The evolution and challenges of new forms of multiple-input and multiple-output(MIMO)technologies in the context of the sixth generation of mobile communications system(6G)are explored.Emerging MIMO technologies exhibit trends toward larger array apertures and denser array elements,encompassing forms such as ultra-large-scale MIMO,holographic MIMO,and ultra-massive intelligent metasurfaces.By analyzing these technologies,the study investigates their current applications and advancements in far-field and near-field channel modeling,near-field channel estimation and beamforming schemes.The results demonstrate that as the communication frequency band expands into higher bands such as millimeter waves and terahertz waves,the extension of the Rayleigh distance leads to significantly increased near-field effects,and various new forms of MIMO technologies have shown unique advantages in addressing these challenges.The future research directions for emerging MIMO technologies and their potential contributions to enhancing 6G performance are summarized in the conclusion.
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