A first communication device transmits a first communication frame that includes scheduling information corresponding to a time period in which an uplink multi-user transmission is to occur. The scheduling information indicates a start time of the time period. The first communication frame includes an indication of a group of two or more second communication devices that are to transmit simultaneously during the time period as part of the uplink multi-user transmission. The first communication device transmits a second communication frame during the time period. The second communication frame is configured to trigger at least some second communication device in the group of two or more second communication devices to transmit simultaneously as part of the uplink multi-user transmission. The first communication device receives the uplink multi-user transmission during the time period, the uplink multi-user transmission being responsive to the second communication frame.