Dpd Introduction And Principle Of Operation - Analog Devices ADRV9029 User Manual

Preliminary Technical
Data

DPD INTRODUCTION AND PRINCIPLE OF OPERATION

DPD is a technique for improving the linearity of a non-linear system such as a power amplifier by introducing precise anti-
distortion into the input waveform that compensates for the PA's in-band nonlinear products. DPD works on the principle of pre-
distorting the transmit data in the digital domain to cancel the distortion caused by PA compression in the analog domain. In this
way, DPD can improve PA power-added efficiency by double or more, allowing the PA to be pushed further into saturation while
maintaining linearity requirements.
A baseband model of the power amplifier is created and trained on the input and output digital-baseband samples that pass
through the PA as shown in Figure 2. The pre-distorter then applies an inverse of the PA model function to input samples before
passing them to the transmitter output. The cascade of the pre-distorter response and the PA response becomes a nearly-linear
system.
Out
Out
Out
In
In
In
-1
PA PA
Figure 2. Concept of Digital Pre-Distortion for Linearizing the PA Response
The intermodulation distortion products between various subcarriers due to PA non-linearities in a wideband transmission
protocol such as LTE/NR manifest as power leaked into adjacent channels. Adjacent Channel Leakage Ratio (ACLR) is defined as
the ratio of the transmitted power on the assigned channel to the power leaked in the adjacent radio channel. The ACLR
performance improvement following the application of DPD to the baseband data is captured in Figure 3. These plots illustrate
how the out of band non-linearities due to intermodulation products of an LTE 20 MHz signal are reduced by 15-20 dB after the
application of DPD.
Figure 3. Power Spectral Density Showing Improvement in ACLR After Application of DPD for a 20 MHz LTE Signal
Rev. PrA | Page 4 of 82