Ack/Nack indication. This brings us to time = 8, which wraps around to time = 0 (there are 8 HARQ processes in the synchronous uplink for LTE). This Ack/Nack indication is decoded at time = 9 (or time = 1 in wraparound) for potential retransmission by the base station at time = 10 (or time = 2 in wraparound). In the synchronous method of operation, the base station knows exactly when to expect the HARQ Ack/Nack indication and no explicit signaling is required: there is a fixed timing relationship between the over-the-air initial transmission (in above example, time = 4) and the transmission of HARQ Ack/Nack by the receiver, also over the air (in above example, time = 8 or wraparound to time = 0). The HARQ context (in 3GPP language: Process ID) not signaled but inferred from the frame and subframe number. Asynchronous HARQ is very similar from the functional point of view, but the timing aspects are different. In Asynchronous HARQ, where the HARQ process ID is assigned by the MAC sched- uler and hence can be different for each (re)transmission. At the cost of explicit signaling of the HARQ process identifier, the retransmissions of a packet happen at any time compared to the initial transmission. Asynchronous HARQ as such allows more flexibility in scheduling. The mechanism for both the sender and receiver to know which HARQ process is used is the Downlink Control Information (DCI) indication. DCI carries the field called HARQ processor number. In LTE, only the DCI for downlink scheduling carries this field (since LTE DL use asynchronous HARQ) and the DCI for uplink scheduling does not carry this field (since LTE UL use synchronous HARQ). 64 Open Radio Access Network (O-RAN) Systems Architecture and Design Slot n Slot n+1 Slot n+K1 PDSCH Ack/Nack on Transmission PUCCH FIGURE 2-21 5G/NR asynchronous HARQ signaling. HARQ, Hybrid Automatic Repeat Request. However in NR, both downlink scheduling DCI (i.e., DCI 1_0, 1_1) and uplink scheduling DCI (i.e., DCI 0_0, 0_1) carry the field HARQ processor number since they bot