  Reconfigurable intelligent surfaces (RISs) are typically used in multi-user
systems to mitigate interference among active transmitters. In contrast, this
paper studies a setting with a conventional active encoder as well as a passive
encoder that modulates the reflection pattern of the RIS. The RIS hence serves
the dual purpose of improving the rate of the active encoder and of enabling
communication from the second encoder. The capacity region is characterized,
and information-theoretic insights regarding the trade-offs between the rates
of the two encoders are derived by focusing on the high- and low-power regimes.
