RIS-Assisted Cell-Free Massive MIMO Systems for High-Speed Train Communications

Cell-free massive multiple-input multiple-output (mMIMO) technology can significantly improve the data transmission rate in high-speed train (HST) communications. In this article, we propose a cell-free mMIMO system for HST communications assisted by reconfigurable intelligent surfaces (RISs) with many low-cost reflecting elements. RISs are deployed nearby the train sensors (TSs) to improve the reception of the desired signals. By leveraging the reflective elements, RISs dynamically adjust the phases and create favorable propagation conditions via reconfigurable reflections in a cost-effective way. Aiming at enhancing the energy efficiency of RIS-assisted cell-free mMIMO systems in HST communications, we formulate an optimization problem for designing the phase shifts at RISs and the transmit beamforming at access points (APs). The formulated problem, characterized by nonconvexity and high complexity, is challenging to address. Thus, we propose a nonmonotone accelerated proximal gradient (NAPG)-based algorithm and a projected gradient ascent (PGA) approach, taking advantage of alternating optimization to obtain the suboptimal solutions. Simulation results demonstrate that the alternating optimization algorithm achieves similar energy efficiency performance to the state-of-the-art scheme, but with significantly reduced computation time.