Comtech EF Data SLM-5650A - Manual (Page 302)

SLM5650A_CDM625 Satellite Modem Revision 8
DoubleTalk Carrier-in-Carrier Option MN-SLM5650A
15–4
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.
15.3.1 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. Fireberd) or internal BER tester when testing CnC
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.