Content for  TS 38.321  Word version:  18.0.0

A MAC PDU consists solely of a MAC SDU whose size is aligned to a TB; as described in Figure 6.1.4-1. This MAC PDU is used for transmissions on PCH, BCH, DL-SCH including BCCH, and SL-BCH.

A MAC PDU consists of one or more MAC subPDUs and optionally padding. Each MAC subPDU consists one of the following:

• a MAC subheader with Backoff Indicator only;
• a MAC subheader with RAPID only (i.e. acknowledgment for SI request);
• a MAC subheader with RAPID and MAC RAR.

A MAC subheader with Backoff Indicator consists of five header fields E/T/R/R/BI as described in Figure 6.1.5-1. A MAC subPDU with Backoff Indicator only is placed at the beginning of the MAC PDU, if included. 'MAC subPDU(s) with RAPID only' and 'MAC subPDU(s) with RAPID and MAC RAR' can be placed anywhere between MAC subPDU with Backoff Indicator only (if any) and padding (if any). A MAC subheader with RAPID consists of three header fields E/T/RAPID as described in Figure 6.1.5-2. Padding is placed at the end of the MAC PDU if present. Presence and length of padding is implicit based on TB size, size of MAC subPDU(s).

A MAC PDU consists of one or more MAC subPDUs and optionally padding. Each MAC subPDU consists one of the following:

• a MAC subheader with Backoff Indicator only;
• a MAC subheader and fallbackRAR;
• a MAC subheader and successRAR;
• a MAC subheader and MAC SDU for CCCH, DCCH or DTCH;
• a MAC subheader and padding.

A MAC subheader with Backoff Indicator consists of five header fields E/T1/T2/R/BI as described in Figure 6.1.5a-1. A MAC subPDU with Backoff Indicator only is placed at the beginning of the MAC PDU, if included. A MAC subheader for fallbackRAR consists of three header fields E/T1/RAPID as described in Figure 6.1.5a-2. A MAC subheader for successRAR consists of eight header fields E/T1/T2/S/R/R/R/R as described in Figure 6.1.5a-3. A MAC subheader for MAC SDU consists of the four header fields R/F/LCID/L as described in Figure 6.1.2-1 and Figure 6.1.2-2. At most one 'MAC subPDU for successRAR' indicating presence of 'MAC subPDU(s) for MAC SDU' is included in a MAC PDU. MAC subPDU(s) for MAC SDU are placed immediately after the 'MAC subPDU for successRAR' indicating presence of 'MAC subPDU(s) for MAC SDU'. If MAC PDU includes MAC subPDU(s) for MAC SDU, the last MAC subPDU for MAC SDU is placed before MAC subPDU with padding as depicted in Figure 6.1.5a-4. Otherwise, the last MAC subPDU in MAC PDU is placed before padding as depicted in Figure 6.1.5a-5. The MAC subPDU with padding includes R/R/LCID MAC subheader as described in Figure 6.1.2-3 and padding. The size of padding in the MAC subPDU with padding can be zero. The length of padding is implicit based on TB size, size of MAC subPDU(s).

A MAC PDU consists of one SL-SCH subheader and one or more MAC subPDUs. Each MAC subPDU consists of one of the following:

• A MAC subheader only (including padding);
• A MAC subheader and a MAC SDU;
• A MAC subheader and a MAC CE;
• A MAC subheader and padding.

The MAC SDUs are of variable sizes. Each MAC subheader except SL-SCH subheader corresponds to either a MAC SDU, a MAC CE, or padding. The SL-SCH subheader is of a fixed size and consists of the seven header fields V/R/R/R/R/SRC/DST.

A MAC subheader except for fixed-sized MAC CE and padding consists of the four header fields R/F/LCID/L as depicted in Figure 6.1.2-1 (with 8-bit L field) and Figure 6.1.2-2 (with 16-bit L field). A MAC subheader for fixed-sized MAC CE and padding consists of the two header fields R/LCID as depicted in Figure 6.1.2-3. SL MAC subPDU(s) with MAC SDU(s) is placed after the SL-SCH subheader and before the MAC subPDU with a MAC CE and the MAC subPDU with padding in the MAC PDU as depicted in Figure 6.1.6-2. SL MAC subPDU with a MAC CE is placed after all the MAC subPDU(s) with MAC SDU and before the MAC subPDU with padding in the MAC PDU as depicted in Figure 6.1.6-2. The size of padding can be zero.

A maximum of one MAC PDU can be transmitted per TB per MAC entity.