rying to use the REs that carried some other UE's more urgent data when decoding the PDSCH that got partially punctured. There is no specified UE behavior for DCI format 21 reception; the UE is free to ignore the message, or use it any way it is able to when decoding the affected PDSCH. The usage of the pre-emption indication by the UE is a trade-off between an increased PDCCH decoding load in the UE as it has to check if the indication was sent or not at every possibility configured for it, and the possibility for increased throughput in the event of pre-emption. Similarly, for the network, the trade-off is between sending the pre-emption indication whenever pre-emption actually occurred versus rescheduling the packets that were affected using the regular HARQ procedure. Physical Layer Procedures The physical layer procedures with 5G have many new elements over LTE. One of the new areas is beamforming, which is addressed in the next section in more detail. The other key physical layer procedures are as follows [3, 4]: Physical layer retransmission handling (HARQ) Power control and power sharing with dual connectivity Timing advance 6 5G Physical Layer 6.11.1 HARQ Procedure The HARQ procedure is based on the stop-and-wait HARQ operation. Once the packet is transmitted from the gNB, the UE tries to decode it, and if the decoding fails, the UE keeps the soft samples in the buffer and requests a retransmission. Compared to LTE, the UE decoding time is clearly faster, and in an ideal case, a retransmission could occur just three scheduling intervals later, as shown in Figure 6.26. In the case of higher bands and when using a shorter scheduling time interval, the operation could be even faster, but then with the TDD case, the uplink (or downlink) resource is not always available, and thus the selected uplink/downlink allocation will impact the HARQ loop latency experienced. There is support for up to 16 HARQ procedures, which allows operation also with different backhaul latencies in case part of the functionality is 