Figure 9-1. Traditional Fdma System (Without Cnc); Figure 9-2. Same System Using Cdm-Qx/Qxl And Doubletalk Carrier-In-Carrier; System Functionality - Comtech EF Data CDM-Qx Installation And Operation Manual

CDM-Qx/QxL Multi-Channel Satellite Modem with DoubleTalk™ Carrier-in-Carrier®
DoubleTalk™ Carrier-in-Carrier® (CnC)
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 9-2. 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.

Figure 9-1. Traditional FDMA System (without CnC)

9–2
Revision 7
MN/CDMQX.IOM