5G NR Sidelink MAC
5G NR sidelink MAC is the MAC-layer behavior used for direct UE-to-UE communication over PC5. It sits in the same MAC layer family as ordinary NR MAC, but the operating model is different because resource-pool use, sidelink grants, sidelink HARQ, SCI, and peer-directed control replace the usual UE-to-gNB scheduling picture.
Use this page as the main reference entry for sidelink MAC in the 3glteinfo MAC library. It summarizes where sidelink MAC fits, how SL-SCH and SL-BCH are handled, how resource selection and resource reselection change the reading of traces, and which sidelink-specific MAC procedures matter most in Release 18. For the broader parent view, start with 5G NR MAC Overview and then move into Sidelink MAC CEs or message-level sidelink pages as needed.
| Technology | 5G NR sidelink |
|---|---|
| Protocol area | MAC over PC5 for direct UE-to-UE communication |
| Main specification | 3GPP TS 38.321 |
| Architecture context | 3GPP TS 38.300 and 3GPP TS 38.331 |
| Release baseline | Release 18 |
| Main channels | SL-BCH and SL-SCH with SBCCH, SCCH, and STCH mapping |
| Core topics | Resource pools, sidelink grants, SCI, sidelink HARQ, resource reselection, sidelink DRX, inter-UE coordination |
| Best paired with | MAC Overview, Logical Channels and Transport Channels, HARQ, MAC PDU Format, Sidelink MAC CEs |
Definition and purpose
Sidelink MAC is the NR MAC behavior used when UEs exchange data and control directly over PC5 instead of sending everything through the gNB user-plane path. It still performs mapping, multiplexing, demultiplexing, HARQ-related handling, and control-element carriage, but it does so in a sidelink-specific channel and grant model.
The important difference is that sidelink MAC is not read with the same assumptions as ordinary uplink scheduling. Resource pools, sidelink grants, SCI, retransmission decisions, peer coordination, and sidelink DRX all change how transmission opportunity and failure should be interpreted.
Where sidelink MAC fits
| Layer or context | Relation to sidelink MAC |
|---|---|
| RRC | Configures sidelink resource pools, logical channels, priorities, DRX-related behavior, reporting triggers, and other PC5 operating rules. |
| RLC | Supplies sidelink SDUs and receives demultiplexed SDUs after SL-SCH reception. |
| MAC | Handles sidelink mapping, multiplexing, resource use, HARQ-related operation, control-element carriage, and reception-side disassembly. |
| PHY | Provides PSCCH, PSSCH, PSFCH, and SL-BCH transmission or reception behavior that MAC depends on. |
Sidelink channels and mapping
Sidelink logical channels map onto sidelink transport channels, and MAC is the layer that makes that mapping operational. This is the sidelink counterpart of the channel model described on the Logical Channels and Transport Channels page.
For high-level reading, keep three layers separate: logical channels describe information type, transport channels describe how MAC exchanges data with PHY, and physical channels describe the actual PC5 radio resources used for sidelink signalling, data, and feedback.
| Channel layer | Main sidelink items | What it means |
|---|---|---|
| Logical channels | SBCCH, SCCH, STCH | Broadcast control, sidelink control, and sidelink traffic information types. |
| Transport channels | SL-BCH, SL-SCH | The transport channels MAC uses for sidelink broadcast and shared sidelink data transfer. |
| Physical channels | PSCCH, PSSCH, PSFCH | PHY-side sidelink control, shared data, and feedback channels that MAC works with indirectly through the sidelink procedures. |
Resource model and grant handling
Sidelink MAC has to be read through the resource-pool model, not only through the ordinary cellular scheduler mindset. Practical questions are: which pool is active, which sidelink bandwidth part is in use, was the transmission network-scheduled or UE-selected, and was the selected or granted resource kept or reselected.
At MAC level, the active transmission path may involve a dynamic sidelink grant, a configured sidelink grant, or a selected sidelink grant. The spec also distinguishes between resource-allocation behavior where the network provides scheduling assistance and behavior where the UE performs autonomous sidelink resource selection.
| Resource path | What MAC needs to handle | Why it matters in traces |
|---|---|---|
| Network-assisted sidelink scheduling | Use of configured or dynamically indicated sidelink resources under configured PC5 rules | The failure picture is closer to grant interpretation and timing. |
| UE-selected sidelink resources | Resource selection, possible reselection, and conflict handling within the configured pool | A missed transmission may be a resource-selection outcome rather than missing demand. |
| Resource reselection | Replacement of a previously usable resource or grant path under current conditions | Repeated change in selected resources can explain unstable latency or bursty delivery. |
| Prioritization across competing transmissions | Decision on whether sidelink transmission can proceed when other transmissions also exist | It explains why data may be queued but not transmitted at the expected moment. |
Main sidelink MAC procedures
| Procedure area | Purpose | Why open next |
|---|---|---|
| SL-SCH data transmission | Transmit sidelink MAC PDUs using the active grant or selected resource. | This is the core transmit-side MAC behavior and the anchor for most sidelink troubleshooting. |
| SCI reception | Read sidelink control information associated with upcoming or active sidelink data handling. | SCI often explains why reception, HARQ, or resource interpretation differs from expectation. |
| Sidelink HARQ operation | Track acknowledgement outcome, retransmission need, and sidelink process state. | Open HARQ next for the general retransmission model, then return to the sidelink-specific differences here. |
| Disassembly and demultiplexing | Parse the received sidelink MAC PDU and deliver content to the right destination. | Useful when a decode succeeds physically but the expected control or data does not appear at the right layer. |
| Sidelink BSR and related reporting | Trigger resource demand signalling and related MAC control for sidelink traffic. | Open Sidelink MAC CEs for the CE view. |
| SL-CSI reporting | Report sidelink channel-state information in the MAC-defined control path. | Important when sidelink quality reporting affects grant demand or radio decisions. |
| Inter-UE coordination | Exchange request or information MAC CEs for coordination between peer UEs. | Important in advanced sidelink deployments and Release 18 reading. |
| Sidelink DRX behaviour | Control when sidelink data should be treated as in or out of active time. | Use with DRX when a sidelink grant is ignored or appears unexpectedly unused. |
| SL-BCH data transfer | Handle broadcast sidelink information on SL-BCH. | Useful when the question is broadcast discovery or synchronization context rather than user data transfer. |
Sidelink HARQ and feedback
Sidelink HARQ remains a MAC and PHY boundary topic, but the feedback path and the process interpretation are different from ordinary UE-to-gNB operation. On the receive side, sidelink HARQ tracks acknowledgement outcome and sidelink process behaviour for SL-SCH reception, while on the transmit side it stays tied to the active sidelink grant and related PSFCH feedback handling.
The main practical point is that sidelink retransmission reading depends on the active grant, the destination context, whether feedback is enabled, whether the resource remains usable, and whether sidelink DRX or prioritization caused a grant to be ignored. Do not reduce sidelink HARQ to a simple ACK or NACK counter.
| Observed pattern | Sidelink HARQ reading |
|---|---|
| No retransmission after missing delivery | Check whether feedback was configured, whether the grant remained available, and whether the transmission path was ignored by sidelink DRX or prioritization. |
| Repeated retransmissions on the same traffic flow | Check resource-pool conditions, peer feedback behavior, and whether the sidelink process is repeatedly reusing stressed resources. |
| HARQ status looks inconsistent across peers | Check Source Layer-2 ID and Destination Layer-2 ID context together with SCI and the corresponding sidelink process. |
MAC PDU and sidelink MAC Control Elements
Sidelink data on SL-SCH is still carried in a MAC PDU, but the sidelink format has its own SL-SCH subheader and LCID space. Read this together with the MAC PDU Format page when decoding captures.
Sidelink operation also uses sidelink-specific MAC Control Elements. Representative examples include sidelink BSR MAC CEs, sidelink configured grant confirmation, sidelink CSI reporting, sidelink DRX Command, sidelink inter-UE coordination request or information, and SL-PRS related MAC CEs. Use the MAC Control Elements Overview and Sidelink MAC CEs page for the CE view.
| Sidelink MAC structure item | Why it matters |
|---|---|
| SL-SCH subheader | It separates sidelink MAC parsing from the ordinary UL-SCH and DL-SCH reading model. |
| LCID for SCCH or logical channels | It identifies whether the carried content is sidelink control, traffic, duplication-related content, or a sidelink-specific MAC CE. |
| Source Layer-2 ID and Destination Layer-2 ID scope | It matters when reading per-peer behaviour, especially in unicast or peer coordination cases. |
| Sidelink MAC CEs | Many practical procedure clues appear as compact CEs rather than as larger messages. |
Reading notes
- Start with the active sidelink resource pool and sidelink BWP before judging whether a transmission should have happened.
- Read SCI, the sidelink grant or selected resource, and the resulting MAC PDU together.
- Keep Source Layer-2 ID and Destination Layer-2 ID context visible during troubleshooting.
- Check whether sidelink DRX, prioritization, or grant unavailability explains ignored transmissions.
- Use Sidelink MAC CEs when the trace summary hides the real trigger inside a compact control element.
Where sidelink MAC appears in real procedures
| Procedure area | Why sidelink MAC matters |
|---|---|
| Direct UE-to-UE data transfer | This is the main path for SL-SCH assembly, transmission, retransmission, and reception-side demultiplexing. |
| PC5 control exchange | SCCH and PC5 signalling depend on correct sidelink control carriage and interpretation. |
| Sidelink unicast or groupcast reliability handling | HARQ, destination context, and peer coordination become visible at MAC level. |
| Sidelink DRX operation | A grant may exist but still be ignored if the destination is not in SL DRX active time. |
| Inter-UE coordination and advanced feature control | Release 18 sidelink behaviour uses more MAC-level coordination and feature-specific control elements. |
Release 18 scope
A Release 18 sidelink MAC page needs more than the basic “UEs exchange direct traffic” view. The active scope includes sidelink DRX, richer sidelink MAC CE families, inter-UE coordination, positioning-related sidelink behaviour such as SL-PRS support, and more detailed handling of advanced control and reporting paths.
That wider scope is why sidelink MAC belongs on its own protocol page instead of as a short subsection under the main MAC hub.
Troubleshooting
| Symptom | Sidelink MAC area to inspect | Why |
|---|---|---|
| Expected sidelink transmission does not occur | Active resource pool, selected or granted resource, and prioritization outcome | The missing transmission may be a resource-selection or prioritization outcome rather than missing data. |
| Repeated sidelink retransmissions | Sidelink HARQ process, peer feedback, and resource reuse or reselection | The same traffic may keep using stressed resources or fail to complete cleanly. |
| Grant appears present but MAC does not use it | SL DRX active time and destination context | The grant can be ignored when the destination is not in active time. |
| Wrong peer seems involved in the decode | Source Layer-2 ID and Destination Layer-2 ID correlation | Per-peer context matters much more in sidelink than in ordinary cellular scheduling. |
| Control looks fine but application data is missing | Disassembly, demultiplexing, LCID interpretation, and MAC CE parsing | The payload may have been carried differently from what the summary view suggests. |
| Advanced sidelink feature behaves inconsistently | Sidelink MAC CEs, SCI, and inter-UE coordination state | Modern sidelink behaviour often depends on compact control that is easy to miss. |
References
- ETSI TS 138 321 V18.2.0 / 3GPP TS 38.321 Release 18 - MAC architecture, sidelink mapping, sidelink procedures, sidelink HARQ, sidelink MAC PDU structure, and sidelink MAC Control Elements.
- ETSI TS 138 300 / 3GPP TS 38.300 - NR and NG-RAN overall architecture context, including sidelink placement in the wider NR system.
- ETSI TS 138 331 / 3GPP TS 38.331 - RRC configuration context for sidelink pools, logical channels, and related PC5 behaviour.
- ETSI TS 138 213 / 3GPP TS 38.213 - physical-layer procedures for control that sidelink MAC depends on for timing, feedback, and resource interpretation.
- ETSI TS 138 214 / 3GPP TS 38.214 - physical-layer procedures for data and radio-resource behaviour that shape sidelink MAC operation.
FAQ
What is 5G NR sidelink MAC?
It is the NR MAC-layer behaviour used for direct UE-to-UE communication over PC5, including sidelink channel mapping, grant handling, control-element carriage, and sidelink HARQ-related operation.
How is sidelink MAC different from ordinary NR MAC?
The basic MAC functions are familiar, but the operating model is different because sidelink uses PC5 channels, resource pools, sidelink grants or selected resources, SCI, peer context, and sidelink-specific control elements.
Which channels matter most in sidelink MAC?
At MAC level, the main sidelink logical channels are SBCCH, SCCH, and STCH, and the main transport channels are SL-BCH and SL-SCH.
Why is sidelink MAC important for troubleshooting?
Because direct UE-to-UE behaviour is often explained by resource-pool choice, grant or resource selection, sidelink HARQ, SCI interpretation, or sidelink DRX rather than by the usual uplink scheduler logic.
Does sidelink MAC include its own MAC Control Elements?
Yes. The sidelink MAC CE space includes reporting, grant-related, DRX-related, inter-UE coordination, positioning-related, and other sidelink-specific controls.
Does Release 18 make sidelink MAC broader?
Yes. Release 18 reading includes richer sidelink control families, sidelink DRX, inter-UE coordination, and advanced sidelink feature behaviour beyond the earliest basic sidelink view.