Technologies for Ultra-Reliable Low-Latency Communication

Release Date:2021-05-26  Author:By Yuan Wenchong,Li Changxiao  Click:

With a decline in wireless network users and traffic dividends, operators cannot simply improve the service experience of existing users when making network evolution plans. The demands for using wireless networks in industrial applications for improved productivity and service capability are also increasing. The 5G network can meet different network service requirements in eMBB, mMTC and URLLC scenarios.
URLLC is ideal for applications with stringent latency and reliability requirements. Typical services can be found in vertical domains such as factory, electric power and transportation. Even in a single vertical industry, different applications have different network requirements. Therefore, when upgrading a network for URLLC, operators need to comprehensively use key technologies such as NodeEngine and network slicing to develop diversified network deployment solutions for different industries and applications.
The latency and reliability indicators for URLLC defined by ITU are as follows:
Latency: The minimum requirement for one-way user plane latency is 1 ms.
Reliability: In the urban macro station scenario, the success probability of transmitting a layer 2 PDU packet of 32 bytes within 1 ms in channel quality of coverage edge is 99.999%.

Low Latency Technology 
To reduce the radio interface latency between mobile phones and base stations, key technologies such as a flexible frame structure, mini-slots, and channel multiplexing, can be introduced. 
Flexible frame structure: 5G new radio (NR) supports LTE subcarrier spacing of 15 kHz and subcarrier spacing of 30 kHz, 60 kHz, 120 kHz and 240 kHz. The wider the subcarrier spacing, the lower the latency. Meanwhile, 5G NR supports frame structure adjustments. Compared with LTE where there are fixed two slots per subframe, NR allows for 1, 2 or 4 slots per subframe and flexible uplink-downlink ratio configurations, thus greatly reducing the latency.
Smaller scheduling period—mini-slot: A timeslot is the minimum scheduling unit. A slot in LTE consists of 14 symbols whereas a mini-slot in 5G NR can contain 2, 4 or 7 symbols (Fig. 1). A shorter slot can reduce the feedback delay. 
Multiplexing of URLLC and eMBB: The data in the low latency scenario is characterized by strong burst but small data volume. Therefore, NR introduces a preemption-based mechanism where the BTS assigns physical resources of eMBB to URLLC service and informs the eMBB UE of the preemption result to ensure low latency for the URLLC service. 
Grant-free configuration: The gNodeB configures a UE to have pre-allocated periodic resources. The UE requests the resources on PUSCH from the BS in advance and is configured by the corresponding parameters. When there are uplink resources, the UE directly uses these resources for transmission without sending a scheduling request and waiting for the feedback from the BS, thus meeting the low latency requirement of URLLC. 
Enhanced HARQ feedback: In R15, a UE transmits only one PUCCH carrying HARQ-ACK information in a slot. When the UE needs to transmit HARQ-ACK information again on the PUCCH in the same slot to reduce the latency, it is not allowed. In R16, multiple PUCCHs within a slot are enabled for HARQ-ACK transmission. To support this design, a R16 UE is required to support at least two HARQ-ACK codebooks.
Edge computing: The 5G network allows the UPF to be placed closer to the user side and to be co-located with the edge computing server. When the UPF identifies that the destination address of a service flow is local, it offloads services to the local edge computing server for processing, reducing the redundant transmission paths for services and the latency.
Integration of time-sensitive networking (TSN) with 5G: Time-sensitive transmission is implemented to ensure clock synchronization. High-precision reference time is transmitted via the PBCH or sent through the RRC layer to ensure accurate synchronization between the master clock and the terminal clock and implement time-sensitive transmission. Because TSN is an Ethernet-based technology, Ethernet frames need to be encapsulated with headers, which reduces transmission efficiency. Therefore, Ethernet frame headers need to be compressed to improve data transmission efficiency and reduce latency.

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Ultra-Reliable Technology 
Ultra-reliable radio technology involves:
Optimizing the MCS/CQI table: The MCS/CQI in LTE cannot meet the system reliability and transmission rate requirements of NR. Therefore, two lower bit rates are added to CQI table values for NR. Correspondingly, two MCS low-frequency options are added to the base station, and a lower bit rate can be selected between the UE and the base station to ensure reliability.
Retransmission of data packets: The HARQ retransmission mechanism at MAC and RLC layers in LTE achieves reliability at the cost of delay. The NR system duplicates data at the PDCP layer and transmit the same data via different PDCP channels to improve reliability.
PDCCH with a high aggregation level: CCE is the basic unit of PDCCH. An LTE PDCCH contains a maximum of eight CCEs, and a NR PDCCH contains a maximum of 16 CCEs. More resources can reduce the transmission coding rate to improve the reliability.
Redundant transmission scheme: Redundant PDU sessions based on UE are transmitted via two redundant N3 tunnels. First, the NG-RAN duplicates the uplink data packets, and sends them to the UPF through two redundant links (N3 interface). Each N3 tunnel corresponds to a PDU session, and two independent N3 tunnels are established to transmit data. The BTS, SMF and UPF will provide different routes for the two links (Fig. 2). 
Retransmission at the mini-slot layer: R15 uses a retransmission mechanism based on timeslot scheduling. R16 further supports mini-slot level retransmission with the maximum number of retransmission reaching 16.

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URLLC Deployment Solution
The URLLC service can be deployed in FDD, TDD and millimeter wave bands. 5G network positioning and URLLC deployment in different frequency bands are discussed below. 
A 5G FDD network can upgrade the key URLLC technologies in an all-round manner to make breakthroughs in industry applications. 5G FDD networks have natural advantages in networking. For example, the 2.1 GHz FDD band, a band refarmed for 5G, is mainly used to improve the coverage of the whole 5G network and supplement the network capacity. The natural advantages of the FDD mode will be more favorable for carrying services requiring ultra-low latency and ultra-high reliability. Therefore, this band can accommodate URLLC applications with extreme performance indicators, and will be the main band for an overall upgrade of the URLLC features in certain areas. 
For a 5G TDD network, the technical features of URLLC can be selectively upgraded to enhance 5G network reliability and optimize network service latency. TDD bands at 2.6 GHz and 3.5 GHz are used in the initial phase of 5G deployment. They are the main bands for continuous coverage in urban areas and target ordinary consumers and some industrial users. Limited by a fixed frame structure, a TDD network has greater difficulty in improving the latency than an FDD network, and allows for more downlink capacity. Therefore, URLLC technology upgrades at the wireless side will be aimed at improving the brand competitiveness and the service experience of an operator's 5G network, and will be based on the service requirements in a coverage area.
Millimeter-wave (mmWave) bands are a new band resource for future development. Although the mmWave bands adopt TDD, they support wider subcarrier spacing, and a mmWave network with coverage discontinuity supports a flexible frame structure. Therefore, the mmWave network can better meet low latency requirements in URLLC than the current 5G TDD system.

5G will break through the limitation of the existing mobile communications industry, and enables in-depth integration of wireless communications and vertical industries. Operators need to dig deeper into industry requirements, tap into their advantages in network construction and O&M management, provide end-to-end solutions that truly match the requirements of industry users, and create new value for 5G networks. At the same time, the cooperation between upstream and downstream partners in the industry are required in exploring new URLLC services.

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