WE CLAIM:
1. A communication system comprising:
a communication terminal device; and
a base station device configured to perform radio communication with the 5 communication terminal device,
wherein the base station device includes:
a plurality of distributed units (DUs) that transmit and receive radio signals; and a central unit (CU) that controls the plurality of DUs,
the CU duplicates a downlink packet addressed to the communication terminal 10 device, and forwards the duplicated downlink packet to each of at least two DUs among the plurality of DUs,
each of the at least two DUs transmits, to the communication terminal device by the radio signal, the downlink packet obtained from the CU, and
upon redundant receipt of the downlink packets, the communication terminal 15 device removes a redundant downlink packet in accordance with a predefined downlink packet removal criterion.
2. The communication system according to claim 1,
wherein the communication terminal device transmits, to each of two or more 20 of the plurality of DUs, a duplicated uplink packet of an uplink packet to be transmitted from the communication terminal device, and
upon redundant receipt of the uplink packets, the base station device removes a redundant uplink packet in accordance with a predefined uplink packet removal criterion.
25 3. 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 base station device includes:
5 a plurality of distributed units (DUs) that transmit and receive radio signals; and
a central unit (CU) that controls the plurality of DUs,
the CU obtains, from a DU connected to the communication terminal device,
delivery complete information on a packet that has already been delivered to the
communication terminal device, and
10 the CU transmits, from the DU or another DU to the communication terminal
device, a packet that is not indicated in the delivery complete information.
4. The communication system according to claim 3,
wherein the delivery complete information is a sequence number of a packet 15 whose delivery completion has been notified from the communication terminal device.
5. The communication system according to claim 3,
wherein the CU obtains the delivery complete information based on a packet data convergence protocol (PDCP) status report notified from the communication 20 terminal device.
6. A communication system comprising:
a communication terminal device; and
a plurality of base station devices configured to perform radio communication 25 with the communication terminal device,

wherein the plurality of base station devices include a master base station device and a secondary base station device which configure bearers for the communication terminal device,
when the secondary base station device that communicates with the
5 communication terminal device is changed from a first secondary base station device to a
second secondary base station device, the master base station device obtains, from the
communication terminal device, delivery information on a packet that has already been
transmitted from the first secondary base station device to the communication terminal
device, and
10 the master base station device transmits, from the second secondary base station
device or the master base station device to the communication terminal device, a packet that is not indicated in the delivery information.
7. A communication system comprising:
15 a communication terminal device; and
a base station device configured to perform radio communication with the communication terminal device,
wherein the base station device includes:
a plurality of distributed units (DUs) that transmit and receive radio signals to 20 and from the communication terminal device; and
a central unit (CU) that controls the plurality of DUs, and
the CU has a function of routing downlink data addressed to the communication terminal device, in or lower than a packet data convergence protocol (PDCP).
25 8. 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 5 device and a secondary base station device which configure bearers for the communication terminal device,
each of the master base station device and the secondary base station device includes:
a plurality of distributed units (DUs) that transmit and receive radio signals to 10 and from the communication terminal device; and
a central unit (CU) that controls the plurality of DUs,
the master base station device receives, from a network device higher than the master base station device, downlink data addressed to the communication terminal device, and routes the downlink data to the master base station device and the secondary 15 base station device,
the CU of the master base station device has:
a function of determining a routing destination DU of the master base station device; and
a function of forwarding, to the determined routing destination DU, the 20 downlink data routed to the master base station device, and the CU of the secondary base station device has:
a function of determining a routing destination DU of the secondary base station device; and
a function of forwarding, to the determined routing destination DU, the 25 downlink data routed to the secondary base station device.

9. A communication system comprising:
a communication terminal device; and
a plurality of base station devices configured to perform radio communication 5 with the communication terminal device,
wherein the plurality of base station devices include a master base station device and a secondary base station device which configure bearers for the communication terminal device,
each of the master base station device and the secondary base station device 10 includes:
a plurality of distributed units (DUs) that transmit and receive radio signals to and from the communication terminal device; and
a central unit (CU) that controls the plurality of DUs,
the master base station device receives, from a network device higher than the 15 master base station device, downlink data addressed to the communication terminal device, and routes the downlink data to the master base station device and the secondary base station device,
the CU of the master base station device has:
a function of determining a routing destination DU of the master base station 20 device;
a function of forwarding, to the determined routing destination DU, the downlink data routed to the master base station device; and
a function of determining a routing destination DU of the secondary base
station device, and
25 the CU of the secondary base station device has a function of forwarding the

downlink data routed to the secondary base station device, to the routing destination DU of the secondary base station device that has been determined by the master base station device.
5 10. 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 10 device and a secondary base station device which configure bearers for the communication terminal device,
each of the master base station device and the secondary base station device includes:
a plurality of distributed units (DUs) that transmit and receive radio signals to 15 and from the communication terminal device; and
a central unit (CU) that controls the plurality of DUs,
the master base station device receives, from a network device higher than the master base station device, downlink data addressed to the communication terminal device, and routes the downlink data to the master base station device and the secondary 20 base station device,
the CU of the master base station device has:
a function of determining a routing destination DU of the master base station device;
a function of forwarding, to the determined routing destination DU, the 25 downlink data routed to the master base station device;

a function of determining a routing destination DU of the secondary base station device; and
a function of forwarding, to the routing destination DU of the secondary base
station device, the downlink data routed to the secondary base station device.
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