Ultra Reliable Low Latency Communication

Description

Ultra Reliable Low Latency Communication (URLLC) is a foundational service category within the 5G system architecture, defined to support applications with stringent requirements for end-to-end latency, reliability, and availability. Unlike enhanced Mobile Broadband (eMBB) which focuses on high data rates, URLLC prioritizes deterministic performance, often targeting latencies as low as 1 millisecond for the radio interface and reliability levels up to 1-10^-5 or 1-10^-6 (99.999% to 99.9999% success probability). The architecture supporting URLLC permeates both the Radio Access Network (RAN) and the Core Network (5GC), requiring coordinated enhancements in scheduling, transmission schemes, and network resource management.

At the physical and MAC layer, URLLC employs several key techniques to achieve low latency. These include grant-free or configured grant uplink transmissions, which allow a User Equipment (UE) to transmit data immediately without waiting for a scheduling grant, drastically reducing signaling delay. Short Transmission Time Intervals (TTIs), such as mini-slots, enable faster packet encoding and decoding. For reliability, robust modulation and coding schemes (MCS), along with techniques like repetition coding, frequency diversity, and multi-connectivity (where a UE is simultaneously connected to multiple gNBs or cells), are utilized. Packet duplication over multiple paths, managed by the Packet Data Convergence Protocol (PDCP) layer, is a critical feature where identical data packets are sent via different radio links or carriers to increase the chance of successful delivery.

In the Core Network, URLLC support involves network functions like the Access and Mobility Management Function (AMF) and Session Management Function (SMF) being aware of URLLC Quality of Service (QoS) profiles. The 5G QoS Identifier (5QI) includes standardized values specifically for URLLC flows, which map to precise packet delay budget, packet error rate, and default priority levels. The core network ensures that user plane functions (UPF) are deployed appropriately, potentially using edge computing (via Multi-access Edge Computing - MEC) to localize traffic processing and further reduce latency. Network slicing is intrinsically linked with URLLC, allowing the creation of dedicated, logically isolated network slices with reserved resources and tailored configurations to guarantee the required performance independently from other service types.

Purpose & Motivation

URLLC was created to address the growing demand for wireless connectivity in industrial and mission-critical applications that cannot tolerate the variable latency and reliability of traditional mobile broadband services. Prior to 5G, cellular networks (2G, 3G, 4G) were optimized for human-centric communication—voice and mobile internet—where delays of tens or hundreds of milliseconds were acceptable. The advent of Industry 4.0, autonomous systems, and remote real-time control exposed the limitations of these networks for applications like factory automation, smart grids, and tele-surgery, where a missed deadline or a lost packet could lead to catastrophic failure, safety hazards, or significant economic loss.

The motivation for standardizing URLLC within 3GPP, starting from Release 14 as a study item and evolving through subsequent releases, was to transform cellular technology into a universal connectivity fabric capable of supporting both human and machine-type communication with guaranteed performance. It solves the problem of providing 'deterministic' wireless communication over a shared, statistical multiplexing medium. By defining clear targets and standardizing the enabling mechanisms across the entire protocol stack, URLLC allows diverse vertical industries to rely on 5G as a replacement for or complement to wired fieldbus systems (like PROFINET, EtherCAT) and proprietary wireless solutions, enabling greater flexibility, mobility, and scalability in automated environments.

Key Features

• Grant-free/Configured Grant uplink transmission for reduced scheduling latency
• Packet duplication at PDCP layer over multiple radio links for enhanced reliability
• Shortened Transmission Time Intervals (TTIs) like mini-slots
• Ultra-reliable low-latency specific 5QI values for QoS flow handling
• Support for Time-Sensitive Communication (TSC) and integration with IEEE TSN
• Enhanced support for uplink pre-emption and prioritization

Evolution Across Releases

Defining Specifications