Wireless communications aided by reconfigurable intelligent surfaces (RISs) is a promising way to improve the coverage for cellular users. The controlled reflection of signals from RISs is especially useful in mm-wave/THz networks when the direct link between a cellular user and its serving base station (BS) is weak or unavailable due to blockages. However, the joint blockage of the user-RIS and the user-BS links may significantly degrade the performance of RIS-aided transmissions. This paper aims to study the impact of joint blockages on downlink performance. When RIS locations are coupled with BS locations, using tools from stochastic geometry, we obtain an optimal placement of RISs to either minimize the joint blockage probability of the user-RIS and the user-BS links or maximize the downlink coverage probability. The results show that installing RISs near the cell edge of BSs usually provides optimal coverage. Moreover, deploying RISs on street intersections improves the coverage probability. For users associated with BSs that are deployed sufficiently close to intersections, the intersection-mounted RISs offer a better coverage performance as compared to BS-coupled RISs.

翻译：可重构智能表面辅助的无线通信是提升蜂窝用户覆盖范围的一种有前景的方式。在毫米波/太赫兹网络中，当蜂窝用户与其服务基站之间的直接链路因遮挡而减弱或不可用时，RIS对信号的可控反射尤为有用。然而，用户-RIS链路与用户-BS链路的联合遮挡可能显著降低RIS辅助传输的性能。本文旨在研究联合遮挡对下行链路性能的影响。当RIS位置与BS位置耦合时，利用随机几何工具，我们获得了RIS的最优部署方案，以最小化用户-RIS链路与用户-BS链路的联合遮挡概率，或最大化下行链路的覆盖概率。结果表明，在BS小区边缘附近安装RIS通常能提供最优覆盖。此外，在街道交叉口部署RIS可提升覆盖概率。对于与部署在足够靠近交叉口的BS相关联的用户，相较于与BS耦合的RIS，安装在交叉口的RIS能提供更好的覆盖性能。