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Appendix L
CDM-760 Advanced High-Speed Trunking Modem
L.2
DPD – Operational Requirements
Predistortion and CFR are typically applied in a digital system collocated with the nonlinear
amplifier. However, in this application these techniques must be applied remotely from a
ground-based modem to compensate for the nonlinearity of the orbiting satellite transponder.
Due to the removed nature of this implementation some operational requirements must be
met.
See Figure L-8. In the local implementation, the transmitting modem must receive its own
outbound (S') for the predistortion training described in Sect. L.1.2. As such, local DPD can only
be employed under the following conditions:
•
The link must include a "loop-back" circuit (i.e. the transmitting station must be able to
receive its own outbound).
•
Local DPD requires the use of the modem's Rx signal chain. Consequently, the CDM-760
will operate in Transmit Only mode when local DPD is enabled. ACM cannot be
employed as a result.
For a remote implementation, see Figure L-9. There must be a full-duplex circuit so the trained
coefficients can be returned to the predistorter. As such, remote DPD can only be employed
under the following condition:
•
There must be a full-duplex circuit to return trained coefficients. Note that sending the
coefficients adds a very low overhead burden and are automatically passed much the
same as ACM messaging.
For both local and remote DPD, if the satellite transponder were processing other carriers, or
other processing was applied, the nonlinear transponder's operation on the carrier of interest
would become unpredictable. Therefore, additional operational restrictions are applicable to
both local and remote DPD:
•
The link must be over a single-channel-per-transponder configuration. That is, multiple
carriers, even in a CnC configuration, cannot be operated on the transponder to which
DPD will be applied.
Figure L-8. Local DPD – Satellite Links
L–6
Revision 4
MN-CDM760
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