Method and apparatus for transmitting downlink control information in wireless communication system

A user equipment (UE) in a carrier aggregation system, the UE comprising:

• a transceiver for transmitting and receiving a radio signal;
• and
• a processor operatively coupled to the transceiver,
• wherein the processor receives, through a physical downlink control channel (PDCCH) of a primary cell, downlink control information (DCI) scheduling a physical downlink shared channel (PDSCH) of a secondary cell, decodes the DCI, receives data through the PDSCH of the secondary cell and transmits acknowledgement/negative-acknowledgement (ACK/NACK) for the data through a physical uplink control channel (PUCCH) of the primary cell,
• wherein the PUCCH is transmitted only on the primary cell among the primary cell and the secondary cell,
• wherein one of the primary cell and the secondary cell is a frequency division duplex (FDD) cell which uses FDD frames and the other is a time division duplex (TDD) cell which uses TDD frames,
• wherein a bit size of a hybrid automatic repeat request (HARQ) process number field in the DCI depends on whether the primary cell is the FDD cell or the TDD cell,
• wherein based on the primary cell being the TDD cell, the DCI includes a downlink assignment index (DAI) field,
• wherein based on the primary cell being the FDD cell, the DCI includes no DAI field and
• wherein based on the primary cell being the FDD cell, a maximum number of HARQ processes of the secondary cell is determined as a maximum number of HARQ processes of the primary cell regardless of whether the secondary cell is the TDD cell or the FDD cell.

A processor for a wireless communication device in a carrier aggregation system, the processor comprising:

• memory, in which program instructions are stored, which when executed control the wireless communication device to:
• receive, through a physical downlink control channel (PDCCH) of a primary cell, downlink control information (DCI) scheduling a physical downlink shared channel (PDSCH) of a secondary cell;
• decode the DCI;
• receive data through the PDSCH of the secondary cell;
• and
• transmit acknowledgement/negative-acknowledgement (ACK/NACK) for the data through a physical uplink control channel (PUCCH) of the primary cell,
• wherein the PUCCH is transmitted only on the primary cell among the primary cell and the secondary cell,
• wherein one of the primary cell and the secondary cell is a frequency division duplex (FDD) cell which uses FDD frames and the other is a time division duplex (TDD) cell which uses TDD frames,
• wherein a bit size of a hybrid automatic repeat request (HARQ) process number field in the DCI depends on whether the primary cell is the FDD cell or the TDD cell,
• wherein based on the primary cell being the TDD cell, the DCI includes a downlink assignment index (DAI) field,
• wherein based on the primary cell being the FDD cell, the DCI includes no DAI field and
• wherein based on the primary cell being the FDD cell, a maximum number of HARQ processes of the secondary cell is determined as a maximum number of HARQ processes of the primary cell regardless of whether the secondary cell is the TDD cell or the FDD cell.