  Reconfigurable Intelligent Surfaces (RISs) are an emerging technology for
future wireless communication systems, enabling improved coverage in an energy
efficient manner. RISs are usually metasurfaces, constituting of
two-dimensional arrangements of metamaterial elements, whose individual
response is commonly modeled in the literature as an adjustable phase shifter.
However, this model holds only for narrowband communications, and when wideband
transmissions are utilized, one has to account for the frequency selectivity of
metamaterials, whose response usually follows a Lorentzian-like profile. In
this paper, we consider the uplink of a wideband RIS-empowered multi-user
Multiple-Input Multiple-Output (MIMO) wireless system with Orthogonal Frequency
Division Multiplexing (OFDM) signaling, while accounting for the frequency
selectivity of RISs. In particular, we focus on designing the controllable
parameters dictating the Lorentzian response of each RIS metamaterial element,
in order to maximize the achievable sum rate. We devise a scheme combining
block coordinate descent with penalty dual decomposition to tackle the
resulting challenging optimization framework. Our simulation results reveal the
achievable rates one can achieve using realistically frequency selective RISs
in wideband settings, and quantify the performance loss that occurs when using
state-of-the-art methods which assume that the RIS elements behave as
frequency-flat phase shifters.
