A method implemented in an access point (AP) used in a wireless communications system including a first uplink (UL) client device, a second UL client device, and a first downlink (DL) client device is disclosed. The method comprises: receiving, from the first DL client device, first composite channel matrix M1 comprising a first channel matrix and a second channel matrix, wherein the first DL client device estimates the first channel matrix and the second channel matrix. Other methods, systems, and apparatuses also are disclosed.