In Spatial Division Multiple Access (SDMA) communications, mobile devices that are transmitting to a base substantially simultaneously during a predetermined communications phase may wait until after an end of the predetermined communications phase to receive acknowledgments to their transmissions, so that a mobile device making a relatively short transmission does not experience an acknowledgement timeout while another mobile device is still completing a relatively long transmission. In some embodiments, the mobile devices may transmit a non-immediate block acknowledgement request during the communications phase.