A method, performed by a user equipment, is described for uplink transmission in a wireless communication system. A first time alignment (TA) to a first cell belonging to a first timing advance group (TAG) is performed. A second TA to a second cell belonging to a second TAG is performed. A determination is made as to whether to transmit or drop a sounding reference signal (SRS) on a subframe. The SRS is determined to be dropped based on whether the first and second TAGs are configured, whether at least one symbol of the subframe is used to transmit the SRS toward the second cell of the second TAG and also used to transmit a physical uplink shared channel (PUSCH) toward the first cell of the first TAG, and whether a total uplink transmission power exceeds a maximum value.