Mobile communication system, base station, and user terminal

1. A mobile communication system that employs a frequency division duplex scheme, comprising:

one or a plurality of user terminals that communicate with a serving cell; and

a base station that constitutes a first cell and a second cell, wherein

the first cell uses a pair of first uplink carrier and downlink carrier for communication,

the second cell uses a pair of a second uplink carrier and the downlink carrier for communication, and

the base station and the user terminal switch a serving cell of the user terminal between the first cell and the second cell by a handover procedure.

2. The mobile communication system according to claim 1 , wherein

the first cell uses a first radio frame for downlink communication,

the second cell uses a second radio frame for downlink communication, and

the base station sets to differ a frame configuration of the first radio frame from a frame configuration of the second radio frame such that the downlink carrier is shared in the first cell and the second cell in time division.

3. The mobile communication system according to claim 2 , wherein

the first radio frame includes a first MBSFN subframe and a first non-MBSFN subframe,

the second radio frame includes a second MBSFN subframe and a second non-MBSFN subframe, and

the base station sets the frame configurations of the first radio frame and the second radio frame such that the first non-MBSFN subframe and the second MBSFN subframe overlap on a time axis, and the second non-MBSFN subframe and the first MBSFN subframe overlap on the time axis.

4. The mobile communication system according to claim 2 , wherein

on the basis of a traffic situation in each of the first cell and the second cell, the base station sets the frame configurations of the first radio frame and the second radio frame again such that a ratio of the time division is adjusted.

5. The mobile communication system according to claim 2 , wherein

the first radio frame includes a first specific subframe that should transmit a downlink broadcast signal,

the second radio frame includes a second specific subframe that should transmit a downlink broadcast signal, and

the base station sets the frame configurations of the first radio frame and the second radio frame such that the first specific subframe and the second specific subframe do not overlap on the time axis.

6. The mobile communication system according to claim 5 , wherein

the first radio frame includes a plurality of first subframes to which subframe numbers are assigned,

the second radio frame includes a plurality of second subframes to which subframe numbers are assigned,

the first specific subframe is a subframe having a specific subframe number, of the plurality of first subframes,

the second specific subframe is a subframe having the specific subframe number, of the plurality of second subframes, and

the base station sets subframe numbers of the plurality of first subframes and subframe numbers of the plurality of second subframes to be shifted by a predetermined subframe number such that the first specific subframe and the second specific subframe do not overlap on the time axis.

7. The mobile communication system according to claim 1 , wherein

the user terminal omits a measurement report process, which is a process for the handover procedure and for reporting a measurement result of a reception state in the user terminal to the serving cell, when the serving cell is switched between the first cell and the second cell.

8. The mobile communication system according to claim 1 , wherein

when a serving cell of the user terminal is the first cell and an interference power level in the first uplink carrier is higher than a predetermined level, the base station controls the serving cell of the user terminal to be switched from the first cell to the second cell.

9. The mobile communication system according to claim 8 , wherein

after the serving cell of the user terminal is switched from the first cell to the second cell, the base station controls the first cell to be deactivated.

10. The mobile communication system according to claim 8 , wherein

when there is a handover request or a connection request for a new user terminal in the middle of switching the serving cell of the user terminal from the first cell to the second cell, the base station controls the new user terminal to be accommodated in the second cell.

11. The mobile communication system according to claim 8 , wherein

when the interference power level in the first uplink carrier is lower than a predetermined level after the serving cell of the user terminal is switched from the first cell to the second cell, the base station activates the first cell and controls the serving cell of the user terminal to be switched from the second cell to the first cell.

12. The mobile communication system according to claim 11 , wherein

the base station switches the serving cell of the user terminal from the second cell to the first cell and controls to deactivate the second cell.

13. The mobile communication system according to claim 11 , wherein

when there is a handover request or a connection request for a new user terminal in the middle of switching the serving cell of the user terminal from the second cell to the first cell, the base station controls the new user terminal to be accommodated in the first cell.

14. The mobile communication system according to claim 11 , wherein

on the basis of a communication state of the user terminal, the base station controls a serving cell of a user terminal with high priority among the user terminals to be preferentially switched.

15. A base station that constitutes a first cell and a second cell in a mobile communication system that employs a frequency division duplex scheme, comprising:

a control unit that controls a serving cell of a user terminal to be switched between the first cell and the second cell by a handover procedure, wherein

the first cell uses a pair of first uplink carrier and downlink carrier for communication, and

the second cell uses a pair of a second uplink carrier and the downlink carrier for communication.

16. A user terminal that communicates with a serving cell in the mobile communication system that employs a frequency division duplex scheme, comprising:

a control unit that controls the serving cell to be switched between a first cell and a second cell by a handover procedure, wherein

the first cell uses a pair of first uplink carrier and downlink carrier for communication, and

the second cell uses a pair of a second uplink carrier and the downlink carrier for communication.