Over-the-Air Computation via Active RIS With Sub-Connected Architecture

Abstract

Over-the-air computation (AirComp) has emerged as a promising technology for wireless data aggregation. However, its performance can be severely impaired by unfavorable channel conditions. To address this issue, we investigate AirComp aided by an active reconfigurable intelligent surface (RIS) with a sub-connected architecture, where multiple elements within each sub-array share one power amplifier to reduce hardware cost. We derive a closed-form expression for the mean squared error (MSE) between the target and estimated function values and formulate an MSE minimization problem that jointly optimizes the devices’ transmit coefficients, the aggregation factor at the access point, the sub-array amplification factors, and the per-element phase shifts. To address this nonconvex problem, we develop an alternating optimization framework that decomposes it into four efficiently solvable subproblems. Simulation results show that the sub-connected architecture retains most of the performance benefit of the fully-connected architecture while requiring considerably fewer power amplifiers.