The memoryless lookup table (LUT) model, which is used for linearization of the Doherty amplifier, as well as all required calculations for the LUT are realized in MATLAB. The used signal, the modulator and the demodulator are realized with basic MATLAB and LabVIEW functions. To control all parameters of the signal chain is important in order to be able to evaluated the gain curve (AM-AM), the phase (AM-PM), the adjacent channel leakage ratio (ACLR) and as well as the EVM with and without DPD.
The gain curve (AM-AM) (including the gain of the preamplifier) and the phase (AM-PM) with and without DPD over the back-off range are presented in Fig. 13 and Fig. 14. These results show improved gain and phase linearity with the memoryless DPD. The 64-QAM constellation (see Fig. 15) shows a reduction of the EVM from 8.8 % without DPD to 2.0 % with the memoryless DPD. Furthermore, the power spectral density (PSD) is presented in Fig. 16 and shows a reduction of the ACLR. Summarizing, the used memoryless DPD linearizes the Doherty amplifier very well for intended application of transmitting a 20 MHz 64-QAM-OFDM signal and for further modulation schemes.
