ProSe Per-Packet Reliability

Description

ProSe Per-Packet Reliability (PPPR) is a Quality of Service (QoS) parameter introduced to complement ProSe Per-Packet Priority (PPPP) for sidelink communications. While PPPP dictates the order of transmission, PPPR dictates the required likelihood of successful packet delivery. It is expressed as a target reliability value, for example, a packet delivery success probability of 90%, 99%, or 99.9%, over a defined channel delay budget. This parameter is crucial for services where mere prioritization is insufficient and a guaranteed level of transmission robustness is required.

PPPR operates by influencing the physical layer and resource management strategies of the transmitting UE. When a packet is tagged with a specific PPPR value, the UE's protocol stack uses this requirement to select appropriate transmission parameters. This can include choosing a more robust Modulation and Coding Scheme (MCS), increasing the transmission power within allowed limits, or employing techniques like packet duplication (transmitting multiple copies of the same packet over different resources in time or frequency). The UE performs a link adaptation process that considers the target PPPR, current channel conditions, and the selected resource pool's characteristics to meet the reliability target.

Architecturally, PPPR is part of the QoS profile for a ProSe service. It is configured alongside PPPP, often by a ProSe Function or via pre-configuration for standardized services like V2X. The network can broadcast or provision information about which reliability levels are supported in a given geographical area or resource pool. In network-scheduled mode (mode 1), the eNB/gNB can consider the UE's reported PPPR requirements when granting sidelink resources. In autonomous mode (mode 2), the UE autonomously selects resources and transmission schemes from a pool that supports its required PPPR level. The interaction between PPPP and PPPR allows for sophisticated traffic management; a high-priority emergency stop message (high PPPP) would also have an extremely high PPPR (e.g., 99.999%) to ensure it is received, potentially at the cost of higher resource consumption or latency.

Purpose & Motivation

PPPR was created to address the stringent reliability requirements of advanced V2X and industrial IoT applications that emerged after the initial ProSe work. While PPPP solved the problem of 'what transmits first,' it did not guarantee 'if the transmission will be successful.' Early D2D and basic V2X services assumed a best-effort reliability model. However, applications like cooperative collision avoidance, automated driving, and remote control of machinery require predictable, ultra-reliable low-latency communication (URLLC) over the sidelink.

The limitation of having only a priority mechanism was that a critical safety message could be scheduled immediately (high PPPP) but still be lost due to a poor channel condition if transmitted with an aggressive, high-efficiency MCS. PPPR introduces a second dimension to sidelink QoS, allowing the system to trade spectral efficiency for reliability when necessary. Its development was motivated by the 3GPP's work on enhancing V2X in Rel-14 and beyond, where meeting the strict performance targets set by automotive and industrial standards became imperative. PPPR enables the sidelink to support service level agreements (SLAs) for packet delivery, which is foundational for trustworthy direct communication between machines and vehicles.

Key Features

• Defines a target packet delivery success probability (e.g., 99.9%) for sidelink packets
• Influences physical layer parameters like MCS selection and transmit power
• Enables transmission techniques such as packet duplication for ultra-reliability
• Works in conjunction with PPPP for two-dimensional QoS (priority + reliability)
• Configurable per-packet or per-service via ProSe policies or pre-configuration
• Essential for supporting URLLC-type requirements over the PC5 sidelink interface

Evolution Across Releases

Defining Specifications