Method for transmitting HARQ ACK/NACK and wireless device using same

1. A method for transmitting hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in a wireless communication system, the method comprising:

receiving, by a wireless device, downlink control information from a first base station on a downlink control channel;

receiving, by the wireless device, a downlink transport block from the first base station on a downlink shared channel according to the downlink control information; and

transmitting, by the wireless device and to a second base station, a first uplink control channel carrying ACK/NACK for the downlink transport block received from the first base station,

wherein the first uplink control channel is configured using a virtual cell identity of the second base station and an indicator for determining a radio resource for the first uplink control channel,

wherein the virtual cell identity of the second base station and the indicator are received from the first base station,

wherein the radio resource for the first uplink control channel carrying the ACK/NACK is separated from a radio resource for a second uplink control channel for the second base station,

wherein a region of the radio resource for the first uplink control channel is determined based on a size of a control region for the downlink control channel, and

wherein the first uplink control channel is transmitted at a first transmission power different from a second transmission power for transmitting the second uplink control channel.

2. The method of claim 1 , further comprising:

receiving, by the wireless device, information regarding a plurality of uplink channel resource candidates from the first base station,

wherein the indicator indicates one of the plurality of uplink channel resource candidates.

3. The method of claim 1 , wherein the indicator indicates an offset of a resource index used to determine a radio resource for the first uplink control channel.

4. The method of claim 1 , wherein the transmitting of the first uplink control channel carrying the ACK/NACK comprises:

modulating the ACK/NACK to generate a modulation symbol;

spreading the modulation symbol to a sequence which is cyclically shifted by a cyclic shift amount; and

transmitting the spread sequence.

5. The method of claim 4 , wherein the cyclic shift amount is determined based on the virtual cell identity.

6. The method of claim 4 , wherein the modulation symbol is modulated by using binary phase shift keying (BPSK) modulation or quadrature phase shift keying (QPSK) modulation.

7. The method of claim 1 , wherein a search space in which the downlink control channel is detected is defined by one or more physical resource block (PRB) pairs.

8. The method of claim 7 , wherein orthogonal frequency division multiplexing (OFDM) symbols in which the downlink control channel is received are partially or entirely overlapped with OFDM symbols in which the downlink shared channel is received.

9. A wireless device for transmitting hybrid automatic repeat request (HARQ) positive-acknowledgement (ACK)/negative-acknowledgement (NACK) in a wireless communication system, the wireless device comprising:

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

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

receive downlink control information from a first base station on a downlink control channel,

receive a downlink transport block from the first base station on a downlink shared channel according to the downlink control information, and

transmit, to a second base station, a first uplink control channel carrying ACK/NACK for the downlink transport block received from the first base station,

wherein the first uplink control channel is configured using a virtual cell identity of the second base station and an indicator for determining a radio resource for the first uplink control channel,

wherein the virtual cell identity of the second base station and the indicator are received from the first base station,

wherein the radio resource for the first uplink control channel carrying the ACK/NACK is separated from a radio resource for a second uplink control channel for the second base station,

wherein a region of the radio resource for the first uplink control channel is deteimined based on a size of a control region for the downlink control channel, and

wherein the first uplink control channel is transmitted at a first transmission power different from a second transmission power for transmitting the second uplink control channel.

10. The wireless device of claim 9 , wherein the processor receives information regarding a plurality of uplink channel resource candidates from the first base station, and

wherein the indicator indicates one of the plurality of uplink channel resource candidates.

11. The wireless device of claim 9 , wherein the processor is further configured to transmit the first uplink control channel carrying the ACK/NACK by:

modulating the ACK/NACK to generate a modulation symbol;

spreading the modulation symbol to a sequence which is cyclically shifted by a cyclic shift amount; and

transmitting the spread sequence.

12. The wireless device of claim 11 , wherein the cyclic shift amount is determined based on the virtual cell identity.