  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. Performance analysis
of the system is carried out on Rayleigh 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.
