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As shown in Table F-1, DoubleTalk Carrier-in-Carrier allows equivalent spectral efficiency using a
lower order modulation and/or FEC Code Rate; CAPEX is therefore reduced by allowing the use
of a smaller BUC/HPA and/or antenna. And, as DoubleTalk Carrier-in-Carrier can be used to
save transponder bandwidth and/or transponder power, it can be successfully deployed in
bandwidth-limited as well as power-limited scenarios.
F.2.1
DoubleTalk Carrier-in-Carrier Cancellation Process
The state-of-the-art signal processing technology employed via DoubleTalk Carrier-in-Carrier
continually estimates and tracks all parametric differences between the local uplink signal and its
image within the downlink. Through advanced adaptive filtering and phase locked loop
implementations, it dynamically compensates for these differences by appropriately adjusting the
delay, frequency, phase and amplitude of the sampled uplink signal, resulting in excellent
cancellation performance.
When a full duplex satellite connection is established between two sites, separate satellite
channels are allocated for each direction. If both directions transmitted on the same channel,
each side would normally find it impossible to extract the desired signal from the aggregate due to
interference originating from its local modulator. However since this interference is produced
locally, it is possible to estimate and remove its influence prior to demodulation of the data
transmitted from the remote location.
For the DoubleTalk Carrier-in-Carrier cancellation, it is necessary to provide each demodulator
with a copy of its local modulator's output.
As shown in Figure F-5, for Modem 1 and Modem 2 transmit signals S1 and S2 respectively, the
satellite receives, translates, and retransmits the composite signal. The downlink signals S1* and
S2*, received at Modem 1 and Modem 2 differ from the transmit signals primarily in terms of
phase, frequency, and delay offsets.
Appendix F
Figure F-5. DoubleTalk Carrier-in-Carrier Signals
F–8
CDM-425 Advanced Satellite Modem
MN-CDM-425
Revision 0
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