Max-Min Energy Efficient Optimization for RIS-Aided Cell-Free MIMO Systems With Statistical CSI

The incorporation of reconfigurable intelligent surface (RIS) into cell-free (CF) networks is a promising paradigm to improve the network coverage and energy efficiency (EE). This letter addresses the fairness-based EE problem in spatially correlated RIS-aided CF networks. A joint design of beamformers at access points (APs) and phase shifts at the RIS is conducted in order to maximize the minimum EE of all users. We first derive the closed-form approximate expression of user ergodic rate based on statistical channel state information (CSI). Then, with this expression, we formulate the problem as a max-min fractional programming and propose an iterative algorithm to solve it under the framework of alternating optimization (AO). Simulation results demonstrate the effectiveness of the proposed algorithm by comparing with a few benchmark schemes.