  Reconfigurable intelligent surface has recently emerged as a promising
technology for shaping the wireless environment by leveraging massive low-cost
reconfigurable elements. Prior works mainly focus on a single-layer metasurface
that lacks the capability of suppressing multiuser interference. By contrast,
we propose a stacked intelligent metasurface (SIM)-enabled transceiver design
for multiuser multiple-input single-output downlink communications.
Specifically, the SIM is endowed with a multilayer structure and is deployed at
the base station to perform transmit beamforming directly in the
electromagnetic wave domain. As a result, an SIM-enabled transceiver overcomes
the need for digital beamforming and operates with low-resolution
digital-to-analog converters and a moderate number of radio-frequency chains,
which significantly reduces the hardware cost and energy consumption, while
substantially decreasing the precoding delay benefiting from the processing
performed in the wave domain. To leverage the benefits of SIM-enabled
transceivers, we formulate an optimization problem for maximizing the sum rate
of all the users by jointly designing the transmit power allocated to them and
the analog beamforming in the wave domain. Numerical results based on a
customized alternating optimization algorithm corroborate the effectiveness of
the proposed SIM-enabled analog beamforming design as compared with various
benchmark schemes. Most notably, the proposed analog beamforming scheme is
capable of substantially decreasing the precoding delay compared to its digital
counterpart.
