Quantum and Evolution Series Installation and Operating Handbook
8-53
In terms of configuration, the link is set up as normal, using a nominal data rate,
modulation and FEC rate that gives the required symbol rate. The
only
other step is
switching ACM on!
The modcod selected by the user on the forward path represents the maximum modcod
to be used when ACM is active. This allows higher order modcods that cause non-linear
distortion in the RF chain to be avoided. ACM will use all modcods up to and including the
modcod set by the user. It is easier to set up an ACM link as normal rather than being
forced to adapt to an alternative such as setting a fixed symbol rate. Uniquely for any
ACM implementation, other than switching ACM on, no other ACM-specific controls are
necessary.
Paradise ACM works at true QEF point. Each modcod operates over a defined range in
relation to carrier-to-noise level, providing true quasi-error-free operation (using Es/No
values that Paradise modems are known to meet). Rapid fading is countered by using
predictive tracking to estimate what the remote Es/No will do before it happens, which
prevents link loss even in sudden deep fades. Should the link be lost then the minimum
modcod is selected in order to re-establish the link. The transmitter can switch between
any two modcods – it does not have to go through them in sequence.
Note that with DVB-S2 ACM, pilots are automatically switched on at operating points
where their use achieves the maximum throughput for a particular Es/No. For example,
for an available Es/No of 6.5dB, it is better to use short-block 8PSK 3/5 with pilots on
(spectral efficiency 1.69bits/s/Hz) than to use the most suitable short-block modcod with
pilots off, i.e. QPSK 5/6 (spectral efficiency 1.6bits/s/Hz).
Note that ACM can be used with AUPC at the same time (providing automatic uplink
power control helps maintain the remote Eb/No at the target level).
Modcod changes are transparent at the receiver and does not involve the demodulator
having to reacquire the signal. Modcod changes can be made instantly whenever
required, maximising throughput at all times.
Paradise ACM is intrinsically tied to an IP traffic shaper and TCP accelerator, allowing
data grooming to match data priority to the available bandwidth. Without this, there would
be no effective data management of a dynamically varying channel, which would become
a free-for-all where you would not be sure which packets will be passed over satellite and
which will be dropped.
Because the symbol rate is kept constant, any change in modcod will change the
terrestrial data rate. An increase in data rate does not need to be explicitly signalled to the
terrestrial network. A decrease does, because it could result in a buffer overflow in the
transmit modem if it receives more data than it can transmit. Upstream buffer control is
achieved by sending Ethernet Pause frames that define a period of time for which
transmission should be suspended. In principle, Pause frames are supported only for full-
duplex links and support for them in vendor equipment is optional, therefore the modem
auto-negotiates this capability with the attached equipment.
Note that ACM works to the full symbol rate of the modem.
