17–1
Chapter 17. ADAPTIVE CODING
AND MODULATION (ACM)
17.1 Introduction
The VersaFEC® Adaptive Coding and Modulation (ACM) feature is a patents-pending technology,
wholly owned and developed by Comtech EF Data and CEFD sister division Comtech AHA Corp. It
serves as a very significant operational enhancement for the CDM-625 Advanced Satellite Modem:
• ACM turns fade margin into increased link capacity – gains of 100% or more are
possible, compared to traditional Constant Coding and Modulation (CCM). This is
accomplished by automatically adapting the modulation type and FEC code rate to give
highest possible throughput.
• ACM maximizes throughput regardless of link conditions (noise or other impairments,
clear sky, rain fade, etc). Initial setup is easy, and then requires no further user
intervention.
• With a CCM system, severe rain fading can cause the total loss of the link, and zero
throughput. ACM keeps the link up (with lower throughput) – and can yield much higher
system availability.
• ACM in the CDM-625 is used in conjunction with VersaFEC and is currently for IP traffic
only.
The VersaFEC ACM feature requires Firmware Version 1.4.0 or later, the VersaFEC plug-in module,
and the appropriate FAST codes.
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