On The Energy-Efficiency Fairness of Reconfigurable Intelligent Surface-Aided Cell-Free Network

With the ability to overcome the inter-cell interferences, cell-free network is a promising network paradigm to achieve high spectrum efficiency for future 6G communications. However, the increasing number of distributed base stations in cell-free network introduces very high power consumption. To address this issue, inspired by the reconfigurable intelligent surface (RIS) technique with low power consumption, the concept of RIS-aided cell-free network has been recently proposed to improve the spectrum efficiency at a low cost of power consumption. In this paper, we take a further step to investigate the energy-efficiency fairness (EEF) of RIS-aided cell-free network. Specifically, we formulate the precoding design problem to maximize the energy efficiency of the worst user in a wideband RIS-aided cell-free network. To solve this problem, we then propose an iterative precoding algorithm by using Lagrangian transform and fractional programming (FP) to tackle the highly decoupled optimization objective. Through alternatingly solving subproblems of subcarrier assignment, power allocation, combining, and precoding, the objective will finally converge. Simulation results demonstrate the effectiveness of the proposed precoding algorithm, and the energy efficiency of users in cell-free network can be efficiently increased by the aid of RISs.