Afterwards, when it is determined that more radio resources are needed, the terminal 100 may search for another cell through an RRC connection reconfiguration procedure and establish the RRC connection with a discovered cell to use the carrier aggregation in which the terminal 100 simultaneously communicates both cells. The carrier aggregation may be set up for the uplink transmission, and may also be set up for the downlink transmission.

The terminal 100 may set up the carrier aggregation for the uplink to use two frequency bands simultaneously (S101). In such a case, the terminal 100 communicates with two cells, each of which serves as the serving cell. The serving cells may be categorized into the primary cell (PCell) and the secondary cell (SCell). In the case that terminal 100 communicates with two serving cells at the same time, one of the serving cells is the primary cell (PCell) and the other serving cell is the secondary cell (SCell). The terminal 100 first establishes the RRC connection to the primary cell to communicate with each other and, if additional radio resources are needed, may establish the RRC connection to the secondary cell also to communicate with both serving cells at the same time.