Beam Defocus Mitigation in Ultra-Wideband Mmwave UCA Systems Based on Frequency Invariant Precoder Design

Artikel i vetenskaplig tidskrift, 2025

Hybrid precoding, as a compromise between low power yet less flexible analog precoding and power hungry but fully flexible digital precoding solutions, has attracted huge attention from industry and academia in recent years for milimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) systems. The adoption of uniform circular arrays (UCA) can provide advantages of a wide angle scanning range and flat array gain, hence considered for massive MIMO hybrid precoding. However, the phase shifters used in hybrid precoder architectures are typically narrowband, which do not support the ultra- wideband bandwidth required in mmWave systems. This limitation can significantly degrade the performance of UCA in ultra- wideband communications, leading to well-known beam defocus effect. In this paper, we first reveal the beam defocus effects in ultra- wideband mmWave UCA precoding systems, where the beamforming gain severely degrades as the frequency deviates from the center. We then propose a novel frequency-invariant precoder to mitigate beam defocus effect in the hybrid precoding architectures, which approximately achieve the same high beamforming gain for the whole wide bandwidth without extra hardware cost. Finally, numerical simulation and outdoors experimental validations with same parameter settings are performed. Both simulation and measurement results are provided to demonstrate the effectiveness and robustness of the proposed frequency-invariant precoding algorithm.