# Power Amplifier Predistortion Using Reduced Sampling Rates in the Forward and Feedback Paths
## 4 Experimental Validation
### Device under Test and Experimental Setup
It is only used in this experimental validation to allow the DPD function and the APD function to operate at different sampling rates. This allows for including the effect of reduced sampling rate in the forward path while operating the APD at a higher sampling rate. The re-sampling block will produce a signal sampled at a sufficiently high rate to allow for the presence of all inter-modulation distortions and spectrum regrowth that will be caused by the APD function. The signal at the output of the re-sampling block is applied to the APD function. In here, the APD function is simulated using an AM/AM look-up table. The signal at the output of the APD is saved and downloaded into the vector signal generator. The VSG generates the corresponding RF signal that will be applied to the device under test made of the driver and the power amplifier. This allows for the realistic evaluation of the predistortion system performance while having a power amplifier prototype in the loop. The signal at the output of the power amplifier is fed into the vector signal analyzer to compute the adjacent channel leakage ratio and assess the performance of the predistortion system and its effectiveness in reducing the spectrum regrowth. The measured complex gain characteristics of the device under test are reported in Figures 8 and 9 for the 20 MHz and the 40 MHz test signals, respectively. The device under test here refers to the cascade made of the driver and the power amplifier. The AM/AM characteristic depicted in Figure 8a shows that the power amplifier has a nonlinear behavior with mild memory effects as illustrated in the spread of the data points. Conversely, the AM/PM characteristic of Figure 8b is mainly dominated by the dispersion due to memory effects and does not manifest any significant nonlinearity. The measured gain characteristics corresponding to the 40 MHz test signal show similar nonlinear behavior to what was observed for the 20 MHz test signal but with a noticeably stronger memory effects, as demonstrated by the wide spread of the data points in Figure 9.  
Figure 7: Simplified functional block diagram of the experimental setup. The experimental validation of the proposed predistorter was performed using each of these two test signals. For each signal, the proposed predistorter was derived while operating the forward path's digital-to-analog converter and the feedback path's analog-to-digital converter at consistent sampling rates. These sampling rates were decreased iteratively and the performance of the proposed predistorter was recorded for the different sampling rates. In order to compare the achieved results to state of the art predistorters, the reverse twin-nonlinear two-box (RTNTB) [31] predistorter and the envelope memory polynomial (EMP) [32] predistorters were used as benchmark. The rationale behind the selection of these models for the comparative analysis is mainly attributed to their relative similarity to the proposed predistorter. In fact, the reverse twin-nonlinear two-box model is a two-box predistorter that uses a memory polynomial function followed by a look-up table. The two differences between the RTNTB predistorter and the proposed hybrid predistorter are related to the type of look-up table sub-function. First, in the reverse twin-nonlinear two-box predistorter, the look-up table is complex-valued and compensates for both the memoryless AM/AM distortions and the memoryless AM/PM distortions. Conversely, in the proposed predistorter, the look-up table sub-function is real-valued and compensates only for the memoryless AM/AM distortions. Second, in the RTNTB model, the look  
Figure 8: Measured instantaneous gain characteristic of the device under test when driven by a 5G test signal with 20 MHz bandwidth: (a) AM/AM characteristic; (b) AM/PM characteristic. Figure 9: Measured instantaneous gain characteristic of the device under test when driven by a 5G test signal with 40 MHz bandwidth: (a) AM/AM characteristic; (b) AM/PM characteristic.