RIS-Based Code Index Modulation for Joint Active/Passive Transmission

In this paper, we employ a reconfigurable intelligent surface (RIS) to spread incident signals and develop RIS-based code index modulation (CIM) for joint active/passive transmission (JAPT). In RIS-CIM, the RIS performs passive beamforming to simultaneously maximize the receive signal-to-noise ratio and spread the incident signals from the active transmission. The indices of spreading codes are used to transmit passive information. We design a low-complexity (LC) yet near-optimal detector to recover the active and passive information in addition to the optimal maximum-likelihood (ML) detector. Upper bounds and approximate expressions are derived in closed-form for the bit error rate of RIS-CIM with the ML and LC detectors, respectively. Subsequently, we extend the idea of RIS-CIM to the scenario of multi-antenna transmission. Simulation results verify the performance analysis and show the superiority of RIS-CIM over the existing RIS-aided JAPT schemes.