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CDD-880 Multi Receiver Router
Appendix F
are powered on, their presence in the dSCPC distribution mesh is constant unless taken out by
operator intervention.
F.2.2
Return Link VersaFEC ACM
F.2.2.1
VersaFEC®
The transmit (Tx) side of the CDM-840 uses a family of short-block LDPC codes called VersaFEC®.
VersaFEC is a patent pending technology wholly owned and developed by Comtech EF Data and
Comtech AHA Enterprise Products Group (the VersaFEC name is a trademark registered to
Comtech AHA).
VersaFEC is ideal for lower data rates that demand the shortest possible latency. The VersaFEC
codes are equally well-suited to Constant Coding and Modulation (CCM) applications.
F.2.2.2
Adaptive Coding and Modulation (ACM)
Adaptive Coding and Modulation (ACM) in the CDM-840 is used in conjunction with VersaFEC
and is currently for IP traffic only.
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 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 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.
With ACM enabled, the CDM-840 offers significant operational enhancements:
•
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.
•
ACM turns fade margin into increased link capacity – gains of 100% or more are possible,
compared to traditional CCM. This is accomplished by automatically adapting the
modulation type and FEC code rate to give highest possible throughput.
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.
F–4
MN-CDD880
Revision 2
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