A wireless network including client stations and an access point. A first set of client stations transmit uplink user data during a simultaneous uplink transmission period without sensing channel prior to transmitting the uplink user data. A second set of client stations transmit the uplink user data by sensing channel prior to transmitting the uplink user data. The access point (i) services the client stations, (ii) instructs the second set of client stations to not transmit the uplink user data during the simultaneous uplink transmission period, and (iii) receives the uplink user data transmitted from two or more of the first set of client stations during the simultaneous uplink transmission period. The access point sequentially transmits respective acknowledgements to the two or more of the first set of client stations in a non-overlapping manner during the simultaneous uplink transmission period.