  We envision a future in which multi-antenna technology effectively exploits
the spatial domain as a set of non-interfering orthogonal resources, allowing
for flexible resource allocation and efficient modulation/demodulation.
Reconfigurable intelligent surface (RIS) has emerged as a promising technology
which allows shaping the propagation environment for improved performance. This
paper studies the ability of three extended types of reconfigurable surface
(RS), including the recently proposed beyond diagonal RIS (BD-RIS), to achieve
perfectly orthogonal channels in a general multi-user multiple-input
multiple-output (MU-MIMO) scenario. We propose practical implementations for
the three types of RS consisting of passive components, and obtain the
corresponding restrictions on their reconfigurability. We then use these
restrictions to derive closed-form conditions for achieving arbitrary
(orthogonal) channels. We also study the problem of optimal orthogonal channel
selection for achieving high channel gain without active amplification at the
RS, and we propose some methods with satisfying performance. Finally, we
provide efficient channel estimation and RS configuration techniques such that
all the computation, including the channel selection, may be performed at the
base station (BS). The numerical results showcase the potential and
practicality of RS channel orthogonalization, thus taking a step towards
orthogonal spatial domain multiplexing (OSDM).
