This letter investigates the reconfigurable intelligent surface (RIS)-aided massive multiple-input multiple-output (MIMO) systems with a two-timescale design. First, the zero-forcing (ZF) detector is applied at the base station (BS) based on instantaneous aggregated CSI, which is the superposition of the direct channel and the cascaded user-RIS-BS channel. Then, by leveraging the channel statistical property, we derive the closed-form ergodic achievable rate expression. Using a gradient ascent method, we design the RIS passive beamforming only relying on the long-term statistical CSI. We prove that the ergodic rate can reap the gains on the order of $\mathcal{O}\left(\log_{2}\left(MN\right)\right)$, where $M$ and $N$ denote the number of BS antennas and RIS elements, respectively. We also prove the striking superiority of the considered RIS-aided system with ZF detectors over the RIS-free systems and RIS-aided systems with maximum-ratio combining (MRC).

翻译：这封信调查了可重新配置的智能表面(RIS) 辅助的大规模多投入多输出(MIMO) 系统,其设计有两时制。 首先,零强制(ZF)探测器根据即时综合CSI(即直接频道和级联用户-RIS-BS频道的叠加)应用在基站(BS),这是直接频道和级联用户-RIS-BS频道的叠加。然后,通过利用频道统计属性,我们得出闭式的可变速率表达。我们使用梯度上升法,设计了仅依靠长期统计CSI(MIMO)的RIS被动被动成形系统。我们证明,ERGedics率可以以美元/mathcal{O ⁇ left(log ⁇ 2 ⁇ left(MN\right)\right)的顺序获得收益,其中分别是M$和$NUS天线和RIS元素的数量。我们还证明,经过考虑的RIS辅助系统与无RIS系统和RIS-A(MC)的ZF探测器具有惊人的优势。