Comtech EF Data DMD-2050E Dokumentacja Pobierz pdf (Strona 76)

DMD2050E Universal Satellite Modem Theory of Operation

MN-DMD2050E Revision 2 3–34

Figure 3-17. DoubleTalk Carrier-in-Carrier Signals

Referring to Figure 3-5: 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.

Referring to Figure 3-6: 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 3-18. 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.

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Comtech EF Data DMD-2050E Dokumentacja Strona 76