Secondary D2D Synchronization Signal

Description

The Secondary D2D Synchronization Signal (SD2DSS) is a critical physical layer signal defined for LTE-based sidelink communication, introduced as part of the Proximity Services (ProSe) and Device-to-Device (D2D) feature in 3GPP Release 12. It forms one half of the D2D synchronization signal pair, complementing the Primary D2D Synchronization Signal (PD2DSS). The primary function of these signals is to enable User Equipments (UEs) to achieve and maintain time and frequency synchronization with a synchronization reference when operating in direct communication mode on the sidelink (PC5 interface). This reference can be another UE (synchronization source UE), an eNodeB (in coverage), or a Global Navigation Satellite System (GNSS).

Technically, the SD2DSS is transmitted along with the PD2DSS in specific subframes within the sidelink synchronization resource pool. The signals have a defined periodicity. The PD2DSS provides coarse timing and helps identify the fundamental synchronization source type (e.g., in-coverage vs. out-of-coverage). The SD2DSS carries additional information that refines the synchronization process. Specifically, the sequence of the SD2DSS conveys the sidelink synchronization identity (SLSS ID), which is a crucial parameter. The SLSS ID, derived from the combination of PD2DSS and SD2DSS sequences, identifies a specific synchronization source and its characteristics. A receiving UE performs a correlation search to detect the PD2DSS first, then detects the associated SD2DSS to decode the full SLSS ID.

Once detected, the UE uses the timing of these signals to align its receiver. The achieved synchronization is vital for the proper operation of all other sidelink physical channels, such as the Physical Sidelink Broadcast Channel (PSBCH) which carries essential system information, and the Physical Sidelink Shared Channel (PSSCH) used for data transmission. Without robust synchronization provided by PD2DSS/SD2DSS, sidelink communication would suffer from severe inter-symbol interference and be unreliable, especially in mobile scenarios like public safety or V2X. The design of SD2DSS, including its sequence generation and mapping to resource elements, was carefully crafted to ensure low detection complexity, good autocorrelation and cross-correlation properties, and coexistence with legacy LTE cellular signals to avoid interference.

Purpose & Motivation

The SD2DSS was created to solve the fundamental challenge of establishing a common timing reference for direct device-to-device communication in LTE networks. Before ProSe standardization, LTE devices could only synchronize to an eNodeB. For direct communication, especially in out-of-coverage scenarios critical for public safety, devices needed a method to synchronize with each other autonomously. A simple extension of cellular synchronization signals was insufficient due to different requirements and potential interference.

The introduction of dedicated D2D synchronization signals (PD2DSS and SD2DSS) was motivated by the need for a flexible, hierarchical synchronization framework for sidelink. The PD2DSS provides the initial anchor, but the SD2DSS is essential for conveying the SLSS ID, which enables several key functionalities. This ID allows devices to distinguish between multiple synchronization sources (e.g., different clusters of devices), select the best source (e.g., based on priority or signal strength), and propagate synchronization in a multi-hop manner across a network of devices. This solves the problem of maintaining network-wide timing in decentralized scenarios, enabling reliable broadcast and groupcast communication for mission-critical services. It formed the synchronization backbone for LTE-based V2X in later releases, ensuring vehicles could coordinate direct communications even without cellular coverage.

Key Features

• Works with PD2DSS to form the D2D synchronization signal pair for LTE sidelink
• Carries part of the Sidelink Synchronization Signal Identity (SLSS ID)
• Enables UEs to achieve time/frequency sync with other UEs or GNSS in direct mode
• Supports synchronization source identification and selection
• Essential for the operation of sidelink broadcast and shared channels (PSBCH, PSSCH)
• Designed for robust detection in high-mobility and out-of-coverage scenarios

Evolution Across Releases

Defining Specifications