HARQ-ACK for a plurality of carrier groups of a downlink slot set

An apparatus comprising:

• a receiver that receives, from a base unit, multiple downlink transmissions in a downlink slot set, wherein the downlink slot set comprises a first number of reference slots and a second number of carriers;
• a processor that:
• groups the multiple carriers into a plurality of carrier groups based on a subcarrier spacing value of each carrier, each carrier group comprising a plurality of carriers, wherein a reference slot is defined for a carrier group and is determined based on a first carrier of the carrier group, wherein each carrier in a carrier group has the same subcarrier spacing value;
• generates a reference-slot-specific codebook corresponding to each reference slot of the first number of reference slots;
• and
• a transmitter that transmits a hybrid automatic repeat request acknowledgement (“HARQ-ACK”) codebook to the base unit for the downlink slot set, wherein the HARQ-ACK codebook is formed from the reference-slot-specific codebooks, wherein the HARQ-ACK codebook for the downlink slot set corresponds to all downlink transmissions in the downlink slot set.