China Mobile Xiongyan Consulting Insights: 5G URLLC Key Technology Research Report

Labs Guide

URLLC "Low Latency High Reliability" is one of the three major application scenarios of 5G, and is also a typical scenario that distinguishes 5G from 2G/3G/4G. As a breakthrough for the mobile communications industry to enter vertical industries, URLLC is critical for the wide application of autonomous driving, industrial manufacturing, Internet of Vehicles, smart grids and other fields, and has been fully enhanced in the 3GPP R16 stage.

The development of smart cities around the world is entering a new stage. 5G, as a new engine for the development of smart cities, is driving cities into a new civilized price range. With the increase in bandwidth and capabilities of wireless mobile communication systems, mobile applications for individuals and industries are developing rapidly, and the mobile communication-related industrial ecology will gradually change. 5G is not only an air interface technology with higher speed, larger bandwidth and stronger capabilities, but also an intelligent network for smart cities.

5G currently includes three typical scenarios: enhanced mobile broadband eMBB, low latency and high reliability URLLC, and massive machine type communications mMTC. The three typical scenarios can help a large number of application technologies including: high-definition video, Internet of Things, drones, AR/VR, etc. The increase in bandwidth has greatly improved the data transmission rate, which is likely to bring a revolution to data processing related industries. With the promotion and construction of 5G networks, various vertical industry fields in the city will be changed. At present, industrial manufacturing, Internet of Vehicles and smart grids have an urgent need for 5G, and the URLLC technology scenario is very suitable for application in the above fields. Therefore, 3GPP proposed some low-latency technical discussions in the early stage of 5G research, and focused on the technical solutions of URLLC scenarios in the R16 stage.

1. 5G network architecture

In order to meet the needs of different application scenarios and applications. The network design of 5G is based on the design concept of elasticity, agility, and flexible reuse. 5G introduces SDN/NFV technology to further virtualize and decouple the software and hardware platforms. The underlying unified NFVI infrastructure is used, and the SDN controller is used to realize flexible scheduling of internal resources. Traditional network elements are divided into more fine-grained functional modules, called network functions (NF). Lightweight API interfaces are used to communicate between network functions to achieve high efficiency, flexibility, and openness of the system.

The 5G network is divided into three layers: access network, transmission network and core network. In the access network, the 5G network adopts a new architecture, new design, new frequency band and new antenna technology. The new architecture means that the new network architecture will be user-centric and the network will be built around users. At the same time, the traditional BBU (Building Base band Unite) will be divided into two network element devices: CU (Centralized Unit) and DU (Distributed Unit). At the same time, the RRU (Remote Radio Unit) and the feeder and antenna form a new AAU (Active Antenna Unit). As the name suggests, CU is a centrally controlled device that mainly processes the SDAP, RRC and PDCP layers, that is, it mainly performs QoS flow processing, wireless resource control and compression, alignment encryption and other functions of upper layer data, and is located at the layer 3 position of the wireless air interface. DU is a distributed control unit that mainly processes RLC, MAC and high-level physical layer protocols, that is, it mainly performs wireless link quality control, logical channel and physical channel mapping and baseband functions. It is located at the layer 2 position of the air interface. AAU is an active antenna unit, which combines the traditional RRU and antenna feeder mainly to process radio frequency signals. At the same time, the parameters and frame structures are more flexible, and the uplink and downlink ratios can be adjusted according to actual needs. The 5G network has a higher frequency band, and currently mainly uses the sub6G frequency band, which will be expanded to the 26Ghz millimeter wave frequency band. The higher frequency band has relatively richer spectrum resources and brings greater bandwidth. At the same time, a new 3D MIMO antenna is used, generally a 64T64R or even 128T128R antenna array, which greatly improves the transmission efficiency compared to the previous 4T4R antenna.

The scope of 5G transmission network is relatively broad, from the fronthaul from AAU to DU, the midhaul between DUCU to the backhaul from CU to the core network, all of which belong to the transmission bearer network. The surge in 5G traffic has brought huge challenges to the transmission network. In response to this, China Mobile has proposed the Slicing Packet Network (SPN) bearer network technology [1].

SPN adopts the ITU-T layered model and can carry Ethernet, IP, and CBR services.

SPN is divided into slice transport layer, slice channel layer and slice grouping layer, and also includes clock synchronization and management modules.

The slice transport layer is a physical layer technology based on 802.3 Ethernet technology and OIF Flex E, which provides bandwidth at the physical level. The slice channel layer uses SE technology to time-slot Ethernet interfaces, optical fiber resources, etc., and performs hard pipeline segmentation based on the TDM principle. The segmentation is hard isolation at the L1 level. The slice packet layer distributes, encapsulates and transmits services, and can provide forwarding capabilities such as L2\L3VPN; provides service identification, grouping, QoS guarantee processing, and provides connection-oriented service bearer channels based on SR-TP technology.

The 5G core network is based on SDN and NFV technologies, and has successfully achieved software and hardware decoupling. Network elements communicate based on TCP/IP, and interfaces are implemented through the https protocol [2]. Compared with the previous 4G core network, the 5G core network architecture has the following main features:

• The control plane is separated from the user plane.
• Network element function virtualization. NFV technology is applied to the core network to decouple software and hardware, and network elements become software function modules.
• Virtual network elements communicate with each other through interfaces, and lightweight RESTful/Http protocols are used between different network elements.
• SBA (Software Based on Architecture) network architecture. The coupling between virtual network elements is low, and other services can quickly access virtual network elements through interfaces, and the entire network architecture can be adjusted according to actual business needs.
• Weak correlation between access network and core network. The 5G core network has no strong correlation with the access network. UE can access the 5G core network through various networks. Even if it is not a 3GPP network, it can also access the 5G core network through the N3IWF network element.
• Virtual network elements are stateless. That is, storage resources are decoupled from computing resources. The control plane functions are mainly handed over to AMF and SMF, while the stored data is mainly placed in UDR and UDSF, realizing the decoupling of computing and storage.

The figure below is a schematic diagram of the 5G core network architecture.

The main network elements of the 5G core network are as follows:

• AMF (Access and Mobility Management Function): responsible for user access and mobility management;
• SMF (Session Management Function): responsible for user session management;
• UPF (User Plane Function): responsible for user plane processing;
• AUSF (Authentication Server Function): responsible for authenticating the user's 3GPP and non-3GPP access;
• PCF (Policy Control Function): responsible for user policy control, including session policy and mobility policy;
• UDM (Unified Data Management): responsible for user contract data management;
• NSSF (Network Slice Selection Function): responsible for selecting the network slice used by user services;
• NRF (Network Function Registration Function): responsible for the registration, discovery and selection of network functions;
• NEF (Network Capability Exposure Function): responsible for opening the capabilities of 5G networks to external systems;
• AF (Application Function): Interoperates with the core network to provide users with third-party applications.

2. URLLC key technologies

In the 3GPP standardization process, URLLC includes three aspects of research: low latency technology, high reliability technology, and URLLC and eMBB multiplexing. At the beginning of R15 research, a work project was established to study latency reduction technologies such as subcarrier spacing, flexible frame structure, and short time slot scheduling. As of R16, 3GPP has completed the performance evaluation of URLLC use cases, the enhancement of each channel in the physical layer, and the research and standardization of technologies such as URLLC and eMBB uplink multiplexing, but there are still many optimization tasks that are expected to be left for R17 research.

In order to achieve the low latency requirements of the uRLLC scenario, 3GPP proposed the following solutions in the R15 stage:

• Support flexible frame structure. 5G NR (New Radio) supports 15KHz carrier spacing of LTE system, and also supports more spacing schemes including 30KHz, 60KHz, 120KHz, and 240KHz. The higher the carrier spacing, the lower the latency performance. At the same time, 5GNR supports adjusting the frame structure. Compared with the LTE system's fixed subframe including 2 time slots, NR can flexibly switch between 1, 2, and 4 time slots and can flexibly configure the uplink and downlink ratio, which greatly reduces the latency.
• Support for smaller scheduling cycles - mini-slots. A slot is the smallest scheduling cycle unit. In the LTE system, a slot consists of 14 symbols, but NR supports mini-slots. Mini-slots can support 2, 3, and 4 symbol lengths. Shorter slots can reduce feedback delays.
• Flexible PDCCH configuration. The search space consists of a set of candidate PDCCHs (Physical Downlink Control Channel). The search space can be configured with parameters such as search type, period, time slot offset, number of time slots, CORESET, DCI format, etc. By configuring a reasonable PDCCH monitoring period and offset value and the PDCCH monitoring pattern in a time slot, a relatively dense PDCCH monitoring opportunity can be achieved. There are multiple PDCCH monitoring moments in a time slot, which can cope with business scenarios with sudden URLLC demand and meet the requirements of low latency. [5]
• URLLC high priority transmission. The data characteristics of URLLC low-latency scenarios are mainly bursty but small in volume, so NR supports URLLC to occupy channel resources in a preemptive manner. When the base station allocates physical resources to eMBB services, the resources of the eMBB services are also allocated to the URLLC services. When URLLC preempts physical resources, NR notifies the UE of the preemption result to ensure the low latency requirements of URLLC.
• Edge computing technology is used. 5G networks can sink the UPF user plane function to the user side. The edge computing server and UPF are co-deployed. When UPF recognizes that the destination address of the service flow is local, it will be diverted to the local edge computing server for service processing, reducing redundant transmission paths for services and reducing latency.

In the R16 stage, 3GPP further proposed the URLLC low-latency enhancement solution:

• Unlicensed configuration: The base station pre-configures periodic resources, and the UE does not need to apply to the base station. The UE pre-applies to the base station for resources used by the PUSCH (Physical Uplink Share Channel) and configures the corresponding parameters. When uplink resources are available, these resources are directly used for transmission, eliminating the time of sending scheduling requests to the base station, applying for resources, and receiving feedback from the base station, ensuring the low latency requirements of URLLC.
• HARQ feedback enhancement: In the R15 stage, the UE can only transmit HARQ-ACK (Hybrid Automatic Repeat Request) once on the PUCCH in a time slot. When the UE needs to send HARQ-ACK again on the PUCCH in the same time slot in order to reduce latency, it is not allowed. In the R16 stage, HARQ-ACK feedback is allowed on multiple PUCCH channels within a time slot. In order to support this design, the R16 terminal requires the UE to support at least two HARQ coding methods and the physical layer can recognize them.
• Support the integration of time-sensitive network TSN and 5G network: realize time-sensitive transmission and ensure clock synchronization. Broadcast high-precision reference time in PBCH or send it in the RRC layer to ensure accurate time synchronization between the master clock and the terminal clock and realize time-sensitive transmission. Because TSN technology is developed from basic Ethernet transmission technology, TSN needs to encapsulate Ethernet frame headers, but this will reduce transmission efficiency, so it is also necessary to compress Ethernet frame headers to improve data transmission efficiency and reduce latency.

In order to achieve high reliability requirements, 3GPP proposed the following solutions in the R15 stage:

• On the physical level, the MCS\CQI table has been optimized. The MCS\CQI of the LTE system cannot meet the requirements of NR for system reliability and transmission rate, so NR has added two lower code rates in the CQI (channel quality indication) table, and the corresponding base station has added two MCS (Modulation and Coding Scheme) low-frequency options. The UE and the base station can choose a lower code rate to ensure reliability.
• Duplicate transmission of data packets: The LTE system proposes a HARQ retransmission mechanism at the MAC and RLC layers, but this reliability comes at the expense of latency. NR proposes to replicate data at the PDCP layer and transmit the same data on different PDCP channels to improve reliability.
• PDCCH with high aggregation level: CCE is the basic unit of PDCCH. LTE PDCCH contains up to 8 CCEs. In the N15 stage, NR can contain up to 16 CCEs. More resources can reduce the transmission coding rate and ensure transmission reliability.

In the R16 stage, 3GPP further proposed URLLC high reliability enhancement solution:

• Redundant transmission scheme, redundant PDU sessions and redundant transmission of N3 interface are established between UEs. Redundant transmission based on N3 interface. First, NG-RAN copies the uplink data packet and then sends it to UPF through two redundant link (N3 interface) channels, where each N3 channel is associated with a PDU session, and two independent N3 channels are established to transmit data. The base station, SMF and UPF will provide different routes for the two links.
• Repeat transmission at the mini-timeslot level. The retransmission mechanism of the R15 version is based on the scheduling of time slots. The R16 stage further supports retransmission at the mini-timeslot level, with a maximum of 16 retransmissions.
• Currently, research is still ongoing on PUCCH, PUSCH, and HARQ enhancement.

Based on R15 and R16, the current URLLC scenarios are mainly applied in industrial automation, Internet of Vehicles, smart grid and AR/VR. Although 3GPP has implemented many key URLLC technical solutions in R15 and R16, there are still technical enhancement issues and applications in Internet of Vehicles and industrial scenarios, which will be solved in R17 and later versions.

3. URLLC main application scenarios

The biggest features of 5G URLLC scenarios are low latency and high reliability. The scope of application of URLLC scenarios is very wide, and the requirements for latency, reliability and bandwidth are different in different scenarios. Specifically, they include power automation "three remote" scenarios, Internet of Vehicles scenarios and industrial manufacturing scenarios. [6]

1 Power Automation Scenario

Differential protection is a self-protection method for power networks. It compares the electrical quantities at both ends of the transmission line to determine the scope of the fault, achieve accurate isolation of the fault, and avoid the expansion of the scope of the power outage. Grid communication is mainly based on optical fiber, but the distribution network below 35kv has not achieved optical fiber coverage, and the deployment scenarios are complex and diverse, requiring wireless networks as communication carriers. 5G URLLC scenarios are very suitable for deployment in power automation scenarios.

2 Industrial Manufacturing Scenarios

Industrial manufacturing has high requirements for technical performance, and high-end manufacturing has very 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 work manufacturing scenarios. Manufacturing equipment accesses the enterprise cloud or field control system through 5G, collects field environmental data and production data, and analyzes production status in real time, making the entire production line unmanned and wireless.

3 Internet of Vehicles Scenarios

Due to its particularity, the Internet of Vehicles has very high requirements for system security, reliability and ultra-low latency. 5G URLLC scenarios are very suitable for deployment in Internet of Vehicles scenarios. At the current stage, the Internet of Vehicles mainly uses vehicle-road collaboration technology, that is, deploying intelligent data collection equipment including intelligent lamp poles and intelligent traffic lights on the roadside infrastructure, and exchanging information with the on-board computer through the 5G network, which greatly increases the vehicle's ability to perceive surrounding affairs, improves driving safety, and effectively solves urban congestion problems.

[This article is an original article by 51CTO columnist "Mobile Labs". Please contact the original author for reprinting.]

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