Synchronization of joint transmissions with multiple access points

A first access point (AP) comprising:

• a wireless network interface device comprising one or more integrated circuit (IC) devices configured to:
• generate a first physical layer (PHY) data unit that for initiating a joint channel sounding procedure between i) a group of APs including the first AP and one or more second APs and ii) one or more client stations, the one or more IC devices being configured to generate the first PHY data unit to include in a first PHY preamble of the first PHY data unit a) a first training signal, and b) an indication that the first training signal is to be used by each of one or more second APs for determining a relative timing offset between the first AP and the second AP in connection with synchronous data transmissions by the group of APs,
• control the wireless network interface device to transmit the first PHY data unit to the one or more second APs to initiate the joint sounding procedure,
• generate a second PHY data unit for initiating synchronous data transmissions by the group of APs to the one or more client stations, the one or more IC devices being configured to generate the second PHY data unit to include in a second PHY preamble of the second PHY data unit i) a second training signal, and ii) an indication that each of the one or more second APs is to determine the relative timing offset between the first AP and the second AP using a) the first training signal in the first PHY preamble of the first PHY data unit and b) the second training signal in the second PHY preamble of the second PHY data unit, and
• control the wireless network interface device to transmit the second PHY data unit to the one or more second APs to initiate synchronous data transmissions by the group of APs to the one or more client stations, wherein i) the synchronous data transmissions are steered to the one or more client stations using beamforming parameters determined based on the joint sounding procedure and ii) a synchronous data transmission by a second AP among the one or more second APs is adjusted based on the relative timing offset determined based on a) the first training signal in the first PHY preamble of the first PHY data unit and b) the second training signal in the second PHY preamble of the second PHY data unit.