16.9.2.2 MAC

The MAC sublayer provides the following services and functions over the PC5 interface in addition to the services and functions specified in clause 6.2.1:

• Radio resource selection;
• Packet filtering;
• Priority handling between uplink and sidelink transmissions for a given UE;
• Sidelink CSI reporting.

With LCP restrictions in MAC, only sidelink logical channels belonging to the same destination can be multiplexed into a MAC PDU for every unicast, groupcast and broadcast transmission which is associated to the destination. NG-RAN can also control whether a sidelink logical channel can utilise the resources allocated to a configured sidelink grant Type 1 (see clause 16.9.3.2). For transmissions to RX UE(s) using SL DRX operation, LCP ensures that a TX UE transmits data in the active time of the RX UE(s).

For packet filtering, a SL-SCH MAC header including portions of both Source Layer-2 ID and a Destination Layer-2 ID is added to each MAC PDU as specified in clause 8.4. LCID included within a MAC subheader uniquely identifies a logical channel within the scope of the Source Layer-2 ID and Destination Layer-2 ID combination.

The following logical channels are used in sidelink:

• Sidelink Control Channel (SCCH): a sidelink channel for transmitting control information (i.e. PC5-RRC and PC5-S messages) and NR sidelink discovery messages from one UE to other UE(s);
• Sidelink Traffic Channel (STCH): a sidelink channel for transmitting user information from one UE to other UE(s);
• Sidelink Broadcast Control Channel (SBCCH): a sidelink channel for broadcasting sidelink system information from one UE to other UE(s).

The following connections between logical channels and transport channels exist:

• SCCH can be mapped to SL-SCH;
• STCH can be mapped to SL-SCH;
• SBCCH can be mapped to SL-BCH.