SHAPING THE FUTURE OF SATELLITE COMMUNICATIONS
9 FEATURES
9.1 FlexACM
9.1.1 Introduction
In a satellite link operated with FlexACM, the modulator and demodulator parts
must exchange signalling information with each other in order to control
dynamically the modulation parameters in function of the instantaneous receive
conditions in the demodulator. This signalling consists of short messages in IP
format. The average bit rate of this communication is very low (only a few hundred
bits per second).
The ACM forward signalling is transmitted over the satellite link (RF). The return
signalling can be transmitted over any communication channel between the
modulator and demodulator, being a satellite link or a terrestrial link.
Since there is by definition always a satellite link between the modulator and
demodulator, the communication from the modulator to the demodulator is in most
cases transmitted in the satellite signal itself. It is possible to send this signalling
message in the same stream as the IP payload, or to isolate it from the payload in
another DVB-S2 stream (multi stream configuration).
For the FlexACM communication between the demodulator and the modulator, the
implementation depends on the topology of the network. For two-way DVB-S2
transmissions, the FlexACM signalling can be transmitted in the DVB-S2 return
link, mixed with the IP payload or in a separate DVB-S2 stream. The FlexACM
signalling can also be transmitted in any other type of satellite return channel (for
example VSAT) or terrestrial link (ADSL, GPRS, etc…).
The ACM signalling is generated and received by the Monitoring and Control
module of the modulator, demodulator, modem, or by the EL860 Controller and
Shaper when such unit is used in front of the modulator.
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