Physical Sidelink Shared Channel

Description

The Physical Sidelink Shared Channel (PSSCH) is a key physical layer channel defined in 3GPP specifications for sidelink (SL) communication, introduced in LTE Release 12 and continued in 5G NR. Sidelink refers to direct communication between User Equipments (UEs) without the data passing through a base station (eNodeB/gNB) or the core network. The PSSCH is the primary channel used to transport user data (transport blocks) and associated sidelink control information (SCI) between UEs in proximity. It operates in the uplink spectrum for LTE-based sidelink (Mode 3 and 4) and in dedicated or shared spectra for NR sidelink.

The PSSCH transmission involves several physical layer procedures. A transmitting UE first sends Sidelink Control Information (SCI) on the Physical Sidelink Control Channel (PSCCH), which is typically mapped to resources adjacent or nearby to the PSSCH resources. This SCI carries crucial information for the receiving UE to decode the subsequent PSSCH transmission, including resource allocation, modulation and coding scheme (MCS), group destination ID, and timing information. The actual user data is then transmitted on the PSSCH using the resources and parameters indicated by the SCI. The channel utilizes similar modulation schemes (QPSK, 16QAM, 64QAM, 256QAM in NR) and coding (Turbo codes in LTE, LDPC in NR) as other shared channels.

In terms of resource allocation, two main modes are defined for LTE V2X: Mode 3 (scheduled) where the eNodeB allocates sidelink resources, and Mode 4 (autonomous) where the UE autonomously selects resources using a sensing and reservation protocol. NR sidelink introduces more advanced modes with greater flexibility. The PSSCH is fundamental for applications requiring low latency and high reliability, such as vehicle platooning, autonomous driving coordination, and public safety D2D communication. Its design includes features for handling high mobility, half-duplex constraints (a UE cannot transmit and receive on the same frequency simultaneously), and interference management in a distributed environment.

Purpose & Motivation

PSSCH was created to support direct device-to-device communication, a capability essential for new use cases beyond traditional cellular communication. The initial driver in LTE Release 12 was Proximity Services (ProSe) for public safety, allowing first responders to communicate directly when network infrastructure is damaged or unavailable. This addressed a critical limitation of conventional cellular networks, which rely entirely on base station coverage.

The motivation expanded significantly with the introduction of Vehicle-to-Everything (V2X) communication in LTE Release 14. Existing vehicular communication standards like IEEE 802.11p (DSRC) had limitations in scalability, coverage, and integration with cellular networks. PSSCH, as part of the 3GPP V2X standard, was designed to provide a more robust, high-capacity, and network-managed alternative for direct vehicle communication. It solves problems related to high-speed mobility, dense scenarios, and quality of service by leveraging the cellular spectrum and physical layer design. The creation of PSSCH enabled low-latency broadcast, groupcast, and unicast communication between vehicles, pedestrians, and infrastructure, forming the foundation for advanced driving safety and autonomous vehicle coordination.

Key Features

• Carries user data and transport blocks for direct UE-to-UE (sidelink) communication
• Works in conjunction with the PSCCH, which carries the necessary Sidelink Control Information (SCI) for decoding
• Supports multiple resource allocation modes: network-scheduled (Mode 3) and UE autonomous (Mode 4) in LTE
• Utilizes advanced modulation (up to 256QAM in NR) and channel coding (LDPC in NR) for high spectral efficiency
• Designed for high mobility scenarios with support for frequency and time synchronization among UEs
• Enables various transmission types: broadcast, groupcast, and unicast for flexible V2X and D2D services

Evolution Across Releases

Defining Specifications