  We present a novel, power- & hardware-efficient, multiuser, multibeam RIS
(Reflective Intelligent Surface) architecture for multiuser MIMO, especially
for very high frequency bands (e.g., high mmWave and sub-THz), where channels
are typically sparse in the beamspace and LOS is the dominant component. The
key module is formed by an active multiantenna feeder (AMAF) with a small
number of active antennas, placed in the near field of a RIS with a much larger
number of passive controllable reflecting elements. We propose a pragmatic
approach to obtain a steerable beam with high gain and very low sidelobes. Then
K independently controlled beams can be achieved by closely stacking K such
AMAF-RIS modules. Our analysis includes the mutual interference between the
modules and the fact that, due to the delay difference of propagation through
the AMAF-RIS structure, the resulting channel matrix is frequency selective
even for pure LOS propagation. We consider a 3D geometry and show that "beam
focusing" is in fact possible (and much more effective in terms of coverage)
also in the far-field, by creating spotbeams with limited footprint both in
angle and in range. Our results show that: 1) simple RF beamforming (BF)
without computationally expensive baseband multiuser precoding is sufficient to
practically eliminate multiuser interference when the users are chosen with
sufficient angular/range separation, thanks to the extremely low sidelobe
beams; 2) the impact of beam pointing errors with standard deviation as large
as 2.5 deg and RIS quantized phase-shifters with quantization bits > 2 is
essentially negligible; 3) The proposed architecture is more power efficient &
much simpler from a hardware implementation viewpoint than standard active
arrays with the same BF performance. We show also that the array gain of the
proposed AMAF-RIS structure is linear with the RIS aperture.
