  Space-time modulated metasurfaces (STMMs) are a newly investigated technology
for next 6G generation wireless communication networks. An STMM augments the
spatial phase function with a time-varying one across the elements, allowing
for the conveyance of information that possibly modulates the impinging signal.
Hence, STMM represents an evolution of reconfigurable intelligent surfaces
(RIS), which only design the spatial phase pattern. STMMs convey signals
without a relevant increase in the energy budget, which is convenient for
applications where energy is a strong constraint. This paper proposes a
mathematical model for STMM-based wireless communication, that creates the
basics for two potential STMM architectures. One has excellent design
flexibility, whereas the other is more cost-effective. The model describes
STMM's distinguishing features, such as space-time coupling, and their impact
on system performance. The proposed STMM model addresses the design criteria of
a full-duplex system architecture, in which the temporal signal originating at
the STMM generates a modulation overlapped with the incident one. The presented
numerical results demonstrate the efficacy of the proposed model and its
potential to revolutionize wireless communication.
