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Content for  TS 38.321  Word version:  16.5.0

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4  GeneralWord‑p. 11

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.
Reproduction of 3GPP TS 38.321, Figure 4.2.2-1: MAC structure overview
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Figure 4.2.2-2 illustrates one possible structure for the MAC entities when MCG and SCG are configured.
Reproduction of 3GPP TS 38.321, Figure 4.2.2-2: MAC structure overview with two MAC entities
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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.
Reproduction of 3GPP TS 38.321, Figure 4.2.2-3: MAC structure overview for sidelink
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4.3  ServicesWord‑p. 13

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.
MAC function Downlink Uplink Sidelink TX Sidelink RX
Mapping between logical channels and transport channels XXXX
Multiplexing XX
Demultiplexing XX
Scheduling information reporting XX
Error correction through HARQ XXXX
Logical Channel prioritization XX
Radio resource selection X
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4.5  Channel structureWord‑p. 14

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.
Transport channel name Acronym Downlink Uplink Sidelink
Broadcast Channel BCHX
Downlink Shared Channel DL-SCHX
Paging Channel PCHX
Uplink Shared Channel UL-SCHX
Random Access Channel RACHX
Sidelink Broadcast Channel SL-BCHX
Sidelink Shared Channel SL-SCHX
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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.
Logical channel name Acronym Control channel Traffic channel
Broadcast Control Channel BCCHX
Paging Control Channel PCCHX
Common Control Channel CCCHX
Dedicated Control Channel DCCHX
Dedicated Traffic Channel DTCHX
Sidelink Broadcast Control Channel SBCCHX
Sidelink Control Channel SCCHX
Sidelink Traffic Channel STCHX
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4.5.4  Mapping of Transport Channels to Logical ChannelsWord‑p. 15

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.
Transport channel UL-SCH RACH
Logical channel
CCCH X
DCCH X
DTCH X
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4.5.4.3  Downlink mapping

The downlink logical channels can be mapped as described in Table 4.5.4.3-1.
Transport channel BCH PCH DL-SCH
Logical channel
BCCH XX
PCCH X
CCCH X
DCCH X
DTCH X
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4.5.4.4  Sidelink mapping |R16|

The sidelink logical channels can be mapped as described in Table 4.5.4.4-1.
Transport channel SL-BCH SL-SCH
Logical channel
SBCCH X
SCCH X
STCH X
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