  Transmission through reconfigurable intelligent surfaces (RISs), which
control the reflection/scattering characteristics of incident waves in a
deliberate manner to enhance the signal quality at the receiver, appears as a
promising candidate for future wireless communication systems. In this paper,
we bring the concept of RIS-assisted communications to the realm of index
modulation (IM) by proposing RIS-space shift keying (RIS-SSK) and RIS-spatial
modulation (RIS-SM) schemes. These two schemes are realized through not only
intelligent reflection of the incoming signals to improve the reception but
also utilization of the IM principle for the indices of multiple receive
antennas in a clever way to improve the spectral efficiency. Maximum
energy-based suboptimal (greedy) and exhaustive search-based optimal (maximum
likelihood) detectors of the proposed RIS-SSK/SM schemes are formulated and a
unified framework is presented for the derivation of their theoretical average
bit error probability. Extensive computer simulation results are provided to
assess the potential of RIS-assisted IM schemes as well as to verify our
theoretical derivations. Our findings also reveal that RIS-based IM, which
enables high data rates with remarkably low error rates, can become a potential
candidate for future wireless communication systems in the context of beyond
multiple-input multiple-output (MIMO) solutions.
