A method implemented by an Access Point (AP) in a Wireless Local Area Network (WLAN) to facilitate an unassociated station (STA) to participate in an uplink multi-user simultaneous transmission. The method includes receiving a request frame from the unassociated STA and generating a response frame, where the response frame includes timing information regarding a future transmission, by the AP, of a trigger frame. The trigger frame includes scheduling information for scheduling an uplink multi-user simultaneous transmission, where at least one transmission resource unit allocated for the uplink multi-user simultaneous transmission allows random access. The method further includes transmitting the response frame to the unassociated STA through a wireless medium.