  Recently, there has been a flurry of research on the use of Reconfigurable
Intelligent Surfaces (RIS) in wireless networks to create dynamic radio
environments. In this paper, we investigate the use of an RIS panel to improve
bi-directional communications. Assuming that the RIS will be located on the
facade of a building, we propose to connect it to a solar panel that harvests
energy to be used to power the RIS panel's smart controller and reflecting
elements. Therefore, we present a novel framework to optimally decide the
transmit power of each user and the number of elements that will be used to
reflect the signal of any two communicating pair in the system (user-user or
base station-user). An optimization problem is formulated to jointly minimize a
scalarized function of the energy of the communicating pair and the RIS panel
and to find the optimal number of reflecting elements used by each user.
Although the formulated problem is a mixed-integer non-linear problem, the
optimal solution is found by linearizing the non-linear constraints. Besides, a
more efficient close to the optimal solution is found using Bender
decomposition. Simulation results show that the proposed model is capable of
delivering the minimum rate of each user even if line-of-sight communication is
not achievable.
