System Functionality And Operational Considerations; Figure 10-2. Conventional Fdma Link - Comtech EF Data CDM-625 Installation And Operation Manual

CDM-625 Advanced Satellite Modem
DoubleTalk Carrier-in-Carrier Option

10.4 System Functionality and Operational Considerations

Figure 10-2 illustrates a conventional, full duplex satellite link where two carriers are placed in
non-overlapping channels. Figure 10-3 shows the same link using the CDM-625 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-625 which then translates the signal to near baseband where it can be filtered
(decimated) and then processed as a complex envelope signal. The CDM-625 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 10-4, 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.

Figure 10-2. Conventional FDMA Link

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