  Large antenna arrays and high-frequency bands are two key features of future
wireless communication systems. The combination of large-scale antennas with
high transmission frequencies often results in the communicating devices
operating in the near-field (Fresnel) region. In this paper, we study the
potential of beam focusing, feasible in near-field operation, in facilitating
high-rate multi-user downlink multiple-input multiple-output (MIMO) systems. As
the ability to achieve beam focusing is dictated by the transmit antenna, we
study near-field signaling considering different antenna structures, including
fully-digital architectures, hybrid phase shifter-based precoders, and the
emerging dynamic metasurface antenna (DMA) architecture for massive MIMO
arrays. We first provide a mathematical model to characterize near-field
wireless channels as well as the transmission pattern for the considered
antenna architectures. Then, we formulate the beam focusing problem for the
goal of maximizing the achievable sum-rate in multi-user networks. We propose
efficient solutions based on the sum-rate maximization task for fully-digital,
(phase shifters based-) hybrid and DMA architectures. Simulation results show
the feasibility of the proposed beam focusing scheme for both single- and
multi-user scenarios. In particular, the designed focused beams are such that
users residing at the same angular direction can communicate reliably without
interfering with each other, which is not achievable using conventional
far-field beam steering.
