# Power Amplifier Predistortion Using Reduced Sampling Rates in the Forward and Feedback Paths
## 4 Experimental Validation
### Device under Test and Experimental Setup
up table sub-function is implemented digitally while in the proposed predistorter the look-up table sub-function is intended for analog implementation. The envelope memory polynomial is selected as a second benchmark DPD since it allows for the compensation of memory effects while being suitable for analog implementation. Hence, the twin-nonlinear two-box model represents a best-case scenario in terms of performance of a fully digital predistorter, and the envelope memory polynomial model represents a best-case scenario in terms of performance of a potential analog predistortion function that can compensate for memory effects. To assess the performance of the proposed predistorter architecture, Figure 10 presents the spectra measured at the output of the power amplifier when linearized using the proposed predistorter, the benchmark digital predistorter (using the RTNTB structure), and the analog predistorter (using a complex-valued look-up table structure). As can be seen from this figure, the analog predistorter performance is quite limited. This is mainly due to its inability to compensate for the memory effects of the device under test. Furthermore, the performance of the proposed predistorter is identical to that of the RTNTB benchmark. Table 2 includes the measured ACLR at the output of the device under test for the test conditions reported in Figure 10. In this table, and throughout the paper, ACLR_L refers to the lower channel(s) ACLR (corresponding to a negative offset frequency) while ACLR_U refers to the upper channel(s) ACLR (corresponding to a positive offset frequency). These ACLR values are consistent with the results observed in the frequency domain and show that the proposed predistorter consistently outperforms the benchmark DPD by a slight margin. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **ACLR** & & \begin{tabular}{c} **No** \\ **Linear** \\ \end{tabular} & \begin{tabular}{c} **APD** \\ **Linearizer** \\ \end{tabular} & \
\begin{tabular}{c} **DPD** \\ **Linearizer** \\ \end{tabular} &
\begin{tabular}{c} **Proposed** \\ **Linearizer** \\ \end{tabular} \\ \hline \multirow{3}{*}{20 MHz} & Adjacent Channel & ACLR\_L & \(-32.5\) & \(-44.0\) & \(-55.4\) & \(-56.2\) \\  & ACLR\_U & \(-32.5\) & \(-46.8\) & \(-55.1\) & \(-56.1\) \\ \cline{2-6}  & Alternate Adjacent Channel & ACLR\_L & \(-51.7\) & \(-59.4\) & \(-59.4\) & \(-60.3\) \\  & ACLR\_U & \(-51.2\) & \(-58.8\) & \(-57.8\) & \(-59.2\) \\ \hline \multirow{3}{*}{40 MHz} & Adjacent Channel & ACLR\_L & \(-32.3\) & \(-43.2\) & \(-49.5\) & \(-50.7\) \\  & ACLR\_U & \(-31.9\) & \(-46.7\) & \(-50.2\) & \(-51.6\) \\ \cline{2-6}  & Alternate Adjacent Channel & ACLR\_L & \(-51.6\) & \(-54.3\) & \(-53.5\) & \(-54.3\) \\  & ACLR\_U & \(-50.9\) & \(-53.2\) & \(-52.7\) & \(-51.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of ACLR performance of the proposed predistorter, an APD and a DPD for the 20 MHz and the 40 MHz test signals. Figure 10: Measured spectra at the output of the device under test when linearized using the proposed predistorter, a digital predistorter, and an analog predistorter: (**a**) Case of the 20 MHz test signal; (**b**) Case of the 40 MHz test signal. The output power versus input power characteristics of the device under test and that of the cascade made of the device under test and the predistorter are reported in Figure 11 for the case where an APD is used and the case where the proposed predistorter is used. These curves clearly show the ability of the proposed predistorter to linearize the transfer characteristic of the device under test. For the case of the analog predistorter, a similar improvement in the linearity of the output power versus input power characteristics is obtained. In order to compare the linearization performance of the proposed predistorter with respect to that of the APD, the output power versus input characteristic of the case made of the linearizer and the DUT is reported in Figure 12 for the case of the APD and that of the proposed DPD.