Energy Efficient Joint Beamforming Design for Hardware-Impaired STAR-RIS-Assisted CF-mMIMO Systems

Debajit Choudhury, Abhay Kumar Sah

Abstract: The alliance of reconfigurable intelligent surfaces (RISs) and cell-free massive multiple-input multiple-output (CF-mMIMO) is a promising architecture for the next-generation wireless networks. The idea is to deploy a large number of distributed access points (APs) assisted by a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) that provides 360° coverage to user equipments (UEs) located on the front and back side of the RIS, over the same time-frequency resources. In this paper, we focus on the joint active and passive beamforming design in a downlink STAR-RIS-assisted CF-mMIMO network so that the energy efficiency of the network can be maximized. For this, we employ fractional programming and Lagrangian dual transform to decouple the active and passive beamformers and solve it via an alternating optimization (AO). To keep the computational complexity in check, we have derived a closed-form update for the active beamformers at the APs and also obtained a computationally inexpensive line-search-based general inertial projected gradient (GIPG) algorithm to optimize the passive beamformer at the STAR-RIS. Using simulations, we corroborate the effectiveness of the proposed beamforming algorithm in maximizing the EE of the network in the presence of hardware imperfections at APs, UEs, and STAR-RIS.

External IDs:dblp:journals/tcom/ChoudhuryS25