eader MAC sub-PDU 1 MAC sub-PDU 2 MAC sub-PDU 3 MAC sub-PDU 4 MAC sub-PDU n Padding (optional) (BI only) (RAPID only) (RAPID and RAR) MAC PDU with MAC RARs Figure 2.9 Structure of MAC subheaders and example MAC PDU with random-access response [9]. channels, including any MAC CEs, are multiplexed to form transport blocks per component carrier, where each component carrier has its own HARQ entity. In order to efficiently utilize the radio resources, the MAC sublayer in gNB includes sche- dulers that allocate radio resources in the downlink/uplink to the active users in the cell. The default mode of operation of a gNB scheduler is dynamic scheduling, in which the gNB makes scheduling decisions, typically once per slot, and sends the scheduling informa- tion to a group of devices. While slot-based scheduling is a common practice, neither the scheduling decisions, nor the actual data transmission are required to start or end at the slot boundaries, which is important for low-latency applications and unlicensed spectrum opera- tion. The downlink/uplink scheduling are independent and scheduling decisions can be inde- pendently made. The downlink scheduler dynamically controls the radio resources allocated to active devices in order to efficiently share the DL-SCH. The selection of transport format which includes transport block size, modulation and coding scheme, and antenna mapping, as well as logi- cal channel multiplexing for downlink transmissions are all managed by the gNB scheduler. New Radio Access Layer 2/3 Aspects and System Operation 211 The uplink scheduler serves a similar purpose, that is, dynamically controlling the transmis- sion opportunities of the active UEs and efficiently sharing of UL-SCH. The scheduling strategy of a gNB is vendor-dependent and implementation specific and is not specified by 3GPP. In general, the ultimate goal of all schedulers is to take advantage of the channel var- iations experienced by various devices and to schedule the transmissions/receptions to/from each device on the radi