Method and device for transmitting ack/nack in tdd-based wireless communication system

A wireless device configured for transmitting an acknowledgement/negative-acknowledgement (ACK) or a negative-acknowledgement (NACK) in a wireless communication system based on Time Division Duplex (TDD) in which M downlink sub-frame(s) are associated with an uplink sub-frame, where M is equal to or greater than one (1), the wireless device comprising:

• a transceiver configured with a plurality of serving cells;
• and
• a processor operatively coupled with the transceiver and configured to:
• control the transceiver to receive at least one downlink block in the M downlink sub-frame(s);
• determine an ACK/NACK response for the at least one downlink block by using one of a plurality of mapping rules, wherein each of the plurality of mapping rules determines the ACK/NACK response based on M, wherein when an uplink grant comprising a downlink assignment index (DAI) field indicating a pre-determined index value is received by the user equipment, M is replaced by the pre-determined index value of the DAI field for determining the ACK/NACK response;
• and
• control the transceiver to transmit the ACK/NACK response.