Radio orbital angular momentum (OAM) communications require accurate alignment between the transmit and receive beam directions. Accordingly, a key feature of OAM receivers is the ability to reliably estimate the angle of arrival (AoA) of multi-mode OAM beams. Considering the limitations of existing AoA estimation techniques, in this paper, we propose an easier-to-implement AoA estimation method based on applying multiple times the estimating signal parameters via rotational invariance techniques (ESPRIT) algorithm to the received training signals in OAM mode and frequency domains, which is denoted as the mode-frequency (M-F) multi-time (MT)-ESPRIT algorithm.With this method, the misalignment error of real OAM channels can be greatly reduced and the performance approaches that of ideally aligned OAM channels.Index Terms-Orbital angular momentum (OAM), uniform circular array (UCA), angle of arrival (AoA) estimation, estimating signal parameter via rotational invariance techniques (ESPRIT). I. INTRODUCTIONSince the discovery in 1992 that vortex light beams can carry orbital angular momentum (OAM) [1], a significant research effort has been focused on vortex electromagnetic (EM) waves [2]. The phase front of a wave carrying OAM rotates with azimuth exhibiting a helical structure e j φ in space, where φ is the transverse azimuth and is an unbounded integer defined as OAM topological charge or OAM mode number [1]. Due to inherent orthogonality among different OAM modes, OAM wireless communications represent a novel approach for multiplexing a set of orthogonal signals on the same

The sixth-generation (6G) wireless communication system is expected to integrate communication, intelligence, sensing, positioning, control, and calculation to adapt to timecritical, ultra-reliable, and energy-saving data delivery, as well as accurate positioning of personnel and equipment, serving the Internet of Things (IoT). On one hand, reconfigurable intelligent surface (RIS) can intelligently manipulate radio waves and is considered to be one of the candidate technologies for the 6G wireless communication. Hence, there are more and more surveys on RIS-assisted communications. On the other hand, the potential of RIS in positioning has attracted growing attention, and articles on RIS-assisted positioning have been blown out. Therefore, it is time to review this literature to understand the potential of RIS positioning, research status, and point out the direction for future research. This paper first explains the working principle and channel model of RIS and summarizes some characteristics of RIS suitable for positioning. Then we give a concise review and classification of existing RIS positioning research. Finally, we put forward our views on the future research challenges and attractive directions for RIS-aided wireless positioning technology.

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