RIS-Assisted Communications: A Comprehensive Study for Far- and Near-Field Scenarios

The emerging technology of reconfigurable intelligent surface (RIS) equipped with numerous passive reflection elements to control wireless communication environment has garnered significant interest from both academic and industrial circles. On the other hand, the technology of near-field communication (NFC) as an emerging physical propagation mechanism for wireless communication, has received widespread attentions due to its ability to improve the degree of freedom (DoF), capacity, and sensing. In this work, we investigate a RIS-aided wireless communication system based on the path loss model millimeter-wave (mmWave) band and deduce the normalized power radiation pattern, where the passive RIS (PRIS) and active RIS (ARIS) are both considered. In particular, for the ARIS scheme, due to massive integrated elements, the overall power constraint is taken into account in the considered system, including the transmit power for the base station (BS) and amplifying power for ARIS. To elucidate the superiority in system performance, the upper bound expressions of achievable capacity for the RIS-NFC systems are respectively derived with the application of Hölder inequality, as well as the impacts of RIS element on the achievable capacity are also investigated. In addition, with these above analyses, we also provide a frontier discussion for RIS deployment in NFC, clarifying its differences from far-field communication (FFC) systems. Simulation and analytical results are provided to confirm the correctness of our derived expressions, showing that RIS-NFC system achieves better performance than that of the conventional FFC system. In addition, it is also clarified that the number of RIS elements, the length of RIS elements, and the angle of transmitted signals from RIS to the user can affect the upper bound of the achievable capacity.