Abstract: The advancement of multiple-input multiple-output (MIMO) technologies results in larger array apertures and more densely packed array elements. This progress has spurred the development of novel technologies like extremely large-scale MIMO, holographic MIMO, and extremely large-scale reconfigurable intelligent surfaces, collectively known as the new-modal MIMO. As the sixth generation of the mobile communi-cation system (6G) moves towards a higher frequency spectrum, including millimeter waves and terahertz, the double evolutions of new-modal MIMO substantially increase the Rayleigh distance, resulting in nonnegligible considerations of near-field communications. This article investigates far-field and near-field channel modeling, estimation, and beamforming techniques tailored for different paradigms of new-modal MIMO, highlighting their technical details and new characteristics. Finally, prospective research directions are discussed for new-modal MIMO, demonstrating its beneficial potential to enhance 6G mobile com-munication systems.

Key words: New-modal MIMO, channel modeling, channel estimation, beamforming