hrough Hybrid Automatic Repeat Request (ARQ) (HARQ) transport format selection By far, the most complex and compute (and development!) intensive function is the scheduling/priority handling one (Fig. 2-19). Next level details on scheduler algorithm design are covered separately and extensively in chapter 7. The figure shows the high-level design with regards to inputs and outputs. Link Adaptation Slot scheduler Primary User MCS selection Selection Frequency MIMO pairing Allocation Power Control Channel UE, RLC RLC Buffer Channels, Quality Targets Status ACK/NACK TB size Physical Layer MAC/RLC HARQ Process Mgr FIGURE 2-19 MAC scheduler interfaces. MAC, Medium Access Control. 62 Open Radio Access Network (O-RAN) Systems Architecture and Design Physical Layer TBs can be of any arbitrary size but are defined from a discrete set of options that is defined by: the number of layers modulation order code rate number of resource elements (allocation in frequency domain) physical transmission time (allocation in time domain) 3GPP 38.214 includes the procedures used for determining the (discrete) TB size from above mentioned parameters. The scheduler algorithm itself thus essentially defines layer/ modulation/code rate/time + frequency resource allocations on a per-user basis. The sched- uler operates every slot and outputs a list of allocated UEs and for each UE. These allocated resources together with an indication of which RLC logical channels the resources should be allocated to. This is input to the MAC PDU processing component that is responsible for composing the TB payload ( = MAC PDU) by building MAC headers/subheaders and RLC PDUs. This MAC PDU processing component is responsible for calling the RLC layer sequen- tially to generate payload that composes the MAC PDU. Wraparound 1 ms subframe 1 ms subframe (2 slots, (2 slots, 14 symbols) 14 symbols) process 1 process 2 process 3 process 4 process 5 process 6 process 7 process 0 process 0 Prepare Transmitter L2 Transmission Low Physical Uplink Downlink Transmitter L1