4 General

36.3213GPPEvolved Universal Terrestrial Radio Access (E-UTRA)Medium Access Control (MAC) protocol specificationRelease 17TS

4.1 Introduction

The objective is to describe the MAC architecture and the MAC entity from a functional point of view. Functionality specified for the UE equally applies to the RN for functionality necessary for the RN. There is also functionality which is only applicable to the RN, in which case the specification denotes the RN instead of the UE. RN‑specific behaviour is not applicable to the UE. For TDD operation, UE behaviour follows the TDD UL/DL configuration indicated by tdd-Config unless specified otherwise.

The introduction of short TTI allows for more than a single instance of a TTI to occur within a 1ms subframe and as such the use of the term "for each TTI" shall be read as meaning that the associated actions shall be executed for all TTIs also in the case of overlapping TTIs (e.g. a UE may read multiple instances of PDCCH in a downlink subframe).

4.2 MAC architecture

The description in this clause is a model and does not specify or restrict implementations.

RRC is in control of configuration of MAC.

4.2.1 MAC Entities

E-UTRA defines two MAC entities; one in the UE and one in the E-UTRAN. These MAC entities handle 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);

– Multicast Channel(s) (MCH);

– Sidelink Broadcast Channel (SL-BCH);

– Sidelink Discovery Channel (SL-DCH);

– Sidelink Shared Channel (SL-SCH).

The exact functions performed by the MAC entities are different in the UE from those performed in the E-UTRAN.

The RN includes both types of MAC entities; one type for communication with UEs and one type for communication with the E-UTRAN.

In Dual Connectivity, two MAC entities are configured in the UE: one for the MCG and one for the SCG. In DAPS handover, two MAC entities are configured in the UE: one MAC entity for the source cell (source MAC entity) and one MAC entity for the target cell (target MAC entity). Each MAC entity is configured by RRC with a serving cell supporting PUCCH transmission and contention based Random Access. In this specification, the term SpCell refers to such cell, whereas the term SCell refers to other serving cells. The term SpCell either refers to the PCell of the MCG or the PSCell of the SCG depending on if the MAC entity is associated to the MCG or the SCG, respectively. A Timing Advance Group containing the SpCell of a MAC entity is referred to as pTAG, whereas the term sTAG refers to other TAGs.

The functions of the different MAC entities in the UE operate independently if not otherwise indicated. The timers and paramenters used in each MAC entity are configured independently if not otherwise indicated. 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 if not otherwise indicated.

If the MAC entity is configured with one or more SCells, there are multiple DL-SCH and there may be multiple UL-SCH and 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.

The physical layer may perform a listen-before-talk procedure, according to which transmissions are not performed if the channel is identified as being occupied or the physical layer may monitor for PUSCH trigger, as specified in TS 36.213 [2], according to which transmissions are not performed if PUSCH trigger B is not received. In both cases a MAC entity considers the transmission to have been performed anyway, unless stated otherwise.

Figure 4.2.1-1 illustrates one possible structure for the UE side MAC entity when SCG is not configured and for each MAC entity during DAPS handover, and it should not restrict implementation.

Figure 4.2.1-1: MAC structure overview, UE side

Figure 4.2.1-2 illustrates one possible structure for the UE side MAC entities when MCG and SCG are configured, and it should not restrict implementation. MBMS reception and SC-PTM reception are excluded from this figure for simplicity.

Figure 4.2.1-2: MAC structure overview with two MAC entities, UE side

Figure 4.2.1-3 illustrates one possible structure for the UE side MAC entity when sidelink is configured, and it should not restrict implementation.

Figure 4.2.1-3: MAC structure overview for sidelink, UE side

4.3 Services

4.3.1 Services provided to upper layers

This clause describes the different services provided by MAC sublayer to upper layers.

– data transfer

– radio resource allocation

4.3.2 Services expected from physical layer

The physical layer provides the following services to MAC:

– data transfer services;

– signalling of HARQ feedback;

– signalling of Scheduling Request;

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

The access to the data transfer services is through the use of transport channels. The characteristics of a transport channel are defined by its transport format (or format set), specifying the physical layer processing to be applied to the transport channel in question, such as channel coding and interleaving, and any service-specific rate matching as needed.

4.4 Functions

The following functions are supported by MAC sublayer:

– 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 from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels;

– scheduling information reporting;

– error correction through HARQ;

– priority handling between UEs by means of dynamic scheduling;

– priority handling between logical channels of one MAC entity;

– Logical Channel prioritisation;

– transport format selection;

– radio resource selection for SL.

The location of the different functions and their relevance for uplink and downlink respectively is illustrated in Table 4.4-1.

Table 4.4-1: MAC function location and link direction association.

MAC function	UE	eNB	Downlink	Uplink	Sidelink tx	Sidelink rx
Mapping between logical channels and transport channels	X		X	X	X	X
		X	X	X
Multiplexing	X			X	X
		X	X
Demultiplexing	X		X			X
		X		X
Error correction through HARQ	X		X	X	X	X
		X	X	X
Transport Format Selection		X	X	X
	X	X			X
Priority handling between UEs		X	X	X
Priority handling between logical channels of one MAC entity		X	X	X
Logical Channel prioritisation	X			X	X
Scheduling information reporting	X			X
Radio Resource Selection	X				X

4.5 Channel structure

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.

4.5.1 Transport Channels

The transport channels used by MAC are described in Table 4.5.1-1 below.

Table 4.5.1-1: Transport channels used by MAC

Transport channel name	Acronym	Downlink	Uplink	Sidelink tx	Sidelink rx
Broadcast Channel	BCH	X
Downlink Shared Channel	DL-SCH	X
Paging Channel	PCH	X
Multicast Channel	MCH	X
Uplink Shared Channel	UL-SCH		X
Random Access Channel	RACH		X
Sidelink Broadcast Channel	SL-BCH			X	X
Sidelink Discovery Channel	SL-DCH			X	X
Sidelink Shared Channel	SL-SCH			X	X

4.5.2 Logical Channels

The MAC layer provides data transfer services on logical channels. A set of logical channel types is defined for different kinds of data transfer services as offered by MAC.

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

MAC provides the control and traffic channels listed in Table 4.5.2-1 below.

Table 4.5.2-1: Logical channels provided by MAC.

Logical channel name	Acronym	Control channel	Traffic channel
Broadcast Control Channel	BCCH	X
Bandwidth Reduced Broadcast Control Channel	BR-BCCH	X
Paging Control Channel	PCCH	X
Common Control Channel	CCCH	X
Dedicated Control Channel	DCCH	X
Multicast Control Channel	MCCH	X
Single Cell Multicast Control Channel	SC-MCCH	X
Dedicated Traffic Channel	DTCH		X
Multicast Traffic Channel	MTCH		X
Single-Cell Multicast Traffic Channel	SC-MTCH		X
Sidelink Traffic Channel	STCH		X
Sidelink Broadcast Control Channel	SBCCH	X

4.5.3 Mapping of Transport Channels to Logical Channels

The mapping of logical channels on transport channels depends on the multiplexing that is configured by RRC.

4.5.3.1 Uplink mapping

The MAC entity is responsible for mapping logical channels for the uplink onto uplink transport channels. The uplink logical channels can be mapped as described in Figure 4.5.3.1-1 and Table 4.5.3.1-1.

Figure 4.5.3.1-1

Table 4.5.3.1-1: Uplink channel mapping.

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

4.5.3.2 Downlink mapping

The MAC entity is responsible for mapping the downlink logical channels to downlink transport channels. The downlink logical channels can be mapped as described in Figure 4.5.3.2-1 and Table 4.5.3.2-1.

Figure 4.5.3.2-1

Table 4.5.3.2-1: Downlink channel mapping.

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

4.5.3.3 Sidelink mapping

The MAC entity is responsible for mapping the sidelink logical channels to sidelink transport channels. The sidelink logical channels can be mapped as described in Figure 4.5.3.3-1 and Table 4.5.3.3-1.

Figure 4.5.3.3-1

Table 4.5.3.3-1: Sidelink channel mapping.

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