Near-Field RIS-Assisted Communications Considering Practical Loss Factors

Near-field (NF) communications with reconfigurable intelligent surfaces (RISs) are one of the promising technologies in next-generation communication systems. Different from farfield communications, each element of the RIS in the NF communication has its own unique features. Accordingly, different elements have different practical impairment factors. Given this background, this paper investigates phase-dependent amplitude variations (PAVs) and polarization losses (PLs) in the NF RIS-aided system. In particular, we first introduce an RIS-aided NF communication model, by using Friis transmission formula. Then, the PAV and the PL are respectively defined as an empirical equation and a position-related expression. Lastly, we propose an integral form spectral efficiency by leveraging Holders inequality.¨ Numerical evaluations show the correctness of the analytical results. The insights of this paper are 1) although the amplitude of each element is different, we can use the expected amplitude to approximate the PAV impact if the coordinates of the RIS and the transceiver are known, 2) the PL factor becomes more serious when the RIS approaches the transceiver, and it can be mitigated through adjusting the height of the RIS, and 3) Holders inequality provides a tight integral form spectral¨ efficiency expression especially when each element is with its own unique reflection coefficient in the NF case.