WE CLAIM:
1. A communication system comprising:
a communication terminal device; and
a plurality of base station devices configured to perform radio communication with the communication terminal device, wherein
the plurality of base station devices include a master base station device and a secondary base station device that are configured to provide at least one bearer for the communication terminal device,
when the secondary base station device detects a data inactive state for all the bearers, the secondary base station device notifies the master base station device of occurrence of the data inactive state regardless of absence of an inquiry from the master base station device, the data inactive state being a state in which downlink data for the communication terminal device is inactive,
when the master base station device receives the notification about the occurrence of the data inactive state, the master base station device transmits a command to the communication terminal device to transition from an RRCCONNECTED state to an RRCJNACTIVE state, and
based on the command from the master base station device, the communication terminal device transitions to the RRC INACTIVE state.
2. The communication system according to claim 1, wherein
at least one of the master base station device and the secondary base station device holds information about connection with the communication terminal device, even after the communication terminal device transitions to the RRCIN ACTIVE state.

3. A base station device configured to perform radio communication with a
communication terminal device, wherein
the base station device operates as a secondary base station device configuring at least one bearer for the communication terminal device with a master base station device, and
when the base station device detects a data inactive state for all the bearers, the base station device notifies the master base station device of occurrence of the data inactive state regardless of absence of an inquiry from the master base station device, the data inactive state being a state in which downlink data for the communication terminal device is inactive.
4. A base station device configured to perform radio communication with a
communication terminal device, wherein
the base station device operates as a master base station device configuring at least one bearer for the communication terminal device with a secondary base station device,
when the secondary base station device detects a data inactive state for all the bearers, the secondary base station device notifies the base station device of occurrence of the data inactive state regardless of absence of an inquiry from the base station device, the data inactive state being a state in which downlink data for the communication terminal device is inactive, and
when the base station device receives the notification about the occurrence of the data inactive state, the base station device transmits a command to the communication terminal device to transition from an RRC_CONNECTED state to an RRC JNACTIVE state.

5. A communication terminal device configured to perform radio
communication with a plurality of base station devices, wherein
the plurality of base station devices include a master base station device and a secondary base station device that are configured to provide at least one bearer for the communication terminal device,
when the secondary base station device detects a data inactive state for all the bearers, the secondary base station device notifies the master base station device of occurrence of the data inactive state regardless of absence of an inquiry from the master base station device, the data inactive state being a state in which downlink data for the communication terminal device is inactive,
when the master base station device receives the notification about the occurrence of the data inactive state, the master base station device transmits a command to the communication terminal device to transition from an RRCCONNECTED state to an RRCJNACTIVE state, and
based on the command from the master base station device, the communication terminal device transitions to the RRCINACTIVE state.
6. A communication system comprising:
a communication terminal device; and
a plurality of base station devices configured to perform radio communication with the communication terminal device, wherein
the plurality of base station devices include a master base station device and a secondary base station device that are configured to provide at least one bearer for the communication terminal device,

when the secondary base station device detects generation of downlink data for the communication terminal device being in an RRC_INACTIVE state, the secondary base station device notifies the master base station device of the generation of the downlink data,
when the master base station device receives the notification about the generation of the downlink data, the master base station device transmits paging to the communication terminal device, and
when the communication terminal device receives the paging, the communication terminal device transmits a request for restoration to an RRCCONNECTED state to the master base station device,
the paging including at least one of
information about connection between the communication terminal device and the master base station device, and
information about connection between the communication terminal device and the secondary base station device.
7. A communication system comprising:
a communication terminal device; and
a plurality of base station devices configured to perform radio communication with the communication terminal device, wherein
the plurality of base station devices include a master base station device and a secondary base station device that are configured to provide at least one bearer for the communication terminal device, and
the communication terminal device being in an RRC_INACTIVE state performs random access processing with the secondary base station device, transmits a

request to the secondary base station device for restoration to an RRCCONNECTED state after the random access processing, and transmits uplink data to the secondary base station device as small data that can be transmitted during the RRCINACTIVE state before the communication terminal device restores to the RRCCONNECTED state.
8. The communication system according to claim 7, wherein
the communication terminal device being in the RRCINACTIVE state performs random access processing with the master base station device, transmits a request to the master base station device for restoration to the RRCCONNECTED state after the random access processing, and transmits uplink data to the master base station device as the small data before the communication terminal device restores to the RRC_CONNECTED state.
9. A communication system comprising:
a communication terminal device; and
a base station device configured to perform radio communication with the communication terminal device, wherein
when an altitude difference between a position of the communication terminal device and a position of the base station device is equal to or more than a threshold, at least one of the communication terminal device and the base station device performs repetition control of repeatedly transmitting a transmission object.
10. The communication system according to claim 9, wherein
the repetition control includes control of causing hopping of a transmission frequency for the transmission object.

11. A communication system comprising:
a communication terminal device; and
a base station device configured to perform radio communication with the communication terminal device, wherein
when an altitude difference between a position of the communication terminal device and a position of the base station device is equal to or more than a threshold, at least one of cell selection, cell re-selection, and handover destination cell determination is performed based on information of path loss.
12. A communication system comprising:
a communication terminal device; and
a base station device configured to perform radio communication with the communication terminal device, wherein
the communication terminal device and the base station device switch between a plurality of communication modes during communication, depending on an altitude difference between a position of the communication terminal device and a position of the base station device.