A method of operating a wireless communication terminal includes receiving one or more downlink control messages that each contain scheduling information scheduling the wireless terminal to receive a downlink transmission on either a primary carrier or a secondary carrier. The method also includes determining, for each of the downlink control messages, whether that message includes scheduling information for the primary carrier or for a secondary carrier. Additionally, the method includes selecting a format for an uplink control message based on whether any of the downlink control messages includes scheduling information for a secondary carrier, generating an uplink control message based on the selected format, and transmitting the uplink control message to the base station.