The timeline in FIG. 3 includes an overhead portion including the sounding and feedback phases “A” and “B” respectively of an Explicit sounding over a representative interval, e.g. 10 milliseconds. The explicit sounding is initiated at time t0 with the sounding transmissions 308 by the WAP 100. The sounding is conducted on a selected one of the communication channels available to the WLAN. The sounding phase is initiated with an NDPA (not shown) which announces the group of target stations selected for the MU-MIMO sounding and subsequent downlink. This announcement packet is followed by an NDP 310. Both the NDPA and the NDP packets are sent using an isotropic RF signal strength 308. Neither the NDPA nor the NDP contains any user data. These packets are explicitly dedicated to soliciting a sounding of the link channel. The header 310A of the NDP contains the VHT-LTF training field which the receiving stations targeted for the MU-MIMO downlink, i.e. stations 302A-B use to characterize the respective link channels Ha and Hb. Each receiving station 302A-B determines the respective link channel Hn, where “n” is the sub-channel or tone index, for each of the OFDM tones or sub-channels of the selected channel. The group of stations 302A-B targeted for the MU-MIMO downlink will then over the interval t1-t2, send the respective channel feedback packets 314, 312 including in the payload portion thereof, the channel feedback. In an embodiment of the invention the sounding feedback conforms to the IEEE 802.11n standard, where each feedback packet 312, 314 will include the H matrix 312B, 314B respectively for each sub-channel or tone of the corresponding WAP-station sounding. The WAP 300 aggregates this received feedback and determines a precoding matrix “Q” 318A therefrom.
