## 4 Experimental Validation and Discussion  
### Experimental Setup and Results Analysis  
The experimental setup is shown in Figure 12. A computer was used to upload the base-band input signal to a signal generator (R&S SMBV100A). Subsequently, the base-band I/Q output signals from the signal analyzer (R&S FSQ26) were recorded by the computer. Table 3 gives the cases where all modes are used as the feedback operation modes. The experiment steps were as follows:  
* Generate the input signal and record the output signal under different operation modes. Load an LTE signal with a bandwidth of 10 MHz, \(x(n)\), from the computer to the signal generator. Excite the PA using the modulated signal and record the output, \(y(n)\), from the signal analyzer to the computer. * Calculate the pre-distorted signal by MPM. When testing Group A, generate the pre-distorted signal, \(x_{\text{DPD}}(n)\), by the MPM with a memory depth of 7 and an order of 5 using the recorded \(x(n)\) and \(y(n)\). When testing Group B, generate the pre-distorted signal, \(x_{\text{DPD}}\left(n\right)\), by the MPM with a memory depth of 7 and an order of 3 using the recorded \(x(n)\) and \(y(n)\). * Load the pre-distorted signal \(x_{\text{DPD}}(n)\) from the computer into the signal generator and excite the PA using \(x_{\text{DPD}}(n)\) under different modes. Record the corresponding pre-distorted output signals and calculate PSD, EVM, and ACPR. When testing and verifying Group A, \(V_{\text{DD}}\) was set to 28, 20, and 12 V, and the input power level iwass selected as the 1 dB compression point, which was calculated based on the simulation. The performance metrics of the experimental linearization, i.e., EVM and ACPR, are also listed in Table 3. It can be seen that the experimental results satisfactorily match the simulations. For both groups, the experimental EVM after the CMUDPD reduced by an order of magnitude. The experimental ACPR is lower than \(-45\) dB on average, and an averaged ACPR suppression of 13 dB was achieved. As CMUDPD2 achieved superior  
Figure 11: Simulated AM/AM and AM/PM curves. (a) Group A’s AM/AM curves with (W) and without (W/O) CMUDPD2. (b) Group A’s AM/PM curves W and W/O CMUDPD2. (c) Group B’s AM/AM curves W and W/O CMUDPD2. (d) Group B’s AM/PM curves W and W/O CMUDPD2. performance for both groups in Table 3, the experimental PSDs with CMUDPD2 for Group A and B are demonstrated in Figure 12(a),b, respectively. The pre-distortion performance is consistent with the simulation results. As shown in Figure 12(b), the proposed CMUDPD provides effective linearization performance even under severe out-band distortion conditions. In order to strengthen the usefulness of the proposed CMUDPD, the experiment setup with multiple PAs was built as exhibited in Figure 14. Four identical PAs were used, denominated as PA\({}_{1}\), PA\({}_{2}\), PA\({}_{3}\), and PA\({}_{4}\) from left to right. A ventilation fan was installed on the heatsink to dissipate the heat generated by the PAs. Due to the limitation due to the unequal power divider lacking in the laboratory, it was not possible to accurately feed different input powers to the PA array in a single trial. Nevertheless, an equal power  
Figure 12: Experiment setup. Figure 13: Experimental results of PSDs with (W) and without (W/O) CMUDPD2. (**a**) Group A: mode a1, a2, and a3 W and W/O CMUDPD2. (**b**) Group B: mode b1, b2, and b3 W and W/O CMUDPD2. divider was implemented in the experiment to excite the PAs, and the different input power levels were achieved by multiple trials. First, the output of PA\({}_{1}\) (operating in mode a1) was recorded from the signal analyzer. The pre-distorted signal was calculated by the aforementioned MPM method. Then, the pre-distorted signal was loaded to the signal generator, which in turn excited PA\({}_{2}\), PA\({}_{3}\), and PA\({}_{4}\), which operated in mode a1, a2, and a3, respectively.