Distributed Stochastic Phase-Shift Optimization in a RIS-Assisted Cellular Network

Reconfigurable intelligent surfaces (RIS) technology, as the name implies, is a grid of many small intelligent surfaces that can be reconfigured. It is a new and promising concept in wireless communications, expected to help realize the requirements of the future cellular generations. Numerous studies have been done to prove its added advantages and to control these surfaces in beneficial ways. However, these control schemes come with difficulties related to efficient and practical implementation. In this paper, we propose to control multiple RISs in a multi-user scheme with an algorithm that leads to a simple implementation. We formulate a stochastic optimization problem and we propose a new method using a distributed stochastic algorithm that allows the optimization to be done locally at each RIS with low computational requirements and without the need for instantaneous channel knowledge at the RIS. Also, the signaling between the base station (BS) and each RIS is limited to the exchange of a scalar only. Simulation results prove the success and efficiency of this algorithm.