  Extracting the neutral hydrogen (HI) signal is a great challenge for
cosmological 21cm experiments, both the astrophysical foregrounds and the
receiver noise are typically several orders of magnitude greater than the 21cm
signal. However, the different properties of the 21cm signal, foreground, and
noise can be exploited to separate these components. The foregrounds are
generally smooth or correlated over the frequency space along a line of sight
(l.o.s.), while both the 21cm signal and the noise varies stochastically along
the same l.o.s. The foreground can be removed by filtering out the smooth
component in the frequency space. The receiver noise is basically uncorrelated
for observations at different times, hence for surveys they are also
uncorrelated in the different directions, while the 21cm signal which traces
the large scale structure are correlated up to certain scales. In this
exercise, we apply Wiener filters in frequency and angular space, to extract
the 21cm signals. We found that the method works well. The inaccurate knowledge
about beam could degrade the reconstruction, but the overall result is still
good, showing that the method is fairly robust.
