In generating a physical layer (PHY) data unit is for transmission in a shared communication medium during a transmit opportunity (TXOP), a wireless communication device generates a TXOP field to indicate a length of time remaining in the TXOP. The wireless communication device generates a physical layer (PHY) preamble of the PHY data unit to include a legacy signal field in a legacy portion of the PHY preamble and a non-legacy signal field in a non-legacy portion of the PHY preamble, the non-legacy signal field generated to include the TXOP field. The wireless communication device generates the PHY data unit to include the PHY preamble and a data portion, and transmits the PHY data unit in the shared communication medium.