  Reconfigurable intelligent surfaces (RISs) have received extensive concern to
improve the performance of wireless communication systems. In this paper, a
subarray-based scheme is investigated in terms of its effects on ergodic
spectral efficiency (SE) and energy efficiency (EE) in RIS-assisted systems. In
this scheme, the adjacent elements divided into a subarray are controlled by
one signal and share the same reflection coefficient. An upper bound of ergodic
SE is derived and an optimal phase shift design is proposed for the
subarray-based RIS. Based on the upper bound and optimal design, we obtain the
maximum of the upper bound. In particular, we analytically evaluate the effect
of the subarray-based RIS on EE since it reduces SE and power consumption
simultaneously. Numerical results verify the tightness of the upper bound,
demonstrate the effectiveness of the optimal phase shift design for the
subarray-based RIS, and reveal the effects of the subarray-based scheme on SE
and EE.
