High-Order Modulation Transmission Through Frequency Quadrupler Using Digital Predistortion

We demonstrate the operation of a frequency quadrupler as a transmitter to accurately generate amplitude-and phase-modulated signals. Digital predistortion (DPD) is used to facilitate reconstruction of the desired transmitted waveforms at the frequency quadrupler output. Such a process enables novel architectures for higher frequency transmissions without high-frequency mixers or modulators, especially for millimeter-wave (mm-wave) communications, at frequencies where signal amplification, modulation, and distribution are challenging. To investigate the frequency quadrupler input–output relationship and the signal characteristics, a prototype frequency quadrupling system at 3.56-GHz band is demonstrated. A quadrupled memory polynomial (Q-MP) model is derived for forward modeling. Subsequently, a quadrupling DPD (Q-DPD) model is developed to find the inverse of the frequency quadrupler for linearization. Further, a memory polynomial (MP) and Q-DPD cascaded technique is proposed to achieve more accurate linearization performance of the frequency quadrupler. The proposed methods are demonstrated using signals with different bandwidths (5, 10, 20 MHz) and different order modulations [64 and 256 quadrature amplitude modulation (QAM)]. Very good adjacent channel power ratios (ACPRs) of around −43 dBc and error vector magnitudes (EVMs) of about ${{1.6}}\%\sim {{2.4}}\%$ are achieved in different tests.