Capacity Enhancement for High-Speed Train Communications With Irregular RIS-Assisted Cell-Free Massive MIMO

The rapid development of railways worldwide has generated significant attention toward high-speed train (HST) communications. Disruptive technologies for the sixth-generation network, such as cell-free massive multiple-input–multiple-output (mMIMO) and reconfigurable intelligent surface (RIS), are set to substantially enhance the data transmission rate in wireless communication systems. In this article, we leverage the potential benefits of cell-free mMIMO and irregular RIS to achieve the ultimate performance benchmark in HST networks. Specifically, we investigate a cell-free mMIMO system supported by irregular RIS with many low-cost reflecting elements, aiming to improve the weighted sum-rate (WSR) in HST communications. The deployment of irregular RIS allows signals to be transmitted through reflected links with additional degrees of freedom, and thus, a more diverse transmission is facilitated to enhance the received signal power and the system capacity. A joint optimization problem is then formulated to maximize the WSR, while satisfying the constraints on access point power. To address the problem characterized by nonconvexity and high complexity, an alternating optimization algorithm is proposed to iteratively optimize the RIS topology, the transmit beamforming, and the phase shifts. The corresponding subproblems are tackled by exploiting improved tabu search, projected subgradient, and nonmonotone accelerated proximal gradient methods, respectively. Extensive simulations confirm the significant advantages of irregular RIS configurations on the WSR.