  In this paper, the optimization of deploying unmanned aerial vehicles (UAVs)
over a reconfigurable intelligent surfaces (RISs)-assisted visible light
communication (VLC) system is studied. In the considered model, UAVs are
required to simultaneously provide wireless services as well as illumination
for ground users. To meet the traffic and illumination demands of the ground
users while minimizing the energy consumption of the UAVs, one must optimize
UAV deployment, phase shift of RISs, user association and RIS association. This
problem is formulated as an optimization problem whose goal is to minimize the
transmit power of UAVs via adjusting UAV deployment, phase shift of RISs, user
association and RIS association. To solve this problem, the original
optimization problem is divided into four subproblems and an alternating
algorithm is proposed. Specifically, phases alignment method and semidefinite
program (SDP) algorithm are proposed to optimize the phase shift of RISs. Then,
the UAV deployment optimization is solved by the successive convex
approximation (SCA) algorithm. Since the problems of user association and RIS
association are integer programming, the fraction relaxation method is adopted
before using dual method to find the optimal solution. For simplicity, a greedy
algorithm is proposed as an alternative to optimize RIS association. The
proposed two schemes demonstrate the superior performance of 34:85% and 32:11%
energy consumption reduction over the case without RIS, respectively, through
extensive numerical study.
