  The distribution of cosmological neutral hydrogen will provide a new window
into the large-scale structure of the Universe with the next generation of
radio telescopes and surveys. The observation of this material, through 21cm
line emission, will be confused by foreground emission in the same frequencies.
Even after these foregrounds are removed, the reconstructed map may not exactly
match the original cosmological signal, which will introduce systematic errors
and offset into the measured correlations. In this paper, we simulate future
surveys of neutral hydrogen using the Horizon Run 4 (HR4) cosmological N-body
simulation. We generate HI intensity maps from the HR4 halo catalogue, and
combine with foreground radio emission maps from the Global Sky Model, to
create accurate simulations over the entire sky. We simulate the HI sky for the
frequency range 700-800 MHz, matching the sensitivity of the Tianlai
pathfinder. We test the accuracy of the fastICA, PCA and log-polynomial fitting
foreground removal methods to recover the input cosmological angular power
spectrum and measure the parameters. We show the effect of survey noise levels
and beam sizes on the recovered the cosmological constraints. We find that
while the reconstruction removes power from the cosmological 21cm distribution
on large-scales, we can correct for this and recover the input parameters in
the noise-free case. However, the effect of noise and beam size of the Tianlai
pathfinder prevents accurate recovery of the cosmological parameters when using
only intensity mapping information.
