1. A method for a User Equipment (UE) operating in a wireless communication system, the method comprising:configuring with a Timing Advance Group (TAG) to which at least two uplink (UL) cells belong;
receiving a Physical Downlink Control Channel (PDCCH) order from an eNodeB (eNB) on a downlink (DL) cell associated with a first UL cell in the TAG, wherein the PDCCH order triggers a random access procedure by informing a random access preamble;
determining whether the first UL cell is available for transmitting the random access preamble;
selecting one of the at least two UL cells which is available for transmitting the random access preamble if the first cell is determined as unavailable; and
transmitting the informed random access preamble on the selected one of the at least two UL cells,
wherein the first UL cell is determined as unavailable if it is occupied by other transmitters as a result of a Listen Before Talk (LBT) procedure.