Comtech EF Data CDM-760 - Manual (Page 335)

G–1
Appendix G. ADAPTIVE CODING
AND
MODULATION (ACM) OPTION
G.1 Overview
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-760 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-760. When enabled with ACM, the
CDM-760 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-760, the benefits of CnC and ACM are available simultaneously.