and calibration are explained in Sec. \[S:MWA\]. In Sec. \[S:method\], we introduce the 21cm-CMB cross power spectrum and calculation methods. In Sec. \[S:FGRM\], we describe our polynomial foreground removal method. In Sec. \[S:system\], we investigate the contamination of ionosphere, the dependence on groups of pointings and on frequency. The resulting auto and cross power spectra are shown in Sec. \[S:results\]. In Sec. \[S:discuss\], we discuss the forecast. Finally, we summarize this work in Sec. \[S:summary\].

Murchison Widefield Array {#S:MWA}
=========================

The MWA is an interferometer constructed within the Murchison Radioastronomy Observatory in Western Australia. A tile consists of 16 dipole antennas and the effective area of antenna is about 20$\rm m^2$ at 150MHz. The observable frequency range is 80MHz to 300MHz, and instantaneous bandwidth is 30.72MHz which is divided by 1.28MHz coarse channels. The maximum baseline of the MWA phase 1 is about 3000m and the maximum resolution is a few arcminutes. The MWA has huge primary beam (400$\rm deg^2$) and large side lobes. The large field of view (FoV) is an advantage of the MWA for the cross correlation because the large FoV can reduce the cross correlation error. Further details are written in [@2013PASA...30....7T]. Note that, until middle 