l stop-and-wait processes. When a transport block is received at the receiver, it attempts to decode the packet and to inform the transmit- ter about the outcome of the decoding process through an ACK bit indicating whether the decoding process was successful or requesting the retransmission of the transport block. The HARQ-ACKs are sent by the receiver based on a specific timing relationship between UL HARQ-ACKs and DL HARQ processes or based on the position of the ACK bit in the HARQ-ACK codebook when multiple ACKs are transmitted simultaneously. An asynchro- nous HARQ protocol is used for both downlink and uplink, that is, an explicit HARQ pro- cess number is used to identify a particular process, since the retransmissions are scheduled in the same way as the initial transmission (explicit scheduling). The use of an asynchro- nous UL HARQ protocol, instead of a synchronous one that was used in LTE, was deemed to be necessary to support dynamic TDD where there is no fixed UL/DL allocation. It also provides more flexibility in terms of prioritization of data flows and devices and is further useful for operation in unlicensed spectrum. The NR supports up to 16 HARQ processes. The larger number of HARQ processes (compared to LTE) was motivated by the disaggre- gated RAN architectures and consideration for remote radio heads and fronthaul transport delay, as well as the use of shorter slot durations at high-frequency bands. It must be noted that the larger number of maximum HARQ processes does not imply a longer roundtrip delay, since the decoding will typically succeed after a few retransmissions depending on the channel conditions. Note that the PDCP sublayer can provide in-sequence delivery; thus this function is not provided by the RLC sublayer in order to reduce the latency. A new feature of the NR HARQ protocol (compared to LTE) is the possibility for retrans- mission of code block groups, which is useful for very large transport blocks or when a transport block is partially preempted by another transmission.