Sidelink Relay Adaptation Protocol

Description

The Sidelink Relay Adaptation Protocol (SRAP) is a key protocol layer within the 5G New Radio (NR) sidelink protocol stack, specifically defined for relay operations between User Equipments (UEs). It operates as a sublayer of the Packet Data Convergence Protocol (PDCP) layer for the sidelink relay user plane. SRAP is responsible for adapting data packets for transmission over the sidelink relay interface, which is a direct device-to-device (D2D) communication link. It performs functions such as adding and removing SRAP headers, which contain necessary information for relay routing and identification, and may handle segmentation and reassembly of data units to match the characteristics of the sidelink radio bearer.

Architecturally, SRAP sits between the higher-layer services (e.g., IP packets or non-IP data) and the lower-layer sidelink radio protocols (like RLC and MAC). It is utilized in scenarios where a remote UE communicates with the network via a relay UE, which acts as an intermediary. The relay UE has a direct connection to the gNB (Uu interface) and also establishes a sidelink (PC5 interface) with the remote UE. SRAP operates on both the remote UE and the relay UE to ensure data packets are correctly formatted for the sidelink leg of the communication path. It works in conjunction with other sidelink protocols, such as Sidelink Radio Link Control (SL-RLC) and Sidelink Medium Access Control (SL-MAC), to provide reliable data transfer.

The protocol's operation involves the SRAP entity processing Service Data Units (SDUs) from the upper layers. It adds a SRAP header that includes fields like a Destination Layer-2 ID and possibly a Source Layer-2 ID for addressing within the sidelink. This header enables the relay UE to identify the target remote UE or to forward data towards the network. On the receiving side, the SRAP entity removes the header and delivers the SDU to the appropriate upper-layer entity. SRAP is designed to be transparent to the core network, meaning the network treats the remote UE as if it is directly connected, simplifying network management. Its role is crucial in enabling efficient relay-based communication, which is a foundational capability for Proximity Services (ProSe), public safety networks, and Vehicle-to-Everything (V2X) scenarios where direct network coverage may be limited.

Purpose & Motivation

SRAP was introduced to standardize and enhance relay functionality in 5G NR sidelink communications, addressing the need for reliable device-to-device relay without continuous network coverage. Prior to 3GPP Release 17, sidelink communications in LTE and early 5G primarily supported direct communication but had limited standardized relay mechanisms, especially for user plane data. This posed challenges for public safety applications, where first responders might operate in areas with damaged or absent network infrastructure, and for V2X scenarios where vehicles need to extend communication range. SRAP provides a protocol framework to efficiently adapt data for relay transmission, solving the problem of seamless connectivity extension.

The creation of SRAP was motivated by the evolution of Proximity Services (ProSe) and the increasing demand for advanced V2X and industrial IoT applications. In Release 16, 5G NR sidelink was enhanced for V2X, but relay support was not fully fleshed out in the user plane protocol stack. SRAP fills this gap by defining a dedicated adaptation layer that handles the intricacies of relay routing over the PC5 interface. It addresses limitations of previous ad-hoc or non-standardized relay approaches by ensuring interoperability, efficient header usage, and integration with the existing NR sidelink architecture. This enables UEs to dynamically act as relays, extending network coverage and improving service reliability in challenging environments.