The function implemented by the predistortion system is shown in Fig. 2-34. The target of the DPD predistorter is to implement the inverse function of the RF PA (considering (non-)linearity, memory effects, etc.), targeting improved PA efficiency as well as spectral compliance (such as ACLR). The DPD subsystem is implemented through two software components: 1. The actuator-this component applies the DPD adaptation weights in the runtime system path of the system and hence performance critical. 2. The DPD adaptation algorithm-this component calculates the DPD adaptation weights and typically runs in a nontime-critical subsystem (e.g., on host ARM cores). DPD is the block which implements a complex nonlinear function that approximates the inverse transfer function G = H-1 of the PA baseband transfer function H. DPD implemen- tations can be either memoryless or memory-based. Memoryless implementations assume the output to depend only on the current input as opposed to memory-based systems, where previous signal levels (last sample, the sample before that, etc.) are also taken into consider- ation. DPD algorithms look at both AM/AM distortion which defines the amplitude of the output signal as a function of the amplitude of the input signal as well as AM/PM distortion where the phase of the output signal is a function of the amplitude of the input signal. Implementation of the predistortion algorithm can be done by Lookup Tables (LUTs) and/or by a mathematical representation such as Volterra series. 82 Open Radio Access Network (O-RAN) Systems Architecture and Design Consumed Power (DC) as Function of PA Output Power at Different Efficiency points 125mW 250mW Output power at antenna (dBm) FIGURE 2-33 DPD justification for power efficiency. DPD, digital predistortion. Predistorter Power Amplifier FIGURE 2-34 Digital predistortion concept. Both implementations rely on characterization of the PA to establish the values that are used in the LUTs or Volterra series. This characterization can be done offline through mea- s