Wideband Precoding for U6G XL-MIMO Systems: Beam Squint Boundaries and Channel Slicing

The unconventionally large aperture of extremely large-scale multiple-input multiple-output (XL-MIMO) arrays, in conjunction with the wider bandwidths in the upper-6 GHz (U6G) frequency bands, will very likely lead to non-negligible beam squint effects. In the context of a limited number of radio frequency (RF) chains, and by adopting hybrid precoding, the beams at different subcarriers may point to different positions and compromise the spectral efficiency (SE). Moreover, the existence of \textit{multiple paths} in U6G XL-MIMO channels also entails practical challenges for wideband precoding. It is therefore essential to ascertain whether the beam squint effect is pronounced for U6G XL-MIMO systems and design efficient wideband precoding schemes. To address these challenges, precise antenna-domain and frequency-domain wideband boundaries are derived from the near-field and far-field perspectives, respectively. These boundaries can inform the design of wideband precoding in future system settings. Subsequently, a channel slicing scheme is proposed for wideband precoding. The process involves the segmentation of U6G XL-MIMO channels into multiple blocks, with the objective of mitigating the beam squint effect for each path. The antenna-domain and frequency-domain slicing methods are developed for multipath and multiuser scenarios, respectively. The simulation results prove that the beam squint effect remains a significant issue for U6G XL-MIMO systems, while the near-field effect invariably precedes the beam squint effect as the array size and the bandwidth increase. In addition, the proposed scheme can greatly improve the SE.