Example methods disclosed herein for a station in a communication system include accessing a response frame received in response to a control frame transmitted to an access point. The response frame indicates a start time for a trigger frame to be transmitted by the access point to trigger the station to transmit an uplink transmission. Disclosed example methods also include controlling a receiver of the station to receive the trigger frame at the indicated start time. Example methods disclosed herein for an access point in a communication system include transmitting a response frame to a station in response to a control frame received from the station. The example response frame indicates a start time for a trigger frame to be transmitted by the access point to trigger the station to transmit an uplink transmission. Disclosed example methods also include transmitting the trigger frame at the indicated start time.