RIS Deployment in Near-Field Communications

Near-field communications (NFC) have gained significant attention with the utilization of extremely large-scale antenna arrays (ELAA) and ultra-high frequency resources, which result in a substantial extension of the Rayleigh distance. It is worth noting that the reconfigurable intelligent surface (RIS) is envisioned to be deployed near the transmitter or receiver to alleviate the double fading from cascaded channels, naturally leading it to be within the NFC range whether from transmitter to RIS or RIS to receiver. However, given the introduced antenna projection aperture from near-field spherical wave transmission, are the RIS deployment strategies investigated in farfield communications (FFC) still effective in NFC? To answer this question, we refine and detail RIS deployment strategies for the NFC and clarify how these strategies differ from FFC across passive RIS (PRIS), active RIS (ARIS), and hybrid RIS (HRIS). Furthermore, the overall power constraint is taken into consideration, due to the significantly increasing amplifying power with the rapidly growing antenna and RIS reflecting element numbers. Based on a rigorous analysis, we derive optimal near-field RIS deployment guidelines as functions of the antenna number, the number of reflecting elements, the available amplifying power, and the path-loss fading. Following the above conclusions, open issues are presented for future practical deployments, such as complexity, cross-field transmission, and the challenges of broadband communications with near-field RIS.