  In this paper, we deploy a full-wave FDTD paradigm to investigate the effect
of reconfigurable intelligent surface (RIS) -- switchable frequency-selective
surfaces (FSS) -- on generic massive MIMO uplink channel's eigenspace
structure. We place an RIS based on two switchable FSS layers in the vicinity
of a 64-element massive MIMO base-station (BS) array, serving a cluster of four
fixed user equipment (UE) units. Utilizing an electromagnetic tool based on
time-averaged Poynting flow developed recently by the authors, we demonstrate
how the illumination of BS-array aperture can be controlled by the intentional
deployment of various switching states in the RIS placed near the BS. We show
that such supplementary RIS structures may assist the wireless link engineer in
deterministically "customizing" the uplink channel behaviour by selectively
enhancing/suppressing certain channel eigenvalues.
