# Power Amplifier Predistortion Using Reduced Sampling Rates in the Forward and Feedback Paths
## 4 Experimental Validation
### Device under Test and Experimental Setup  
Figure 6 shows a photograph of the experimental setup. The MS2530A test instrument, from Anritsu, Kanagawa, Japan is used as a vector signal generator (VSG) and a vector signal analyzer (VSA). The baseband digital waveform is synthesized in a computer where all digital signal processing is performed. The baseband waveform is downloaded into the vector signal generator and the corresponding RF signal is produced. The RF signal is applied to the driver (ZHL 42 from Mini-Circuits) in order to increase its power to be able to drive the power amplifier into its nonlinear region. The signal at the output of the power amplifier is attenuated and then fed into the VSA. The signal down-conversion and digital demodulation are performed within the VSA, and the resulting complex baseband waveform is then saved into the computer for further processing (time-delay estimation and alignment, and predistortion functions identification). The test instrument used in this work has a maximum sampling rate of 120 MSa/s in the signal generator part, and 200 MSa/s in the signal analysis part. These correspond to a maximum vector modulation bandwidth of 120 MHz, and a maximum signal analysis bandwidth of 125 MHz. This sets an upper limit on the bandwidth of the test signals that can be used for experimental validation. The power amplifier used in this work is a 10 W class AB commercial gallium nitride (GaN) power amplifier (model CGH40010F-AMP from Wolfspeed, Durham, NC, USA). The amplifier is designed to operate in the sub-6 GHz frequency band. During the experiments, the amplifier was operated around a carrier frequency of 2425 MHz using two 5G test signals. The two test signals used in this work were both generated according to the 5G new radio (NR) standard. The first signal has a bandwidth of 20 MHz, while the second one has a bandwidth of 40 MHz. The peak to average power ratios of these signals are 10.7 dB and 10.9 dB, respectively. During the tests, including characterization and performance assessment of the proposed predistorter, the power levels were set such that the peak power at the output of the power amplifier is 42 dBm, which corresponds to the saturation power of the CHG40010F-AMP device from Wolfspeed, Duham, NC, USA. A functional block diagram of the experimental setup is presented in Figure 7. For the experimental validation performed in this work, the DPD and APD functions were implemented in MATLAB Software. During the first step of the process (referred to as APD loop in Figure 5), in which the APD function is identified, the DPD function, the re-sampling block, and the APD are bypassed. The input signal waveform is generated at the reduced sampling rate specified in the test case, as will be described in the next subsection. The output of the power amplifier measured under this condition is also acquired at the reduced sampling rate and is used along with the input signal to identify the APD function. In the second step, referred to as DPD loop in the flow chart of Figure 5, the DPD function is bypassed, and the input signal waveform generated at the reduced sampling rate is fed into the re-sampling block, which will increase the sampling rate to its highest value. This signal is then applied into the APD and downloaded into the vector signal generator. The corresponding RF signal is applied to the device under test, and the partially linearized output is acquired at the reduced sampling rate. The input signal waveform and the VSA signal waveform are then used in the predistortion functions identification block to derive the DPD function. Figure 6: Photograph of the experimental setup. Figure 7 shows that the input signal waveform is first fed into the DPD function. A re-sampling block is placed at the output of the DPD function. It is important to note that the signal re-sampling block is not needed for the implementation of the proposed predistorter.