A method is provided for transmitting a signal by a station in a wireless local area network (WLAN). The station obtains a transmission opportunity (TXOP) indicating a time duration used for transmission of a plurality of data units. The station transmits, during the TXOP, an initial data unit of the plurality of data units. The initial data unit comprises a control field indicating a transmit bandwidth of the initial data unit. The station selects a transmit bandwidth of a non-initial data unit of the plurality of data units to be narrower than the transmit bandwidth of the initial data unit. The station transmits, during the TXOP, the non-initial data unit. The non-initial data unit comprises a control field indicating the transmit bandwidth of the non-initial data unit.