sidelink

Sidelink refers to direct communication between two or more user equipment (UE) without routing through the base station or core network. In cellular technology, sidelink is used to enable device-to-device communication with the goals of reducing latency, saving spectrum, and supporting proximity-based services such as public safety, vehicle-to-everything (V2X), and cooperative sensing.

In LTE, sidelink was introduced as part of Proximity Services (ProSe). Communications use the PC5 interface and can be coordinated or autonomous. There are two modes: Mode 1, where the network assigns resources for sidelink transmissions; Mode 2, where devices autonomously select resources. Discovery mechanisms broadcast beacons or queries allow nearby devices to learn about each other. Sidelink supports unicast, multicast, and broadcast transmissions, with traffic types ranging from small control messages to larger payloads depending on capability.

In 5G NR, sidelink is extended to NR sidelink (often referred to as NR V2X). It maintains the basic D2D concept while offering enhanced resource allocation, higher data rates, and better reliability. NR sidelink adds improved discovery, quality-of-service signaling, and security features, enabling vehicle-to-vehicle, vehicle-to-infrastructure, and public safety applications. It can operate with network assistance or in standalone fashion, depending on deployment and regulatory constraints.

Implementation considerations include interference management, power control, device capability, energy consumption, and security and privacy. The standardization work is carried out by 3GPP across multiple releases, evolving sidelink to better support automotive, industrial, and consumer use cases.