CDM-625 Advanced Satellite Modem Revision 13
DoubleTalk
Carrier-in-Carrier Option MN-CDM625
10–4
4) Link Asymmetries: Various asymmetries in the forward and return link can produce
differences in the relative power of the two received signal components. These can be
both deterministic (static) or random (and time varying). An example of the former
would be the 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
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