Comtech EF Data CDM-760 - Manual (Page 381)

Appendix J Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
J–3
This performance is achieved through advanced signal processing algorithms that provide superior
cancellation while tracking and compensating for these common link impairments:
1) Time varying delay: In addition to the static delays of the electronics and the round-trip
delay associated with propagation to the satellite and back, there is a time-varying
component due to movement of the satellite. The CnC module tracks and compensates for
this variation.
2) Frequency offset and drift: Common sources are satellite Doppler shift, up and down converter
frequency uncertainties, and other drift associated with the electronics in the CDM-760 itself.
The CnC module tracks and compensates for this frequency offset and drift.
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.
Figure J-1. Conceptual Block Diagram
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: