Description
The SL-RNTI is a crucial identifier within the 3GPP sidelink communication framework, specifically defined for LTE-based Device-to-Device (D2D) and Vehicle-to-Everything (V2X) services and later carried into NR sidelink. It is a 16-bit value configured by the network via RRC signaling or derived from pre-configured parameters for out-of-coverage operation. The UE uses this RNTI to monitor the Physical Downlink Control Channel (PDCCH) for sidelink grants. When the network schedules a sidelink transmission, it sends a Downlink Control Information (DCI) format scrambled with the specific UE's SL-RNTI. The UE performs a cyclic redundancy check (CRC) on the received DCI using its assigned SL-RNTI; if the check passes, the UE decodes the grant information, which contains resource allocation details for the Physical Sidelink Shared Channel (PSSCH) or Physical Sidelink Control Channel (PSCCH). This mechanism ensures that sidelink resource assignments are directed to the correct UE, enabling efficient and collision-avoided direct communication. In Mode 1 (network-scheduled) operation, the gNB or eNB has full control over resource allocation, using the SL-RNTI to address UEs. For autonomous resource selection (Mode 2/3/4), the SL-RNTI may still be used for specific control signaling or in scenarios with partial network coverage. Its role is analogous to the C-RNTI used for uplink/downlink but is dedicated to the sidelink interface, separating the control plane for direct communication from traditional cellular links.
Purpose & Motivation
The SL-RNTI was introduced to enable network-controlled scheduling for direct device-to-device communication, a foundational requirement for Proximity Services (ProSe) and V2X. Prior to its introduction, 3GPP standards lacked a mechanism for the network to efficiently allocate and manage radio resources for direct UE-to-UE transmissions. Without such an identifier, the base station could not address individual UEs for sidelink grants, forcing reliance solely on contention-based autonomous resource selection, which leads to potential collisions, unpredictable latency, and inefficient spectrum use in high-density scenarios. The SL-RNTI solves this by providing a unique, temporary handle for the network to schedule sidelink resources with the same reliability and control as it schedules uplink resources. This was particularly critical for public safety applications where predictable communication is required, and for V2X where low latency and high reliability are paramount. It allows the network to coordinate interference, prioritize traffic, and integrate sidelink communication seamlessly into the overall radio resource management strategy, bridging the gap between traditional cellular and ad-hoc D2D networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (60 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-12, normative work from Rel-15.
In Release 15, the SL-RNTI function was enhanced through corrections and clarifications to V2X sidelink communication procedures, specifically within the MAC and RRC layers. These updates included corrections to the logical channel selection in the sidelink Logical Channel Prioritization (LCP) procedure and to MAC PDU reception in sidelink. Furthermore, the release introduced clarifications for sidelink processes such as PDCP duplication and the booking processes for resource allocation.
- Introduction of New Radio Access Technology in TS 36.300 TS 36.300CR0998
- Correction on V2X sidelink communication in TS 36.300 TS 36.300CR1199
- Correction on the logical channel selection in sidelink LCP TS 36.321CR1330
- Correction on V2X sidelink communication in TS 36.321 TS 36.321CR1365
- Clarification to Sidelink PDCP Duplication TS 36.321CR1396
- Clarification to TX Carrier (re)-Selection for V2X Sidelink Communication TS 36.321CR1397
+ 5 more changes
In Release 16, the SL-RNTI function was extended to support the new 5G V2X services over the NR Sidelink interface, a capability not present in previous releases. This introduction required new procedures for dynamic sidelink grant handling and prioritization, particularly between uplink and sidelink transmissions for V2X. Furthermore, the release specified corrections and clarifications for associated mechanisms like the sidelink BSR procedure and radio capability mapping.
- Introduction of 5G V2X with NR Sidelink TS 36.300CR1271
- Introduction of 5G V2X with NR Sidelink TS 36.321CR1467
- Introduction of 5G V2X with NR Sidelink TS 38.321CR0701
- Correction for NR sidelink communication TS 36.300CR1287
- Introducing UE Radio Capability Mapping procedure for EN-DC TS 36.300CR1314
- Clarification on LTE DAPS and sidelink on 36.300 TS 36.300CR1338
+ 15 more changes
In Release 17, the SL-RNTI function was enhanced to support new sidelink procedures, including Sidelink Relay operations and Sidelink CSI reporting. These enhancements required specific MAC layer corrections for handling configured sidelink grants and for the cancellation criteria of random access procedures triggered by sidelink BSR and CSI reporting. Furthermore, clarifications and corrections were made regarding the Buffer Size field in Sidelink BSR formats and the usage of default CBR values for NR sidelink.
- Introduction of Sidelink Relay TS 38.321CR1194
- CR of TS 38.321 for Sidelink enhancement TS 38.321CR1206
- Correction for sidelink relay in MAC TS 38.321CR1277
- Miscellaneous corrections for NR Sidelink Relay (rapporteur CR) TS 38.321CR1318
- Corrections on MAC reset regarding configured sidelink grant TS 38.321CR1605
- Correction on the usage of default CBR values for NR sidelink TS 38.321CR1611
+ 8 more changes
In Release 18, the SL-RNTI function was updated to support new NR sidelink relay enhancements, as indicated by the primary work item. The release also included multiple Medium Access Control (MAC) layer corrections and procedural clarifications for sidelink evolution, such as corrections to random resource selection and the Maximum Consecutive Slot (MCSt) parameter for sidelink.
- Introduction of NR sidelink relay enhancements TS 38.321CR1703
- Clarification of satellite identifiers TS 36.300CR1430
- MAC corrections on Rel-18 NR sidelink evolution TS 38.321CR1788
- Correction on Release-18 Sidelink evolution TS 38.321CR1830
- MAC correction on Release-18 Sidelink evolution TS 38.321CR1922
- Correction on Co-channel coexistence for LTE sidelink and NR sidelink TS 38.321CR1942
+ 6 more changes
In Release 19, the SL-RNTI function was updated to support the new NR Sidelink Multi-hop Relay capability, enabling extended device-to-device communication. Additionally, corrections were made to the processing of sidelink grants when using a Dedicated SL-PRS resource pool to ensure proper operation.
Explore further
Broader topics and technologies where SL-RNTI plays a role.
Defining Specifications
3GPP specifications that define or reference SL-RNTI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |