System Functionality And Operational Considerations - Comtech EF Data DMD50 Installation And Operation Manual

DMD50 Universal Satellite Modem

3.14.4 System Functionality and Operational Considerations

Figure 3-2 illustrates a conventional, full duplex satellite link where two carriers are placed in
non-overlapping channels.
Figure 3-3 shows the same link using the Radyne DMD50 equipped with the DoubleTalk
Carrier-in-Carrier option. Note that now only 50% of the bandwidth is being used, as now both
carriers are occupying the same bandwidth.
The transponder downlinks the composite signal containing both carriers on the same band to the
Radyne DMD50 which then translates the signal to near baseband where it can be filtered
(decimated) and then processed as a complex envelope signal. The Radyne DMD50 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 3-4, 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.
The Radyne DMD50 CnC module operates on the near-zero signal before the demodulator, and is
waveform agnostic. This means that no prior knowledge of the underlying modulation, FEC, or
any other waveform specific parameter is required in order to perform the signal suppression
operation. The only caveat to this is that the waveform must be sufficiently random.
MN-DMD50– Revision 3
DMD50 Satellite Modem
Figure 3-11. Conventional FDMA Link
Theory of Operation
DMD50 Satellite Modem
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