Comtech EF Data CDM-760 - Manual (Page 382)

Appendix J Revision 2
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
J–4
• 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;
• 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:
• 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 0-300 ms,
where 0 ms is used when doing loopback or bench testing.
• Use the CDM-760’s Internal BERT Tester when evaluating CnC performance.
J.4 System Functionality and Operational Considerations
Figure J-2 illustrates a conventional, full duplex satellite link where two carriers are placed in
non-overlapping channels. Figure J-3 shows the same link using CDM-760s equipped with the
CnC option. Note now that, with CnC used, only 50% of the bandwidth is being used, as both
carriers are occupying the same bandwidth.
The transponder downlinks the composite signal containing both carriers on the same band to
the CDM-760 which then translates the signal to near baseband where it can be filtered
(decimated) and then processed as a complex envelope signal. The CDM-760 then suppresses