Comtech EF Data CDM-750 - Manual (Page 242)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix C MN-CDM750
C–6
C.3 ACM System Description
A generic example of ACM‐over‐Satellite is shown in Figure C‐3. There are a number of essential
requirements to enable this scheme:
• A near side modulator and FEC encoder that can instantaneously, when commanded,
change either modulation type, FEC encoder rate or both. This needs to be
accomplished without the corruption of data anywhere in the path. The ModCod must
remain fixed throughout an entire frame and may change to any other ModCod on the
next frame.
• The near side modulator is required to send the value of ModCod at the start of each
frame to inform the far side demodulator/decoder how to configure for the correct
ModCod.
• A far side demodulator/decoder must be capable of demodulating and decoding the
signal transmitted by the near side modulator without any prior knowledge of when a
change has taken place, based purely on the value of ModCod seen at the start of each
Frame. Again, this needs to be accomplished without the corruption of data anywhere in
the path.
• The far side receiver needs to derive an estimate of the link quality, the received Es/No,
and then communicate this estimate, via a return channel, back to the near side
modulator.
• The near side modulator needs to process the link quality metric from the far side
demodulator and, based upon a pre‐determined algorithm, adapt the data rate and
change the ModCod sent to the far side receiver. This closes the loop and maximizes the
data rate for the current link conditions.