Hi,
The code for PA fitting and the DPD model is shipping with RF Blockset.
If you have access to R2020b, you will find it as part of one of the shipping examples.
Power Amplifier Characterization
Digital Predistortion to Compensate for Power Amplifier Nonlinearities
The DPD feedback look implemented using the ADI9371 transceiver can be downloaded here:
You will also find similar examples in the priovious releases.
With best regards,
Giorgia