Note The DPD measurement requires Power Amplifier license activation. If you install RFmx SpecAn but do not activate the Power Amplifier license, you will have 30 days from the first time you use the measurement to evaluate it.

One of the fundamental assumptions in the study and design of signal processing and communication systems is that the systems under investigation are more or less linear. While the linearity assumption holds in digital subsystems, the response of analog subsystems can only be treated as being approximately linear over a certain range of input power or frequency band.

An example of a nonlinear subsystem used in the ubiquitous wireless communication systems is a power amplifier (PA). While the overall response of the PA is clearly nonlinear, many applications where the region of operation is limited to a relatively linear portion of the PA response do not face any serious problem due to the nonlinear nature of the PA. This is true for cellular standards such as GSM or EDGE.

The following figure shows the typical input-output response of a PA.

With the rapid migration of cellular and connectivity technologies towards multicarrier modulation schemes, such as HSPA+, LTE-Advanced and IEEE 802.11ac where the RF envelope fluctuations vary from 6 dB to 12 dB relative to the average signal power, it has become impractical to back-off the operating point to continue to operate the PA in a relatively linear region because operating efficiency of PAs diminishes as the operating point is backed-off. Technologies such as envelope tracking and digital predistortion have been developed in order to linearize power amplifier response without resorting to power back-off and thus continue to operate the PA in a high efficiency mode.

Digital predistortion (DPD) is a generic name for all power amplifier linearization techniques that modify the complex baseband representation of the RF signal at the input to the PA under test in such a way that the RF signal at the output of the PA is as close to the expected output as possible, had the PA been a linear memoryless device. The exact implementation of individual DPD techniques may require PA characterization or PA modeling.

The following figure shows a simplified representation of the signal processing chain where PA is preceded by digital predistortion.