Comtech EF Data CDM-625A - Manual (Page 683)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix M Revision 3
M–3
M.3 Requirements for ACM
A generic example of ACM-over-Satellite is shown in Figure M-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.