Low-Complexity Codebook Design for Near-Field Wideband Precoding and Its Experimental Validation

Massive multiple input multiple output (MIMO) and millimeter-wave (mmWave) technologies leverage abundant spectrum and large antenna arrays with beamforming capabilities to overcome high propagation loss in sparse channels and enable high data rate wireless communications. However, the increased antenna aperture shifts electromagnetic propagation from conventional far-field into the near-field region, leading to performance degradation of traditional codebooks based on the far-field plane wave assumption. Meanwhile, wideband transmission in mmWave systems introduces spatial-wideband effect that degrades the performance of traditional narrowband-based codebooks. In this communication, we propose a novel distance–frequency-invariant codebook, using mode excitation techniques to compensate for both near field and wideband effects. For experimental validation, we conducted measurement campaign in a hall with 20 users using a mmWave massive MIMO system. The results show that near-optimal rates can be achieved through beam training with low training overhead and no additional hardware cost. The proposed beam training framework can be directly implemented in mmWave massive MIMO systems to enhance beam management performance.