h use asynchronous HARQ. Given that the MAC scheduler controls what DCI are generated, the MAC scheduler can control allocation of both DL and UL HARQ process on a per-link basis. This gives the scheduler the tools to manage HARQ associated context store/restore bandwidth. In LTE: Uplink HARQ is synchronous with 8 HARQ processes. Downlink HARQ is implemented asynchronous with 8 HARQ processes. In 5G/NR: Uplink HARQ is asynchronous with < 16 (RRC configured bound/number) HARQ processes. Note there is no explicit Ack/Nack signaling for UL HARQ in 5G/NR. Indication to the UE that it can start a new transmission is done by the base station through toggling the New Data Indicator signaling bit. Downlink HARQ is implemented asynchronous with VI 16 (RRC configured bound/ number) HARQ processes. Downlink HARQ timing in 5G/NR is signaled through parameter K1 by the base station to the client as shown in Fig. 2-21. K1 defines the number of slots between the base station transmission and the expected Ack/Nack from the UE. In a so-called self-contained slot scenario, K1 can be as small as 0. K1 minimum capabilities are signaled by the client to the base station during connection establishment. K1 and HARQ timing capabilities on both client and base station side are defined by multiple parameters: Transmitter and receivers' Physical Layer capabilities: Chapter 2 System components, requirements, and interfaces 65 decoding, encoding latencies buffering capabilities modem sensitivity 3GPP specifications: slot structures (UL/DL split, TDD structure) Transmission Time Interval (TTI) duration: number of symbols per slot (e.g., mini slots), symbol duration/subcarrier spacing (SCS) Higher layers: MAC scheduler radio bearer management max number of retransmissions Deployment scenario: cell size/propagation delays number of users per cell propagation channel conditions Note that supporting uplink synchronous HARQ has been limiting the deployment of O-RAN systems for 4G/LTE in conditions where the fronthaul is nonideal with regards to i