Comtech EF Data SLM-5650A Installation And Operation Manual page 475

SLM-5650A Satellite Modem
Revision 10
For the CnC cancellation, it is necessary to provide each demodulator with a copy of its local
modulator's output.
As shown in Figure G-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 G-5. DoubleTalk Carrier-in-Carrier Signals
: For round trip delay estimation (Figure G-6), 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 G-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.
Unwanted interfering signal suppression of 30 dB or more has been achieved in commercial
products with minimal degradation of the demodulator performance.
Appendix H G–9 MN-SLM-5650A