  The multi-user Holographic Multiple-Input and Multiple-Output Surface
(MU-HMIMOS) paradigm, which is capable of realizing large continuous apertures
with minimal power consumption, has been recently considered as an
energyefficient solution for future wireless networks, offering increased
flexibility in impacting electromagnetic (EM) wave propagation according to the
desired communication, localization, and sensing objectives. The tractable
channel modeling in MU-HMIMOS wireless systems is one of the most critical
research challenges, mainly due to the coupling effect induced by the
excessively large number of closely spaced patch antennas. In this paper, we
focus on this challenge for the downlink of multi-user MIMO communications and
extend an EM-compliant channel model to multiuser case, which is expressed in
the wavenumber domain using the Fourier plane wave approximation. Based on the
presented channel model, we investigate the spectral efficiency of maximumratio
transmission and Zero-Forcing (ZF) precoding schemes. We also introduce a novel
hardware efficient ZF precoder, leveraging Neumann series (NS) expansion to
replace the required matrix inversion operation, which is very hard to be
computed in the conventional way due to the extremely large number of patch
antennas in the envisioned MU-HMIMOS communication systems. In comparison with
the conventional independent and identical Rayleigh fading channels that ignore
antenna coupling effects, the proposed EM-compliant channel model captures the
mutual couplings induced by the very small antenna spacing. Our extensive
performance evaluation results demonstrate that our theoretical performance
expressions approximate sufficiently well ...
