  This paper considers the reconfigurable intelligent surface (RIS)-assisted
multi-user communications, where an RIS is used to assist the base station (BS)
for serving multiple users. The RIS consisting of passive reflecting elements
can manipulate the reflected direction of the incoming electromagnetic waves by
adjusting the phase shifts of the reflecting elements. Alternating optimization
(AO) based approach is commonly used to determine the phase shifts of the RIS
elements. While AO-based approaches have shown significant gain of RIS, the
complexity is quite high due to the coupled structure of the cascade channel
from the BS through RIS to the user. In addition, the sub-wavelength structure
of the RIS introduces spatial correlation that may cause strong interference to
users. To handle severe multi-user interference over correlated channels, we
consider adaptive user grouping previously proposed for massive mutli-input and
multi-output (MIMO) systems and propose two low-complexity beamforming design
methods, depending on whether the grouping result is taken into account.
Simulation results demonstrate the superior sum rate achieved by the proposed
methods than that without user grouping. Besides, the proposed methods can
perform similarly to the AO-based approach but with much lower complexity.
