URLLC service was introduced in 5G to support use cases with stringent requirements for extremely low latency (e.g., 1ms) and high reliability (e.g., 99.999%). Examples include public safety, remote diagnosis/surgery, emergency response, autonomous driving, industrial automation, and smart energy and grid.
Mini-slot Based Scheduling
Based on a scalable numerology, a shorter transmission can be achievable with a larger subcarrier spacing. More efficient scheduling for URLLC services is possible by introducing a scheduling unit smaller than a slot, referred to as a mini-slot. This enables to quickly schedule a URLLC transmission starting at any OFDM symbol to meet the stringent latency requirements.
Extreme Low Latency Support
Mini-slot based transmission can preempt ongoing transmission for immediate transmission of data requiring very low latency. For example, when there is no resource immediately available for URLLC data, the URLLC data transmission can be scheduled on resources overlapped with the ongoing transmissions for other type of services (e.g., enhanced mobile broadband). The preempted ongoing transmission can be handled by Hybrid Automatic Repeat reQuest (HARQ) and/or preemption indication. In 5G NR, HARQ is enhanced with Code Block Group (CBG)-based HARQ retransmission that allows to retransmit only a code block group interrupted. Also, the punctured resource of interrupted transmission can be indicated by a preemption indicator that is transmitted at the beginning of a slot after the time of preemption.
Extreme High Reliability Support
In 5G NR, the Low-Density Parity Check (LDPC) code is adopted for fast turnaround HARQ thanks to its parallelized decoding process. URLLC data can be transmitted with a number of repetitions using pre-configured periodic grants without the need for a dynamic uplink grant. The resources of a pre-configured grant can be isolated for URLLC services by logical channel prioritization restrictions. Reliability can be further enhanced by Packet Data Convergence Protocol (PDCP) packet duplication.
Flexible and Scalable 5G Core
5G core is also designed in a flexible and scalable way to meet the requirements of diverse services including the URLLC service. Network slicing and Mobile Edge Computing (MEC) are key enablers supporting URLLC services in the 5G core network. The network slicing enables URLLC services to be served by dedicated network resources, and the MEC can dramatically eliminate network delays by hosting services close to a user’s access point. This results in users being in close proximity of contextual information with low latency and real-time awareness of local environments.
