Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix F F–9 MN-CDM-425
As shown in Figure F-6, for round trip delay estimation, a search algorithm is utilized that
correlates the received satellite signal to a stored copy of the local modulator’s transmitted signal.
The interference cancellation algorithm uses the composite signal and the local copy of S1 to
estimate the necessary parameters of scaling (complex gain/phase), delay offset and frequency
offset. The algorithm continuously tracks changes in these parameters as they are generally time-
varying in a satellite link.
Figure F-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.
F.2.2 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-425 Advanced Satellite Modem, the additional margin requirements are as follows:
Table F-2. Margin Requirements
Modulation Nominal Margin*
BPSK 0.3 dB
QPSK/OQPSK 0.3 dB
8-PSK 0.5 dB
8-QAM 0.4 dB
16-QAM 0.6 dB
* Equal power and equal symbol rate for the interfering carrier and the desired carrier, i.e., 0 dB PSD ratio. Measured
at IF with AWGN, +10 dBc Adjacent Carriers, 1.3 spacing.