Comtech EF Data CDM-625 - Manual (Page 338)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–8
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.
Referring to Figure 10-5: 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.
Figure 10-5. DoubleTalk Carrier-in-Carrier Signals
Referring to Figure 10-6: For round trip delay estimation, a search algorithm is utilized that
correlates the received satellite signal to a stored copy of the local modulator’s transmitted
signal. The interference cancellation algorithm uses the composite signal and the local copy of
S1 to estimate the necessary parameters of scaling (complex gain/phase), delay offset and
frequency offset. The algorithm continuously tracks changes in these parameters as they are
generally time-varying in a satellite link.
Figure 10-6. Carrier-in-Carrier Signal Processing Block Diagram
The resulting estimate of the unwanted interfering signal is then subtracted from the composite
signal. In practical applications, the estimate of the unwanted signal can be extremely accurate.