  Index modulation (IM) is one of the candidate technologies for the upcoming
sixth generation (6G) wireless communications networks. In this paper, we
propose a space-time-modulated reconfigurable intelligent metasurface (RI-MTS)
that is configured to implement various frequency-domain IM techniques in a
multiple-input multiple-output (MIMO) array configuration. Unlike prior works
which mostly analyze signal theory of general RI-MTS IM, we present novel
electromagnetics-compliant designs of specific IMs such as sub-carrier index
modulation (SIM) and MIMO orthogonal frequency-domain modulation IM
(MIMO-OFDM-IM). Our full-wave electromagnetic simulations and analytical
computations establish the programmable ability of these transceivers to vary
the reflection phase and generate frequency harmonics for IM. Our experiments
for bit error rate show that RI-MTS-based SIM and MIMO-OFDM-IM are lower than
conventional MIMO-OFDM.
