Sidelink Radio Bearer

Description

The Sidelink Radio Bearer (SLRB) is the radio bearer abstraction used over the PC5 interface to transport sidelink data between User Equipments (UEs) without traversing the network infrastructure. It was introduced in 3GPP Release 12 as part of the Proximity Services (ProSe) work, which defined direct device-to-device communication for public safety and commercial use cases. An SLRB is conceptually analogous to a Data Radio Bearer (DRB) on the Uu interface, but it operates peer-to-peer between UEs rather than between a UE and a base station. The SLRB configuration is defined in the ASN.1 of TS 36.323 for LTE sidelink and TS 38.323 for NR sidelink, which specify the PDCP layer used for sidelink data transfer.

Architecturally, an SLRB sits above the sidelink RLC, MAC, and PHY layers. Each SLRB is associated with a PDCP entity that performs header compression (ROHC), ciphering, and integrity protection where applicable. In LTE V2X (Rel-14), SLRBs were largely pre-configured or provisioned by the network via SIB messages, with a fixed mapping between the ProSe Per-Packet Priority (PPPP) and the radio bearer. In NR V2X (Rel-16), the SLRB model was significantly enhanced: the network can configure SLRBs with QoS parameters (PC5 5QI, PQI) and the UE performs autonomous SLRB establishment based on the QoS flow to which a packet belongs. This mirrors the SDAP/QoS flow framework of the Uu interface, allowing differentiated treatment of sidelink traffic.

How it works: when a UE has sidelink data to send, the application or V2X layer maps the packet to a PC5 QoS flow. The UE then selects or establishes an SLRB whose QoS profile matches that flow. The SLRB's PDCP entity processes the packet (compression, ciphering), passes it to the sidelink RLC entity, which segments it and forwards to the MAC layer. The MAC layer multiplexes data from multiple SLRBs onto sidelink transport channels (SL-SCH), and the PHY transmits on the PC5 interface using configured sidelink resource pools. On the receiving side, the reverse process reassembles the data and delivers it to the correct SLRB and QoS flow.

SLRBs support both unicast, groupcast, and broadcast transmission modes in NR sidelink. For unicast, a PC5-RRC connection is established between peer UEs, and SLRB configuration is negotiated via RRC signaling. For groupcast and broadcast, SLRB configuration is typically pre-configured or provided by the network. The SLRB identity (SLRB ID) is used to distinguish bearers, and in NR it is scoped within a PC5 unicast link. Security for SLRBs includes ciphering and integrity protection keys derived during PC5 unicast link establishment.

The SLRB concept is central to V2X, public safety, and emerging sidelink applications such as NR sidelink relay and positioning. It enables QoS differentiation on the sidelink, which is essential for safety-critical V2X messages that require low latency and high reliability, while allowing best-effort traffic to share the same PC5 resources.

Purpose & Motivation

The SLRB exists to provide a structured, QoS-aware data transport mechanism on the PC5 sidelink interface, analogous to the DRB on the Uu interface. Before SLRBs, early device-to-device communication lacked a standardized bearer concept, making it difficult to apply differentiated treatment to different types of sidelink traffic. The introduction of SLRBs in Rel-12 allowed ProSe direct communication to be managed with the same rigor as cellular bearers, including security and header compression.

As V2X use cases matured, the limitations of the original LTE SLRB model became apparent. The fixed PPPP-to-bearer mapping could not express the richer QoS requirements of advanced V2X services such as platooning, extended sensors, and remote driving. NR V2X in Rel-16 therefore redefined SLRBs with a QoS flow framework, PC5 5QI, and network-configurable or UE-autonomous bearer establishment. This solved the problem of mapping diverse application requirements onto sidelink radio resources.

The SLRB also addresses the need for security and reliability on direct links. By anchoring ciphering and integrity protection at the PDCP layer of the SLRB, 3GPP ensured that sidelink communication could be protected end-to-end between UEs without relying on network intermediation. This is critical for public safety and V2X scenarios where infrastructure coverage may be absent.