01 Introduction

URLLC, short for Ultra Reliable & Low Latency Communication, is one of the three major application scenarios of 5G and a key technology for empowering industrial applications. In June 2018, with the release of R15, ultra-reliable and low-latency communication technology was introduced, mainly targeting power automation and AR/VR (entertainment industry), and the standardization of new network architecture and basic functions was completed. In July 2020, with the release of R16, URLLC was enhanced, accelerating the application of 5G in scenarios such as autonomous driving, industrial Internet, and remote control.

SageRAN actively participates in cutting-edge communication technologies, deeply explores the implementation and landing methods of ultra-reliable and low-latency communication technology. This article will elaborate on 5G URLLC from the application scenarios of URLLC and SageRAN's related technical planning, and look forward to collaborating with more industry partners to participate in the innovation and landing of new technologies.

02 Application Scenarios and Challenges

The biggest feature of 5G URLLC scenarios is low latency and high reliability. The scope of URLLC scenarios is large, and the requirements for latency, reliability, and bandwidth are different in different scenarios. Generally speaking, it mainly includes three major scenarios: industrial automation, power automation, and connected vehicles.

Table 1: Latency and reliability requirements for different URLLC services

A. Industrial Automation

Factory automation is a typical application scenario of industrial manufacturing, and high-end manufacturing has extremely high requirements for the latency and stability of workshop equipment. The low latency and high reliability of 5G URLLC are very suitable for application in the manufacturing scene. Manufacturing equipment can access enterprise clouds or on-site control systems through 5G, collect on-site environmental data and production data, analyze production status in real-time, and ultimately achieve the digitization and automation of the entire production line.

Figure 1: Smart Factory

B. Power Automation Scenario

The communication of the power grid is mainly based on optical fibers, and the communication nodes are sunk to the edge. The coverage cost of the communication network has increased dramatically, and the deployment scenarios are complex and diverse. Wireless networks are the most economical choice. Since power grid access requires phase alignment, it is necessary to adjust the phase of the generator unit to match the overall network, and high-precision measurement, low-latency feedback, and control are more conducive to achieving phase alignment. In the long run, the demand for URLLC in power application scenarios will gradually increase and is on a high-speed development trajectory.

Figure 2: Power System Automation

C. V2X

With the arrival of R16 URLLC, intelligent vehicle networking applications gradually have the conditions for landing. URLLC can be used for automated control of road traffic infrastructure, that is, intelligent collection devices such as smart streetlights and intelligent traffic lights are deployed on the roadside, and interact with the vehicle's onboard computer through the 5G network to greatly increase the vehicle's perception of surrounding affairs, and not be affected by adverse weather conditions, improving the speed and safety of intelligent driving, thereby improving active safety driving, effectively alleviating urban road congestion, improving the efficiency of traffic resource allocation, increasing travel rates, and achieving smart urban transportation.

Figure 3: 5G Connected Vehicles

3. SageRAN`s Plan for URLLC Scenarios

SageRAN has always been actively involved in promoting the evolution of cutting-edge communication. SageRAN 's self-developed 5G protocol stack has high flexibility and scalability. The following is SageRAN 's technical judgment and planning for URLLC implementation, which will be explained from the perspectives of reducing latency and improving reliability:

(1) Achieving Low Latency Characteristics of URLLC

Mainly from the perspective of reducing latency to ensure the ultra-low latency requirements of URLLC scenarios, including non-slot scheduling, uplink grant-free scheduling, downlink semi-static period scheduling (DL SPS), uplink control channel enhancement, asynchronous HARQ, and HARQ enhancement. Specifically:

a. Mini-slot

The R15 version improves the flexibility of the schedulable OFDM symbol configuration, and supports multiple symbol lengths for both uplink and downlink scheduling. Using mini-slot scheduling enables URLLC business data to be sent immediately, reducing waiting and scheduling latency to some extent.

b. Uplink grant-free scheduling

By reducing the delay of signal interaction through uplink grant-free scheduling, the demand for scheduling control information is reduced, indirectly reducing the impact of control information reliability on URLLC business reliability.

c. Uplink control channel enhancement

According to the R16 version standard, the Shijue network protocol stack can support multiple feedback HARQ within one time slot, which can timely retransmit low-latency physical downlink data, indirectly reducing latency.

d. Support Time-Sensitive Networks (TSN) and 5G Integration

Broadcasting high-precision reference time in PBCH or sending it in the RRC layer to ensure accurate time synchronization between the master clock and terminal clock, achieving time-sensitive transmission and ensuring clock synchronization.

(2). Achieving High Reliability of URLLC

SageRAN considers ensuring ultra-high reliability requirements of URLLC scenarios from the perspective of improving reliability, including new modulation and coding schemes, downlink control channel enhancement, slot aggregation, and various redundancy transmission schemes.

a. New Modulation and Coding Schemes 

Adjusting lower-rate MCS schemes makes the overall modulation order of URLLC lower, up to 64QAM, reducing the number of constellation points, enhancing the fault tolerance of modulation and demodulation, improving data transmission reliability, and also improving the anti-interference ability of URLLC.

b. Enhanced PUCCH

The R16 version protocol enhances PUCCH in three aspects: supporting two different priority feedback codebooks, introducing sub-slots for supporting multiple PUCCH transmissions within one time slot.

c. Slot Aggregation

Slot aggregation is similar to retransmission, transmitting the same data block in multiple consecutive slots, and multiple bound slots are treated as the same resource. Different HARQ redundant versions of the same data block are transmitted in different slots, improving reliability for small packet business scenarios.

d. Various Redundancy/Retransmission

By using PDCP retransmission, user plane redundancy transmission, and N3/N9 redundancy transmission, gain is obtained through redundancy transmission, improving reliability.

4. Summary and Outlook

As a key application scenario empowered by 5G, URLLC has been highlighted by SageRAN in terms of air interface latency, and the planning of low-latency and high-reliability technologies for URLLC. However, in actual networks, factors that affect end-to-end latency for users include transmission delay and business platform delay, in addition to air interface latency. Therefore, building a low-latency network is a systematic engineering project that requires joint efforts from wireless, transmission, core network, and business platform links. In the future, SageRAN will continue to work with industry partners to jointly improve the technical system of URLLC, promote the maturity and implementation of URLLC technology, and promote the rapid emergence of large-scale and systematic industry applications empowered by 5G, thereby contributing to the development of various industries.