On Phase Mode Selection in the Frequency-Invariant Beamformer for Near-Field mmWave Channel Characterization

The large-scale antenna array and wide system bandwidth are envisioned to be widely utilized in millimeter-wave (mmWave) communication. The resulting wideband near-field conditions challenge channel characterization, since most traditional algorithms were proposed under narrowband far-field assumptions and hence fail to work properly. The frequency-invariant beamformer (FIBF) based on the uniform circular array (UCA) is promising for wideband 3-D near-field mmWave channel characterization, due to its capability to achieve the invariant beam pattern with respect to the frequency, elevation angle, and distance. However, this work finds out that the FIBF with existing phase mode selection would suffer from performance deterioration in practical 3-D near-field scenarios, which is overlooked in the literature. Suitable guidance for the generic application of the FIBF is still missing. In this work, we first demonstrate how the FIBF performance is deteriorated by improper phase mode selection through both simulation and theoretical derivation. Then, a novel and unified phase mode-selection guideline is proposed to ensure the robustness of the FIBF in various mmWave scenarios, that is, narrow/wide-band 2-D/3-D far/near-field scenarios, through theoretical analysis and a two-stage brute-force method. Finally, a numerical simulation and three wideband channel measurements with different parameter settings, that is, frequencies (28 and 15 GHz) and UCA radii (0.5 and 0.24 m), are performed to demonstrate the effectiveness and generality of the guideline, where excellent estimation results are achieved. Our proposed phase mode-selection guidance will be valuable for UCA-based FIBF design for accurate and robust mmWave channel characterization.