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CDM-625 Advanced Satellite Modem
DoubleTalk Carrier-in-Carrier Option
The resulting estimate of the unwanted interfering signal is then subtracted from the composite
signal. In practical applications, the estimate of the unwanted signal can be extremely accurate.
Unwanted interfering signal suppression of 30 dB or more has been achieved in commercial
products with minimal degradation of the demodulator performance.
10.4.2
Margin Requirements
Typical interfering signal cancellation is 28 to 35 dB (depending on the product). The residual
interfering signal appears as noise causing a slight degradation of the Eb/No. To compensate for
the residual noise, a small amount of additional link margin is required to maintain the BER.
Margin requirements depend on the product, modulation and power ratios:
For the CDM-625, the additional margin requirements are as follows:
Modulation
BPSK
QPSK/OQPSK
8-PSK
8-QAM
16-QAM
10.4.3
Carrier-in-Carrier Latency
Carrier-in-Carrier has no measurable impact on circuit latency.
10.4.4
Carrier-in-Carrier and Adaptive Coding and Modulation
Carrier-in-Carrier is fully compatible with VersaFEC Adaptive Coding and Modulation (ACM)
mode of operation in the CDM-625.
Carrier-in-Carrier combined with VersaFEC ACM can provide 100-200% increase in average
throughput.
10.4.5
Carrier-in-Carrier Link Design
Carrier-in-Carrier link design involves finding the FEC and modulation combination that provides
optimal bandwidth utilization. Just like conventional link design, it is an iterative process that
involves trying different FEC and modulation combinations with Carrier-in-Carrier until an
optimal combination is found.
For optimal Carrier-in-Carrier performance, it is recommended that the two carriers have similar
symbol rate and power. This can be achieved by selecting appropriate ModCods as shown in
following sections.
Nominal Margin*
0.3 dB
* Equal power and equal symbol rate for the
0.3 dB
interfering carrier and the desired carrier, i.e., 0 dB
PSD ratio. Measured at IF with AWGN, +10 dBc
0.5 dB
Adjacent Carriers, 1.3 spacing.
0.4 dB
0.6 dB
10–9
Revision 13
MN-CDM625
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