Beam Training and Alignment for RIS-Assisted Millimeter-Wave Systems: State of the Art and Beyond

Reconfigurable intelligent surface (RIS) has recently emerged as a promising paradigm for future cellular networks. Specifically, due to its capability in reshaping the propagation environment, RIS was introduced to address the blockage issue in millimeter-wave (mmWave) or even terahertz communications. The deployment of RIS, however, complicates the system architecture and poses a significant challenge for beam training (BT)/beam alignment (BA), a process that is required to establish a reliable link between the transmitter and the receiver. In this article, we first review several state-of-the-art beam training solutions for RIS-assisted mmWave systems and discuss their respective advantages and limitations. We also present a new multidirectional BT method, which can achieve decent BA performance with only a small amount of training overhead. Finally, we outline several important open issues in BT for RIS-assisted mmWave systems.