Online RIS Configuration Learning for Arbitrary Large Numbers of 1-Bit Phase Resolution Elements

Reinforcement Learning (RL) approaches are lately deployed for orchestrating wireless communications empowered by Reconfigurable Intelligent Surfaces (RISs), leveraging their online optimization capabilities. Most commonly, in RL-based formulations for realistic RISs with low resolution phase-tunable elements, each configuration is modeled as a distinct reflection action, resulting to inefficient exploration due to the exponential nature of the search space. In this paper, we consider RISs with 1-bit phase-resolution elements and model the reflection action as a binary vector including the feasible reflection coefficients. We then introduce two variations of the well-established Deep Q-Network (DQN) and Deep Deterministic Policy Gradient (DDPG) agents, aiming for effective exploration of the binary action spaces. For the case of DQN, we make use of an efficient approximation of the Q-function, whereas a discretization post-processing step is applied to the output of DDPG. Our simulations consider large-scale RISs, where existing tuning methods are largely impractical, and showcase that the proposed techniques greatly outperform the baseline in terms of the rate maximization objective. In addition, when dealing with moderate-scale RIS sizes, where the conventional DQN relying on configuration-based action spaces is feasible, the performance of the latter technique is similar to the proposed learning approach.