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.

翻译：可重构智能表面（RIS）因其提升无线通信系统性能的潜力而受到广泛关注。本文研究了基于子阵列方案对RIS辅助系统中遍历频谱效率（SE）和能量效率（EE）的影响。在该方案中，相邻单元被划分为一个子阵列并由同一信号控制，共享相同的反射系数。推导了遍历SE的上界，并针对基于子阵列的RIS提出了最优相位偏移设计。基于该上界与最优设计，我们得到了上界的最大值。特别地，由于子阵列型RIS同时降低了SE和功耗，我们定量评估了其对EE的影响。数值结果验证了上界的紧致性，证明了所提最优相位偏移设计对子阵列型RIS的有效性，并揭示了子阵列方案对SE和EE的影响。