A client station of a wireless local area network (WLAN) determines respective availabilities at the client station of a plurality of subchannels for a multi-user (MU) transmission, and generates a media access control (MAC) data unit (MPDU) that includes a MAC header. The MAC header includes a control field, and the control field includes a subfield having i) an identification (ID) that indicates the subfield includes subchannel availability information, and ii) a bitmap that indicates the respective availabilities at the client station of the plurality of subchannels. The client station transmits the MPDU to an access point of the WLAN for a subsequent allocation by the access point of wireless bandwidth for the MU transmission.