PC5 QoS Identifier

Description

The PC5 QoS Identifier (PQI) is a scalar value that serves as a reference to a standardized QoS profile defined in 3GPP specifications for communications over the PC5 reference point. The PC5 interface enables direct communication between User Equipments (UEs), known as sidelink communication, which is pivotal for Vehicle-to-Everything (V2X), public safety, and commercial D2D services. Each PQI value corresponds to a specific combination of QoS parameters, including resource type (e.g., Guaranteed Bit Rate or Non-GBR), priority level, packet delay budget, packet error rate, and optionally, a default maximum data burst volume and averaging window.

In operation, when an application on a UE initiates a communication session over PC5, it requests a certain QoS level. This request is often translated into a PQI value. The PQI is used during the PC5 unicast link establishment or modification procedures, as defined in specifications like TS 23.287. The initiating UE signals the desired PQI to the peer UE. Both UEs then use this identifier to locally derive the full set of QoS characteristics from the standardized table. This derivation informs critical functions such as packet classification, marking, scheduling, and admission control on the sidelink radio bearers.

The architecture leverages PQI to abstract complex QoS parameter sets into a simple integer, simplifying signaling and ensuring consistency. The QoS profile referenced by a PQI dictates how the data flow should be treated across the protocol layers, particularly at the Access Stratum (AS). For example, a PQI associated with a high-priority, low-latency resource type will influence how the MAC layer schedules transmissions and selects resources in the sidelink resource pool, potentially using more reliable modulation and coding schemes or more frequent transmission opportunities.

PQI is a cornerstone of the QoS framework for NR sidelink and evolved LTE sidelink (when used in a 5G context). It works in tandem with the PQFI (PC5 QoS Flow ID). While the PQI defines the "what"—the QoS characteristics template—the PQFI identifies the "which"—the specific data flow instance to which those characteristics are applied. This separation allows multiple flows (each with a unique PQFI) to share the same PQI if they have identical QoS requirements, or to have different PQIs if their requirements differ. Management entities, such as the ProSe Function or the 5G Core Network's Policy Control Function (PCF), can provision or authorize the use of specific PQI values for different services or applications, enabling network-controlled QoS even for direct UE-to-UE communication.

Purpose & Motivation

The PQI was created to standardize and streamline QoS management for direct PC5-based communication, which became essential with the introduction of advanced V2X services in 3GPP Release 14 and its significant enhancement in the 5G context from Release 16. Earlier D2D mechanisms lacked a comprehensive, standardized QoS identifier, making it difficult to ensure predictable and interoperable service quality across different manufacturers' devices, especially for life-critical applications like autonomous driving.

It solves the problem of complex and inefficient signaling of full QoS parameter sets for every communication session. Without PQI, UEs would need to negotiate and signal numerous individual parameters (delay budget, error rate, etc.) each time, increasing signaling overhead and setup latency. PQI condenses this information into a single, well-known index, drastically simplifying session establishment and modification. This efficiency is crucial for dynamic V2X environments where communication sessions may be brief and established rapidly.

The motivation stems from the need to extend the robust QoS framework of 5G, which uses the 5QI for Uu interface, to the sidelink interface. This ensures a unified approach to service quality across all access types. PQI allows the network to exert policy control over sidelink communications by authorizing which PQI values a UE can use for specific services. It addresses the limitations of earlier ProSe Per-Packet Priority (PPPP), which was a simpler priority indicator, by providing a full set of QoS characteristics necessary to support the diverse and stringent requirements of 5G-enabled verticals like automotive and industrial IoT.