HARQ ACK/NACK is used in both downlink (enodeb to UE) and uplink (ue to eNodeB) directions.
In case of downlink, UE receives data from PDSCH and sends HARQ ACK/NACK using PUCCH.
While in case of uplink, UE sends data over the PUSCH and it receives HARQ ACK/NACK over PHICH.
