# Reduced Complexity Sequential Digital Predistortion Technique for 5G Applications
## 3 Performance Assessment of the Proposed Digital Predistortion System  
For the experimental validation of the proposed DPD, an inverse Class-F power amplifier operating around 900 MHz was used. The functional block diagram of the experimental setup is reported in Figure 6. The DPD signal processing unit includes the functionalities related to time delay alignment, DPD function identification and predistortion of the input signal. The baseband digital waveform at the output of the DPD signal processing unit is fed into the vector signal generator (VSG) which implements the digital to analog conversion and the frequency up-conversion. The signal at the output of the VSG is pre-amplified using a driver before being applied at the input of the main power amplifier. The signal at the output of the power amplifier is first attenuated and then fed into the vector  
Figure 5: Proposed sequential DPD system. signal analyzer (VSA). The VSA performs the frequency down-conversion, the analog to digital conversion, and the demodulation of the signal. The resulting digital waveform corresponding to the PA's output signal is used by the DPD signal processing block to synthesize the predistortion function. A photograph of the experimental setup is presented in Figure 7.  
The PA was driven by a test signal having a bandwidth of 20 MHz. This bandwidth is largest that can be handled by the available experimental setup. The output signal was acquired at various sampling rates using the vector signal analyzer. The sampling rates used were 100 MSa/s, 50 MSa/s, 40 MSa/s, 30 MSa/s and 25 MSa/s. The in-phase and quadrature components of the baseband input signal \((I_{int},Q_{int})\) and the in-phase and quadrature components of the baseband output signal \((I_{out},Q_{out})\) were used to compute the instantaneous complex gain of the device under test and derive its AM/AM (amplitude-modulation to amplitude-modulation) and AM/PM (amplitude-modulation to phase-modulation) characteristics. The AM/AM and AM/PM characteristics represent the magnitude and the phase of the complex instantaneous gain, respectively. The measured AM/AM and AM/PM characteristics of the device under test derived from the 100 MSa/s sampling rate data acquisition are presented in Figure 8.  
Then, the five acquisitions of the DUT's output signal were performed using sampling rates of 100 MSa/s, 50 MSa/s, 40 MSa/s, 30 MSa/s and 25 MSa/s. In these tests, since the input waveform has a sampling rate of 100 MSa/s, acquisitions performed at lower sampling rates required the use of a resampling step in order to up-sample the acquired output signal to the same 100 MSa/s rate as the input signal. This resampling step is accomplished before performing the time delay estimation and alignment as well as the pre-distortion function synthesis. From each of the five acquired output signals, a memoryless look-up table was built. All predistorters were derived from the measured input and output  
Figure 6: Functional block diagram of the experimental setup. Figure 7: Photograph of the experimental setup. data using the same exponentially weighted moving average algorithm for the LUT DPD synthesis. These LUT-based DPDs were then used to linearize the power amplifier when driven by the 20 MHz input signal. The spectra obtained at the output of the linearized power amplifier are reported in Figure 9 for all the memoryless predistorters along with the output spectra without linearization. This figure shows that the LUT derived from the measured output signal at 100 MSa/s leads to the best linearization performance. However, residual distortions can be seen in this spectrum. This is expected since the DUT exhibits memory effects, and therefore a memoryless DPD will not be able to fully compensate for the DUT's distortions. Moreover, considering the spectra (of Figure 9) corresponding to DPDs derived from lower sampling rates, additional performance degradation is observed. This performance deterioration is more pronounced as the sampling rate decreases.