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9–1
Chapter 9. DoubleTalk™
Carrier-in-Carrier
®
OPERATION
The Modem optionally incorporates a proprietary technology licensed from Applied
Signal Technology, Inc. called DoubleTalk™, hereafter referred to as DoubleTalk
Carrier-In-Carrier (CnC) when implemented in Comtech EF Data modems. CnC
essentially allows two carriers to share the same channel.
Traditional full-duplex links utilize frequency division multiplexing to allow
communications in two directions. This requires allocating two frequency bands, one for
each direction (A and B). CnC allows the two carriers to reside on the same center
frequency allowing the duplex link to consume half the bandwidth as that of FDMA.
All of the following requirements must be met before carrier in carrier will function
correctly.
1. Both of the earth stations must be able to see each other’s carriers.
2. The modulator and demodulator must be “grouped” together for CnC to operate.
3. The relative power level between the two carriers must be < ± 10 dB of each
other. For optimal performance the carriers should be < ± 7 dB of each other.
4. The symbol rate ratio of the two carriers should be < 3:1 (either way).
It is recommended that the user start with the traditional FDMA configuration as shown
in Figure 10-1. This allows the user to establish the links, make sure the modems and RF
equipment are all functioning correctly and that a reasonable Eb/No has been set in each
direction for the modulation and code rate selected.
Then the modem generating the “B” carrier in this example should be relocated in
frequency to be “on top” of carrier “A”. At this point the demodulator-receiving carrier
“A” will loose lock. The carrier in carrier function must now be enabled in the Rx
configuration menus. Once CnC is turned on the demodulator will perform a search for a