ACM
Pronunciation: A-C-M
Also written as: ACM — Adaptive Coding and Modulation
A technique in which the modulation scheme and forward error correction code rate of a satellite link are dynamically adjusted in response to changing signal conditions, optimising throughput while maintaining link reliability.
Full Definition
ACM allows a satellite modem to vary its modulation order (e.g., QPSK, 8PSK, 16APSK) and FEC code rate in real time based on measured link quality, such as the signal-to-noise ratio reported by the receiving terminal. Under clear-sky conditions, higher-order modulation delivers maximum throughput; during rain fade, the system falls back to more robust, lower-order modes to maintain connectivity. ACM is standardised in DVB-S2 and DVB-S2X. Editors encounter ACM in modem specifications, service descriptions, and HTS system design documents. It must be distinguished from ULPC, which adjusts transmit power rather than the waveform.
Usage
Usage note: VCM (Variable Coding and Modulation) is an open-loop precursor to ACM; the two are related but not synonymous. In DVB-S2X, ACM operates across a wider range of modulation orders.
In Context
- "With ACM enabled, the HTS terminal maintained connectivity throughout the rain event by stepping down from 16APSK to QPSK." — System performance case study
- "The datasheet states that ACM supports modulation orders from QPSK to 32APSK as defined in DVB-S2X." — Modem product specification