4 General

38.3213GPPMedium Access Control (MAC) protocol specificationNRRelease 17TS

4.1 Introduction

The objective of this clause is to describe the MAC architecture and the MAC entity of the UE from a functional point of view.

4.2 MAC architecture

4.2.1 General

This clause describes a model of the MAC i.e. it does not specify or restrict implementations.

RRC is in control of the MAC configuration.

4.2.2 MAC Entities

The MAC entity of the UE handles the following transport channels:

– Broadcast Channel (BCH);

– Downlink Shared Channel(s) (DL-SCH);

– Paging Channel (PCH);

– Uplink Shared Channel(s) (UL-SCH);

– Random Access Channel(s) (RACH).

When the UE is configured with SCG, two MAC entities are configured to the UE: one for the MCG and one for the SCG.

When the UE is configured with DAPS handover, two MAC entities are used by the UE: one for the source cell (source MAC entity) and one for the target cell (target MAC entity).

The functions of the different MAC entities in the UE operate independently unless otherwise specified. The timers and parameters used in each MAC entity are configured independently unless otherwise specified. The Serving Cells, C-RNTI, radio bearers, logical channels, upper and lower layer entities, LCGs, and HARQ entities considered by each MAC entity refer to those mapped to that MAC entity unless otherwise specified.

If the MAC entity is configured with one or more SCells, there are multiple DL-SCH and there may be multiple UL-SCH as well as multiple RACH per MAC entity; one DL-SCH, one UL-SCH, and one RACH on the SpCell, one DL-SCH, zero or one UL-SCH and zero or one RACH for each SCell.

If the MAC entity is not configured with any SCell, there is one DL-SCH, one UL-SCH, and one RACH per MAC entity.

Figure 4.2.2-1 illustrates one possible structure of the MAC entity when SCG is not configured and for each MAC entity during DAPS handover.

Figure 4.2.2-1: MAC structure overview

Figure 4.2.2-2 illustrates one possible structure for the MAC entities when MCG and SCG are configured.

Figure 4.2.2-2: MAC structure overview with two MAC entities

In addition, the MAC entity of the UE handles the following transport channel for sidelink:

– Sidelink Shared Channel (SL-SCH);

– Sidelink Broadcast Channel (SL-BCH).

Figure 4.2.2-3 illustrates one possible structure for the MAC entity when sidelink is configured.

Figure 4.2.2-3: MAC structure overview for sidelink

4.3 Services

4.3.1 Services provided to upper layers

The MAC sublayer provides the following services to upper layers:

– data transfer;

– radio resource allocation.

4.3.2 Services expected from physical layer

The MAC sublayer expects the following services from the physical layer:

– data transfer services;

– signalling of HARQ feedback;

– signalling of Scheduling Request;

– measurements (e.g. Channel Quality Indication (CQI)).

4.4 Functions

The MAC sublayer supports the following functions:

– mapping between logical channels and transport channels;

– multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels;

– demultiplexing of MAC SDUs to one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels;

– scheduling information reporting;

– error correction through HARQ;

– logical channel prioritization;

– priority handling between overlapping resources of one UE;

– radio resource selection.

The relevance of MAC functions for uplink, downlink, and sidelink is indicated in Table 4.4-1.

Table 4.4-1: The link direction association of MAC functions.

MAC function	Downlink	Uplink	Sidelink TX	Sidelink RX
Mapping between logical channels and transport channels	X	X	X	X
Multiplexing		X	X
Demultiplexing	X			X
Scheduling information reporting		X	X
Error correction through HARQ	X	X	X	X
Logical Channel prioritization		X	X
Radio resource selection			X

4.5 Channel structure

4.5.1 General

The MAC sublayer operates on the channels defined below; transport channels are SAPs between MAC and Layer 1, logical channels are SAPs between MAC and RLC.

4.5.2 Transport Channels

The MAC sublayer uses the transport channels listed in Table 4.5.2-1 below.

Table 4.5.2-1: Transport channels used by MAC

Transport channel name	Acronym	Downlink	Uplink	Sidelink
Broadcast Channel	BCH	X
Downlink Shared Channel	DL-SCH	X
Paging Channel	PCH	X
Uplink Shared Channel	UL-SCH		X
Random Access Channel	RACH		X
Sidelink Broadcast Channel	SL-BCH			X
Sidelink Shared Channel	SL-SCH			X

4.5.3 Logical Channels

The MAC sublayer provides data transfer services on logical channels. To accommodate different kinds of data transfer services, multiple types of logical channels are defined i.e. each supporting transfer of a particular type of information.

Each logical channel type is defined by what type of information is transferred.

The MAC sublayer provides the control and traffic channels listed in Table 4.5.3-1 below.

Table 4.5.3-1: Logical channels provided by MAC.

Logical channel name	Acronym	Control channel	Traffic channel
Broadcast Control Channel	BCCH	X
Paging Control Channel	PCCH	X
Common Control Channel	CCCH	X
Dedicated Control Channel	DCCH	X
Dedicated Traffic Channel	DTCH		X
MBS Control Channel	MCCH	X
MBS Traffic Channel	MTCH		X
Sidelink Broadcast Control Channel	SBCCH	X
Sidelink Control Channel	SCCH	X
Sidelink Traffic Channel	STCH		X

4.5.4 Mapping of Transport Channels to Logical Channels

4.5.4.1 General

The MAC entity is responsible for mapping logical channels onto transport channels. This mapping depends on the multiplexing that is configured by RRC.

4.5.4.2 Uplink mapping

The uplink logical channels can be mapped as described in Table 4.5.4.2-1.

Table 4.5.4.2-1: Uplink channel mapping.

Transport channel Logical channel	UL-SCH	RACH
CCCH	X
DCCH	X
DTCH	X

4.5.4.3 Downlink mapping

The downlink logical channels can be mapped as described in Table 4.5.4.3-1.

Table 4.5.4.3-1: Downlink channel mapping.

Transport channel Logical channel	BCH	PCH	DL-SCH
BCCH	X		X
PCCH		X
CCCH			X
DCCH			X
DTCH			X
MCCH			X
MTCH			X

4.5.4.4 Sidelink mapping

The sidelink logical channels can be mapped as described in Table 4.5.4.4-1.

Table 4.5.4.4-1: Sidelink channel mapping.

Transport channel Logical channel	SL-BCH	SL-SCH
SBCCH	X
SCCH		X
STCH		X