# Power Amplifier Predistortion Using Reduced Sampling Rates in the Forward and Feedback Paths
## 4 Experimental Validation
### Case I--Validation Using 20 MHz Test Signal  
In the first test, the 5G signal having a 20 MHz bandwidth was used to assess the performances of the proposed predistorter as well as the two benchmark predistorters. Initially, the digital-to-analog converter in the forward path and the analog-to-digital converter in the feedback path were operated at a sampling rate of 122.88 MSa/s. The device under test was first linearized using the proposed hybrid predistorter as well as the RTNTB predistorter and the EMP predistorter using the reference sampling rate of 122.88 MSa/s. Then, the sampling rates used in the forward and feedback paths were reduced successively to 61.44 MSa/s and 49.15 MSa/s. These correspond to a reduction of 50% and 60% when compared to the reference sampling rate, respectively. The spectra measured at the output of the power amplifier using these five predistorters as well as the spectra measured before linearization are reported in Figure 13. This figure shows that before linearization, the power amplifier had significant distortions. These distortions are reduced by approximately 20 dB when the RTNTB predistorter is used to linearize the amplifier. Similar results are obtained with the proposed predistorter when a sampling rate of 122.88 MSa/s is used in the forward and feedback paths. Conversely, the EMP-based predistorter, even though derived from 122.88 MSa/s sampling rate, leads to noticeable residual nonlinear distortions in the adjacent channels, hence, clearly depicting the limits of this predistorter. Figure 13 also shows the spectra measured at the output of the power amplifier when the proposed predistorter is derived and applied using the 61.44 MSa/s and 49.15 MSa/s sampling rates. These results show that reducing the sampling rate in the forward and the feedback paths by 50% from 122.88 MSa/s to 61.44 MSa/s does not have any noticeable effect on the proposed predistorter's performance. However, a further reduction in the sampling rate to 49.15 MSa/s clearly impacts the performance of the predistorter and its ability to reduce spectrum regrowth. The results reported in Figure 13 were derived for the case where the memory polynomial function in the RTNTB predistorter as well as the proposed predistorter has a total of 15 coefficients (that is three branches with a fifth order nonlinearity in each). Here, it is important to note that the coefficients of the memory polynomial function in these two DPDs are identified independently. Moreover, the EMP results reported are also for the same nonlinearity order and memory depth. To illustrate the advantage of the proposed predistorter in relaxing the sampling rate of the digital-to-analog converter in the signal forward path, the signals expected at the  
Figure 13: Measured spectra at the output of the linearized amplifier using benchmark digital predistorters and the proposed predistorter derived and implemented using various reduced sampling rates (Case I: 20 MHz test signal). input of the DAC of the forward path in each of the three considered predistorters were observed. Figure 14 includes the spectra of the signals at the input of the DAC used in the forward path for the two benchmark predistorters as well as the three versions of the proposed predistorter (derived using sampling rates of 122.88 MSa/s, 61.44 MSa/s, and 49.15 MSa/s). The results shown in this figure clearly demonstrate that for the proposed predistorter, the signals present at the input of the DAC have much narrower bandwidth and therefore can be accommodated with reduced sampling rates without significant loss of performance. It also shows that for the proposed predistorter versions corresponding to sampling rates of 122.88 MSa/s and 61.44 MSa/s, the signals are almost identical.