lush the soft buffer of an HARQ process 7 5G Radio Protocols or to fill it with new symbols. But unlike in LTE, asynchronous HARQ is used in both the uplink and downlink for increased flexibility. Synchronous HARQ and non-adaptive retransmissions are therefore not supported, and retransmissions always rely on explicit scheduling on the PDCCH (see Chapter 6) for simplified operations in the MAC sub- layer. Initial transmissions are either explicitly scheduled via the PDCCH or pre-configured via RRC. The pre-configuration of grants was already introduced in LTE as semi-persistent scheduling (SPS) to reduce the scheduling overhead for services with fixed transmission properties such as voice services (one voice frame every 20 ms). Once an SPS grant is activated by the PDCCH, it follows a fixed pattern configured by RRC. In 5G, the same mechanism is used, but has been given a different name in the uplink: configured uplink grant type 2. Why type 2? Because 5G also introduces a configured uplink grant type 1. Configured uplink grant type 1 allows quick transmissions of small data on contention-based resources signaled by RRC. Unlike type 2, no PDCCH is required to activate the grant. Type 1 is especially suited for URLLC services. Finally, the downlink was also enhanced in 5G to make it possible for the gNB to pre-empt ongoing downlink transmissions on the PDSCH with latency-critical trans- missions, not necessarily for the same UEs (as discussed in Chapter 6). This is achieved by using a different RNTI on the PDCCH. 7.6.5 Scheduling Request The scheduling request procedure is triggered by the UE whenever resources are needed for a new transmission on the UL-SCH. When PUCCH resources are available, an SR is sent on the PUCCH; otherwise, a random access procedure is used (see Section 7.6.3). An SR is considered as pending until resources are granted by the gNB and is repeated on the PUCCH as long as it is pending. When the number of SRs sent on the PUCCH reaches a maximum configured by the higher layer, SR failure is d