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Appendix J
CDM-760 Advanced High-Speed Trunking Modem
3) Atmospheric effects: Fading and scintillation can affect amplitude, phase, and spectral
composition of the signal and the degree to which it correlates with the original signal. The
CnC module tracks and compensates for these atmospheric related impairments.
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. Both
are exposed to adjacent carriers, cross-polarization, and rain fade, and exhibit impairments
when any of these become too great. CnC additionally operates in an environment where:
•
Carriers intentionally occupy the same spectral slot, and
•
Performance depends upon desired and co-located interfering carrier. The interfering
carrier is canceled, leaving the desired carrier for demodulation.
J.3
Operational Details
The rules for CnC operation are summarized thusly:
•
Both earth stations share the same satellite footprint, so each sees both carriers;
•
CnC carriers are operated in pairs;
•
One outbound with multiple return carriers is not allowed;
•
Asymmetric operation is allowed up to a 3:1 ratio of symbol rate;
•
The ratio of CnC (ratio of interferer power to desired power) is normally within ±7 dB;
Figure J-1. Conceptual Block Diagram
J–3
Revision 4
MN-CDM760
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