This invention relates with a method for detecting a downlink control structure for carrier aggregation in communication network in which data transmission is scheduled by a physical downlink control channel (PDCCH). An UE receives higher layer signaling enabling carrier aggregation for the UE. The UE reads the PDCCHs of component carriers (CCs), wherein the downlink control information (DCI) in the PDCCHs of each CC is read according to one of a plurality of predefined formats derived from the higher layer signaling.