Comtech EF Data CDM-625 - Manual (Page 334)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–4
differences resulting from antenna size/gain variations between the two ends of the link. An
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.
10.3.1 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 say 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