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

Apparatuses, methods, and systems are disclosed for communicating HARQ-ACK feedback for a plurality of carrier groups of a downlink slot set. One apparatus includes a transmitter that transmits, to a remote unit, multiple downlink transmissions in a downlink slot set. The downlink slot set comprises a first number of reference slots and a second number of carriers grouped into a plurality of carrier groups based on a subcarrier spacing value of each carrier, such that each carrier in a carrier group has the same subcarrier spacing value. The apparatus includes a receiver that receives a HARQ-ACK codebook from the remote unit for the downlink slot set, wherein the HARQ-ACK codebook for the downlink slot set corresponds to all downlink transmissions in the downlink slot set.