Reconfigurable Intelligent Surface Code Index Modulation With Quadrature Receive Spatial Modulation

This paper introduces a novel reconfigurable intelligent surface (RIS)-aided wireless communication scheme, termed RIS code index modulation with quadrature receive spatial modulation (RIS-CIM-QRSM), designed to enhance the error performance of RIS-aided index modulation (IM) systems. The proposed RIS-CIM-QRSM scheme combines M-pulse amplitude modulation (PAM) with the indices of spreading codes (SCs) and receive antennas (RAs) to convey information effectively. In this system, the RIS is partitioned into two parts for in-phase and quadrature operations, with each part dynamically configured at a rate faster than the symbol period, enabling code index modulation (CIM). In RIS-CIM-QRSM, orthogonal SCs are generated at the RIS for increased IM bits, achieved by introducing a π/2 phase shift. Additionally, both maximum-likelihood (ML) and low-complexity (LC) detectors are proposed and analyzed for RIS-CIM-QRSM, where the LC detector offers a computationally efficient detection performance. Comparative error performance, spectral and energy efficiency analyses, supported by extensive computer simulations, demonstrate the superior performance of RIS-CIM-QRSM over benchmark schemes, RIS-CIM and RIS-CIM-RSM, across various scenarios and system configurations.