Comtech EF Data CDM-750 - Manual (Page 262)

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.