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diff --git a/dpd/README.md b/dpd/README.md index 173b4c6..b5a6b81 100644 --- a/dpd/README.md +++ b/dpd/README.md @@ -1,21 +1,56 @@ -Digital Predistortion for ODR-DabMod -==================================== +Digital Predistortion Calculation Engine for ODR-DabMod +======================================================= -This folder contains work in progress for digital predistortion. It requires: +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. -- ODR-DabMod with enabled dpd_port, and with a samplerate of 8192000 samples per second. + 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. + +File format for coefficients +---------------------------- +The coef file contains the polynomial coefficients used in the predistorter. The file format is +very similar to the filtertaps file used in the FIR filter. It is a text-based format that can +easily be inspected and edited in a text editor. + +The first line contains the number of coefficients as an integer. The second and third lines contain +the real, respectively the imaginary parts of the first coefficient. Fourth and fifth lines give the +second coefficient, and so on. The file therefore contains 2xN + 1 lines if it contains N +coefficients. + TODO ---- -Implement a PA model that updates the predistorter. -Implement cases for different oversampling for FFT bin choice +Implement a PA model. +Implement cases for different oversampling for FFT bin choice. +Fix loads of missing and buggy aspects of the implementation. |