Physical HARQ Indicator Channel (PHICH)
The PHICH is an LTE downlink channel that tells a device whether its uplink data arrived correctly.
What is Physical HARQ Indicator Channel (PHICH)?
HARQ (hybrid automatic repeat request) allows fast retransmissions. In LTE, the base station uses the PHICH to acknowledge uplink transmissions. 5G NR has no PHICH and signals this through downlink control information instead.