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CDM-625A Advanced Satellite Modem
Appendix M
M.3
Requirements for ACM
A generic example of ACM-over-Satellite is shown in
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. Recently,
a specific nomenclature has emerged to describe a combination of a modulation type
and code rate – namely, 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.
Figure
M–3
Revision 0
MN-CDM625A
M-1. There are a number of
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