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：大规模天线阵列在高频段的应用被广泛视为后5G无线网络的发展方向。这导致近场区域中波前不再为平面波而呈现球面波特性，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球面波模型提升至更精确的电磁传播模型。基于完整电磁传播模型，针对三种电场类型（矢量电场、标量电场与整体标量电场）建立了具备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，采用克拉美罗界评估可达估计精度。通过结合电磁传播理论与估计理论，推导了不同电场观测下克拉美罗界的表达式。当终端位于接收天线表面中心垂线上时，可进一步获得闭合表达式。此外，上述讨论被扩展至多接收天线系统。在此情形下，推导了采用不同电场类型时的克拉美罗界，并深入研究了不同接收天线数量的影响。数值结果量化了克拉美罗界并验证了解析结论，同时评估了电场类型、波长、接收天线尺寸及天线数量等不同系统参数的影响。