Primary D2D Synchronization Signal

Description

The Primary D2D Synchronization Signal (PD2DSS) is one of the two synchronization signals defined for the LTE sidelink (SL), the other being the Secondary D2D Synchronization Signal (SD2DSS). It is a fundamental physical layer signal used for Device-to-Device communication, particularly within Proximity Services (ProSe). The PD2DSS is transmitted by a UE that acts as a synchronization source. Its core function is to provide the initial coarse timing and frequency synchronization for receiving UEs. This allows discovering UEs to align their receivers in time and frequency with the transmitting UE, which is the essential first step before attempting to decode any data or control channels, such as the associated PD2DSCH.

Architecturally, the PD2DSS is designed to be robust and easily detectable. It is based on a Zadoff-Chu (ZC) sequence, similar to the Primary Synchronization Signal (PSS) used in the downlink for cellular synchronization with an eNodeB. However, to avoid confusion and interference, different root indices and sequence lengths are used for D2D. The PD2DSS is transmitted in specific subframes within a configured sidelink synchronization signal resource pool. A transmitting UE sends the PD2DSS along with the SD2DSS (which provides additional information like the sidelink identity) and the PD2DSCH in a synchronized block. The combination allows a receiving UE to determine the 504 unique sidelink synchronization identities.

The operational procedure involves a UE scanning for PD2DSS signals. Upon detection, it performs correlation to find the timing offset and estimate the carrier frequency offset. Once the PD2DSS is detected and coarse synchronization is achieved, the UE proceeds to detect the SD2DSS to identify the specific synchronization source ID and to achieve finer timing. Finally, it decodes the PD2DSCH to obtain the Sidelink Master Information Block (SL-MIB). The PD2DSS is crucial for establishing a common time reference among a group of UEs, enabling technologies like UE-to-Network relaying and direct communication in public safety scenarios where devices may form multi-hop chains, with each hop re-synchronizing using PD2DSS from a peer.

Purpose & Motivation

PD2DSS was created to solve the problem of initial synchronization between UEs in the absence of a cellular network (eNodeB). In traditional LTE, UEs synchronize to the eNodeB's PSS/SSS. For direct D2D communication, especially for public safety use cases where network infrastructure may be damaged or unavailable, an alternative synchronization mechanism was required. PD2DSS provides this UE-originated synchronization beacon.

The historical context is the 3GPP Rel-12 work on Proximity Services (ProSe). A key requirement was to support communication for first responders 'off-network.' PD2DSS, along with SD2DSS, forms the D2D synchronization signal pair that mimics the function of PSS/SSS but for the sidelink interface. It addresses the limitation of devices being unable to coordinate in time and frequency, which would lead to inefficient, collision-prone communication. By enabling UEs to become synchronization references, it allows the formation of self-organizing networks, extends coverage, and is the enabling physical layer signal for all subsequent D2D discovery and communication procedures. Its design ensures it can be distinguished from downlink signals to prevent mis-synchronization.

Key Features

• Provides initial time and frequency synchronization for D2D/sidelink communication
• Based on a Zadoff-Chu sequence, distinct from the downlink PSS to avoid confusion
• Transmitted by a UE acting as a synchronization source (eNB-synced, UE-synced, or GNSS-synced)
• Part of a synchronization signal block that includes SD2DSS and PD2DSCH
• Enables identification of one of 504 unique sidelink synchronization identities (in combination with SD2DSS)
• Fundamental for out-of-coverage, partial-coverage, and UE-to-Network relaying scenarios

Evolution Across Releases

Defining Specifications