Chapter 9. Doubletalk Carrier-In-Carrier (Cnc); What Is Doubletalk Carrier-In-Carrier; Application Requirements - Comtech EF Data CDM-Qx Installation And Operation Manual

BEFORE ATTEMPTING TO COMMISSION A SATELLITE LINK USING CARRIER-IN-
CARRIER, THE USER
NORMAL OPERATION. ONLY WHEN THIS HAS BEEN DONE – AND ALL SYSTEM
ISSUES (E.G.,
SATELLITE INTERFERENCE, ETC.) HAVE BEEN RESOLVED – SHOULD THE USER
ATTEMPT THE USE OF CARRIER-IN-CARRIER.
9.1

What is DoubleTalk Carrier-in-Carrier?

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).
These standard carriers must occupy non-overlapping spectral segments with no more than one
carrier in the same space.
The CDM-Qx/QxL DoubleTalk Carrier-in-Carrier option utilizes a patented (US 6,859,641) signal
processing algorithm developed by Applied Signal Technology, Inc. that allows both the forward
and reverse carriers of a full duplex link to share the same segment of transponder bandwidth, using
adaptive cancellation. Applied Signal uses the term DoubleTalk™, and Comtech EF Data refers to
it as DoubleTalk Carrier-in-Carrier (CnC).
CnC allows the two carriers to reside on the same center frequency, a practice that is disastrous
for normal carries. When carriers share common bandwidth, up to 50% savings in transponder
utilization is possible.
9.2

Application Requirements

The following conditions are necessary in order to operate DoubleTalk Carrier-in-Carrier:
•
Both of the earth stations must be able to see each other's carriers.
•
The modulator and demodulator must be "grouped" together for CnC to operate.
• The relative power level between the two carriers must be < ± 10dB of each other.
For optimal performance the carriers should be < ± 7dB of each other.
•
The symbol rate ratio of the two carriers should be < 3:1 (either way).
Chapter 9. DoubleTalk
Carrier-in-Carrier
MUST
ENSURE THAT THE LINK IS ROBUST ENOUGH FOR
ANTENNA-POINTING,
9–1
CABLING, TERRESTRIAL
TM
®
(CnC)
INTERFERENCE,