Comtech EF Data SLM-5650A - Manual (Page 306)

SLM5650A_CDM625 Satellite Modem Revision 8
DoubleTalk Carrier-in-Carrier Option MN-SLM5650A
15–8
For the CnC cancellation, it is necessary to provide each demodulator with a copy of its local
modulator’s output.
As shown in Figure 15-5: 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 15-5. DoubleTalk Carrier-in-Carrier Signals
Referring to Figure 15-6: For round trip delay estimation, a search algorithm is used 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 15-6. 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.