Sidelink

Description

Sidelink is the 3GPP-defined direct radio interface between User Equipment (UE) that allows devices to communicate with each other without routing user data through the base station or core network. It is standardised as the PC5 interface and is specified across both LTE (E-UTRA) and NR (New Radio) access technologies. The interface supports three main families of functionality: sidelink communication (unicast, groupcast and broadcast data transfer between UEs), sidelink discovery (procedures by which a UE discovers nearby UEs or services), and V2X sidelink communication (vehicular communication for safety, traffic efficiency and automated driving use cases). Sidelink operates in dedicated or shared spectrum, and can be used both in-coverage (under network control) and out-of-coverage (autonomous operation), which is essential for public safety and disaster scenarios where infrastructure may be unavailable.

Architecturally, sidelink sits at the radio access level but interacts with the core network for authorisation, provisioning and policy. In LTE, sidelink resource allocation is controlled by the eNB through either scheduled (Mode 1) or UE-autonomous (Mode 2) resource selection. In NR, sidelink introduces more advanced designs: Mode 1 (network-scheduled) and Mode 2 (UE-autonomous) resource allocation, a flexible numerology, a physical sidelink control channel (PSCCH), physical sidelink shared channel (PSSCH), physical sidelink feedback channel (PSFCH) for HARQ feedback, and a sidelink synchronisation signal (SLSS) with a physical sidelink broadcast channel (PSBCH). NR sidelink also supports unicast and groupcast with PC5-RRC signalling, beamforming, and quality-of-service (QoS) management through PC5 5QI (PQI).

The protocol stack for sidelink mirrors the Uu interface but is adapted for direct communication. At the physical layer, sidelink uses SC-FDMA in LTE and CP-OFDM/DFT-s-OFDM in NR. Above the physical layer, the MAC layer handles scheduling, multiplexing and HARQ, the RLC layer provides segmentation and reassembly, and the PDCP layer handles ciphering and integrity protection. For NR sidelink, a PC5-RRC protocol was introduced to support unicast link establishment, capability exchange and radio link management. The sidelink also defines a PC5-S signalling protocol for link establishment, security and discovery procedures.

Sidelink is central to several 3GPP service enablers. In public safety (MCPTT, MCVideo, MCData), it provides off-network direct communication when coverage is lost. In V2X, it enables vehicles to exchange CAM/DENM messages directly, supporting cooperative awareness and collision avoidance. In Rel-16 and beyond, NR sidelink is being extended to support advanced V2X (eV2X), industrial IoT, and even relay and positioning use cases. Rel-17 and Rel-18 further enhance sidelink with features such as sidelink relay, power saving, and improved resource allocation for reduced capability devices.

From a network perspective, sidelink reduces latency and offloads traffic from the Uu interface, while enabling new proximity-based services. It requires careful coordination with the Uu interface to avoid interference and to manage resource pools. The core network, through the PCF and UDM, provides authorisation and policy for sidelink usage, and the NG-RAN configures sidelink resource pools and parameters via broadcast or dedicated signalling. Overall, sidelink represents a fundamental shift from purely network-centric communication to direct device-to-device interaction, and it continues to evolve as a key enabler for 5G-Advanced and 6G use cases.

Purpose & Motivation

Sidelink was created to enable direct communication between devices without relying on network infrastructure. Before its introduction, all UE-to-UE communication had to traverse the base station and core network, which introduced latency, consumed network resources, and failed entirely when coverage was unavailable. Public safety organisations needed a way for first responders to communicate in disaster zones where cellular infrastructure might be damaged or congested. Similarly, vehicular communication required low-latency, high-reliability message exchange between nearby vehicles, which is difficult to guarantee through the network alone.

Sidelink addresses these limitations by providing a direct radio path between UEs, known as the PC5 interface. It allows devices to discover each other and exchange data even when out of network coverage, using pre-configured resource pools and synchronisation procedures. This makes it ideal for mission-critical push-to-talk, V2X safety applications, and emerging use cases like industrial automation and extended reality. By offloading traffic from the Uu interface, sidelink also improves spectral efficiency and reduces core network load.

The evolution from LTE sidelink to NR sidelink reflects the growing demand for higher data rates, lower latency, and more advanced topologies. NR sidelink introduces unicast and groupcast with feedback, beamforming, and QoS differentiation, enabling a broader range of services beyond basic broadcast. As 5G-Advanced and 6G evolve, sidelink is expected to play a central role in device-to-device, vehicle-to-everything, and even integrated sensing and communication scenarios.