RIS-Assisted XL-MIMO for Coexistence of Near-Field and Far-Field Communications

Extremely large-scale multiple-input multiple-output (XL-MIMO) is a transformative technology to achieve spectral-efficient and energy-saving wireless communication. However, XL-MIMO leads to the near-field propagation becoming dominant. In this paper, we examine a reconfigurable intelligent surface (RIS)-assisted XL-MIMO communication system to serve two distinct groups of users, namely near-field users (NFUEs), directly served by the XL-MIMO, and far-field users (FFUEs), served with the assistance of a RIS. We derive the signal-to-interference-plus-noise ratio (SINR) expressions for whole-array-based precoders, including maximum-ratio transmission (MRT), and zero-forcing (ZF). Moreover, we take into account the spatially, non-stationary channel characteristics, indicating that user terminals may only have visibility of a specific portion of the array, referred to as the visibility region (VR). To further leverage the VR for complexity reduction, we propose a heuristic algorithm designed to determine the VR, while also guaranteeing the individual SINR requirements for both NFUEs and FFUEs. Simulation results indicate that utilizing VR with our proposed heuristic algorithm yields performance comparable to a benchmark utilizing the whole-array, albeit with a notable reduction in the number of antennas and computational complexity.