CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix F Revision 4
F–4
example of the latter would be transient power differences due to different levels of
atmospheric fading in the uplinks. CnC compensates for the asymmetries, up to a certain
extent.
In a number of ways, CnC carriers behave similar to conventional carriers in satellite links. They
are both exposed to adjacent carriers, cross-polarization and rain fade, and exhibit impairments
when any of these become too great. In addition, CnC operates in an environment where:
• Carriers intentionally occupy the same spectral slot;
• Performance depends upon desired and co-located interfering carrier.
F.1.3 Operational Recommendations
The rules for CnC operation are summarized as follows:
• Both earth stations share the same footprint so each sees both carriers
• CnC carriers are operated in pairs
• One outbound with multiple return carriers is not allowed
• Asymmetric data rates are allowed (no restrictions)
• The ratio of power spectral density is normally less than 11 dB
• CnC operates with modems – not modulators only or demodulators only
In addition, to minimize ‘false’ acquisition, observe the following:
• Use of IESS-315 V.35 Scrambler is highly recommended
• Keep the search delay range as narrow as possible – once the modem has reported the
search delay, narrow the search delay range to the nominal reported value ±5 ms. For
example, if the modem reported delay is 245 ms, narrow the search range to 240-250 ms
• Use external data source (e.g. Firebird) or internal BER tester when testing Carrier-in-Carrier
performance
• To prevent self-locking in case the desired carrier is lost, it is recommended that the two
carriers have some configuration difference – for example, use different settings for
Spectrum Inversion.
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