An acknowledgment (ACK) frame has two parts: a group ACK part and a scheduling information part. In the group ACK part, an ACK/NACK indicator is included for a previous uplink (UL) transmission per station (STA). In the scheduling information part, information included for the following UL transmission per selected STA may comprise, for example, one or more of: a number of selected STAs, a media access control (MAC) address of each selected STA, a data transmission duration per selected STA, a transmission rate per selected STA, a request transmission duration, a pilot transmission order, or a STA transmission order.