Multiple trigger frames are generated at a first communication device to trigger an uplink orthogonal frequency multiple access (OFDMA) transmission by multiple second communication devices. The multiple trigger frames include a broadcast trigger frame that includes information to indicate transmission parameters for a first subset of the second communication devices, and one or more unicast trigger frames, each of the one or more unicast trigger frame including information to indicate transmission parameters for a particular second communication device in a second subset of the second communication devices. The broadcast trigger frame is transmitted, in a first frequency portion of a downlink OFDMA transmission, to the first subset of the second communication devices, and respective unicast trigger frames are transmitted, in respective second frequency portions of the downlink OFDMA transmission, to the second subset of the second communication devices.