Over the last two years, Adaptive Coding and Modulation (ACM) has become a widely used feature in DVB-S2 links. Although the change of ModCods within the signal is standardised, Jörg Rockstroh, Senior R&D Engineer at WORK Microwave, goes beyond theory to share his firsthand experience of ACM installations, touching on several important characteristics that ensure the most value out of this feature.
Prioritise ACM messages:
It is crucial that the status messages from the receivers reach the modulator. As most systems are likely to be IP-based, correct routing and prioritisation in the network must be assured in order to guarantee the delivery of messages. Otherwise a notable drop in link bandwidth will occur due to lost messages — even if the signal quality is in good shape.
Manage outbound IP traffic:
Equally important is the management of outbound traffic, which faces a non-constant available bandwidth. Many times, this turns out to be an unknown scenario for IP specialists, so interaction between ACM and the IP traffic shaping must be a consideration.
Limit channels based on the symbol rate:
In point-to-multipoint links a symbol rate-based traffic management approach will prevent the IP links from affecting each other during changing conditions. Without this limitation, changing one ModCod could impact all present channels, as baseband frames of different modulations require a different length on the carrier (in symbols).
Consider an Es/N0 offset:
Most links require a small offset to the theoretical Es/N0 margins, typically 0.5 to 1.0 dB. That offset will cover delay times within the system (i.e. return channel over satellite and time between ACM messages) and accuracy limitations of the Es/N0 measurement. Without an offset, frame loss may occur when the signal quality degrades. Dynamic offsets and a hysteresis could be used to support link stability and efficiency.
Do not only focus on numbers:
Some effects of satellite links are not represented to their full extent in the Es/N0 value, especially non-linearities or narrow band interference. Changing the offset will help overcome the influence immediately (i.e. as long as the interference is present) and maintain the operation of the link.
Start conservatively, approach the optimum:
ACM settings are hard to predict beforehand. When setting up a link it is wise to start out implementing very conservative settings until a stable link has been established. Even if the “first shot” does not have the desired bandwidth efficiency, an incremental approach is the best way to optimise the link once it is up and stable. Due to numerous parameters and conditions affecting the Es/N0, the best settings will be reached by analysing the results, making an adjustment, and then testing the new settings until they have been optimised.
Keep an eye on the environmental conditions:
Even after the link is set up, optimised, and running, the ACM controller cannot predict where the remote units are installed. Offset and hysteresis values for the control loop should be configured to account for worst case scenarios, such as snow or dust storms, heavy rain, and dish mount stability issues due to heavy winds. All of these situations can have a different effect (i.e. in different frequency bands) on the Es/N0 and therefore the ACM.