Comtech EF Data CDM-750 - Manual (Page 237)

C–1
Appendix C. ADAPTIVE CODING
AND
MODULATION (ACM) OPTION
C.1 Introduction
Adaptive Coding and Modulation (ACM) is a technique that allows for automatic change in
modulation and FEC Code Rate in response to changing link conditions. ACM may be used in
packet‐based satellite links (as when using the CDM‐750 with Ethernet Interface) to boost
system throughput. The basic goal of ACM is to capture historically unused satellite system link
margin and convert this margin into additional data throughput.
An ACM system is set up using a constant physical layer symbol rate – and therefore occupied
bandwidth – and power, but with a varying assortment of modulation and coding combinations
called ModCods. The ModCods are selected to span a range of Es/No (Energy per Symbol to
Noise density ratio) so that if a system detects a change in link margin – e.g., fading – it can use
a different ModCod to preserve the link, albeit at a different user throughput rate. Therefore, in
an ACM system, as Es/No increases or decreases, so does the user data rate.
ACM operation is a FAST‐enabled option in the CDM‐750 Advanced High‐Speed Trunking
Modem. When enabled with ACM, the CDM‐750 offers significant operational enhancements:
• 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, clear sky, rain fade,
inclined orbit, antenna pointing error, or other impairments). Initial setup is easy and
requires no further user intervention.
• With a CCM system, severe rain fading can cause the total loss of the link – the result
being zero throughput. ACM can keep the link active during heavy fades with lower
throughput, and can yield much higher system availability.
• As Comtech EF Data’s DoubleTalk Carrier‐in‐Carrier (CnC) operates in conjunction with
ACM in the CDM‐750, the benefits of CnC and ACM are available simultaneously.