A method for transmitting an orthogonal frequency division multiple access (OFDMA) data unit is described. Respective sub-channels of an orthogonal frequency division multiplexing (OFDM) communication channel are allocated to first and second communication devices for simultaneous OFDM transmission. A first sub-channel is allocated to the first communication device and a second sub-channel is allocated to the second communication device. Indications of the respective allocated sub-channels are transmitted to the first and second communication devices. A medium access control (MAC) protocol data unit (MPDU) is generated that omits i) a MAC address of a transmitter, and ii) a MAC address of the first communication device. An OFDMA data unit including the MPDU is transmitted by the transmitter via the first sub-channel to the first communication device.