The reconfigurable intelligent surface (RIS) has drawn considerable attention for its ability to enhance the performance of not only the wireless communication but also the indoor localization with low-cost. This paper investigates the performance limits of the RIS-based near-field localization in the asynchronous scenario, and analyzes the impact of each part of the cascaded channel on the localization performance. The Fisher information matrix (FIM) and the position error bound (PEB) are derived. Besides, we also derive the equivalent Fisher information (EFI) for the position-related intermediate parameters. Enabled by the derived EFI, we verify that both the ranging and bearing information of the user can be obtained when the near-field model is considered for the RIS-User equipment (UE) part of the channel, while only the direction of the UE can be inferred in the far-field scenario. This result is well known in the scenario that the curvature of arrival (COA) is directly sensed by the traditional active large-scale array, and we prove that it still holds when the COA is sensed passively by the large RIS. For the base station (BS)-RIS part of the channel, we reveal that this part of the channel determines the type of the gain provided by the BS antenna array. Besides, in the single-carrier, single snapshot case, it requires both the BS-RIS and the RIS-UE part of the channel works in the near-field scenario to localize the UE. We also show that the well-known focusing control scheme for RIS, which maximizes the received SNR, is not always a good choice and may degrade the localization performance in the asynchronous scenario. The simulation results validate the analytic work. The impact of the focusing control scheme on the PEB performances under synchronous and asynchronous conditions is also investigated.

翻译：可重构智能表面（RIS）因其不仅能提升无线通信性能，还能以低成本增强室内定位能力而备受关注。本文研究了异步场景下基于RIS的近场定位性能极限，并分析了级联信道各组成部分对定位性能的影响。推导了费舍尔信息矩阵（FIM）和位置误差界（PEB）。此外，我们还推导了与位置相关中间参数的等效费舍尔信息（EFI）。借助所推导的EFI，我们验证了当信道中RIS-用户设备（UE）部分采用近场模型时，可以同时获取用户的距离信息和方位信息，而在远场场景下仅能推断UE的方向。这一结果在传统有源大规模阵列直接感知到达曲率（COA）的场景中已是共识，我们证明当COA由大型RIS被动感知时该结论仍然成立。对于基站（BS）-RIS信道部分，我们揭示了该部分信道决定了BS天线阵列提供的增益类型。此外，在单载波单快照情况下，需要信道中BS-RIS和RIS-UE部分均处于近场场景才能实现对UE的定位。我们还表明，经典的RIS聚焦控制方案（以最大化接收信噪比为目标）并非总是最佳选择，在异步场景下可能会降低定位性能。仿真结果验证了理论分析。同时研究了同步和异步条件下聚焦控制方案对PEB性能的影响。