Channel Capacity Saturation Point and Beamforming Acceleration for Near-Field XL-MIMO Multiuser Communications

One of the most important technologies in the fifth generation (5G) and the sixth generation (6G) is massive multiple input multiple outputs (MIMO) or extremely large-scale MIMO (XL-MIMO). With the evolving high-frequency technologies in millimeter band or teraHz band, the communication scene is changing into near-field rather than the conventional far-field scenario. In this letter, instead of advertising the XL-MIMO in the near-field, we appeal that a limit should be set on the size of the antenna array, beyond which the channel capacity will not show a significant increase. We show capacity saturation point can be analytically determined. Moreover, we propose a new beamforming algorithm that relieves the heavy computation due to the large antenna size even around the saturation point. Numerical results are provided to validate our analysis and show the performance of our newly proposed beamforming scheme.