Hybrid Near-Field and Far-field XL-MIMO: Channel Characterization, Channel Estimation, and Future Directions

Extremely large-scale multiple-input multiple- output (XL-MIMO) systems are expected to be one of the key enablers in the forthcoming sixth generation (6G) wireless networks, where numerous antenna elements are integrated into a compact space to provide high beamforming gain. The enlarged aperture of the antenna array, however, also leads to an expanded near-field region. Therefore, users and scatterers are distributed in both the near-field and far-field of the antenna array, thus leading to hybrid near-field and far-field communications. In this article, we propose a channel estimation scheme for hybrid near-field and far-field XL-MIMO communication systems. Starting from the characterization of hybrid-field channels, we present a channel estimation scheme that takes into account power diffusion effects in hybrid near-field and far-field communications. To demonstrate the effectiveness and feasibility of the proposed algorithm, we then conduct numerical comparisons with conventional methods and explore its potential applications in real-world scenarios. Finally, future research directions on hybrid-field XL-MIMO communications are discussed.