Digital Predistortion (Dpd); Background; Adrv9001 Dpd Function - Analog Devices ADRV9005 Reference Manual

Reference Manual ADRV9001

DIGITAL PREDISTORTION (DPD)

BACKGROUND

One 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 PA can only maintain linearity up to a specific input level, beyond which the gain starts to lower, and the PA enters
into a nonlinear or compression region, as shown in Figure 209. Most low-power linear amplifiers operate in the linear region, as shown in the
"LINEAR REGION" circle. The PA that operates mostly in the linear region has lower efficiency. PA efficiency is defined as the ratio of the output
RF power to the DC supply power. Therefore, it is desirable to operate the PA at high efficiency to save DC power and reduce heat dissipation.
To achieve higher PA efficiency, the highest input signal peak is usually set at around 1dB (P1dB) compression region, as shown in the "1dB
COMPRESSION REGION" circle in Figure 209. However, compression of the peak signals produces harmonics and hence intermodulations.
Some intermodulations fall back right into or next to the carrier spectrum, 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 can
degrade, and the spectral regrowth can interfere with adjacent channels, resulting in worse than required adjacent channel power ratio (ACPR)
performance. Digital predistortion (DPD) mitigates this problem.
Figure 209. Ideal Power Amplifier Output vs. Actual Power Amplifier Output

ADRV9001 DPD FUNCTION

The ADRV9001 device provides a fully integrated DPD function that supports both narrowband and wideband applications. It is a hardware/soft-
ware combined solution that linearizes the PA by predistorting the digital transmit signal with the inverse of the PA's nonlinear characteristics.
After amplifying by the PA, the predistortion compensates for the PA's nonlinearity. So, the amplified RF transmit signal becomes linear.
Therefore, the integrated DPD solution allows the PA to operate at very high efficiency while achieving a satisfactory EVM and adjacent channel
power ratio (ACPR) performance.
Figure 210 depicts a high-level block diagram of the DPD algorithm and shows that before the PA, a "Predistortor" block is added in the transmit
datapath, which distorts the transmit signal d(t) with the inverse of the PA's nonlinear characteristics, as shown by the first input/output figure
curve. Spectral regrowth is introduced after the predistortion. However, after the "predistorted" transmit signal x(t) is amplified by the PA, the
PA nonlinear characteristics, as shown by the second input/output figure curve, cancel out the predistortion. Therefore, the PA y(t) output
becomes linear, as shown by the third input/output figure curve. In addition, the spectral regrowth after predistortion is also corrected. The "DPD
Coefficients Computation" block is used to compute the predistortion parameters using the "Predistortor" output signal x(t) as well as the PA
output signal y(t) through a feedback path. It models the behavior of the PA in the reverse direction, i.e., from output to input. Therefore, it
characterizes the inverse of the PA nonlinearity and then feeds the parameters to the "Predistortor."
Figure 210. High Level Block Diagram of DPD Algorithm
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