A method of performing channel sounding performed by an access point (AP) in a wireless LAN system is provided. The method includes transmitting an NDPA (NDP Announcement) frame informing the NDP (null data packet) to be transmitted, transmitting the NDP, and receiving a feedback frame from the station (STA). The NDP includes a signal field. The signal field of the NDP is transmitted through two Orthogonal Frequency Division Multiplexing (OFDM) symbols. One of the two OFDM symbols is a repeated OFDM symbol in which the remaining one is repeated.
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