  Reconfigurable surface (RS) has been shown to be an effective solution for
improving wireless communication links in general multi-user multiple-input
multiple-output (MU-MIMO) setting. Current research efforts have been largely
directed towards the study of reconfigurable intelligent surface (RIS), which
corresponds to an RS made of passive reconfigurable elements with only phase
shifting capabilities. RIS constitutes a cost- and energy- efficient solution
for increased beamforming gain since it allows to generate constructive
interference towards desired directions, e.g., towards a base station (BS).
However, in many situations, multiplexing gain may have greater impact on the
achievable transmission rates and number of simultaneously connected devices,
while RIS has only been able to achieve minor improvements in this aspect.
Recent work has proposed the use of alternative RS technologies, namely
amplitude-reconfigurable intelligent surface (ARIS) and fully-reconfigurable
intelligent surface (FRIS), to achieve perfect orthogonalization of MU-MIMO
channels, thus allowing for maximum multiplexing gain at reduced complexity. In
this work we consider the use of ARIS and FRIS for simultaneously
orthogonalizing a MU-MIMO channel, while embedding extra information in the
orthogonalized channel. We show that the resulting achievable rates allow for
full exploitation of the degrees of freedom in a MU-MIMO system with excess of
BS antennas.
