Sidelink Synchronisation Signal

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.