The invention relates to a plasma source whose plasma is ignited by means of an electric voltage. An undershootovershoot eliminator is coupled to the MOS for the MOS changing from totally on state to totally off state or vice versa softly when switching the MOS. The serving BS determines the power availability for transmit antennas in the floating CoMP cell that are shared with other floating CoMP cells, and scales the tentative precoding weights based on the power availability of the shared transmit antennas to determine final precoding weights so that the power constraints of the shared transmit antennas will not be violated. A sacrificial spacer is formed on each inner wall of the first groove by etching back the second insulating layer and forming a second groove in the first insulating layer in communication with the first groove to expose a surface of the polysilicon layer at the bottom of the second groove.