A first access point (AP) generates and transmits a first physical layer (PHY) data unit for initiating a joint channel sounding procedure between a group of APs, including the first AP and one or more second APs, and one or more client stations. The first PHY data unit is generated to indicate that the first PHY data unit is of joint channel sounding type. The first AP generates and transmits a second PHY data unit for initiating synchronous data transmissions by the group of APs. The second PHY data unit is generated to indicate that the second PHY data unit is of a joint data transmission type. The synchronous data transmissions initiated by the second PHY data unit are steered using beamforming parameters determined based on the joint sounding procedure. A synchronous data transmission by a second AP is adjusted based on the relative timing offset.