We Claim:
1. A mobile communication system in which a first base station and a second
base station communicate with a user equipment in parallel,
wherein the first base station transmits, to the second base station, a notification regarding communication by the first base station and the second base station with the user equipment in parallel,
the second base station transmits, to the first base station, a notification regarding communication by the first base station and the second base station with the user equipment in parallel,
the first base station transmits, to the user equipment, a notification regarding communication by the first base station and the second base station with the user equipment in parallel, and
the user equipment transmits, to the first base station, a notification regarding communication by the first base station and the second base station with the user equipment in parallel.
2. A mobile communication system in which a first base station and a second
base station communicate with a radio network controller in parallel,
wherein the first base station transmits, to the radio network controller, a notification regarding communication by the first base station and the second base station with theradio network controller in parallel, and
the radio network controller transmits, to the first base station, a notification regarding communication by the first base station and the second base station with theradio network controller in parallel.

3. The mobile communication system as claimed in claim 2,
wherein the radio network controller includes:
a mobility management devicethat communicates with the first base station in a control plane; and
a gateway stationthat communicates with the second base station in a user plane,
themobility management device transmits, to the gateway station, a notification regarding communication by the first base station and the second base station with theradio network controller in parallel, and
the gateway station transmits, to themobility management device, a notification regarding communication by the first base station and the second base station with theradio network controller in parallel.
4. The mobile communication system as claimed in claim 2,
wherein in a control plane, the first base station communicates with the user equipment and the first base station communicates with the radio network controller, and
in a user plane, the second base station communicates with the user equipment and the second base station communicates with the radio network controller.
5. The mobile communication system as claimed in claim 4, wherein in the
user plane, the second base station communicates directly with the radio network
controller.

6. The mobile communication system as claimed in claim 4, wherein in the user plane, the first base station communicates directly with the user equipment and the second base station communicates with the radio network controller via the first base station.
7. A base station that communicates with a user equipment in parallel with another base station,
wherein the base station transmits, to the other base station, a notification regarding communication by the base station with the user equipment in parallel with the other base station,
the base station receives, from the other base station, a notification regarding communication by the base station with the user equipment in parallel with the other base station,
the base station transmits, to the user equipment, a notification regarding communication by the base station with the user equipment in parallel with the other base station, and
the base station receives, from the user equipment, a notification regarding communication by the base station with the user equipment in parallel with the other base station.
8. A base station that communicates with a user equipment in parallel with
another base station,
wherein the base station receives, from the other base station, a notification regarding communication by the base station with the user equipment in parallel with the other base station, and

the base station transmits, to the other base station, a notification regarding communication by the base station with the user equipment in parallel with the other base station.
9. A user equipmentthat communicates with a first base station and a second base station in parallel,
wherein the user equipment receives, from the first base station, a notification regarding communication by the user equipmentwiththefirst base station and the second base station in parallel, and
the user equipment transmits, to the first base station, a notification regarding communication by the user equipmentwiththefirst base station and the second base station in parallel.