In LTE and 5G NR, retransmissions can be adaptive in uplink and downlink. The use of code blocks is common to LTE and 5G NR but 5G NR has introduced CBG-based retransmissions.
In 5G NR, HARQ is asynchronous in downlink and uplink. In 4G/LTE, HARQ is asynchronous in downlink and synchronous in the uplink. However, Release 13 introduced asynchronous UL HARQ that’s used by License-Assisted Access (LAA). Synchronous HARQ is not suitable in 5G NR due to dynamic TDD and operation in unlicensed spectra.
In LTE synchronous UL HARQ, the same process ID is used every eighth subframe. An LTE UE responds with HARQ ACK 3ms after receiving the downlink data. 5G NR has more flexibility to respond faster for URLLC use cases. In fact, first transmission and its retransmission can happen within 1ms.
In LTE, Physical HARQ Indicator Channel (PHICH) is a dedicated downlink channel to carry HARQ ACK/NACK for uplink traffic carried on PUSCH. Such a channel is not required in 5G NR since HARQ is asynchronous.
In LTE, the number of HARQ processes was limited to 8 but this is increased to 16 in 5G NR.