Acm And Dmesh; Acm And Dpc (Dynamic Power Control - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix L
The requirements for an ACM system that approaches the minimum possible latency are:
•
The shortest possible LDPC codes that give performance at or very close to DVB-S2, in order
to minimize latency, and which do not use interleaving.
•
Design of the encoder to further reduce latency to the minimum possible.
• A constant number of symbols per block, to reduce the demodulator and decoder
complexity, and significantly also reduces latency in the ACM case.
•
The elimination of the need for pilot symbols for carrier tracking at low SNR by substitution
of other modulation techniques. This further reduces the complexity of the demodulator.
•
A reduction in the number of ModCods to further reduce complexity.
•
The inclusion, at the physical layer, of an overhead channel to permit the reporting of SNR
metrics back to the originating end. Note that this does not have to be enabled or disabled –
it is part of the fundamental frame structure of VersaFEC ACM, and has been take into
account in the code rate.
L.2.2.2.1

ACM and dMesh

Although ACM and dMesh are independent of one another, ACM is supported in a dMesh
environment.
Return link ACM for the remote CDM-840 to the hub CDD-880 link is modeled after the Comtech
EF Data CDM-625 Advanced Satellite Modem's ACM approach and framing, while adapting
control techniques to operate in a dynamic point-to-multipoint network that includes a mesh
environment.
L.2.2.2.2

ACM and DPC (Dynamic Power Control)

Appendix M. DPC (VMS DYNAMIC POWER CONTROL)
The DPC (Dynamic Power Control) feature provided in Comtech EF Data's Advanced VSAT
products provides a mechanism whereby remote satellite uplinks have their transmit power
levels adjusted in order to optimize the receive signal quality as measured by the corresponding
demodulator E
S
DPC and ACM function independent of one another, whereby power is modified first or second
in combination with ACM. Depending on conditions, DPC and ACM can be operated exclusively
or in combination, sharing the same resources for checks and balances. When DPC and ACM are
operating in combination, the CDM-840/CDM-850 receives the LQRM comparing value to
current settings and depending on conditions adjusts power or modifies ModCod.
/N (the ratio of energy per symbol to noise density).
0
L–5
MN-CDM840
Revision 3