ETSI TS 138 321 V16.22.0 (2026-02)

5G; NR; Medium Access Control (MAC) protocol specification (3GPP TS 38.321 version 16.22.0 Release 16)

RTS/TSGR-0238321vgm0

General Information

• Status: Not Published
• Technical Committee: 3GPP RAN 2 - Radio layer 2 specification and Radio layer 3 RR specification
• Current Stage: 12 - Citation in the OJ (auto-insert)
• Completion Date: 24-Feb-2026

Standards Content (Sample)

TECHNICAL SPECIFICATION
5G;
NR;
Medium Access Control (MAC) protocol specification
(3GPP TS 38.321 version 16.22.0 Release 16)

3GPP TS 38.321 version 16.22.0 Release 16 1 ETSI TS 138 321 V16.22.0 (2026-02)

Reference
RTS/TSGR-0238321vgm0
Keywords
5G
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ETSI
3GPP TS 38.321 version 16.22.0 Release 16 2 ETSI TS 138 321 V16.22.0 (2026-02)
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ETSI
3GPP TS 38.321 version 16.22.0 Release 16 3 ETSI TS 138 321 V16.22.0 (2026-02)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 7
1 Scope . 8
2 References . 8
3 Definitions, symbols and abbreviations . 9
3.1 Definitions . 9
3.2 Abbreviations . 10
4 General . 11
4.1 Introduction . 11
4.2 MAC architecture . 11
4.2.1 General . 11
4.2.2 MAC Entities . 11
4.3 Services . 13
4.3.1 Services provided to upper layers . 13
4.3.2 Services expected from physical layer . 13
4.4 Functions . 13
4.5 Channel structure . 14
4.5.1 General . 14
4.5.2 Transport Channels . 14
4.5.3 Logical Channels . 14
4.5.4 Mapping of Transport Channels to Logical Channels . 15
4.5.4.1 General . 15
4.5.4.2 Uplink mapping . 15
4.5.4.3 Downlink mapping . 15
4.5.4.4 Sidelink mapping . 15
5 MAC procedures . 16
5.1 Random Access procedure . 16
5.1.1 Random Access procedure initialization . 16
5.1.1a Initialization of variables specific to Random Access type . 20
5.1.2 Random Access Resource selection . 22
5.1.2a Random Access Resource selection for 2-step RA type . 24
5.1.3 Random Access Preamble transmission . 26
5.1.3a MSGA transmission . 27
5.1.4 Random Access Response reception. 29
5.1.4a MSGB reception and contention resolution for 2-step RA type . 31
5.1.5 Contention Resolution . 34
5.1.6 Completion of the Random Access procedure . 36
5.2 Maintenance of Uplink Time Alignment . 36
5.3 DL-SCH data transfer . 37
5.3.1 DL Assignment reception . 37
5.3.2 HARQ operation . 39
5.3.2.1 HARQ Entity. 39
5.3.2.2 HARQ process . 39
5.3.3 Disassembly and demultiplexing . 40
5.4 UL-SCH data transfer . 41
5.4.1 UL Grant reception . 41
5.4.2 HARQ operation . 44
5.4.2.1 HARQ Entity. 44
5.4.2.2 HARQ process . 47
5.4.3 Multiplexing and assembly . 49
5.4.3.1 Logical Channel Prioritization . 49
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5.4.3.1.1 General . 49
5.4.3.1.2 Selection of logical channels . 49
5.4.3.1.3 Allocation of resources . 50
5.4.3.2 Multiplexing of MAC Control Elements and MAC SDUs . 52
5.4.4 Scheduling Request. 52
5.4.5 Buffer Status Reporting . 55
5.4.6 Power Headroom Reporting . 57
5.4.7 Pre-emptive Buffer Status Reporting . 60
5.5 PCH reception . 60
5.6 BCH reception . 61
5.7 Discontinuous Reception (DRX) . 61
5.8 Transmission and reception without dynamic scheduling . 65
5.8.1 Downlink . 65
5.8.2 Uplink . 66
5.8.3 Sidelink . 67
5.9 Activation/Deactivation of SCells . 69
5.10 Activation/Deactivation of PDCP duplication. 71
5.11 MAC reconfiguration . 71
5.12 MAC Reset . 72
5.13 Handling of unknown, unforeseen and erroneous protocol data . 73
5.14 Handling of measurement gaps . 73
5.15 Bandwidth Part (BWP) operation . 73
5.15.1 Downlink and Uplink. 73
5.15.2 Sidelink . 77
5.16 SUL operation . 78
5.17 Beam Failure Detection and Recovery procedure . 78
5.18 Handling of MAC CEs . 80
5.18.1 General . 80
5.18.2 Activation/Deactivation of Semi-persistent CSI-RS/CSI-IM resource set . 80
5.18.3 Aperiodic CSI Trigger State Subselection . 81
5.18.4 Activation/Deactivation of UE-specific PDSCH TCI state . 81
5.18.5 Indication of TCI state for UE-specific PDCCH . 81
5.18.6 Activation/Deactivation of Semi-persistent CSI reporting on PUCCH . 81
5.18.7 Activation/Deactivation of Semi-persistent SRS and Indication of spatial relation of SP/AP SRS . 82
5.18.8 Activation/Deactivation of spatial relation of PUCCH resource . 82
5.18.9 Activation/Deactivation of semi-persistent ZP CSI-RS resource set . 82
5.18.10 Recommended Bit Rate . 83
5.18.11 Void . 83
5.18.12 Void . 83
5.18.13 Void . 83
5.18.14 Update of Pathloss Reference RS of SRS . 83
5.18.15 Update of Pathloss Reference RS of PUSCH . 83
5.18.16 Indication of spatial relation of SRS resource for a Serving Cell set . 84
5.18.17 Activation/Deactivation of Semi-Persistent Positioning SRS . 84
5.18.18 Timing offset adjustment for IAB . 84
5.18.19 Guard symbols for IAB . 84
5.19 Data inactivity monitoring . 85
5.20 Void . 85
5.21 LBT operation . 85
5.21.1 General . 85
5.21.2 LBT failure detection and recovery procedure . 86
5.22 SL-SCH Data transfer . 87
5.22.1 SL-SCH Data transmission . 87
5.22.1.1 SL Grant reception and SCI transmission . 87
5.22.1.2 TX resource (re-)selection check . 92
5.22.1.2a Re-evaluation and Pre-emption . 93
5.22.1.3 Sidelink HARQ operation . 94
5.22.1.3.1 Sidelink HARQ Entity . 94
5.22.1.3.1a Sidelink process . 96
5.22.1.3.2 PSFCH reception . 97
5.22.1.3.3 HARQ-based Sidelink RLF detection . 98
5.22.1.4 Multiplexing and assembly . 99
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5.22.1.4.0 General . 99
5.22.1.4.1 Logical channel prioritization . 99
5.22.1.4.1.1 General . 99
5.22.1.4.1.2 Selection of logical channels. 99
5.22.1.4.1.3 Allocation of sidelink resources . 100
5.22.1.4.2 Multiplexing of MAC Control Elements and MAC SDUs . 101
5.22.1.5 Scheduling Request . 101
5.22.1.6 Buffer Status Reporting . 102
5.22.1.7 CSI Reporting . 104
5.22.2 SL-SCH Data reception . 105
5.22.2.1 SCI reception. 105
5.22.2.2 Sidelink HARQ operation . 105
5.22.2.2.1 Sidelink HARQ Entity . 105
5.22.2.2.2 Sidelink process . 106
5.22.2.3 Disassembly and demultiplexing . 107
5.23 SL-BCH data transfer . 107
5.23.1 SL-BCH data transmission . 107
5.23.2 SL-BCH data reception . 107
6 Protocol Data Units, formats and parameters . 108
6.1 Protocol Data Units . 108
6.1.1 General . 108
6.1.2 MAC PDU (DL-SCH and UL-SCH except transparent MAC and Random Access Response) . 108
6.1.3 MAC Control Elements (CEs) . 111
6.1.3.1 Buffer Status Report MAC CEs . 111
6.1.3.2 C-RNTI MAC CE . 115
6.1.3.3 UE Contention Resolution Identity MAC CE . 115
6.1.3.4 Timing Advance Command MAC CE . 115
6.1.3.4a Absolute Timing Advance Command MAC CE . 116
6.1.3.5 DRX Command MAC CE . 116
6.1.3.6 Long DRX Command MAC CE . 116
6.1.3.7 Configured Grant Confirmation MAC CE . 116
6.1.3.8 Single Entry PHR MAC CE . 116
6.1.3.9 Multiple Entry PHR MAC CE . 118
6.1.3.10 SCell Activation/Deactivation MAC CEs . 120
6.1.3.11 Duplication Activation/Deactivation MAC CE. 121
6.1.3.12 SP CSI-RS/CSI-IM Resource Set Activation/Deactivation MAC CE . 121
6.1.3.13 Aperiodic CSI Trigger State Subselection MAC CE . 122
6.1.3.14 TCI States Activation/Deactivation for UE-specific PDSCH MAC CE . 123
6.1.3.15 TCI State Indication for UE-specific PDCCH MAC CE . 124
6.1.3.16 SP CSI reporting on PUCCH Activation/Deactivation MAC CE . 124
6.1.3.17 SP SRS Activation/Deactivation MAC CE . 125
6.1.3.18 PUCCH spatial relation Activation/Deactivation MAC CE. 126
6.1.3.19 SP ZP CSI-RS Resource Set Activation/Deactivation MAC CE . 127
6.1.3.20 Recommended bit rate MAC CE . 127
6.1.3.21 Timing Delta MAC CE . 128
6.1.3.22 Guard Symbols MAC CEs . 129
6.1.3.23 BFR MAC CEs . 129
6.1.3.24 Enhanced TCI States Activation/Deactivation for UE-specific PDSCH MAC CE . 131
6.1.3.25 Enhanced PUCCH Spatial Relation Activation/Deactivation MAC CE . 132
6.1.3.26 Enhanced SP/AP SRS Spatial Relation Indication MAC CE . 133
6.1.3.27 SRS Pathloss Reference RS Update MAC CE . 134
6.1.3.28 PUSCH Pathloss Reference RS Update MAC CE . 135
6.1.3.29 Serving Cell Set based SRS Spatial Relation Indication MAC CE . 135
6.1.3.30 LBT failure MAC CEs . 137
6.1.3.31 Multiple Entry Configured Grant Confirmation MAC CE . 137
6.1.3.32 Duplication RLC Activation/Deactivation MAC CE . 138
6.1.3.33 Sidelink Buffer Status Report MAC CEs . 138
6.1.3.34 Sidelink Configured Grant Confirmation MAC CE . 139
6.1.3.35 Sidelink CSI Reporting MAC CE . 139
6.1.3.36 SP Positioning SRS Activation/Deactivation MAC CE . 140
6.1.4 MAC PDU (transparent MAC) . 143
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6.1.5 MAC PDU (Random Access Response) . 143
6.1.5a MAC PDU (MSGB) . 144
6.1.6 MAC PDU (SL-SCH) . 145
6.2 Formats and parameters . 146
6.2.1 MAC subheader for DL-SCH and UL-SCH . 146
6.2.2 MAC subheader for Random Access Response. 148
6.2.2a MAC subheader for MSGB . 149
6.2.3 MAC payload for Random Access Response . 149
6.2.3a MAC payload for MSGB . 150
6.2.4 MAC subheader for SL-SCH . 152
7 Variables and constants . 152
7.1 RNTI values . 152
7.2 Backoff Parameter values . 154
7.3 DELTA_PREAMBLE values . 154
7.4 PRACH Mask Index values. 155
Annex A (informative): Change history . 156
History . 161

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Foreword
This Technical Specification has been produced by the 3rd Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
ETSI
3GPP TS 38.321 version 16.22.0 Release 16 8 ETSI TS 138 321 V16.22.0 (2026-02)
1 Scope
The present document specifies the NR MAC protocol.
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 38.300: "NR; Overall description; Stage 2".
[3] 3GPP TS 38.322: "NR; Radio Link Control (RLC) protocol specification".
[4] 3GPP TS 38.323: "NR; Packet Data Convergence Protocol (PDCP) protocol specification".
[5] 3GPP TS 38.331: "NR; Radio Resource Control (RRC); Protocol specification".
[6] 3GPP TS 38.213: "NR; Physical Layer Procedures for control".
[7] 3GPP TS 38.214: "NR; Physical Layer Procedures for data".
[8] 3GPP TS 38.211: "NR; Physical channels and modulation".
[9] 3GPP TS 38.212: "NR; Multiplexing and channel coding".
[10] Void.
[11] 3GPP TS 38.133: "NR; Requirements for support of radio resource management".
[12] 3GPP TS 36.133: "Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for
support of radio resource management".
[13] 3GPP TS 26.114: "Technical Specification Group Services and System Aspects; IP Multimedia
Subsystem (IMS); Multimedia Telephony; Media handling and interaction".
[14] 3GPP TS 38.101-1: "NR; User Equipment (UE) radio transmission and reception; Part 1: Range 1
Standalone".
[15] 3GPP TS 38.101-2: "NR; User Equipment (UE) radio transmission and reception; Part 2: Range 2
Standalone".
[16] 3GPP TS 38.101-3: "NR; User Equipment (UE) radio transmission and reception; Part 3: Range 1
and Range 2 Interworking operation with other radios".
[17] 3GPP TS 36.213: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer
Procedures".
[18] 3GPP TS 37.213: "Physical layer procedures for shared spectrum channel access".
[19] 3GPP TS 23.287: "Architecture enhancements for 5G System (5GS) to support Vehicle-to-
Everything (V2X) services ".
[20] 3GPP TS 23.285: "Architecture enhancements for V2X services".
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[21] 3GPP TS 36.331: "Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource
Control (RRC); Protocol specification".
[22] 3GPP TS 36.321: "Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access
Control (MAC); Protocol specification".
[23] 3GPP TS 37.355: "Evolved Universal Terrestrial Radio Access (E-UTRA); LTE Positioning
Protocol (LPP)".
[24] 3GPP TS 38.215: "NR; Physical layer measurements".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in TR 21.905 [1] and the following apply. A
term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
Dormant BWP: The dormant BWP is one of downlink BWPs configured by the network via dedicated RRC signalling.
In the dormant BWP, the UE stop monitoring PDCCH on/for the SCell, but continues performing CSI measurements,
Automatic Gain Control (AGC) and beam management, if configured.
DRX group: A group of Serving Cells that is configured by RRC and that have the same DRX Active Time.
HARQ information: HARQ information for DL-SCH, for UL-SCH, or for SL-SCH transmissions consists of New
Data Indicator (NDI), Transport Block Size (TBS), Redundancy Version (RV), and HARQ process ID.
IAB-donor: gNB that provides network access to UEs via a network of backhaul and access links.
IAB-node: RAN node that supports NR access links to UEs and NR backhaul links to parent nodes and child nodes.
Listen Before Talk: A procedure according to which transmissions are not performed if the channel is identified as
being occupied, see TS 37.213 [18].
Msg3: Message transmitted on UL-SCH containing a C-RNTI MAC CE or CCCH SDU, submitted from upper layer
and associated with the UE Contention Resolution Identity, as part of a Random Access procedure.
NR backhaul link: NR link used for backhauling between an IAB-node and an IAB-donor, and between IAB-nodes in
case of a multi-hop backhauling.
NR sidelink communication: AS functionality enabling at least V2X Communication as defined in TS 23.287 [19],
between two or more nearby UEs, using NR technology but not traversing any network node.
PDCCH occasion: A time duration (i.e. one or a consecutive number of symbols) during which the MAC entity is
configured to monitor the PDCCH.
Serving Cell: A PCell, a PSCell, or an SCell in TS 38.331 [5].
Sidelink transmission information: Sidelink transmission information included in an SCI for an SL-SCH transmission
as specified in clause 8.3 and 8.4 of TS 38.212 [9] consists of Sidelink HARQ information including NDI, RV, Sidelink
process ID, HARQ feedback enabled/disabled indicator, Sidelink identification information including cast type
indicator, Source Layer-1 ID and Destination Layer-1 ID, and Sidelink other information including CSI request, a
priority, a communication range requirement and Zone ID.
Special Cell: For Dual Connectivity operation the term Special Cell 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. Otherwise the term Special
Cell refers to the PCell. A Special Cell supports PUCCH transmission and contention-based Random Access, and is
always activated.
Timing Advance Group: A group of Serving Cells that is configured by RRC and that, for the cells with a UL
configured, using the same timing reference cell and the same Timing Advance value. A Timing Advance Group
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containing the SpCell of a MAC entity is referred to as Primary Timing Advance Group (PTAG), whereas the term
Secondary Timing Advance Group (STAG) refers to other TAGs.
V2X sidelink communication: AS functionality enabling V2X Communication as defined in TS 23.285 [20], between
nearby UEs, using E-UTRA technology but not traversing any network node.
NOTE 1: A timer is running once it is started, until it is stopped or until it expires; otherwise it is not running. A
timer can be started if it is not running or restarted if it is running. A Timer is always started or restarted
from its initial value. The duration of a timer is not updated until it is stopped or expires (e.g. due to BWP
switching). When the MAC entity applies zero value for a timer, the timer shall be started and
immediately expire unless explicitly stated otherwise.
NOTE 2: In this version of the specification, the SRS in the procedural description includes Positioning SRS and
Positioning SRS is treated the same as SRS by the UE unless explicitly stated otherwise.
3.2 Abbreviations
For the purposes of the present document, the abbreviations given in TR 21.905 [1] and the following apply. An
abbreviation defined in the present document tak
...