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CDM-625 Advanced Satellite Modem
®
DoubleTalk
Carrier-in-Carrier
Worst case example:
Outbound interferer = 12.5 Msymbols/sec Desired Inbound = 18 ksymbols/sec
Ratio of power spectral density = +11 dB
Absolute power ratio (CnC Ratio) = +11dB +(10 log Outbound/desired symbol rate) = +38.4 dB
It is therefore necessary to take into account the symbol rate ratio to properly estimate the CnC
ratio, although the modem does this automatically. For estimating the link parameters, the CnC
ratio is adjusted by 10 Log (Symbol Rate Ratio). If the symbol rate ratio is 2.0 then the narrower
carrier has a 3 dB CnC ratio when the CnC carriers have the same spectral density.
During CnC operation, the interfering carrier is removed by the CDM-625 using a stored version
of the transmitted carrier to adaptively cancel it from the composite received signal. The desired
carrier remaining after the cancellation process is delivered to the demodulator and decoder to
recover the data.
When the CnC ratio increases, the level of the interferer rises relative to the desired carrier and
degradation grows. As the CnC ratio decreases, the desired carrier dominates and degradation
becomes negligible.
CnC Ratio = 0 dB
Equal Interfering And
Desired Carriers
Figure 10-4 represents two cases of the CnC Ratio when the interfering and desired carriers are
equal, and when the interferer is 6 dB stronger than the desired carrier. This representation is
artificial because a real spectral plot displays only the composite power of the combined carriers
and is unable to distinguish two carriers, but it is instructive to describe the underlying principle.
Figure 10-5 shows the case where the outbound symbol rate is 2 times the inbound. As can be
seen, it's all about spectral density and ratio of bandwidths.
®
Option
A
Figure 10-4. CnC Ratio – Equal Symbol Rates
Spectral density ratio = 0 dB
CnC ratio = + 3dB
A
Figure 10-5. CnC Ratio – Unequal Symbol Rates
MN-0000036 (Ref MN/CDM625.IOM)
+6 dB
CnC Ratio = 6 dB
Interferer >
Desired Carrier
B
CnC ratio = + 9dB
Spectral density ratio = 6 dB
+6 dB
Interferer symbol rate =
2 x Desired Carrier
B
10–6
Revision 1
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