A method of transmitting an acknowledgement/non-acknowledgement, ACK/NACK, in a 3 rd Generation Partnership Project, 3GPP, communication system based on Time Division Duplex, TDD, in which M, M=4, downlink subframes are associated with an uplink subframe, comprising: receiving, by a user equipment, zero or more downlink transport blocks in the M downlink subframes from a first serving cell; determining, by the user equipment, an ACK/NACK state {hybrid automatic repeat request-ACK, HARQ-ACK,HARQ-ACK,HARQ-ACK, HARQ-ACK} for the first serving cell; determining, by the user equipment, an ACK/NACK response based on the ACK/NACK state; and transmitting, by the user equipment, the ACK/NACK response in the uplink subframe, wherein each downlink transport block is received on each physical downlink shared channel, PDSCH, and a PDSCH is indicated by a downlink resource assignment in a downlink grant on a corresponding physical downlink control channel, PDCCH, and the downlink grant includes a downlink assignment index, DAI, which indicates a accumulative number of PDCCH with assigned PDSCH transmission, wherein the ACK/NACK state {HARQ-ACK,HARQ-ACK,HARQ-ACK, HARQ-ACK} is selected among (A, A, A, N/D), (A, A, N/D, any), (A, D, D, D), (A, A, A, A), (N/D, any, any, any) and (A, N/D, any, any) except for (A, D, D, D), wherein four different ACK/NACK states are mapped to a Physical Uplink Control Channel, PUCCH, resource and a constellation having 2-bit information, wherein a first state of the four different ACK/NACK states is (A, A, A, N/D), wherein a second state of the four different ACK/NACK states is (A, A, N/D, any), wherein a third state of the four different ACK/NACK states is (A, D, D, D) or (A, A, A, A), and wherein a fourth state of the four different ACK/NACK states is (N/D, any, any, any) or (A, N/D, any, any) except for (A, D, D, D), and where 'A' indicates ACK for corresponding downlink transport block, 'N' indicates NACK for corresponding downlink transport block, 'D' indicates no transmission, DTX, in corresponding downlink subframe, and 'any' indicates any of ACK, NACK and DTX, and wherein HARQ-ACK(j), j=1,2,3,4, is ACK/NACK/DTX for corresponding downlink transport block on PDSCH with a DAI value j in the corresponding PDCCH if there is no PDSCH transmission without corresponding PDCCH. receiving, by a user equipment, zero or more downlink transport blocks in the M downlink subframes from a first serving cell; determining, by the user equipment, an ACK/NACK state {hybrid automatic repeat request-ACK, HARQ-ACK,HARQ-ACK,HARQ-ACK, HARQ-ACK} for the first serving cell; determining, by the user equipment, an ACK/NACK response based on the ACK/NACK state; and transmitting, by the user equipment, the ACK/NACK response in the uplink subframe, wherein each downlink transport block is received on each physical downlink shared channel, PDSCH, and a PDSCH is indicated by a downlink resource assignment in a downlink grant on a corresponding physical downlink control channel, PDCCH, and the downlink grant includes a downlink assignment index, DAI, which indicates a accumulative number of PDCCH with assigned PDSCH transmission, wherein the ACK/NACK state {HARQ-ACK,HARQ-ACK,HARQ-ACK, HARQ-ACK} is selected among (A, A, A, N/D), (A, A, N/D, any), (A, D, D, D), (A, A, A, A), (N/D, any, any, any) and (A, N/D, any, any) except for (A, D, D, D), wherein four different ACK/NACK states are mapped to a Physical Uplink Control Channel, PUCCH, resource and a constellation having 2-bit information, wherein a first state of the four different ACK/NACK states is (A, A, A, N/D), wherein a second state of the four different ACK/NACK states is (A, A, N/D, any), wherein a third state of the four different ACK/NACK states is (A, D, D, D) or (A, A, A, A), and wherein a fourth state of the four different ACK/NACK states is (N/D, any, any, any) or (A, N/D, any, any) except for (A, D, D, D), and where 'A' indicates ACK for corresponding downlink transport block, 'N' indicates NACK for corresponding downlink transport block, 'D' indicates no transmission, DTX, in corresponding downlink subframe, and 'any' indicates any of ACK, NACK and DTX, and wherein HARQ-ACK(j), j=1,2,3,4, is ACK/NACK/DTX for corresponding downlink transport block on PDSCH with a DAI value j in the corresponding PDCCH if there is no PDSCH transmission without corresponding PDCCH.