Comtech EF Data CDM-750, Installation And Operation Manual

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CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix D MN-CDM750
D–4
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.
D.3 Operational Details
The rules for CnC operation can be 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;
•
CnC operates with modems – not modulators only or demodulators only.
•
During CnC acquisition, neither modem passes data. Only after both modems complete
the CnC acquisition process does data flow.
For best operation, observe the following:
•
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 +/ ‐ 10 ms. For
example , if the modem reported delay is 245 ms, narrow the search range to 235 ‐ 255 ms.
Factory default for search delay is 230 ‐ 290 ms. Search delay programmable range is
0 ‐ 400 ms, where 0 ms is used when doing loop ‐ back or bench testing.
•
Use the CDM ‐ 750’s Internal BERT Tester when evaluating CnC performance.