5G URLLC between Release 15 and 16

Introduction

Ultra-reliable low-latency communications (URLLC), one of the different use cases supported by the 5G NR standards, is a key technology for enabling driving automation. It is also an important enabler of other mission-critical applications, including industrial IoT (IIoT), smart grids, remote surgery, and tactile internet, which enables real-time human and machine interaction via the internet.

URLLC System Target

URLLC guarantees highly secure communications with a latency of no more than one millisecond (ms) and connection reliability of at least 99.999%. To achieve this level of performance, URLLC places much more quality of service (QoS) demands on the network.
The 5G NR standards — Release 15 and Release 16 — from the 3rd Generation Partnership Project (3GPP) include several enhancements designed to attain the QoS required for URLLC. These enhancements include time-sensitive networking, a new frame structure, flexible numerologies, dynamic time division duplex (TDD), and other physical layer procedures for data transmission.

3GPP has been using a brute-force approach centered on system-level simulations to meet the 99.999% (5-nine) reliability and 1 ms latency targets

URLLC in 5G Release 15

3GPP has been working on URLLC solutions since the beginning of Rel-15 New Radio (NR) work. Reliability is defined as the success probability of transmitting a small packet within a certain latency. As the first step within the radio access network (RAN) group, the targeted URLLC reliability requirement for data is 10^-5 for 32 bytes with a user plane latency of 1 ms.

Enhanced hybrid ARQ or Blind repetitions that send multiple copies of a given packet rather than waiting for the network to request retransmission when a packet is lost or arrives with errors. Blind repetitions are a redundancy feature designed to increase the chances that a packet arrives at the intended location.
Frequency diversity sends the same information at different frequencies or using different antennas, a technique known as spatial diversity. Like blind repetitions, this is a redundancy feature designed to increase the chances that the transmission arrives at the intended location.
Slot aggregation for the physical uplink shared channel (PUSCH) and the physical downlink shared channel (PDSCH) enables an uplink or downlink transmission to span multiple slots to facilitate improved coverage.
New numerology, short Transmission Time Interval (TTI)/mini-slot, Short TTI size to ensure the fast transmission of data packets, and new numerologies with flexible subcarrier spacing and bandwidth leading to the shorter slot; moreover mini-slot (1-13 OFDM symbols) based operation can further reduce latency.
Bi-directional slots for time division duplex (TDD), Bi-directional slots are a feasible solution to achieve low latency communication in TDD where multiple switching points can be configured within one slot. The time-division multiplexing of DL/UL control and data symbols in one slot allows fast and energy-efficient pipeline processing of control and user data in the receiver.
UE and gNB processing time, Reduced UE and gNB processing time to ensure fast creation of transport blocks for transmission as well as fast processing at the receiver for fast feedback transmission by, for example, placing dedicated pilot sequences at the beginning of the scheduling resource.
Macro-diversity: multi-connectivity, Data duplication transmission from multiple cells to the same UE to improve reliability through robustness against shadow fading, blocking effects, and cell failures. Packet data convergence protocol (PDCP) layer duplications have been specified in Rel-15.

URLLC in 5G Release 16

Release 15 laid the groundwork for the URLLC use case, but it did not implement all elements required. Release 16 will enhance and build on the URLLC components included in Release 15 and cater to new use cases that have appeared since the completion of Release 15.

Release 16 introduces enhancements to meet the requirements imposed by these new use cases and the initial vision for 5G network capability.
URLLC enhancements expected in Release 16 will support the multiplexing of enhanced mobile broadband (eMBB) and URLLC traffic.

URLLC services can reduce latency by preempting uplink eMBB transmissions. It supports the use of downlink channel information (DCI) to cancel uplink transmissions, assigning higher priority to URLLC traffic. This mechanism offers better reliability and lower latency compared with power-boosting, another transmission scheme.
Release 16 has uplink grant-free transmissions to reduce latency to enable the UE to transmit right away when needed, without waiting for an explicit indication from the network.
Release 16 introduces the compact DCI format to improve DCI reliability. The compact DCI format is used for specific use cases that require reliability improvements. It features a smaller payload to reduce the code rate of the DCI. There is one compact DCI format for uplink grants and one compact DCI format for downlink grants. Both can enable configurable sizes for some fields.
Release 16 supports increased physical downlink control channel (PDCCH) monitoring on the maximum number of non-overlapped control channel elements per slot for channel estimation. Increasing the PDCCH monitoring capability reduces latency.
Release 16 supports the use of more than one physical uplink control channel (PUCCH) for hybrid automatic repeat request acknowledgment (HARQ-ACK) transmission within a slot. This capability underpins different service types for a UE, such as eMBB and URLLC.

Conclusion

URLLC has been identified as one of the three main usage scenarios for 5G. With URLLC, the key performance metrics are low latency and high reliability. Generally, ultra-low latency refers to a few milliseconds or even lower, e.g., 1 ms end-to-end latency between client and server on the user plane. With ultra-reliability, mostly we are talking about at least 99.999%, 99.9999%, or even higher successful packet delivery between device and server. In addition, network availability, referring to the downtime of the network components such as base stations and core network elements, is also crucial for many applications.

References

Keysight Technologies.
5G Americas.
3GPP TR22.804, “Study on Communication for Automation in Vertical domains (Release 16)”, V16.0.0, June 2018.
3GPP TR 23.725, v16.1.0, Study on the enhancement of Ultra-Reliable Low-Latency Communication (URLLC) support in the 5G Core Network (5GC), March 2019.