk
®
Carrier-in-Carrier
®
Option MN-0000036 (Ref MN/CDM625.IOM)
Parameters Site A Site B
Satellite EIRP (dBW) 37 37
Satellite BOo (dB) 6 6
Satellite BOi (dB) 10 10
Satellite SFD (dbW/m
2
) -78 -78
Satellite G/T (dB/K) 0 0
E/S Antenna (meters) 4.5 2.4
Data Rate (kbps) 192 192
Carrier Spacing Factor 1.3 1.3
Conventional Link 8PSK 2/3 TCM 8PSK 2/3 TCM
Occupied BW1 for 2 Carriers (kHz) 274.6 274.6
% of Transponder Power 0.55 0.16
CnC Link QPSK 3/4 Turbo QPSK 3/4 Turbo
Occupied BW2 for 2 Carriers (kHz) 166.4 166.4
% of Transponder Power 0.37 0.11
CnC Ratio (dB) +5.3 -5.3
Expected Eb/No Degradation (dB) -0.1 0.0
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.
10–10