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Appendix J
CDM-760 Advanced High-Speed Trunking Modem
•
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
the version of the near end carrier on the downlink side and then passes the desired carrier to
the demodulator for normal processing.
To further illustrate in Figure J-4, without CnC, the two carriers in a typical full duplex satellite
link are adjacent to each other. With CnC, only the composite signal is visible when observed on
a spectrum analyzer. Carrier 1 and Carrier 2 (shown here for reference only) are overlapping,
thus sharing the same spectrum.
Figure J-2. Conventional FDMA Link
J–4
Revision 4
MN-CDM760
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