ETSI TS 138 321 V19.2.0 (2026-04)

5G; NR; Medium Access Control (MAC) protocol specification (3GPP TS 38.321 version 19.2.0 Release 19)

RTS/TSGR-0238321vj20

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: 02-Apr-2026

Standards Content (Sample)

TECHNICAL SPECIFICATION
5G;
NR;
Medium Access Control (MAC) protocol specification
(3GPP TS 38.321 version 19.2.0 Release 19)

3GPP TS 38.321 version 19.2.0 Release 19 1 ETSI TS 138 321 V19.2.0 (2026-04)

Reference
RTS/TSGR-0238321vj20
Keywords
5G
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ETSI
3GPP TS 38.321 version 19.2.0 Release 19 2 ETSI TS 138 321 V19.2.0 (2026-04)
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ETSI
3GPP TS 38.321 version 19.2.0 Release 19 3 ETSI TS 138 321 V19.2.0 (2026-04)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 9
1 Scope . 10
2 References . 10
3 Definitions, symbols and abbreviations . 11
3.1 Definitions . 11
3.2 Abbreviations . 14
4 General . 15
4.1 Introduction . 15
4.2 MAC architecture . 15
4.2.1 General . 15
4.2.2 MAC Entities . 16
4.3 Services . 17
4.3.1 Services provided to upper layers . 17
4.3.2 Services expected from physical layer . 17
4.4 Functions . 18
4.5 Channel structure . 18
4.5.1 General . 18
4.5.2 Transport Channels . 18
4.5.3 Logical Channels . 19
4.5.4 Mapping of Transport Channels to Logical Channels . 19
4.5.4.1 General . 19
4.5.4.2 Uplink mapping . 19
4.5.4.3 Downlink mapping . 19
4.5.4.4 Sidelink mapping . 20
5 MAC procedures . 20
5.1 Random Access procedure . 20
5.1.1 Random Access procedure initialization . 20
5.1.1a Initialization of variables specific to Random Access type . 27
5.1.1b Selection of the set of Random Access resources for the Random Access procedure . 30
5.1.1c Availability of the set of Random Access resources . 38
5.1.1d Selection of the set of Random Access resources based on feature prioritization . 39
5.1.1e Selection of Msg1 repetition for SI request . 39
5.1.2 Random Access Resource selection . 39
5.1.2a Random Access Resource selection for 2-step RA type . 44
5.1.3 Random Access Preamble transmission . 45
5.1.3a MSGA transmission . 47
5.1.4 Random Access Response reception. 49
5.1.4a MSGB reception and contention resolution for 2-step RA type . 54
5.1.5 Contention Resolution . 57
5.1.6 Completion of the Random Access procedure . 60
5.2 Maintenance of Uplink Time Alignment . 61
5.2a Maintenance of UL Synchronization . 66
5.2b Maintenance of UL Synchronization for CLTM candidate cell . 67
5.3 DL-SCH data transfer . 67
5.3.1 DL Assignment reception . 67
5.3.2 HARQ operation . 69
5.3.2.1 HARQ Entity. 69
5.3.2.2 HARQ process . 70
5.3.3 Disassembly and demultiplexing . 72
5.4 UL-SCH data transfer . 72
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5.4.1 UL Grant reception . 72
5.4.2 HARQ operation . 78
5.4.2.1 HARQ Entity. 78
5.4.2.2 HARQ process . 82
5.4.3 Multiplexing and assembly . 84
5.4.3.1 Logical Channel Prioritization . 84
5.4.3.1.1 General . 84
5.4.3.1.2 Selection of logical channels . 84
5.4.3.1.3 Allocation of resources . 85
5.4.3.2 Multiplexing of MAC Control Elements and MAC SDUs . 87
5.4.4 Scheduling Request. 87
5.4.5 Buffer Status Reporting . 93
5.4.6 Power Headroom Reporting . 96
5.4.7 Pre-emptive Buffer Status Reporting . 103
5.4.8 Timing Advance Reporting . 104
5.4.9 Delay status reporting . 105
5.5 PCH reception . 106
5.6 BCH reception . 106
5.7 Discontinuous Reception (DRX) . 106
5.7a Discontinuous Reception (DRX) for MBS Broadcast . 116
5.7b Discontinuous Reception (DRX) for MBS Multicast . 116
5.8 Transmission and reception without dynamic scheduling . 120
5.8.1 Downlink . 120
5.8.1a Downlink for Multicast . 121
5.8.2 Uplink . 121
5.8.3 Sidelink . 126
5.9 Activation/Deactivation of SCells . 128
5.10 Activation/Deactivation of PDCP duplication. 131
5.11 MAC reconfiguration . 131
5.12 MAC Reset . 132
5.12a Void . 134
5.13 Handling of unknown, unforeseen and erroneous protocol data . 134
5.14 Handling of measurement gaps . 134
5.15 Bandwidth Part (BWP) operation . 135
5.15.1 Downlink and Uplink. 135
5.15.2 Sidelink . 139
5.16 SUL operation . 141
5.17 Beam Failure Detection and Recovery procedure . 141
5.18 Handling of MAC CEs . 145
5.18.1 General . 145
5.18.2 Activation/Deactivation of Semi-persistent CSI-RS/CSI-IM resource set . 146
5.18.3 Aperiodic CSI Trigger State Subselection . 146
5.18.4 Activation/Deactivation of UE-specific PDSCH TCI state . 147
5.18.5 Indication of TCI state for UE-specific PDCCH . 147
5.18.6 Activation/Deactivation of Semi-persistent CSI reporting on PUCCH . 147
5.18.7 Activation/Deactivation of Semi-persistent SRS and Indication of spatial relation of SP/AP SRS . 148
5.18.8 Activation/Deactivation of spatial relation of PUCCH resource . 148
5.18.9 Activation/Deactivation of semi-persistent ZP CSI-RS resource set . 149
5.18.10 Recommended Bit Rate . 149
5.18.11 Void . 150
5.18.12 Void . 150
5.18.13 Void . 150
5.18.14 Update of Pathloss Reference RS of SRS . 150
5.18.15 Update of Pathloss Reference RS of PUSCH . 150
5.18.16 Indication of spatial relation of SRS resource for a Serving Cell set . 150
5.18.17 Activation/Deactivation of Semi-Persistent Positioning SRS . 150
5.18.18 Timing offset adjustments for IAB . 151
5.18.19 Guard symbols for IAB . 151
5.18.20 Positioning Measurement Gap Activation/Deactivation Command . 152
5.18.21 PPW Activation/Deactivation Command . 152
5.18.22 Update of PUCCH Power Control Set for multiple TRP PUCCH repetition . 153
5.18.23 Unified TCI States Activation/Deactivation MAC CE . 153
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5.18.24 Update of Differential Koffset . 153
5.18.25 BFD-RS Indication MAC CE . 153
5.18.26 Restricted and recommended beam indication for IAB . 153
5.18.27 DL TX power adjustment for IAB . 154
5.18.28 UL PSD range adjustment for IAB . 154
5.18.29 Timing case indication for IAB . 155
5.18.30 Case-6 Timing Request . 155
5.18.31 Backhaul Link Beam Indication for NCR. 155
5.18.32 Access Link Beam Indication for NCR . 156
5.18.33 Enhanced Unified TCI States Activation/Deactivation MAC CE . 156
5.18.34 Activation/deactivation of PSI-based SDU discard . 156
5.18.35 (Enhanced) LTM Cell Switch Command . 156
5.18.36 Candidate Cell TCI States Activation/Deactivation . 157
5.18.37 Activation/Deactivation of Aggregated Semi-Persistent Positioning SRS . 157
5.18.38 Activation/Deactivation of Semi-Persistent CSI-RS/CSI-IM resource set for candidate cell . 158
5.18.39 Update of Pathloss Offset . 158
5.18.40 UL Rate Control . 158
5.18.41 Activation/deactivation of semi-persistent CLI measurement resource set . 159
5.18.42 On-demand SSB Activation/Deactivation . 159
5.19 Data inactivity monitoring . 159
5.20 Void . 160
5.21 LBT operation . 160
5.21.1 General . 160
5.21.2 LBT failure detection and recovery procedure . 160
5.22 SL-SCH Data transfer and SL-PRS transmission . 161
5.22.1 SL-SCH Data and SL-PRS transmission . 161
5.22.1.1 SL Grant reception and SCI transmission . 161
5.22.1.2 TX resource (re-)selection check . 184
5.22.1.2a Re-evaluation and Pre-emption . 185
5.22.1.2b Re-selection for using a received resource conflict indication . 186
5.22.1.2c Resource re-selection from SL LBT Failure indication . 187
5.22.1.3 Sidelink HARQ operation and SL-PRS transmission . 188
5.22.1.3.1 Sidelink HARQ Entity . 188
5.22.1.3.1a Sidelink process not associated with Dedicated SL-PRS resource pool . 191
5.22.1.3.2 PSFCH reception . 193
5.22.1.3.3 HARQ-based Sidelink RLF detection . 194
5.22.1.3.4 Processing of sidelink grant on Dedicated SL-PRS resource pool . 194
5.22.1.3.5 Sidelink process associated with Dedicated SL-PRS resource pool . 195
5.22.1.4 Multiplexing and assembly . 196
5.22.1.4.0 General . 196
5.22.1.4.1 Logical channel prioritization . 196
5.22.1.4.2 Multiplexing of MAC Control Elements and MAC SDUs . 201
5.22.1.5 Scheduling Request . 201
5.22.1.6 Buffer Status Reporting . 202
5.22.1.7 CSI Reporting . 204
5.22.1.8 Void. 205
5.22.1.9 IUC-Request transmission . 205
5.22.1.10 IUC-Information Reporting. 206
5.22.1.10.1 General . 206
5.22.1.10.2 Reception of IUC-Information Reporting. 207
5.22.1.11 TX carrier (re-)selection . 207
5.22.1.12 SL-PRS Resource Request . 209
5.22.2 SL-SCH Data and SL-PRS reception. 209
5.22.2.1 SCI reception. 209
5.22.2.2 Sidelink HARQ operation and SL-PRS reception on Shared SL-PRS resource pool . 210
5.22.2.2.1 Sidelink HARQ Entity . 210
5.22.2.2.2 Sidelink process . 211
5.22.2.3 Disassembly and demultiplexing . 212
5.22.2.4 SL-PRS reception on Dedicated SL-PRS resource pool . 212
5.23 SL-BCH data transfer . 213
5.23.1 SL-BCH data transmission . 213
5.23.2 SL-BCH data reception . 213
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5.24 Handling of PRS Processing Window . 213
5.25 Positioning Measurement Gap Activation/Deactivation Request. 213
5.26 Positioning SRS transmission in RRC_INACTIVE . 214
5.26.1 General . 214
5.26.2 TA validation for SRS transmission in RRC_INACTIVE. 214
5.27 Small Data Transmission . 215
5.27.1 General . 215
5.27.2 TA Validation for CG-SDT . 217
5.28 Sidelink Discontinuous Reception (DRX) . 218
5.28.1 General . 218
5.28.2 Behaviour of UE receiving SL-SCH Data . 218
5.28.3 Behaviour of UE transmitting SL-SCH Data . 222
5.29 Activation/Deactivation of SCG . 222
5.30 Handling of FR2 UL gap . 223
5.31 Sidelink LBT operation . 224
5.31.1 General . 224
5.31.2 Sidelink LBT failure detection and recovery procedure . 224
5.32 Void . 225
5.33 First PUSCH transmission of RACH-less handover . 225
5.34 Cell-Level Energy Saving . 226
5.34.1 General . 226
5.34.2 Cell Discontinuous Transmission . 226
5.34.3 Cell Discontinuous Reception. 227
5.35 L1 measurement and event triggered report . 228
5.35.1 Introduction. 228
5.35.2 Performing measurement . 229
5.35.3 Measurement report triggering . 230
5.35.3.1 General . 230
5.35.3.2 Event LTM2 (Beam of serving cell becomes worse than threshold) . 231
5.35.3.3 Event LTM3 (Beam of candidate cell becomes offset better than beam of serving cell) . 232
5.35.3.4 Event LTM4 (Beam of candidate cell becomes better than absolute threshold) . 233
5.35.3.5 Event LTM5 (Beam of serving cell becomes worse than threshold1 and Beam of candidate cell

becomes better than threshold2) . 233
5.35.4 Measurement report . 234
5.36 Conditional LTM . 236
5.36.1 Introduction. 236
5.36.2 L1 measurement based Conditional LTM triggering condition evaluation . 236
5.36.3 Conditional LTM execution . 236
6 Protocol Data Units, formats and parameters . 239
6.1 Protocol Data Units . 239
6.1.1 General . 239
6.1.2 MAC PDU (DL-SCH and UL-SCH except transparent MAC and Random Access Response) . 239
6.1.3 MAC Control Elements (CEs) . 242
6.1.3.1 Buffer Status Report MAC CEs . 242
6.1.3.2 C-RNTI MAC CE . 250
6.1.3.3 UE Contention Resolution Identity MAC CE . 250
6.1.3.4 Timing Advance Command MAC CE . 250
6.1.3.4a Absolute Timing Advance Command MAC CE . 251
6.1.3.4b LTM Candidate Timing Advance Command MAC CE . 251
6.1.3.5 DRX Command MAC CE . 252
6.1.3.6 Long DRX Command MAC CE . 252
6.1.3.7 Configured Grant Confirmation MAC CE . 252
6.1.3.8 Single Entry PHR MAC CE . 252
6.1.3.9 Multiple Entry PHR MAC CE . 253
6.1.3.10 SCell Activation/Deactivation MAC CEs . 256
6.1.3.11 Duplication Activation/Deactivation MAC CE. 257
6.1.3.12 SP CSI-RS/CSI-IM Resource Set Activation/Deactivation MAC CE . 257
6.1.3.12a SP CSI-RS/CSI-IM Resource Set Activation/Deactivation for Candidate Cell MAC CE . 258
6.1.3.13 Aperiodic CSI Trigger State Subselection MAC CE . 259
6.1.3.14 TCI States Activation/Deactivation for UE-specific PDSCH MAC CE . 260
6.1.3.15 TCI State Indication for UE-specific PDCCH MAC CE . 260
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6.1.3.16 SP CSI reporting on PUCCH Activation/Deactivation MAC CE . 261
6.1.3.17 SP SRS Activation/Deactivation MAC CE . 262
6.1.3.18 PUCCH spatial relation Activation/Deactivation MAC CE. 263
6.1.3.19 SP ZP CSI-RS Resource Set Activation/Deactivation MAC CE . 264
6.1.3.20 Recommended bit rate MAC CE . 264
6.1.3.21 Timing Delta MAC CE . 265
6.1.3.22 Guard Symbols MAC CEs . 266
6.1.3.23 BFR MAC CEs . 266
6.1.3.24 Enhanced TCI States Activation/Deactivation for UE-specific PDSCH MAC CE . 268
6.1.3.25 Enhanced PUCCH Spatial Relation Activation/Deactivation MAC CE . 269
6.1.3.26 Enhanced SP/AP SRS Spatial Relation Indication MAC CE . 270
6.1.3.27 SRS Pathloss Reference RS Update MAC CE . 271
6.1.3.28 PUSCH Pathloss Reference RS Update MAC CE . 272
6.1.3.29 Serving Cell Set based SRS Spatial Relation Indicatio
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