  The disruptive reconfigurable intelligent surface (RIS) technology is
steadily gaining relevance as a key element in future 6G networks. However, a
one-size-fits-all RIS hardware design is yet to be defined due to many
practical considerations. A major roadblock for currently available RISs is
their inability to concurrently operate at multiple carrier frequencies, which
would lead to redundant installations to support multiple radio access
technologies (RATs). In this paper, we introduce FABRIS, a novel and practical
multi-frequency RIS design. FABRIS is able to dynamically operate across
different radio frequencies (RFs) by means of frequency-tunable antennas as
unit cells with virtually no performance degradation when conventional
approaches to RIS design and optimization fail. Remarkably, our design
preserves a sufficiently narrow beamwidth as to avoid generating signal leakage
in unwanted directions and a sufficiently high antenna efficiency in terms of
scattering parameters. Indeed, FABRIS selects the RIS configuration that
maximizes the signal at the intended target user equipment (UE) while
minimizing leakage to non-intended neighboring UEs. Numerical results and
full-wave simulations validate our proposed approach against a naive
implementation that does not consider signal leakage resulting from
multi-frequency antenna arrays.
