Comtech EF Data CDM-425, Инструкция по установке и эксплуатации

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix F F–4 MN-CDM-425
In a number of ways, CnC carriers behave similar to conventional carriers in satellite links. They
are both exposed to adjacent carriers, cross-polarization and rain fade, and exhibit impairments
when any of these become too great. In addition, CnC operates in an environment where:
•
Carriers intentionally occupy the same spectral slot;
•
Performance depends upon desired and co-located interfering carrier.
F.1.3 Operational Recommendations
The rules for CnC operation are summarized as follows:
•
Both earth stations share the same footprint so each sees both carriers
•
CnC carriers are operated in pairs
•
One outbound with multiple return carriers is not allowed
•
Asymmetric data rates are allowed (no restrictions)
•
The ratio of power spectral density is normally less than 11 dB
•
CnC operates with modems – not modulators only or demodulators only
In addition, to minimize ‘false’ acquisition, observe the following:
•
Use of IESS-315 V.35 Scrambler is highly recommended
•
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 ±5 ms. For
example, if the modem reported delay is 245 ms, narrow the search range to 240-250 ms
•
Use external data source (e.g. Firebird) or internal BER tester when testing Carrier-in-
Carrier performance
•
To prevent self-locking in case the desired carrier is lost, it is recommended that the two
carriers have some configuration difference – for example, use different settings for
Spectrum Inversion.
F.2 System Functionality and Operational Considerations
Figure F-2 illustrates a conventional, full duplex satellite link where two carriers are placed in non-
overlapping channels. Figure F-3 shows the same link using the CDM-425 Advanced Satellite
Modem equipped with the DoubleTalk
Carrier-in-Carrier option. Note that only 50% of the
bandwidth is being used, as both carriers are now occupying the same bandwidth.
The transponder downlinks the composite signal containing both carriers on the same band to the
CDM-425 which then translates the signal to near baseband where it can be filtered (decimated)
and then processed as a complex envelope signal. The CDM-425 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, as shown in Figure F-4, without DoubleTalk Carrier-in-Carrier, the two
carriers in a typical full duplex satellite link are adjacent to each other. With DoubleTalk Carrier-in-
Carrier, 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.