A method for carrying out hybrid automatic repeat request, HARQ, in a long-term evolution, LTE, wireless communication system supporting carrier aggregation, the method comprising: setting a secondary cell having a first time division duplex, TDD, uplink-downlink, UL-DL, configuration and a primary cell having a second TDD UL-DL configuration different from the first TDD UL-DL configuration; receiving data through a DL subframe of the secondary cell, wherein the data is scheduled by control information of the primary cell; and storing at least a portion of the data in a HARQ soft buffer when decoding of the data fails, characterized in that a size of soft channel bits that need to be stored per component carrier in the HARQ soft buffer is determined by using the following expression: ⌊ N soft ' C ⋅ ∑ n = 0 N cells DL − 1 min M DL_HARQ.n, M limit ⋅ K MIMO ⌋, where N' soft is the total number of soft channel bits according to user equipment capability, C is the number of code blocks, N cells DL is the number of configured cells, M DL_HARQ.n is the maximum number of HARQ processes for a cell #n, M limit is a constant value equal to 8, K MIMO is 1 or 2, and └ ┘ is a flooring function, wherein M DL_HARQ.n of the primary cell is determined as the number of DL or special subframes of the primary cell within a maximum HARQ round trip time, RTT, of the second TDD UL-DL configuration, wherein M DL_HARQ.n of the secondary cell is determined as the number of DL or special subframes of the secondary cell within a maximum HARQ RTT of a TDD UL-DL configuration having a smallest number of DL subframes among one or more TDD UL-DL configurations in which subframes set to DL in the primary cell or the secondary cell are all set to DL, provided that at least one subframe of the secondary cell in which the secondary cell is DL and the primary cell is UL is excluded when M DL_HARQ.n of the secondary cell is determined. setting a secondary cell having a first time division duplex, TDD, uplink-downlink, UL-DL, configuration and a primary cell having a second TDD UL-DL configuration different from the first TDD UL-DL configuration; receiving data through a DL subframe of the secondary cell, wherein the data is scheduled by control information of the primary cell; and storing at least a portion of the data in a HARQ soft buffer when decoding of the data fails, characterized in that a size of soft channel bits that need to be stored per component carrier in the HARQ soft buffer is determined by using the following expression: ⌊ N soft ' C ⋅ ∑ n = 0 N cells DL − 1 min M DL_HARQ.n, M limit ⋅ K MIMO ⌋, where N' soft is the total number of soft channel bits according to user equipment capability, C is the number of code blocks, N cells DL is the number of configured cells, M DL_HARQ.n is the maximum number of HARQ processes for a cell #n, M limit is a constant value equal to 8, K MIMO is 1 or 2, and └ ┘ is a flooring function, wherein M DL_HARQ.n of the primary cell is determined as the number of DL or special subframes of the primary cell within a maximum HARQ round trip time, RTT, of the second TDD UL-DL configuration, wherein M DL_HARQ.n of the secondary cell is determined as the number of DL or special subframes of the secondary cell within a maximum HARQ RTT of a TDD UL-DL configuration having a smallest number of DL subframes among one or more TDD UL-DL configurations in which subframes set to DL in the primary cell or the secondary cell are all set to DL, provided that at least one subframe of the secondary cell in which the secondary cell is DL and the primary cell is UL is excluded when M DL_HARQ.n of the secondary cell is determined. N' soft is the total number of soft channel bits according to user equipment capability, C is the number of code blocks, N cells DL is the number of configured cells, M DL_HARQ.n is the maximum number of HARQ processes for a cell #n, M limit is a constant value equal to 8, K MIMO is 1 or 2, and └ ┘ is a flooring function, wherein M DL_HARQ.n of the primary cell is determined as the number of DL or special subframes of the primary cell within a maximum HARQ round trip time, RTT, of the second TDD UL-DL configuration, wherein M DL_HARQ.n of the secondary cell is determined as the number of DL or special subframes of the secondary cell within a maximum HARQ RTT of a TDD UL-DL configuration having a smallest number of DL subframes among one or more TDD UL-DL configurations in which subframes set to DL in the primary cell or the secondary cell are all set to DL, provided that at least one subframe of the secondary cell in which the secondary cell is DL and the primary cell is UL is excluded when M DL_HARQ.n of the secondary cell is determined.