Description
The Sidelink Synchronisation Signal (SLSS) is a critical physical layer signal defined in 3GPP for Proximity Services (ProSe) and sidelink communication, which is the direct radio link between user equipment (UEs). It consists of two main components: the Primary Sidelink Synchronisation Signal (PSSS) and the Secondary Sidelink Synchronisation Signal (SSSS). These signals are broadcast by a UE acting as a synchronization source, often referred to as a SyncRef UE. The primary function of the SLSS is to provide timing and frequency synchronization for other UEs (synchronization followers) within proximity, enabling them to correctly receive and decode subsequent sidelink control and data channels.
Architecturally, SLSS transmission can be network-controlled or autonomous. In coverage, an eNB/gNB can instruct a UE to transmit SLSS and provide the necessary synchronization configuration. Out of coverage, UEs can autonomously become synchronization sources based on predefined rules, creating a synchronization chain. The SLSS carries a sidelink synchronization identity (SLSS ID), which indicates the type of synchronization source (e.g., from the network, from another UE in coverage, or from an independent UE). The receiving UE performs correlation-based detection of the PSSS and SSSS to achieve symbol timing, radio frame timing, and frequency offset correction.
How it works involves a UE scanning for SLSS in designated resource pools. Upon detection, it derives the timing reference and adjusts its internal clock. This synchronized timing is essential because sidelink communications use Time Division Duplexing (TDD) with specific subframes allocated for control and data. Without a common time reference, transmissions would collide, and receivers would not know when to listen. The SLSS also facilitates the discovery of synchronization sources and helps in maintaining a stable synchronization hierarchy in dynamic environments, such as vehicular networks where UEs move at high speeds. Its role is foundational for reliable direct communication in public safety, V2X, and commercial D2D applications.
Purpose & Motivation
SLSS was introduced to solve the fundamental challenge of establishing and maintaining synchronization in decentralized, direct device-to-device communications. Prior to its specification, LTE synchronization was solely between the UE and the base station (eNB). For direct sidelink communication, especially for out-of-coverage scenarios like public safety operations or platooning vehicles, a new synchronization mechanism independent of the network infrastructure was required.
Its creation was motivated by the 3GPP work on Proximity Services (ProSe) starting in Release 12 and later V2X in Release 14. The limitations of relying solely on GNSS for timing (e.g., indoor/unavailability, cost, power consumption) necessitated a cellular-based synchronization signal. SLSS enables the formation of local synchronization clusters, ensuring that all transmitting UEs in an area are time-aligned, which minimizes interference and allows for efficient shared resource usage. It addresses the problem of chaotic, unsynchronized transmissions that would lead to poor reliability and capacity in critical direct communication scenarios.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (79 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 specification introduced corrections and clarifications for sidelink synchronization signalling, specifically addressing the sidelink measurement periodical triggering condition and providing a correction for the Sl-offsetIndicator parameter within the sidelink resource pool configuration. These changes refined the existing SLSS procedures to ensure more reliable synchronization for V2X and other sidelink communications over the PC5 interface. The updates were part of broader enhancements for sidelink operation, including dedicated signalling for resource pools as defined in SystemInformationBlockType18, Type19, and Type26.
- Signalling for euCA (Enhancing LTE CA Utilization) TS 36.331CR3391
- CR on signalling introduction of UE overheating support in NR SA scenario TS 38.331CR0729
- Additional capability signalling for 1024QAM support TS 36.331CR4031
- Additional capability signalling for 1024QAM support TS 38.331CR1120
- Correction for sidelink measurement periodical triggering condition TS 36.331CR3544
- CR on carrier frequency indication in SidelinkUEInformation TS 36.331CR3700
+ 17 more changes
In Release 16, the SLSS function was extended to support the new NR sidelink for 5G V2X communication, as introduced in the associated CRs to TS 36.331 and TS 38.331. This included specific corrections and enhancements to sidelink-related RRC procedures and parameters, such as those for configured grant usage during handover. Furthermore, the release introduced new signalling support for high-speed train scenarios and added a sidelink power class capability indication for UEs.
- Introduction of 5G V2X with NR Sidelink in TS 36.331 TS 36.331CR4222
- Introduction of signalling for high-speed train scenarios TS 36.331CR4326
- Introduction of 5G V2X with NR sidelink TS 38.331CR1493
- Introduction of signalling for high-speed train scenarios TS 38.331CR1464
- Duty cycle signalling for power class 1.5 TS 38.331CR2817
- CR to 36.331 on SLSS ID TS 36.331CR4425
+ 12 more changes
In Release 17, the enhancements for the SLSS function included corrections to sidelink synchronization measurement procedures and introduced new UE capability signaling specifically for Rel-17 sidelink features. Furthermore, the release provided clarifications and corrections on sidelink relay configurations and the conditions for sidelink discovery initiation, such as upon reception of SIB12. These updates were part of broader sidelink enhancements enabling more reliable out-of-coverage synchronization and operation.
- Introduction of Rel-17 sidelink enhancements TS 37.985CR0001
- RRC CR for NR Sidelink enhancement TS 38.331CR2902
- Introduction of RRC signaling for measurement gap enhancement TS 38.331CR2913
- CR to TS 38.331 on Network assistant signalling for Rel-17 CRS interference mitigation TS 38.331CR3021
- Introduction of UE capability for Rel-17 sidelink TS 36.331CR4781
- Introduction of capability filter for Rel-17 sidelink TS 36.331CR4824
+ 8 more changes
In Release 18, the SLSS function saw no specific new feature introductions; the work was focused on corrections and enhancements to existing sidelink RRC signalling, particularly for positioning (SL-PRS) and resource pool configuration. This included corrections to the `SidelinkUEInformationNR` message and the signalling for SL-PRS shared resource pools. The updates aimed to improve the reliability and accuracy of sidelink synchronization and positioning procedures.
- Introduction of sidelink CA and dynamic resource pool sharing for NR V2X TS 37.985CR0007
- Signaling support for intra-band non-collocated NR-CA, EN-DC TS 38.331CR4396
- Introduction of network RRC signalling for advanced receiver TS 38.331CR4488
- Correction on network RRC signalling for advanced receiver TS 38.331CR4585
- Correction on RRC signalling for advanced receiver TS 38.331CR4673
- RRC correction on NR sidelink positioning TS 38.331CR4940
+ 16 more changes
In Release 19, the SLSS function was enhanced to support the new capability of NR sidelink multi-hop relay, extending direct device-to-device synchronization chains. This advancement builds upon the existing sidelink operation definitions for the PC5 interface, enabling more complex relayed synchronization scenarios. The update facilitates improved synchronization signaling support for these extended multi-hop network topologies.
Explore further
Broader topics and technologies where SLSS plays a role.
Defining Specifications
3GPP specifications that define or reference SLSS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TR 37.985 vj00 | Overview of V2X features in LTE and NR | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |