1. A method of transmitting a signal by a station (STA) operating in a wireless LAN system, the method comprising:receiving, from an access point (AP), a first frame for switching a power saving (PS) mode to an awake mode; and
transmitting, to the AP, a second frame indicating the awake mode of the STA in response to the first frame,
wherein the second frame is a Null Data Packet (NDP) frame including a legacy field and a high efficiency-signal (HE-SIG) field, and
wherein the legacy field has a fixed symbol size regardless of a size of a resource allocated to the STA and the HE-SIG field comprises a fixed size field having a fixed symbol size regardless of the size of the resource allocated to the STA and a variable size field having a variable symbol size based on the size of the resource allocated to the STA.