Comtech EF Data DMD-2050E - Manual (Page 73)

DMD2050E Universal Satellite Modem Theory of Operation
MN-DMD2050E Revision 2 3–31
Figure 3-11 shows the same link using the DMD2050E 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
DMD2050E which then translates the signal to near baseband where it can be filtered
(decimated) and then processed as a complex envelope signal. The DMD2050E 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 DMD2050E 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 .
Figure 3-15. Same Link Using DMD2050E and DoubleTalk Carrier-in-Carrier