Technology to determine a Hybrid Automatic Repeat reQuest-ACKnowledge (HARQ-ACK) codebook size for inter-band time division duplex (TDD) carrier aggregation (CA) is disclosed. In an example, a user equipment (UE) operable to determine a HARQ-ACK codebook size for inter-band TDD CA can include computer circuitry configured to: Determine a HARQ bundling window for inter-band TDD CA including a number of downlink (DL) subframes using HARQ-ACK feedback; divide the HARQ bundling window into a first part and a second part; and calculate the HARQ-ACK codebook size based on the first part and the second part. The first part can include DL subframes of configured serving cells that occur no later than the DL subframe where a downlink control information (DCI) transmission for uplink scheduling on a serving cell is conveyed, and the second part can include physical downlink shared channel (PDSCH) subframes occurring after the DCI transmission of the serving cells.