RIS-Based Composite Phase Shift Keying: An Efficient Rectangular QAM Approach

The reconfigurable intelligent surface (RIS)-based carrier modulation technology, which operates without conventional radio-frequency chains, offers hardware simplicity and cost efficiency. In particular, most current RIS-based amplitude modulation schemes rely on the ON/OFF state switching or amplifier gain control of RIS elements, and ambiguity-free high-order phase modulation requires correspondingly highresolution phase shifts. However, practical hardware limitations often impose a low-resolution phase shift and near-constant reflecting amplitude of RIS elements, complicating high-order modulation. To address this, we propose an efficient composite phase shift keying (CPSK) scheme to generate virtual highorder rectangular quadrature amplitude modulation (RQAM) signals at the receiver, referred to as CPSK-RQAM. By decomposing high-order RQAM signals into multiple low-order quadrature phase shift keying signal components, with each component transmitted by a distinct RIS group, CPSK-RQAM eliminates the need for additional amplitude control and ensures robustness to phase-shift quantization errors. Unlike the similar schemes based on conventional I/Q decomposition and equal grouping, CPSK-RQAM achieves a significantly larger minimum Euclidean distance by employing a tailored grouping criterion. Furthermore, we derive the closed-form expressions for the approximate symbol error probability (SEP) of CPSK-RQAM under maximum likelihood detection. Simulation results validate the theoretical analysis and demonstrate the superiority of CPSKRQAM over the state-of-the-art schemes.