Characterizations of Millimeter-Wave Reconfigurable Intelligent Surfaces in the Near-Field Region

Reconfigurable intelligent surfaces (RISs) have been envisaged as a key and promising technology for evolved millimeter-wave 5G and upcoming 6G applications to enhance wireless communication channel capacity and quality. RISs are, in general, implemented with an electrically large size that is much larger than the wavelength at the frequency of interest. In some specific indoor application scenarios, such as intelligent factories, intelligent offices, intelligent shopping malls, etc., the users or targets are usually located at the near-field regions of the RISs. In these application scenarios, using RISs to perform near-field communications rather than conventional far-field communications might bring significant benefits. This paper will demonstrate our findings on the characterizations of millimeter-wave reconfigurable intelligent surfaces in the near-field regions, where it is found and concluded that using a near-field focusing RIS can indeed gain some benefits for wireless communications. Some figures of the metric of benefit distance and near-field gain are defined to quantitatively evaluate the benefits gained by the near-field focusing RIS. The findings will provide system-level reference and thumb of the rule of RISs for some indoor application scenarios, where the users are in the near-field region of the RIS.