Dpd - Operational Recommendations; Dpd Link Design - Comtech EF Data CDM-760 Installation And Operation Manual

Appendix L
CDM-760 Advanced High-Speed Trunking Modem
•
The satellite must be a "non-processing" satellite (i.e. does not demodulate/remodulate
the signal).
L.3 DPD – Operational Recommendations
The DPD function has not been shown to provide significant benefit with smaller constellations
of QPSK or 8-PSK since these are not affected as much by nonlinear distortion. The technique
does not harm these links but doesn't provide significant benefit either. Nonetheless, DPD is
strongly recommended for use with larger constellations.
In addition, to minimize "False" feedback acquisition, keep the search delay range as narrow as
possible:
• Once the modem has reported the search delay, narrow the search delay range to the
nominal reported value +/- 5 ms, e.g. if the modem reported delay is 245 ms, narrow
the search range to say 240 –250ms.
Use an external data source (e.g. Fireberd) or internal BER tester when testing DPD
performance.
L.4

DPD Link Design

DPD link design involves finding the FEC, modulation combination, and transmit power level that
provides optimal bandwidth utilization. Just like conventional link design, it is an iterative
process that involves trying different FEC, modulation, and transmit power combinations with
DPD until an optimal combination is found. This process is described in the next section.
Figure L-9. Remote DPD – Satellite Links
L–7
Revision 4
MN-CDM760