A method implemented by a user equipment, the method comprising: determining a set of aggregated serving cells; grouping the aggregated serving cells into a set of cell groups; determining , for an uplink, UL, subframe, a set of downlink, DL, subframes Y comprising subframes ( y 1 , y 2 ,..., y M ) for which a corresponding hybrid automatic repeat request acknowledgement, HARQ-ACK, is to be transmitted in the UL subframe; receiving a cell-domain downlink assignment indicator, C-DAI, and a cell-domain total assignment indicator, C-TAI, in each downlink control information, DCI, scheduling a physical downlink shared channel, PDSCH; determining a HARQ-ACK codebook based on the C-DAI and C-TAI for each combination of a cell group and a subframe, the cell group belonging to the set of cell groups and the subframe belonging to the set of DL subframes; and concatenating the HARQ-ACK codebooks from all cell groups and all subframes, in response to the set of cell groups containing more than one cell group or the set of DL subframes Y containing more than one subframe; wherein, for a serving cell x of the aggregated serving cells, the corresponding C-DAI indicates an accumulated number of serving cells with PDSCH up to the serving cell x in a subframe y m and in the cell group containing serving cell x , and wherein the corresponding C-TAI indicates the total number of serving cells with PDSCH in the subframe y m and in the cell group containing serving cell x, wherein the serving cells in the cell group and in the subframe y m are ordered according to a pre-determined rule; and characterised in that grouping the aggregated serving cells into cell groups comprises grouping the aggregated serving cells into cell groups based on a maximum number of transport blocks, TBs, transmittable on the PDSCH in a serving cell within a subframe y m , wherein the serving cells in the same cell group have the same maximum number of TBs transmittable on the PDSCH. determining a set of aggregated serving cells; grouping the aggregated serving cells into a set of cell groups; determining , for an uplink, UL, subframe, a set of downlink, DL, subframes Y comprising subframes ( y 1 , y 2 ,..., y M ) for which a corresponding hybrid automatic repeat request acknowledgement, HARQ-ACK, is to be transmitted in the UL subframe; receiving a cell-domain downlink assignment indicator, C-DAI, and a cell-domain total assignment indicator, C-TAI, in each downlink control information, DCI, scheduling a physical downlink shared channel, PDSCH; determining a HARQ-ACK codebook based on the C-DAI and C-TAI for each combination of a cell group and a subframe, the cell group belonging to the set of cell groups and the subframe belonging to the set of DL subframes; and concatenating the HARQ-ACK codebooks from all cell groups and all subframes, in response to the set of cell groups containing more than one cell group or the set of DL subframes Y containing more than one subframe; wherein, for a serving cell x of the aggregated serving cells, the corresponding C-DAI indicates an accumulated number of serving cells with PDSCH up to the serving cell x in a subframe y m and in the cell group containing serving cell x , and wherein the corresponding C-TAI indicates the total number of serving cells with PDSCH in the subframe y m and in the cell group containing serving cell x, wherein the serving cells in the cell group and in the subframe y m are ordered according to a pre-determined rule; and characterised in that grouping the aggregated serving cells into cell groups comprises grouping the aggregated serving cells into cell groups based on a maximum number of transport blocks, TBs, transmittable on the PDSCH in a serving cell within a subframe y m , wherein the serving cells in the same cell group have the same maximum number of TBs transmittable on the PDSCH. <EOS>