  Orthogonal multi-user multiple-input multiple-output (MU-MIMO) channels allow
for optimum performance with simplified precoding/equalization, and they
achieve maximum multiplexing gain which is shared fairly among users.
Reconfigurable intelligent surface (RIS) constitutes a promising cost-efficient
solution to improve the wireless channel, since they consist of passive
reflecting elements able to adjust the phases of the incoming waves. However,
it is still widely unclear how these surfaces can improve spatial-multiplexing.
In fact, the common RIS model cannot achieve perfect orthogonalization of
MU-MIMO channels with a reasonable number of elements. Furthermore, efficient
channel estimation algorithms for RIS, which are key for taking advantage of
its benefits, are still a matter of research. We study two types of
reconfigurable surfaces (RSs), namely amplitude-reconfigurable intelligent
surface (ARIS) and fully-reconfigurable intelligent surface (FRIS), with
extended capabilities over RIS. We show how these RSs allow for perfect channel
orthogonalization, and, by minimizing the applied power, we show that they can
potentially be implemented without the need of amplification. We also present
an efficient channel estimation method for each of them that allows the base
station (BS) to select the desired propagation channel.
