  Reconfigurable intelligent surfaces (RISs) have attracted wide interest from
industry and academia since they can shape the wireless environment into a
desirable form with a low cost. In practice, RISs have three types of
implementations: 1) reflective, where signals can be reflected to the users on
the same side of the base station (BS), 2) transmissive, where signals can
penetrate the RIS to serve the users on the opposite side of the BS, and 3)
hybrid, where the RISs have a dual function of reflection and transmission.
However, existing works focus on the reflective type RISs, and the other two
types of RISs are not well investigated. In this letter, a downlink multi-user
RIS-assisted communication network is considered, where the RIS can be one of
these types. We derive the system sum-rate, and discuss which type can yield
the best performance under a specific user distribution. Numerical results
verify our analysis.
