Multi-User Tracking in Reconfigurable Intelligent Surface Aided Near-Field Wireless Communications System  
[PERSON]  
1 College of Electronics and Information Engineering, Tongji University, Shanghai 201804, China; [EMAIL_ADDRESS] (Y.M. ); erwu. direct signals towards the desired direction, thus significantly enhancing the virtual line-of-sight (VLoS) links [11; 16]. From a positioning perspective, RISs can serve as an extra reference point, offering supplementary measurements that help to avoid dependence on the LoS path [17]. Introducing RISs into positioning recently sparked widespread interest due to its promising benefits. For instance, [PERSON] et al. [18] analyzed a downlink orthogonal frequency division multiplexing (OFDM) scenario aided by RIS from both the communication and positioning perspectives. Similarly, [PERSON] et al. [19] derived the Cramer-Rao lower bound (CRLB) for positioning error in a downlink setting, emphasizing how deploying RIS in the far field can outperform conventional methods relying solely on naturally occurring scatterers for better positioning accuracy. [PERSON] et al. [20] explored the upper limits of positioning performance of an uplink multiple-input multiple-output (MIMO) system, proposed a near-field propagation model, and demonstrated the significant promotion of positioning accuracy brought by a single RIS. [PERSON] et al. [21] applied a RIS-assisted user positioning scheme based on RSS in an indoor scenario, where a high-resolution signal intensity map was built to localize the target through iterative optimization of RIS phase shift. [PERSON] et al. [22] introduced a beam searching method based on hierarchical codebook design to achieve accurate positioning indirectly by utilizing RIS-assisted channel parameters. Similarly, the user position was estimated from the channel information of RIS, and channel angle parameters were obtained through the maximum likelihood method [23]. For MIMO-OFDM systems, [PERSON] et al. [24] implemented a twin-RIS structure and applied advanced signal processing techniques to extract relevant information from the received signals. [PERSON] et al. [25] considered a single-anchor positioning problem under NLoS conditions characterized by frequent applications. More practical RIS-assisted localization schemes can be found in [26; 27]. The design of the RIS is the most critical part of assisting either positioning or communication. Literature concerning the optimization of RIS-assisted communications under various channel state information (CSI) and a priori location knowledge is abundant [28; 29; 30]. However, existing optimization solutions for RIS aimed at improving communication are not necessarily practical or optimal in improving positioning. RIS optimization focusing on positioning is less frequently investigated. [PERSON] et al. [31] proposed a new RIS conformal structure that can be used to estimate the AoD and AoA of RISs and solve scatterer and user localization problems based on it. [PERSON] et al. [32] studied joint positioning and synchronization problems and optimized the BS precoding and RIS phase shift based on their proposed novel beamforming codebook. [PERSON] et al. [33] and [PERSON] et al. [34] optimized a RIS by minimizing positioning error bounds (PEB), where the blockage of LoS and hardware limitations were considered. [PERSON] et al. [35] proposed an optimization scheme of RIS based on CRLB for positioning and transmit power minimization and adopted a semi-positive semi-definite relaxation method for the power minimization part. Most existing works involving RIS-assisted localization consider snapshot localization, and only a few works address the user tracking problem. [PERSON] et al. [36] proposed a Bayesian tracking scheme which is realized indirectly by estimating AoAs. If the user's initial location information is known, better positioning accuracy can be achieved through directional beamforming on the BS side [32].