Mocomtech CDM-QX - Manual (Page 187)

9–1
Chapter 9. DoubleTalk
TM
Carrier-in-Carrier
®
(CnC)
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).
BEFORE ATTEMPTING TO COMMISSION A SATELLITE LINK USING CARRIER-IN-
CARRIER, THE USER
MUST ENSURE THAT THE LINK IS ROBUST ENOUGH FOR
NORMAL OPERATION. ONLY WHEN THIS HAS BEEN DONE – AND ALL SYSTEM
ISSUES (E.G., ANTENNA-POINTING, CABLING, TERRESTRIAL INTERFERENCE,
SATELLITE INTERFERENCE, ETC.) HAVE BEEN RESOLVED – SHOULD THE USER
ATTEMPT THE USE OF CARRIER-IN-CARRIER.