A method of transmitting a hybrid automatic repeat request, HARQ, acknowledgement/negative-acknowledgement, ACK/NACK, in a wireless communication system, the method performed by a user equipment , UE, and comprising: generating (S840) ACK/NACK bits for a plurality of downlink subframes related to a plurality of serving cells, wherein a number of the ACK/NACK bits is determined for the plurality of serving cells and the plurality of downlink subframes for which the UE needs to feedback ACK/NACK in each of the plurality of serving cells; arranging the ACK/NACK bits in an ascending order of cell indexes of the plurality of serving cells; generating (S850) encoded ACK/NACK bits by encoding the ACK/NACK bits; generating (S860) ACK/NACK symbols by modulating the encoded ACK/NACK bits; and transmitting (S870) the ACK/NACK symbols to a base station , characterized in that the ACK/NACK bits are arranged in order of a first series of ACK/NACK bits for a plurality of downlink subframes of a first cell having a lowest cell index and, appended at the end of the first series of the ACK/NACK bits for the first cell, a second series of ACK/NACK bits for a plurality of downlink subframes of a second cell having a cell index subsequent to the lowest cell index and, appended at the end of the second series of the ACK/NACK bits for the second cell, a third series of ACK/NACK bits for a plurality of downlink subframes of a third cell having a cell index subsequent to the cell index of the second cell, and, appended at the end of the third series of the ACK/NACK bits for the third cell, a fourth series of ACK/NACK bits for a plurality of downlink subframes of a fourth cell having a cell index subsequent to the cell index of the third cell, and wherein each series of the ACK/NACK bits includes at least two ACK/NACK bits for the downlink subframes of the corresponding serving cell and is ordered in ascending order of the downlink subframe indexes associated with each corresponding serving cell. generating (S840) ACK/NACK bits for a plurality of downlink subframes related to a plurality of serving cells, wherein a number of the ACK/NACK bits is determined for the plurality of serving cells and the plurality of downlink subframes for which the UE needs to feedback ACK/NACK in each of the plurality of serving cells; arranging the ACK/NACK bits in an ascending order of cell indexes of the plurality of serving cells; generating (S850) encoded ACK/NACK bits by encoding the ACK/NACK bits; generating (S860) ACK/NACK symbols by modulating the encoded ACK/NACK bits; and transmitting (S870) the ACK/NACK symbols to a base station , characterized in that the ACK/NACK bits are arranged in order of a first series of ACK/NACK bits for a plurality of downlink subframes of a first cell having a lowest cell index and, appended at the end of the first series of the ACK/NACK bits for the first cell, a second series of ACK/NACK bits for a plurality of downlink subframes of a second cell having a cell index subsequent to the lowest cell index and, appended at the end of the second series of the ACK/NACK bits for the second cell, a third series of ACK/NACK bits for a plurality of downlink subframes of a third cell having a cell index subsequent to the cell index of the second cell, and, appended at the end of the third series of the ACK/NACK bits for the third cell, a fourth series of ACK/NACK bits for a plurality of downlink subframes of a fourth cell having a cell index subsequent to the cell index of the third cell, and wherein each series of the ACK/NACK bits includes at least two ACK/NACK bits for the downlink subframes of the corresponding serving cell and is ordered in ascending order of the downlink subframe indexes associated with each corresponding serving cell. <EOS>