A first communication device allocates respective frequency sub-channels for subsequent orthogonal frequency division multiple access (OFDMA) communications with two or more second communication devices, including allocating a first frequency sub-channel, a second frequency sub-channel, and a third frequency sub-channel between the first frequency sub-channel and the second frequency sub-channel. The first communication device generates and transmits a first downlink OFDMA data unit configured to prompt the two or more second communication devices to transmit as part of a multi-user transmission that spans the first frequency sub-channel, the second frequency sub-channel, and the third frequency sub-channel. The first communication device receives an uplink OFDMA transmission and determines that the uplink OFDMA transmission did not include a transmission within the third frequency sub-channel. In response, the first communication device does not transmit within the third frequency sub-channel when transmitting one or more subsequent downlink OFDMA data units via the communication channel.