Reconfigurable Intelligent Surface-Empowered Code Index Modulation for High-Rate SISO Systems

In this study, a novel index modulation-based communication system is proposed by combining the recently popular code index modulation-spread spectrum (CIM-SS) and reconfigurable intelligent surface (RIS) techniques. This technique is called CIM-RIS in short. In this proposed system, in addition to the traditional modulated symbols, the spreading code indices also carry data by being embedded in the signal, and the reflection/scattering properties of the signals are voluntarily controlled via the RIS technique. Consequently, the proposed system consumes little energy while transmitting extra bits of information compared to the traditional RIS. Average bit-error error (ABER) analysis of the proposed system is carried out and the system complexity, energy efficiency, and throughput analyses are obtained. The capacity analysis of the proposed CIM-RIS system is derived and it is shown that it provides higher capacity than the RIS-RSM and RIS systems. Performance analysis of the system is carried out on both Rayleigh fading channels and Rician fading channels for the M-ary quadrature amplitude modulation (QAM) technique. It has been shown by computer simulations that the CIM-RIS scheme has better error performance, faster data transmission speed, and lower transmission energy, compared to traditional RIS, transmit spatial modulation aided RIS (TSM-RIS), and transmit quadrature spatial modulation-based RIS (TQSM-RIS) techniques. In addition, the performance of the proposed system under conditions of imperfect channel state information (CSI) has also been obtained and illustrated.