Adaptive Coding and Modulation (ACM)

Some advice..

In ACM the symbol rate normally remains constant and therefore the satellite bandwidth required remains constant.

The modulation method BPSK, QPSK, 8QAM, 16QAM etc and FEC rate varies so that the best customer bit rate is obtained according to the current weather (rain fade etc).  The throughput varies according to the weather.

If you are designing a system and want to get the highest throughput per unit bandwidth then you need the highest quality link. You need to choose a satellite with a high power, a small coverage area spot beam (with high G/T on the uplink), adjustable gain on the satellite transponder and a large receive earth station at the other end.  You might use 16QAM modulation and FEC=0.583. This gives about 3.5 bits per Hz.

Read: http://www.comtechefdata.com/files/articles_papers/wp-cdm625_acm_white_paper.pdf

If you choose a specific fixed bandwidth and then use ACM, you will get the best bit rate under clear sky conditions, with perfect dish pointing etc. As the weather deteriorates the customer bit rate will go down.  You need the latest modems, such as CDM625 with ACM options enabled.

If you don't have ACM and set up a lower bit rate in clear sky by setting say a 3 dB link margin and presetting a fixed modulation and coding, your system will work fine in clear sky and also work fine in some rain but will fail when the degradation is worse than 3 dB.  Old style modems, with a choice of several fixed MODCODs,  are suitable. Most older satellite links work like this and all have a link margin of several dBs, to cope with rain fades.

If you don't have ACM and actually adjust for the best bit rate in clear sky by presetting the highest possible modulation and coding your system will work fine only in clear sky, but will fail as soon as it rains etc.  Old style modems, with a choice of several fixed MODCODs,  are suitable.  During days when it is raining you will get no service.I don't know anyone who operates like this as most customers want 24/7 service.

Best regards, Eric