Uplink MAC PDU R/F/LCID/L R/LCID Fixed-sized R/F/LCID/L Variable-sized MAC SDU subheader subheader MAC CE subheader MAC CE MAC sub-PDU MAC sub-PDU MAC sub-PDU including MAC sub-PDU including MAC sub-PDU including including MAC SDU including MAC SDU MAC CE 1 MAC CE 2 padding (optional) Figure 2.6 Example structures of downlink/uplink MAC PDUs [9]. A MAC PDU is a bit string that is octet-aligned and consists of one or more MAC sub- PDUs. Each MAC sub-PDU may consist of a subheader only (including padding); a subheader and a MAC SDU; a subheader and a MAC CE; or a subheader and padding. The MAC SDUs have variable sizes, where each MAC subheader corresponds to either a MAC SDU, a MAC CE, or padding. A MAC subheader typically consists of four header fields R/F/LCID/L (see Fig. 2.7). However, a MAC subheader for fixed-sized MAC CE, padding, and a MAC SDU containing uplink common control channel (CCCH), consists of two header fields R/LCID. MAC control elements are placed together. The downlink MAC subPDU(s) with MAC CE(s) is placed before any MAC subPDU with MAC SDU and MAC subPDU with padding. The uplink MAC subPDU(s) with MAC CE(s) is placed after all MAC subPDU(s) with MAC SDU and before the MAC subPDU with padding in the MAC PDU. Note that the size of padding can be zero. At most one MAC PDU can be transmitted per transport block per MAC entity. The aforementioned subheader fields are defined as follows [9]: LCID: The LCID field identifies the logical channel instance of the corresponding MAC SDU or the type of the corresponding MAC CE or padding. There is one LCID field per MAC subheader whose size is 6 bits. L: The length field indicates the length of the corresponding MAC SDU or variable- sized MAC CE in bytes. There is one L field per MAC subheader except for subheaders corresponding to fixed-sized MAC CEs, padding, and MAC SDUs containing uplink CCCH. The size of the L field is indicated by the F field. New Radio Access Layer 2/3 Aspects and System Operation 205 Octet 1 Octet 1 Octet 2 Octet 1 Octet 2 Octet 3 