1. A communication system comprising:a base station device and a communication terminal device configured to communicate with each other in a first cell configured on a plurality of predetermined first carriers,
wherein the base station device includes a second cell configured on a second carrier different from the plurality of first carriers, and the base station device and the communication terminal device are configured to communicate with each other in the second cell configured on the second carrier,
the second carrier is associated with the plurality of first carriers, the first carriers belonging to the same frequency band as a frequency band of the second carrier,
an associated carrier, being one of the plurality of first carriers associated with the second carrier, is used in notifying the communication terminal device of carrier information, the carrier information being information on the second carrier, the associated carrier being determined based on the communication terminal device.