The adoption of large-scale antenna arrays at high-frequency bands is widely envisioned in the beyond 5G wireless networks. This leads to the near-field regime where the wavefront is no longer planar but spherical, bringing new opportunities and challenges for communications and positioning. In this paper, we improve the near-field positioning technology from the classical spherical wavefront model (SWM) to the more accurate and true electromagnetic propagation model (EPM). A generic near-field positioning model with different observation capabilities for three electric field types (vector, scalar, and overall scalar electric field) is developed based on the complete EPM. For these three observed electric field types, the Cram\'er-Rao bound (CRB) is adopted to evaluate the achievable estimation accuracy. The expressions of the CRBs for different electric field observations are derived by combining electromagnetic propagation concepts with estimation theory. Closed-form expressions can be further obtained as the terminal is assumed to be on the central perpendicular line (CPL) of the receiving antenna surface. Moreover, the above discussions are extended to the system with multiple receiving antennas. In this case, the CRBs using various electric field types are derived, and the effect of different numbers of receiving antennas is deeply investigated. Numerical results are provided to quantify the CRBs and validate the analytical results. Also, the impact of different system parameters, including electric field type, wavelength, size of the receiving antenna, and number of antennas, is evaluated.

翻译：在超5G无线网络中，高频段大规模天线阵列的广泛应用已被广泛展望。这带来了近场区域，在该区域中波前不再是平面波而是球面波，为通信与定位带来了新的机遇与挑战。本文将近场定位技术从经典的球面波前模型（SWM）改进为更精确且真实的电磁传播模型（EPM）。基于完整的EPM，针对三种电场类型（矢量、标量和总体标量电场）建立了具有不同观测能力的通用近场定位模型。针对这三种观测电场类型，采用克拉美-罗界（CRB）来评估可达估计精度。通过将电磁传播概念与估计理论相结合，推导了不同电场观测下CRB的表达式。当终端假设位于接收天线表面的中心垂直线（CPL）上时，可进一步获得闭式表达式。此外，上述讨论被扩展至多接收天线系统。在该情况下，推导了使用不同电场类型时的CRB，并深入研究了不同接收天线数量的影响。提供了数值结果以量化CRB并验证分析结果。同时，评估了不同系统参数（包括电场类型、波长、接收天线尺寸和天线数量）的影响。