Abstract
This paper presents a novel, power- and hardware-efficient, multiuser,
multibeam RIS (Reflective Intelligent Surface) architecture for
multiuser MIMO, especially suited to operate in very high frequency
bands (e.g., high mmWave and sub-THz), where channels are typically
sparse in the beamspace and line-of-sight (LOS) Â 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 in the presence of 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)Â also in the far-field, by creating
spotbeams with limited footprint both in angle and in range. Our
results show that: 1) Â simple RF beamforming without computationally
expensive baseband digital multiuser precoding is sufficient to
practically eliminate multiuser interference when the users are chosen
with sufficient angular/range separation, thanks to the extremely low
sidelobes of the proposed module; 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 and
much simpler from a hardware implementation viewpoint than standard RF
beamforming active arrays with the same beamforming performance. As a
side result, we show also that the array gain of the proposed AMAF-RIS
structure grows linearly with the RIS aperture, in line with classical
results for standard reflector antennas.