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CDM-625 Advanced Satellite Modem
®
DoubleTalk
Carrier-in-Carrier
of atmospheric fading in the uplinks. CnC compensates for the asymmetries, up to a
certain extent.
In a number of ways, CnC carriers behave similar to conventional carriers in satellite links. They
are both exposed to adjacent carriers, cross-polarization and rain fade, and exhibit impairments
when any of these become too great. In addition, CnC operates in an environment where:
•
Carriers intentionally occupy the same spectral slot
•
Performance depends upon desired and co-located interfering carrier.
The rules for CnC operation are summarized below:
•
Both earth stations share the same footprint so each sees both carriers
•
CnC carriers are operated in pairs
•
One outbound with multiple return carriers is not allowed
•
Asymmetric data rates are allowed (no restrictions)
•
The ratio of power spectral density is normally less than 11 dB
•
CnC operates with modems not modulators only or demodulators only.
10.4 Symbol Rate and DoubleTalk
In the CDM-625 implementation of CnC, there is no restriction on the ratio of symbol rates for
the outbound interferer and the desired inbound carrier. There could, therefore, be a significant
difference in symbol rates.
The CnC Ratio represents the difference in absolute power between the co-located outbound
interfering carrier and the desired inbound carrier, in dB.
CnC Ratio =
In the simplest case, where the symbol rates are equal, then for a CnC ratio of 0 dB there will be
equal power and equal power spectral density.
However, if the outbound interfering carrier symbol rate is now 2 times larger than the symbol
rate of the desired inbound carrier, for a power spectral density ratio of 0 dB, there will be equal
power spectral density, but 3 dB more power overall, so the CnC ratio would be +3 dB.
It is very important to understand that the CnC ratio refers to the ratio of
absolute power, and NOT to the ratio of Power Spectral Density. This is
illustrated in the following examples:
IMPORTANT
Example:
Outbound interferer = 1 Msymbols/sec
Ratio of power spectral density = +10 dB
Absolute power ratio (CnC Ratio) = +10dB +(10 log Outbound/desired symbol rate) = +23 dB
®
Option
Interferer Power Spectral Density - Desired Carrier Power Spectral Density +
10 log (Interferer symbol rate/Desired symbol rate) dB
®
Carrier-in-Carrier
Desired Inbound = 50 ksymbols/sec
10–5
MN-0000036 (Ref MN/CDM625.IOM)
®
Ratio
Revision 1
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