Channel Capacity Saturation Point and Beamforming Acceleration for Near-Field XL-MIMO Multiuser Communications
Xiangyu Cui, Ki-Hong Park, Mohamed-Slim Alouini
TL;DR
The work tackles capacity saturation in near-field XL-MIMO multiuser uplinks by deriving a closed-form capacity-saturation framework for a LOS PNUSW channel and identifying power-saturation points that bound useful antenna sizes. It proves that ergodic saturation points converge to the rectangle corners of the user distribution, and introduces a computationally light beamforming approach (modified ZF) that substitutes expensive inner products with closed-form estimates of diagonal and off-diagonal terms, preserving performance near and beyond saturation. The combination of analytic saturation thresholds, ergodic insights, and a fast beamforming method offers practical guidelines for cost-efficient XL-MIMO deployments at high frequencies. Together, these results enable reliable near-field XL-MIMO operation with manageable hardware and processing requirements while maintaining high spectral efficiency.
Abstract
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 tereHz 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 relieve 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.