Appendix D. Versafec Return Link Acm (Adaptive Coding And Modulation) Option; Functional Overview - Comtec CDM-840 Installation And Operation Manual

Appendix D. V
LINK ACM (ADAPTIVE CODING
D.1

Functional Overview

VersaFEC® is a technology that is wholly owned and developed by Comtech EF Data
and Comtech AHA Enterprise Products Group. VersaFEC is a registered trademark of
Comtech AHA Enterprise Products Group.
Satellite users have traditionally relied on worst-case link margin to overcome rain fade and
other impairments, which leads to significant inefficiencies. ACM (Adaptive Coding and
Modulation) allows modulation and code rate (ModCod) to change on a frame-by-frame basis
subject to current link conditions. As ACM converts available link margin into increased capacity,
average capacity gain of 100% or more is possible. This maximizes each remote's throughput
under all conditions which, in turn, maximizes network efficiency and availability.
Advanced VSAT supports ACM in the forward direction as well as the return direction:
•
With CCM (Constant Coding and Modulation) systems, 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 turns fade margin into increased link capacity by automatically adapting the
modulation type and FEC code rate to give highest possible throughput. Gains of 100% or
more are possible, compared to traditional CCM.
•
ACM maximizes throughput regardless of link conditions (noise or other impairments, clear
sky, rain fade, etc). After initial setup, ACM requires no further user intervention.
You may use ACM with VersaFEC for modems running Firmware Version 1.5.2.X or later.
ERSA
MODULATION) OPTION
D–1
FEC RETURN
AND