Method and apparatus for transmitting sounding reference signal

1. A method for transmitting a sounding reference signal (SRS) in a wireless communication system, performed by a user equipment that is configured with a plurality of serving cells including a time division duplex (TDD) cell and a frequency division duplex (FDD) cell, the method comprising:

receiving downlink control information from a first serving cell of the plurality of serving cells, the downlink control information including a carrier indicator and a downlink assignment, the carrier indicator indicating a second serving cell of the plurality of serving cells, the downlink assignment indicating a downlink resource assignment at the second serving cell; and

transmitting an SRS through the second serving cell if the downlink control information includes an aperiodic SRS request that indicates a triggering of an SRS transmission,

wherein:

if the first serving cell is the FDD cell and the second serving cell is the TDD cell, the downlink control information includes the aperiodic SRS request, and

if the first serving cell is the TDD cell and the second serving cell is the FDD cell, the downlink control information does not include the aperiodic SRS request.

2. The method of claim 1 , further comprising:

determining whether the downlink control information further includes the aperiodic SRS request.

3. The method of claim 1 , wherein the aperiodic SRS request has one bit.

4. The method of claim 3 , wherein, when the aperiodic SRS request is set to one, the aperiodic SRS request indicates the triggering of the SRS transmission.

5. The method of claim 1 , wherein the first serving cell is a primary cell and the second serving cell is a secondary cell.

6. The method of claim 1 , wherein the downlink control information is received on a physical downlink control channel (PDCCH) of the first serving cell.

7. The method of claim 1 , further comprising:

receiving downlink data from the second serving cell based on the downlink resource assignment.

8. The method of claim 1 , further comprising:

receiving an SRS configuration for the SRS transmission, the SRS configuration including a bandwidth of the SRS and a subframe offset of the SRS.

9. A device configured to transmit a sounding reference signal (SRS) in a wireless communication system and configured with a plurality of serving cells including a time division duplex (TDD) cell and a frequency division duplex (FDD) cell, the device comprising:

a radio frequency (RF) unit configured to receive and transmit radio signals; and

a processor operatively coupled with the RF unit and configured to:

instruct the RF unit to receive downlink control information from a first serving cell of the plurality of serving cells, the downlink control information including a carrier indicator and a downlink assignment, the carrier indicator indicating a second serving cell of the plurality of serving cells, the downlink assignment indicating a downlink resource assignment at the second serving cell; and

instruct the RF unit to transmit an SRS through the second serving cell if the downlink control information includes an aperiodic SRS request that indicates a triggering of an SRS transmission,

wherein:

if the first serving cell is the FDD cell and the second serving cell is the TDD cell, the downlink control information includes the aperiodic SRS request, and

if the first serving cell is the TDD cell and the second serving cell is the FDD cell, the downlink control information does not include the aperiodic SRS request.

10. The device of claim 9 , wherein the aperiodic SRS request has one bit.

11. The device of claim 10 , wherein, when the aperiodic SRS request is set to one, the aperiodic SRS request indicates the triggering of the SRS transmission.

12. The device of claim 9 , wherein the first serving cell is a primary cell and the second serving cell is a secondary cell.