f HARQ feedback, and scheduling request (SR), as well as con- ducting certain link-level measurements. The MAC sublayer provides a mapping between logical channels and transport channels; multiplexing of MAC SDUs from one or different logical channels to transport blocks to be delivered to the physical layer on transport chan- nels; demultiplexing of MAC SDUs to one or different logical channels from transport blocks delivered from the physical layer on transport channels; scheduling measurement reporting; forward error correction through HARQ; and logical channel prioritization. The MAC sublayer further provides data transfer services via logical channels. To accommodate different types of data transfer services, various logical channels are defined, each support- ing transfer of a particular type of information. Each logical channel is defined by the type of information which is transferred [9]. The MAC entity maps logical channels to transport channels in the uplink and downlink. This mapping depends on the multiplexing that is con- figured by RRC sublayer. The priority handling among multiple logical channels, where each logical channel has its own RLC entity, is supported by multiplexing the logical channels to one transport chan- nel. The MAC entity at the receiving side handles the corresponding demultiplexing and forwards the RLC PDUs to their respective RLC entity. To enable the demultiplexing function at the receiver, a MAC header is used. In NR, instead of putting the entire MAC header information at the beginning of a MAC PDU as LTE does, which implies that the assembly of a MAC PDU cannot start until the scheduling decision is available, the sub- header corresponding to a certain MAC SDU is placed immediately in front of the SDU, as shown in Fig. 2.6. This allows the PDUs to be processed before a scheduling decision is received. If necessary, padding can be used to align the transport block size with those supported in NR. A MAC subheader contains the identity of the logical channel (LCID) from which