  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.
