On the Performance of Frequency-Mixing Reconfigurable Intelligent Surfaces-Aided System: Achievable Rates and Reflective Patterns

Abstract

Frequency mixing reconfigurable intelligent surface (FMx-RIS) is an innovative concept within the realm of RIS technology. It distinguishes itself from conventional RIS by continuously modifying the phase of incident electromagnetic waves, thereby inducing frequency shifts. By uniquely associating FMx-RIS elements with a specific frequency, this approach allows receivers to distinguish channels from each propagation path by detecting the frequency shifts. This decoupling feature enables channel estimation and augments diversity gain in the frequency domain. In this paper, we explore the architectural aspects of FMx-RIS in both single-carrier and multi-carrier systems to validate its practicality. For single-carrier systems, we derive closed-form lower bounds for achievable data rates under scenarios with perfect and imperfect channel state information, alongside the corresponding power scaling laws. For multi-carrier systems, our focus lies in the scheduling of reflective patterns for FMx-RISs, recognizing that frequency mixing operations introduce additional inter-carrier interference among users. We introduce two scheduling algorithms designed to maximize the minimum user signal-to-interference-plus-noise ratio. The numerical results confirm the analytical achievable rates and demonstrate that the FMx-RIS-aided system surpasses conventional RIS-aided systems. Furthermore, we assess the effectiveness of the proposed scheduling algorithms, where the ADMM-based scheme exhibits similar performance to the SoTA scheme.