Margin Requirements; Carrier-In-Carrier Latency; Carrier-In-Carrier And Adaptive Coding And Modulation (Acm); Figure 8-6. Carrier-In-Carrier Signal Processing Block Diagram - Comtech EF Data CDM-750 Installation And Operation Manual

CDM-750 High Speed Trunking Modem
DoubleTalk Carrier-in-Carrier Option
offset. The algorithm continuously tracks changes in these parameters as they are generally time-
varying in a satellite link.

Figure 8-6. Carrier-in-Carrier Signal Processing Block Diagram

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.

8.4.2 Margin Requirements

Depending on the product, typical interfering signal cancellation is 28 to 35 dB. The residual
interfering signal appears as noise, causing a slight degradation in received Es/No. To
compensate for the residual noise, a small amount of additional link margin is required to
improve the Es/No and maintain the QEF performance. Margin requirements depend on the
product, modulation and power ratios; for the CDM-750, these additional margin requirements
are as follows:
Modulation
QPSK
8PSK
16APSK
32APSK
* Equal power and equal symbol rate for the interfering carrier and the desired carrier, i.e., 0 dB
CnC ratio. Measured at IF with AWGN, +7 dBc Adjacent Carriers, 1.3 spacing.

8.4.3 Carrier-in-Carrier Latency

DoubleTalk Carrier-in-Carrier has no measurable impact on circuit latency.

8.4.4 Carrier-in-Carrier and Adaptive Coding and Modulation (ACM)

DoubleTalk Carrier-in-Carrier is fully compatible with the Adaptive Coding and Modulation
(ACM) mode of operation in the CDM-750.
CnC combined with ACM can provide 100 – 200% increase in average throughput.
Nominal Margin*
0.3 dB
0.3 dB
0.6 dB
1.0 dB
8–9
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MN-CDM750