A method in a wireless access point (AP) for controlling medium access includes: transmitting a beacon frame defining (i) a beacon header interval (BHI), and (ii) a data transmission interval (DTI) divided into a predetermined number of scheduled periods, each scheduled period containing a set of uplink allocation request sub-periods; sending, to a client device, an uplink allocation request assignment indicating an assigned uplink allocation request sub-period from the set, that corresponds to the client device; during a current one of the scheduled periods, receiving an uplink allocation request from the client device during the assigned uplink allocation request sub-period; determining an uplink allocation sub-period for the client device based on the uplink allocation request; and during a next one of the scheduled periods, sending an indication of the uplink allocation sub-period to the client device.