Preliminary Technical Data UG-1828
Rev. PrC | Page 197 of 338
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 PA can only maintain linearity up to a certain input level beyond which the gain
starts to lower and the PA enters into a nonlinear or compression region as shown in Figure 182. For most low-power linear amplifiers,
they operate in the linear region as shown in the “LINEAR REGION” circle. Unfortunately, a PA that operates mostly in the linear region
has lower efficiency. PA efficiency is defined as the ratio of output RF power to the DC supply power. Therefore, it is desirable to operate
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 182. 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.
Figure 182. 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 PA by pre-distorting the digital transmit
signal with the inverse of the PA’s nonlinear characteristics. After amplifying by the PA, the pre-distortion compensates PA’s nonlinearity
so the amplified RF transmit signal becomes linear. Therefore, the integrated DPD solution allows PA to operate at very high efficiency
while achieving a satisfactory EVM and ACPR performance.
Figure 183 depicts a high level block diagram of the DPD algorithm. As shown in this figure, 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 pre-distortion. However, after the “pre-distorted” transmit
signal x(t) being amplified by the PA, the PA nonlinear characteristics, as shown by the second Input/Output figure curve cancels out the
pre-distortion. Therefore, the output of the PA 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 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”.
PA OUTPUT
1dB
COMPRESSION
REGION
PA INPUT
LINEAR REGION
IDEAL PA OUTPUT
ACTUAL PA OUTPUT
1dB
24159-140
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