  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.
