On the Digital Predistortion of Wideband mmWave Communication Systems With Beam Squint

Large-scale antenna arrays in millimeter wave (mmWave) systems are the cornerstone for next-generation Internet of Things (IoT) infrastructure. However, wideband mmWave systems suffer from frequency-dependent channel responses, known as beam squint. With beam squint effect, the far-field over-the-air (OTA) signal may exhibit frequency selective fading, rendering ineffective digital predistortion (DPD) design based on an OTA feedback structure. Therefore, existing mmWave DPD feedback architectures need to be examined carefully. In this article, we analyze the impact of beam squint on DPD and propose a proper DPD feedback architecture. To the best of our knowledge, this is the first work to address DPD in wideband mmWave systems with beam squint. Our results indicate that, with beam squint, the nonlinearity observed at the far-field OTA side differs from that at the power amplifier (PA) output side. We demonstrate that the far-field OTA received signal is no longer suitable as the feedback signal for DPD estimation. Moreover, we propose an effective DPD scheme for mmWave systems with beam squint. In this scheme, analog beamforming coefficients are fixed for DPD identification. Compared to existing solutions, hardware complexity of the proposed DPD scheme is much reduced. Numerical results validate the effectiveness of the proposed DPD scheme.