Digital Predistortion Calculation Engine for ODR-DabMod
=======================================================

This folder contains work in progress for digital predistortion.

Concept
-------

ODR-DabMod makes outgoing TX samples and feedback RX samples available for an external tool. This
external tool can request a buffer of samples for analysis, can calculate coefficients for the
polynomial predistorter in ODR-DabMod and load the new coefficients using the remote control.

The *dpd/main.py* script is the entry point for the *DPD Calculation Engine* into which these
features will be implemented. The tool uses modules from the *dpd/src/* folder:

- Sample transfer and time alignment with subsample accuracy is done by *Measure.py*
- Estimating the effects of the PA using some model and calculation of the updated
  polynomial coefficients is done in *Model.py*
- Finally, *Adapt.py* loads them into ODR-DabMod.

These modules themselves use additional helper scripts in the *dpd/src/* folder.

Requirements
------------

- USRP B200.
- Power amplifier.
- A feedback connection from the power amplifier output, at an appropriate power level for the B200.
  Usually this is done with a directional coupler and additional attenuators.
- ODR-DabMod with enabled *dpd_port*, and with a samplerate of 8192000 samples per second.
- Synchronous=1 so that the USRP has the timestamping set properly, internal refclk and pps
  are sufficient for this example.
- A live mux source with TIST enabled.

See dpd/dpd.ini for an example.

The DPD server port can be tested with the *dpd/show_spectrum.py* helper tool, which can also display
a constellation diagram.

TODO
----

Implement a PA model.
Implement cases for different oversampling for FFT bin choice.
Fix loads of missing and buggy aspects of the implementation.