RIS Deployment: The Differences from NFC to FFC

In this paper, we investigate the reconfigurable intelligent surface (RIS) deployment problem in near-field communication (NFC), including the cases of single- and multi-antenna base station (BS). To comprehensively clarify the differences in RIS deployments between NFC and far-field communication (FFC), we consider two practical RIS deployment scenarios. For the first scenario, the location of BS is considered to move far away from RIS, while the distance between user and passive RIS (PRIS) is fixed, and the location of user is considered to move far away from RIS, while the distance between BS and PRIS is fixed. In the second scenario, RIS moves between BS and user, while the distance between BS and user is fixed. We also study the impact of the distance, number of RIS elements, and RIS element size components on system performance for NFC and FFC. Furthermore, we discuss the differences between NFC and FFC from a physical perspective in two proposed scenarios. Numerical results validate the correctness of our theoretical results, revealing that compared to the deployment strategy with RIS near the multi-antenna BS, the achievable rate is larger when RIS is near the single-antenna user in NFC. For NFC scenario with a single-antenna BS and FFC scenario with a single-antenna BS or a multi-antenna BS, it is demonstrated that deploying RIS near the single-antenna user or near the multi-antenna BS will achieve an equivalent rate regardless of the variation of transmission power or RIS element count. In addition, we also analyze the achievable rates of active RIS (ARIS) and hybrid RIS (HRIS), and their deployment conclusions are consistent with that of PRIS.