Digital Predistortion; Background; Adrv9001 Dpd Function - Analog Devices ADRV9001 User Manual

UG-1828 Preliminary Technical Data

DIGITAL PREDISTORTION

BACKGROUND

It is well known that one of the main criteria of a power amplifier (PA) operation is its ability to maintain linearity, i.e. the gain is
constant regardless of the input amplitude. However, in practice, a power amplifier can only maintain linearity up to a certain input level
beyond which the gain starts to lower and the power amplifier enters into a nonlinear or compression region as shown in Figure 139. For
most low-power linear amplifiers, they operate in the linear region as shown in the red circle. Unfortunately, a power amplifier that
operates mostly in the linear region has lower efficiency. power amplifier efficiency is defined as the ratio of output RF power to the DC
supply power. Therefore, it is desirable to operate power amplifier at . high efficiency to save DC power and reduce heat dissipation.
To achieve higher power amplifier efficiency, the highest input signal peak is usually set at around 1dB (P1dB) compression region as
shown in the blue circle in Figure 139. However, compression of the peak signals produces harmonics and hence intermodulations. Some
of the intermodulations fall back right into or adjacent to the carrier spectrum, therefore not only distorting the transmit signal but also
widening the spectrum of the transmit signal, so called spectral regrowth. If left untreated, the error vector magnitude (EVM)
performance of the transmit signal would be degraded and the spectral regrowth would interfere adjacent channels, resulting in worse
than required adjacent channel power ratio (ACPR) performance. Digital Pre-Distortion (DPD) is designed to mitigate this problem.
1bB
COMPRESSION
REGION
1bB
LINEAR REGION
IDEAL PA OUTPUT
ACTUAL PA OUTPUT
PA INPUT
Figure 139. Ideal Power Amplifier Output vs. Actual Power Amplifier Output

ADRV9001 DPD FUNCTION

The ADRV9001 device provides a fully integrated DPD function that supports both narrow-band (NB) and wide-band (WB)
applications. It is a hardware/software combined solution which performs linearization of the power amplifier by pre-distorting the
digital transmit signal with the inverse of the power amplifier's nonlinear characteristics. After amplifying by the power amplifier, the
pre-distortion compensates power amplifier's nonlinearity so the amplified RF transmit signal becomes linear. Therefore, the integrated
DPD solution allows power amplifier to operate at very high efficiency while achieving a satisfactory EVM and ACPR performance.
Figure 140 depicts a high level block diagram of the DPD algorithm. As shown in this figure, before the power amplifier, a "Predistortor"
block is added in the transmit datapath which distorts the transmit signal d(t) with the inverse of the power amplifier's nonlinear
characteristics, as shown by the first Input/Output figure curve. Spectral regrowth is introduced after the pre-distortion. However, after
the "pre-distorted" transmit signal x(t) being amplified by the power amplifier, the power amplifier nonlinear characteristics, as shown
by the second Input/Output figure curve cancels out the pre-distortion. Therefore, the output of the power amplifier y(t) becomes linear,
as shown by the third Input/Output figure curve. In addition, the spectral regrowth after pre-distortion is also corrected. The "DPD
Coefficients Computation" block is used to compute the pre-distortion parameters by utilizing "Predistortor" output signal x(t) as well as
the power amplifier output signal y(t) through a feedback path. It models the behavior of the power amplifier in the reverse direction, i.e.
from output to input, therefore, it characterizes the inverse of the power amplifier nonlinearity and then feeds the parameters to the
"Predistortor".
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