A mobile station and a method in a mobile station, the method comprising: establishing communication in a multi-carrier wireless communication system using a primary carrier; receiving a first control signaling via the primary carrier, the first control signaling assigning at least one secondary carrier; performing sleep mode operation, the sleep mode having a sleep window during which the mobile station is not listening for a traffic indication and a listening window during which the mobile station is listening for a traffic indication; receiving a traffic indication via the primary carrier during the listening window; determining that the traffic indication corresponds to the secondary carrier; receiving timing information corresponding to the primary carrier; and transmitting uplink data via the secondary carrier, the timing of the uplink transmission based on timing information corresponding to the primary carrier.