This resource is designed to help you optimise and refine the DPD implementation for improved performance your application. Whether you're new to DPD or looking to fine-tune advanced parameters, this guide provides the insights and tools you need to achieve better linearization and efficiency in your signal chain. Digital Pre-Distortion is a technique used to correct non-linearities in power amplifiers, enabling higher efficiency and maintaining signal integrity. By adjusting key parameters such as delays, time filter coefficients, and normalization thresholds, you can significantly enhance system performance and stability.
Below we will go through some pre-calibration and post-calibration parameters to help get the most out of the DPD.
Pre-calibration Parameters
- Enable/Disable
- LUT size and pre-LUT scale
- Model Tap polynomial Terms
Enable and disabling the DPD can be done from the tracking calibrations tab in the TES. This allows the user to stop the updating of the calibration co-efficients when needed. If the DPD is stable and the enviornmental conditions are stable then there might be no need for iterating the co-efficients and the option is there to pause the calculation. The coefficients currently in use will be kept in this scenario until the DPD is enabled again and the calculation engine calculates new co-efficients.
LUT size has an effect on the DPD coefficients. A smaller LUT will have slightly coarser mapping between the input and the coefficient output. It will however save on memory and potentially be slightly faster to calculate coefficients.
Model Tap Polynomial Terms can be turned on or off and tested out to see what good stable performance for the signal under test. The values on each of the terms is calculated by the DPD engine and this array in the TES aloows the user to chose what taps are used in the calculation or not.
Post Calibration Parameters
- Number of samples captured
- Capture Delay
- Time filter co-efficient/LUT switching
- Normalisation Thresholds
- Activation conditions
- Additional power scale
The number of samples captured can be up to 4096 consecutive samples.
In the case where the frame might be smaller than the maximum amount of samples you can reduce the cature length.
In the other event where th frame is veryy long (seconds) or in FDD mode when its constantly on the DPD will capture every 2 seconds.
The Capture delay can be used to delay the start of the DPD capture. In scenario 1 the gaurd time might be taken into account with null data so its possible to push the capture further into the frame to ensure you only get valid data points.
The Time filter coefficient allows te DPD to gradually make changes over time. Instead of taking in a new capture and updating the coefficients based solely off the incoming data the time coefficient aloows a blend from the current DPD coefficients to the next ones by adding them and multiplying by the time coefficient. This will mean there is no major jump from one iteration to the next and the DPD will take longer to settle as a result.
Normalisation thresholds are important to declare the linear region to normalise the data to. This should be above the noise and below the compression region.
The polynomial function creates multiple copies of the the signal based on the order of the non-linearity and memory depth. Additional Power scale is used to to reduce the corelation of the signal to prevent overfitting. The default value is 4.
Peak detection activation is another control that will allow the system to make decision on whether to update the DPD coefficients or not. Users can set the threshold and make a decision based on how many times the peak of the signal has crossed that threshold. For example you can set the threshold at -10dBFS and allow the DPD update if the number of points in a capture above the threshold is above 100. If these conditions are not met then the data is discarded and the previous coefficients remain in place.
Crest factor reduction needs to be applied to the signal before transmitting, there are no internal CFR blocks in the ADRV9002. Signals like LTE will have higher peak to average power ratios (PAPR). This can cause inefficient use of the LUTs since high peaks take up more resources when compared to lower power samples. There is a trade off needed here to keep the signal integrity but also reduce the peaks to increase the efficiency.
