# Power Amplifier Predistortion Using Reduced Sampling Rates in the Forward and Feedback Paths
## 4 Experimental Validation
### Case II--Validation Using 40 MHz Test Signal  
The same tests performed in the case of the 20 MHz test signal were repeated for the 40 MHz test signal. First, the proposed predistorter was derived at various sampling rates and then used to linearize the power amplifier driven by the 40 MHz 5G signal. The two benchmark predistorters were also applied to linearize the power amplifier operating under these conditions. The spectra measured at the output of the linearized power amplifier are reported in Figure 16. This figure also includes the spectrum measured at the output of the power amplifier before linearization. The results reported in this figure correspond to predistorters where the polynomial function has 15 coefficients corresponding to a nonlinearity order of 5 and a memory depth of 2. First, the results of Figure 16 show that the EMP predistorter fails to enhance the linearity of the power amplifier and achieves very limited improvement. This is interesting since this model is the only potential candidate for analog predistortion with memory effects compensation. Moreover, this figure shows that the performance of the proposed predistorter is similar to that of the RTNTB benchmark for sampling rate reductions of up to 30% in both the forward and feedback paths. In the experimental validation, it was observed that a further reduction in the sampling rate of the forward and feedback paths results in performance degradation for the proposed predistorter. To further investigate this, the spectra of the signals at the input of the forward path's digital-to-analog converter were derived and are reported in Figure 17. This figure shows that the use of the proposed predistorter leads to a significant reduction in the spectrum regrowth of the signal fed into the digital-to-analog converter. Moreover, comparing the spectra in this figure to its equivalent for the 20 MHz signal (Figure 14), it appears that the spectrum regrowth in the case of the 40 MHz signal is more pronounced with a significant imbalance. This can be  
Figure 15: Measured adjacent channel leakage ratio at the output of the linearized amplifier as a function of the number of coefficients in the memory polynomial function for the benchmark digital predistorters and the proposed predistorter derived and implemented using various reduced sampling rates (Case I: 20 MHz test signal): (**a**) ACLR in the adjacent channel; (**b**) ACLR in the alternate adjacent channel. attributed to the stronger memory effects observed when the power amplifier is driven by the second test signal having wider bandwidth. This pronounced spectrum regrowth unavoidably impacts the maximum reduction in the sampling rate, which can be used while maintaining satisfactory linearization capability. The results of the ACLR measured at the output of the linearized amplifier while using various predistortion function settings are reported in Figure 18. These measurements were performed for the different number of coefficients in the memory polynomial sub-function of the predistorter. These results show that the EMP-based predistorter performance is consistently a few dBs worse than the other predistorters independently of the number of coefficients. Moreover, the performance of the proposed predistorter is comparable to that of the RTNTB benchmark. Most importantly, even when the sampling rate in the forward and feedback paths was reduced from 153.60 MSa/s to 107.52 MSa/s, the performance  
Figure 16: Measured spectra at the output of the linearized amplifier using benchmark digital pre-distorters and the proposed predistorter derived and implemented using various reduced sampling rates (Case II: 40 MHz test signal). Figure 17: Measured spectra at the input of the digital-to-analog converter in the signal forward path for the benchmark digital predistorters and the proposed predistorter derived and implemented using various reduced sampling rates (Case II: 40 MHz test signal). of the proposed predistorter was unaffected and remained within 2 dBs from that of the RTNTB benchmark.