  The capacity of commercial massive multiple-input multiple-output (mMIMO)
systems is constrained by the limited array aperture at the base station, and
cannot meet the ever-increasing traffic demands of wireless networks. Given the
array aperture, holographic MIMO with infinitesimal antenna spacing can
maximize the capacity, but is physically unrealizable. As a promising
alternative, reconfigurable mMIMO is proposed to harness the unexploited power
of the electromagnetic (EM) domain for enhanced information transfer.
Specifically, the reconfigurable pixel antenna technology provides each antenna
with an adjustable EM radiation (EMR) pattern, introducing extra degrees of
freedom for information transfer in the EM domain. In this article, we present
the concept and benefits of availing the EMR domain for mMIMO transmission.
Moreover, we propose a viable architecture for reconfigurable mMIMO systems,
and the associated system model and downlink precoding are also discussed. In
particular, a three-level precoding scheme is proposed, and simulation results
verify its considerable spectral and energy efficiency advantages compared to
traditional mMIMO systems. Finally, we further discuss the challenges,
insights, and prospects of deploying reconfigurable mMIMO, along with the
associated hardware, algorithms, and fundamental theory.
