Comtech EF Data CDM-425, Installation And Operation Manual

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CDM-425 Advanced Satellite Modem
Revision 0
Appendix K K–3 MN-CDM-425
K.3 Requirements for ACM
A generic example of ACM-over-Satellite is shown in Figure K-1. There are a number of essential
requirements for enabling this scheme:
1. A modulator and FEC encoder that can instantaneously, when commanded, change
either modulation type (order) or FEC encoder rate, or both. This needs to be
accomplished without the corruption of data anywhere in the path. Block FEC codes are
considered to be the most practical in achieving the required synchronization. A specific
nomenclature has emerged to describe a combination of a modulation type and code rate
–
ModCod (also referred to as Mod/Code ). The modulator is required to send the value
of ModCod at the start of each code block to signal the demodulator/decoder how to
configure for the correct modulation type and FEC code rate.
2. A receiver that is capable of demodulating and decoding the signal transmitted by a)
without any a priori knowledge of when a change has taken place, but based purely on
the value of ModCod seen at the start of each FEC block. Again, this needs to be
accomplished without the corruption of data anywhere in the path.
3. The receiver in b) needs to derive an estimate of the link quality (in terms of Eb/No, SNR,
etc.) and then communicate this estimate, via a return channel, to the modulator in a).
4. The modulator in a) needs to able to process the link quality metric from the demodulator
in b), and then, based upon a pre-determined algorithm, adapt the data rate and change
the ModCod sent to the receiver at the distant end. Thus, the data rate on the link can be
maximized, given the current link noise conditions.