A method for adjusting an arrival time of a first orthogonal frequency division multiplex (OFDM) data unit of an uplink multi-user (MU) data unit is described. Respective round trip times (RTTs) are determined by a first communication device for multiple communication devices that communicate with the first communication device. A first time offset for a second communication device of the multiple communication devices is determined by the first communication device based on the respective RTTs. The first OFDM data unit is to be transmitted by the second communication device. A first timing control message that indicates the first time offset is generated by the first communication device. The first timing control message is caused by the first communication device to be transmitted to the second communication device.