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C
BEFORE ATTEMPTING TO COMMISSION A SATELLITE LINK USING CARRIER-IN-
CARRIER (CnC), THE USER
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.
8.1
Introduction
Space segment costs are typically the most significant operating expense for any satellite-based
service, having a direct impact on the viability and profitability of the service. For a satellite
transponder that has finite resources in terms of bandwidth and power, the leasing costs are
determined by bandwidth and power used. Therefore, a satellite circuit should be designed for
optimal utilization to use a similar share of transponder bandwidth and power.
The traditional approach to balancing a satellite circuit – once the satellite and earth station
parameters are fixed – involves trade-off between modulation and coding. A lower order
modulation requires less transponder power while using more bandwidth; conversely, higher order
modulation reduces required bandwidth, albeit at a significant increase in power.
Comtech EF Data has added a new dimension to satellite communication optimization: DoubleTalk
Carrier-in-Carrier.
8.2
What is DoubleTalk Carrier-in-Carrier?
The CDM-750 High Speed Trunking Modem's 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 patented Adaptive Cancellation. Applied Signal uses the term
DoubleTalk™, and Comtech EF Data refers to it as DoubleTalk Carrier-in-Carrier (CnC).
CnC was first introduced in Comtech EF Data products in the CDM-Qx Satellite Modem and,
more recently, in the CLO-10 Link Optimizer and CDM-625 Advanced Satellite Modem.
This innovative technology provides a significant improvement in bandwidth and power
utilization, beyond what is possible with FEC and modulation alone, allowing users to achieve
Chapter 8. D
-
ARRIER
IN
MUST
ENSURE THAT THE LINK IS ROBUST ENOUGH FOR
8–1
OUBLE
-C
ARRIER
T
ALK
OPTION
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