Queue-Aware Hybrid RIS-Assisted Cell-Free Communication Systems

In this paper, we investigate the cross-layer design in hybrid reconfigurable intelligent surface (RIS)-assisted cell-free systems. To maximize long-term energy efficiency while ensuring network stability, we formulate a queue-aware stochastic problem that jointly optimizes the beamforming matrices at the remote antenna units (RAUs) and hybrid RIS, along with the operation modes of RIS elements. By utilizing the Lyapunov theory, the formulated stochastic optimization problem is decomposed into a sequence of slot-level problems that maximize energy efficiency while minimizing the Lyapunov drift. To tackle the formulated non-convex slot-level problem, we propose an alternating optimization algorithm to transform the slot-level problem into transmission and element mode switching subproblems. In particular, for efficient RIS element selection, we first derive the number upper bound of the elements that can be activated given the RIS power budget, and then the element selection optimization is transformed into multiple tractable smaller problems that can be solved in parallel by utilizing a greedy algorithm. Simulation results demonstrate that the proposed algorithm outperforms the baseline schemes in terms of energy efficiency and can effectively improve system stability as well as user fairness.