Array (MWA, [@2013PASA...30....7T; @2013PASA...30...31B]). Meanwhile, the construction of future instruments such as the Hydrogen Epoch of Reionization Array (HERA, [@2016icea.confE...2D]) and the low frequency Square Kilometre Array (SKA\_LOW, [@2013ExA....36..235M]) is planned to begin in a few years. The current suite of interferometers have enough sensitivity for a statistical detection of the 21cm line signal. Although upper limits on the 21cm power spectrum at various scales and redshifts have been given by many groups [e.g. @2015ApJ...809...61A; @2015PhRvD..91l3011D; @2014ApJ...788..106P; @Jacobs2015; @2016ApJ...833..102B; @2017ApJ...838...65P], the redshifted 21cm line signal has not been detected yet because of the insufficient sensitivity, complicated instrumental systematics and extremely bright foregrounds.

Typically, the foreground fluctuation is 3 $\sim$ 4 orders of magnitude larger than the 21cm line signal. To measure the 21cm line signal, one has to remove the synchrotron emission from our Galaxy and extragalactic point sources with high precision. As an useful character of the synchrotron radiation, the spectrum is expected to be smooth along the frequency axis. Thus, a classical method of foreground removal is the polynomial fitting along the line of sight. Although this method could be ineffective when the foregrounds have variance on the spectra, it is simple and pretty effective in the ideal 