its, the effective code rate is lowered by each retransmission as shown in Fig. 4.54. The IR is based on low-rate code and the different redundancy versions are generated by punc- turing the channel coder output. In the example shown in Fig. 4.54, the basic code rate is R and one-third of the coded bits are transmitted in each retransmission. Aside from increasing the received signal to noise ratio Eb/No by each retransmission due to 542 Chapter 4 combining, there is a coding gain 13 attained as a result of each retransmission. It must be noted that chase combining is a special case of HARQ-IR where the retransmissions are identical copies of the original coded bits. 4.1.8.2 UE Processing Times, HARQ Protocol and Timing 3GPP NR uses an asynchronous HARQ-IR scheme in the downlink and uplink. The gNB provides the UE with the HARQ-ACK feedback timing either dynamically in the DCI or semi-statically through RRC configuration messages. The gNB schedules each uplink trans- mission and retransmission using the uplink grant on DCI. In LTE, the basic mode of opera- tion for uplink HARQ is synchronous retransmission, which can be used to reduce the scheduling overhead for retransmissions. In this case, HARQ ACK/NACK is carried on PHICH as a short and efficient message. In NR, asynchronous HARQ is supported. In order to support asynchronous HARQ, a straightforward solution for the gNB is to send an explicit uplink grant through PDCCH for the retransmission in the same way that is done for transmissions in LTE. In some sense, the explicit grant can imply an implicit ACK/ NACK. For example, an explicit scheduling grant of a retransmission may imply a NACK for the initial transmission. The maximum number of HARQ processes in the downlink and uplink per cell is 16. The number of HARQ processes is separately configured for the UE for each cell by RRC parameter mnrofHARQ-processesForPDSCH. In the absence of any configuration, the UE may assume a default number of 8 HARQ processes. The UE must provide a valid HARQ-ACK message, if 