RIS-based Dual-Functional Access Point for Energy Efficiency in Cell-Free Systems

To unleash the potential of reconfigurable intelligent surface (RIS), in this paper we propose a novel dual-functional access point (DF-AP) for enhancing energy efficiency (EE) in the cell-free network. The DF-AP can dynamically switch between transmitter and reflector modes based on the wireless environment and the communication requirement. In the transmitter mode, the DF-AP works as a RIS-based transmitter, which can directly send data to users to improve the throughput of the system and ensure the quality of service (QoS). While in the reflector mode, the DF-AP operates as a conventional RIS, which passively reflects signals towards desired directions to increase coverage in an energy-efficient way. We propose a fractional programming (FP)-manifold-heuristic method to design the active beamforming of access points (APs) and DF-APs in the transmitter mode, the passive beamforming of DF-APs in the reflector mode, and the operation mode of DF-APs to maximize the system EE. The superiority of this DF-AP architecture and the effectiveness of the proposed design algorithm are then validated by simulation.