A first communication device generates a multi-user (MU) physical layer (PHY) data unit having independent data for multiple second communication devices. The first communication device also generates respective control frames to prompt second communication devices to transmit an orthogonal frequency division multiple access (OFDMA) PHY data unit to acknowledge that the second communication devices received the MU PHY data unit. The MU PHY data unit and the respective control frames are transmitted to the multiple second communication devices. The first communication device receives the OFDMA PHY data unit from at least some of the second communication devices. The first communication device processes the OFDMA PHY data unit to determine whether the multiple second communication devices received the MU PHY data unit.