Chapter 10. DoubleTalk
®
Carrier-in-Carrier
®
OPTION
10.1 What is DoubleTalk
®
Carrier-in-Carrier
®
?
The CDM-625 DoubleTalk
®
Carrier-in-Carrier
®
option utilizes a patented (US 6,859,641) signal
processing algorithm developed by Applied Signal Technology, Inc. that allows both the forward
and reverse carriers of a full duplex link to share the same segment of transponder bandwidth,
using adaptive cancellation. Applied Signal uses the term DoubleTalk™, and Comtech EF Data
refers to it as DoubleTalk
®
Carrier-in-Carrier
®
(CnC).
This technology was first introduced in Comtech products in the CDM-Qx Satellite Modem, and
more recently in the CLO-10 Link Optimizer.
The implementation of DoubleTalk
®
Carrier-in-Carrier
®
in the CDM-625 has been further refined,
and some of the limitations that existed in the CDM-Qx implementation have been overcome.
Summary: When carriers share common bandwidth, up to 50% savings in transponder
utilization is possible.
10.2 System Functionality
Figure 10-1
illustrates a conventional, full duplex satellite link where two carriers are placed in
non-overlapping channels.
Figure 10-2
shows the same link using the CDM-625 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-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.
10–1
