Hybrid Near-Field and Far-Field Localization with Multiple Holographic MIMO Surfaces

Localization methods based on holographic multiple input multiple output (HMIMO) have gained much attention for its potential to achieve high accuracy. By deploying multiple HMIMOs, we can improve the link quality and system coverage. As the scale of HMIMO increases to improve beam control capability, the near-field (NF) region of each HMIMO expands. However, existing multiple HMIMO-enabled methods mainly focus on the far-field (FF) of each HMIMO, which leads to low localization accuracy when applied in the NF. In this paper, a hybrid NF and FF localization method aided by multiple RISs, a low cost implementation of HMIMO, is proposed. In such a scenario, it is difficult to achieve user localization and RIS optimization since the equivalent NF of all RISs expands, which results in high complexity, and we need to handle the interference caused by multiple RISs. To tackle this challenge, we propose a two-phase RIS-enabled localization method that first estimate the relative locations of the user to each RIS and fuse the results to obtain the global estimation. In this way, the algorithm complexity is reduced. We formulate the RIS optimization problem to keep the RIS sidelobe as low as possible to minimize the interference. The effectiveness of the proposed method is verified through simulations.