RIS-based Dual-Mode Receive Spatial Modulation for Low-Complexity Wireless Access Points

The emerging technology of reconfigurable intelligent surface-based access points (RIS-APs) shows excellent potential as it effectively replaces conventional RF front ends with passive RISs. This paper introduces RIS-based dual-mode receive spatial modulation (RDM-RSM), a low-complexity scheme designed to improve the spectral efficiency and error performance of RIS-APs. Both versions of the proposed RDM-RSM scheme configure the RIS to simultaneously enhance the RF signals at two different receive antennas while synthesizing distinct modulation symbols from two separate constellations at each antenna, thereby enabling efficient transmission of two parallel, conventionally-modulated data-streams. Additionally, by selecting two antennas and assigning each a distinct constellation mode, more bits can be encoded per index-modulated symbol. The first version, RDM-RSM-Independent (RDM-RSMI), operates by maximizing the energy of the synthesized signals independently for each antenna. In contrast, RDM-RSM-Joint (RDM-RSMJ) optimizes the reflection phase of each RIS element to maximize an objective function that considers the signals at both antennas jointly, resulting in stronger received signals compared to RDM-RSMI, where only half of the elements can serve each antenna. A low-complexity detector is proposed, with its theoretical performance rigorously analyzed by modeling the conditional reflection phases of the RIS elements as von-Mises distributed, providing a tractable and insightful framework.