r). The MAC PDU (Transport Block) size is defined according to the char- acteristics of the Orthognal Frequency Division Multiplexing (OFDM) frame and the allocated time/ frequency resources by the scheduler and the selected Modulation Coding Scheme (MCS). Large SDUs may be beyond the allocated MAC PDU size and need to be segmented to multiple RLC PDUs. If the RLC SDU is small, or the allocated resources are large, several RLC SDUs may be transmitted in a single MAC PDU. In 4G/LTE systems, this packing (concatenation) is performed by the RLC layer. In 5G/NR, this is done by the MAC layer. MAC protocol The Medium Access Control (MAC) scheduler if often interpreted independently from the MAC (and RLC) PDU generation protocol aspects which are different in nature (packet pro- cessing centric as opposed to algorithm centric) from the scheduler itself. References like1 include abstractions of the interface between the scheduler and the remainder components of the MAC layer to allow for implementations by separate teams or companies. Input to the scheduler are: Cell configuration information including bandwidths, antenna count, MIMO configuration Client configuration information such as User Equipment (UE) identity (RNTI), allocated logical channels, supported MIMO configurations, Discontinuous Reception (DRX) and HARQ, RLC, MAC Control Element, Paging, RACH, Scheduling Request and similar buffer information that can be used to identify requirements for user time/frequency allocation Physical layer information such as Signal to Interference and Noise Ratio (SINR), HARQ retransmission rates and Channel Quality Indicator (CQI) Triggers to start operation The output of the MAC scheduler is a downlink and uplink slot definition: A trigger to the remainder MAC to start building of Downlink (DL) MAC PDUs and the associated slot configuration A set of uplink scheduling decisions in the format of DCIs or an equivalent metaformat With this information set, the MAC scheduler can be implemented using algorithms that we describe e