Mocomtech CDM-QX - Manual (Page 188)

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier® Revision 7
DoubleTalk™ Carrier-in-Carrier® (CnC) MN/CDMQX.IOM
9–2
9.3 System Functionality
Figure 9-1 illustrates a conventional FDMA system, where two carriers are placed in non-
overlapping channels.
Figure 9-2 shows the same link using the CDM-Qx/QxL 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
CDM-Qx/QxL, which then translates the signal to near baseband where it can be filtered
(decimated) and then processed as a complex envelope signal. The CDM-Qx/QxL 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.
Figure 9-1. Traditional FDMA System (without CnC)
Figure 9-2. Same System Using CDM-Qx/QxL and DoubleTalk Carrier-in-Carrier
It is recommended that the user start with the traditional FDMA configuration as shown in Figure
9-1. This allows the user to establish the links, make sure the modems and RF equipment are all
functioning correctly, and that a reasonable Eb/No has been set in each direction for the modulation
and code rate selected.