  The cosmological global (sky-averaged) 21-cm signal is a powerful tool to
probe the evolution of the intergalactic medium (IGM) in high-redshift Universe
($z \leq 6$). One of the biggest observational challenges is to remove the
foreground spectrum which is at least four orders of magnitude brighter than
the cosmological 21-cm emission. Conventional global 21-cm experiments rely on
the spectral smoothness of the foreground synchrotron emission to separate it
from the unique 21-cm spectral structures in a single total-power spectrum.
However, frequency-dependent instrumental and observational effects are known
to corrupt such smoothness and complicates the foreground subtraction. We
introduce a polarimetric approach to measure the projection-induced
polarization of the anisotropic foreground onto a stationary dual-polarized
antenna. Due to Earth rotation, when pointing the antenna at a celestial pole,
the revolving foreground will modulate this polarization with a unique
frequency-dependent sinusoidal signature as a function of time. In our
simulations, by harmonic decomposing this dynamic polarization, our technique
produces two separate spectra in parallel from the same observation: (i) a
total sky power consisting both the foreground and the 21-cm background, (ii) a
model-independent measurement of the foreground spectrum at a harmonic
consistent to twice the sky rotation rate. In the absence of any instrumental
effects, by scaling and subtracting the latter from the former, we recover the
injected global 21-cm model within assumed uncertainty. We further discuss
several limiting factors and potential remedies for future implementation.
