Techniques are disclosed for controlling carrier aggregation appropriately in a radio communication system having a mixture of harmonic measure equipped user equipments and harmonic measure unequipped user equipments. One aspect of the present invention relates to user equipment having a carrier aggregation function. The user equipment manages requirements regarding transmission and reception characteristics supported by the user equipment for respective combinations of carrier aggregation frequency bands and uses an identifier for identifying a combination of channel bandwidths supported for a combination of inter-band carrier aggregation capable frequency bands to report the requirements as capability information to a base station.