Comtech EF Data DMD-2050E - Manual (Page 75)

DMD2050E Universal Satellite Modem Theory of Operation
MN-DMD2050E Revision 2 3–33
QPSK 3/4 1.50 3.00
QPSK 7/8 1.75 3.50
8-QAM 2/3 2.00 4.00
8-QAM 3/4 2.25 4.50
8-QAM 7/8 2.63 5.25
16-QAM 3/4 3.00 6.00
16-QAM 7/8 3.50 7.00
As shown here, 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.
3.11.5 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.