L.1.2 Predistortion - Comtech EF Data CDM-760 Installation And Operation Manual

Appendix L
CDM-760 Advanced High-Speed Trunking Modem
L.1.2 Predistortion
Predistortion preemptively compensates the uplink waveform by expanding the signal peaks
that will be compressed by the satellite amplifier. Because a nonlinear satellite channel is not a
fully invertible system, residual distortion will still be present even when the predistorter is
applied. Note also that, while the linear filtering effects of the link are compensated by DPD, the
effects cannot be fully removed due to inherent limits in computational capacity.
Since the best predistorter for a particular satellite transponder is unknown, one must be
identified through a training process. See Figure L-4. The CDM-760 can train the predistorter
according to either of these approaches:
•
Locally, using a single modem and ground station, or
•
Remotely, using two modems and ground stations.
The Local training approach requires the CDM-760 to "listen" to its own outbound undergoing
distortion imparted primarily by the non-linearity, along with the input/output multiplexing
filters, on the satellite.
The Remote training approach does not require the full signal loopback. Instead, additional
processing is required at the remote end. Using the same messaging approach as Adaptive
Modulation and Coding (ACM) control, a return link is required to the original transmit modem
to pass trained coefficients.
Under both approaches, the predistorter is trained according to the 'indirect learning'
architecture. Figure L-5 and Figure L-6 illustrate the training procedure for local and remote
training, respectively.
Figure L-4. Satellite Training Link
L–3
Revision 4
MN-CDM760