Q-Flex Satellite Modem Installation and Operating Handbook
7-32
7.8.13 Adaptive Coding and Modulation (ACM)
Adaptive Coding and Modulation (ACM) uses feedback from the receiver to the
transmitter to respond to changes in channel conditions to optimise throughput, by
providing only the level of error protection that is required.
ACM can be used in both directions at the same time, maximising throughput for
outbound and return. Deployed links have reported throughput gains of up to 100%.
ACM works in conjunction with DVB-S2/S2X (although the return channel does not have
to be DVB-S2/S2X unless ACM operation is required for both outbound and inbound
carriers).
By varying the error correction strength to match atmospheric conditions, link margin is
converted into useful bandwidth. Modulation and FEC rate (modcod) is dynamically
matched to the current Es/No returned from the remote modem. Symbol rate is kept
constant, changing the terrestrial data rate up or down with Es/No. ACM operation
automatically accounts for all other link impairments including antenna alignment error,
inclined orbit, phase noise and other degradations.
ACM can be used on point-to-point IP links. ACM must be on in Tx on the transmitting
modem and Rx in the receiving modem. It must be on in both Tx and Rx in both modems
if ACM is to be actively used for both outbound and inbound carriers. It requires a satellite
return channel (either DVB-S2 or SCPC). The return channel can be used for user data
as well and no special set up is required – the modem will automatically ensure Es/No
information is returned to the transmitting modem with no material impact on bandwidth.
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 points. All DVB-S2 modcods from QPSK ¼ to 16APSK
9/10 are available and both short block and normal block sizes are supported. Each
modcod operates over a defined range in relation to carrier-to-noise level, providing true
quasi-error-free operation. 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 automatically
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. Since ACM can work all the
way down to a negative Es/No, i.e. below what the link was designed for, it effectively
gives 100% link availability (albeit at a reduced data rate).
