Hybrid RIS-Based Reflective Index Modulation With Imperfect CSI

Reconfigurable intelligent surface (RIS) is a promising technology that can modify the wireless propagation environment to improve system performance. Similar to index modulation technique, RIS can exploit reflection coefficients and reflecting element indices to convey information, further improving spectral efficiency. However, the application potential of conventional passive RIS is limited by the multiplicative fading effect. To address this limitation, we present a hybrid RIS-based reflective index modulation (HRIS-RIM) scheme where active and passive reflecting elements coexist on the RIS. In the scheme, information is transmitted through the number of active reflecting element groups of the RIS and the antenna indices at the receiver. Then, the transmission model of the HRIS-RIM under imperfect channel state information (CSI) is proposed. Subsequently, maximum likelihood detection and greedy detection for the HRIS-RIM scheme with imperfect CSI are formulated, and their theoretical bit error probabilities are derived. Finally, the achievable rate of the system is derived based on an information theoretic approach. Simulation results validate the theoretical derivations and show the impact of imperfect CSI on the system performance. Additionally, the results demonstrate that the proposed HRIS-RIM scheme outperforms reference schemes in terms of bit error performance, achievable rate, and energy efficiency.