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Ultra-Reliable and Low-Latency Communications in 5G Downlink: Physical Layer Aspects

Hyoungju Ji, Sunho Park, Jeongho Yeo, Younsun Kim, Juho Lee, Byonghyo Shim

arXiv:1704.05565v6cs.IT

TL;DR

URLLC requires simultaneous ultra-high reliability and low latency, creating physical-layer challenges for 5G downlink. This paper surveys those challenges and the enabling technologies discussed for 3GPP NR Release 15. It presents an overview of requirements, packet and frame structures, scheduling schemes, coexistence issues, and reliability techniques, including reported gains over LTE HARQ and reactive strategies.

  • Problem

    URLLC must satisfy stringent latency and reliability requirements, requiring changes to the 5G NR air interface and mitigation of coexistence challenges with other services.

  • Method

    The paper provides an up-to-date overview of URLLC physical-layer challenges and solutions in 5G NR downlink, including structures, scheduling, and reliability techniques.

  • Results

    The paper reports a 3.1 dB gain at 10^-5 PER and a 32% throughput gain over LTE HARQ, while proactive strategy incurs about 17% throughput loss over reactive strategy.

  • Takeaways & Limitations

    The presented physical-layer techniques can serve as a starting point for satisfying stringent URLLC latency and reliability requirements in 5G NR.

Abstract

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Ultra reliable and low latency communications (URLLC) is a new service category in 5G to accommodate emerging services and applications having stringent latency and reliability requirements. In order to support URLLC, there should be both evolutionary and revolutionary changes in the air interface named 5G new radio (NR). In this article, we provide an up-to-date overview of URLLC with an emphasis on the physical layer challenges and solutions in 5G NR downlink. We highlight key requirements of URLLC and then elaborate the physical layer issues and enabling technologies including packet and frame structure, scheduling schemes, and reliability improvement techniques, which have been discussed in the 3GPP Release 15 standardization.

I. INTRODUCTION

5G introduces URLLC alongside eMBB and mMTC for applications with stringent latency and reliability requirements. The paper surveys physical-layer challenges and enabling technologies for URLLC downlink in 3GPP NR Release 15.

  • 5G service categories include URLLC, mMTC, and eMBB, supporting increasingly diverse communication requirements.
  • URLLC targets low latency and ultra-high reliability, but improving reliability can increase latency through signaling, retransmissions, redundancy, and parity.
  • Small-packet URLLC can trade achievable rate for reliability because many applications do not impose stringent throughput requirements.
  • URLLC physical-layer design is complicated by the interplay among throughput, latency, and reliability requirements.
  • The paper overviews URLLC technical challenges and solutions in 3GPP NR downlink, covering packet and frame structures, scheduling, and reliability improvement.

II. THREE SERVICE CATEGORIES IN 5G

5G defines eMBB, mMTC, and URLLC as service categories with distinct performance priorities. Their coexistence requires different physical-layer and access strategies.

  • A. eMBB: eMBB serves bandwidth-intensive applications such as high-resolution video, virtual reality, and augmented reality, emphasizing throughput.
  • B. mMTC: mMTC supports large numbers of machine-type devices for sensing, tagging, metering, and monitoring, requiring high connection density and energy efficiency.
  • B. mMTC: mMTC approaches include NB-IoT, SigFox, and LoRa, with differing deployment compatibility and service models.
  • C. URLLC: URLLC and other services may require non-orthogonal or aggressive connection strategies when devices substantially outnumber transmission resources.

C. URLLC

URLLC supports latency-sensitive and mission-critical applications whose wireless and end-to-end delays exceed their requirements. The section identifies overhead and control signaling as key downlink obstacles.

  • URLLC targets remote control, autonomous driving, remote surgery, and tactile internet applications requiring transmission times of tens to hundreds of microseconds.
  • 4G LTE end-to-end latency remains 30–100 ms because the backbone commonly uses best-effort delivery that is not optimized for mission-critical services.
  • Reducing URLLC end-to-end latency requires fundamental changes in both the wireless link and backbone network.
  • Virtual network slicing can provide dedicated URLLC connections and significantly reduce backbone-link latency.
  • For a 0.5 ms short packet, more than 60% of resources can be wasted on control overhead in current LTE systems.

III. URLLC SERVICE REQUIREMENTS

URLLC latency comprises multiple physical-layer components, while reliability requirements demand accurate estimation, suitable coding, and additional transmission resources. These constraints motivate new frame structures and transmission schemes.

  • A. Latency Requirement: Physical-layer latency consists of time-to-transmit, propagation, processing, retransmission, and signaling components.
  • A. Latency Requirement: URLLC requires packet transmission times on the order of hundreds of microseconds, unlike the 1 ms period of current LTE systems.
  • A. Latency Requirement: Channel estimation and feedback can bottleneck URLLC, motivating transmission schemes that do not rely on channel information.
  • B. Ultra-High Reliability: URLLC reliability targets at least 0.99999 within 1 ms and as high as 1−10^-7 for mission-critical applications.
  • B. Ultra-High Reliability: Reliability improvement uses better channel estimation, additional frequency, antenna, and spatial resources, short-packet coding, and repetitive time-domain transmission.

C. Coexistence with eMBB and mMTC

URLLC must be transmitted immediately even when eMBB or mMTC traffic is ongoing, creating a coexistence problem because interrupted transmissions can degrade non-URLLC reception. The section examines physical-layer mechanisms and scheduling trade-offs for protecting ongoing services while meeting URLLC latency and reliability requirements.

  • When URLLC begins during eMBB or mMTC transmission, the base station stops ongoing packets and immediately transmits the URLLC packet.
  • Interrupting an eMBB transport block can replace symbols in its third codeblock with URLLC symbols without notifying mobile devices.
  • Reception quality of ongoing eMBB and mMTC services is degraded by unreported URLLC interruption.
  • 3GPP-related coexistence solutions address interrupted-service degradation while physical-layer designs jointly consider latency, reliability, multiplexing, and service coexistence.

A. Packet Structure

URLLC packet design targets low processing and transmission latency while preserving reliability for short packets. The paper discusses stretched packet layouts, pipelined processing, and sparse vector coding as physical-layer approaches for this trade-off.

  • The key packet-design objective is minimizing processing latency Tproc, while short-packet reliability requires advanced channel coding.
  • A non-square packet stretched along the frequency axis is used in 5G NR as a baseline because it minimizes transmission latency Tttt.
  • Grouping pilot, control, and data components enables pipelined channel acquisition, control decoding, and data detection.
  • Sparse vector coding maps information to sparse-vector positions and uses compressed sensing to recover the vector from fewer resources.
  • 3.1 dB at 10^-5 PER: PDCCH with SVC outperforms convolution-coded PDCCH in LTE-Advanced for short-packet transmission.

B. Frame Structure and Latency-sensitive Scheduling Schemes

5G NR introduces a unified frame structure and user-scheduling mechanism intended to cover diverse frequency bands and service categories. These designs support flexible allocation for latency-sensitive URLLC traffic.

  • 5G NR aims to provide a unified frame structure across a wide range of frequency bands and service categories.
  • Flexible frame structures and user-scheduling mechanisms are introduced to support that unified design goal.
  • Reducing time-to-transmit latency Tttt can be pursued directly by reducing the symbol period.

1) Flexible Frame for URLLC:

URLLC frame and scheduling designs reduce transmission latency through flexible numerology, short transmission intervals, and rapid resource access. The reported results show a latency advantage for instant and dynamic reservation strategies, with corresponding throughput or overhead trade-offs.

  • 1) Flexible Frame for URLLC:: Doubling subcarrier spacing from 15 kHz to 30 kHz halves symbol length from 72 µs to 36 µs and subframe transmission time from 1 ms to 0.5 ms.
  • 1) Flexible Frame for URLLC:: Below 6 GHz, reducing symbol duration may be undesirable because of large delay spread, so alternative latency reductions are needed.
  • 1) Flexible Frame for URLLC:: Mini-slot transmission using 2∼3 symbols can reduce Tttt to 142 µs, while slot-level transmission using 7 symbols yields 241 or 500 µs.
  • 1) Flexible Frame for URLLC:: Controlling symbol period and packet symbol count enables Tttt below 1 ms, supported by fast tracking, synchronization, and simultaneous decoding.
  • 2) Scheduling Schemes:: Instant scheduling gives lower average URLLC latency than reservation-based scheduling but causes eMBB throughput loss.
  • 2) Scheduling Schemes:: Dynamic reservation has lower latency than semi-static reservation because of fast resource adaptation, but it requires additional control overhead and reliability protection for signaling.

C. Solutions to the Coexistence Problem

URLLC coexistence with ongoing services is challenging because interrupted-service degradation must be mitigated under implementation complexity and random packet arrivals. The 5G NR standardization discusses reactive and proactive strategies for this problem.

  • Mitigating degradation of interrupted services is an important coexistence-design problem for URLLC.
  • Flexible frame structures may ease coexistence problems, but implementation complexity and random URLLC packet arrivals require a more deliberate deployment solution.

1) Reactive strategy:

Reactive and proactive coexistence strategies protect ongoing services when URLLC interrupts transmissions. They use selective retransmission, interruption signaling, robustness enhancement, and resource sharing, with distinct throughput trade-offs.

  • Reactive strategy:: Reactive coexistence gives priority to URLLC packets while addressing the errors and packet portions affected by interruption.
  • Reactive strategy:: A preemption indicator reports the time and/or frequency resources used by URLLC so scheduled users can identify the interruption affecting their packets.
  • Reactive strategy:: Selected codeblocks can be retransmitted with combining or flush-out indicators, while lowering the retransmission code rate can provide additional coding gain.
  • Reactive strategy:: When URLLC transmissions occur frequently, spatial layer division or power-domain non-orthogonal transmission can support simultaneous eMBB and URLLC channels.
  • Proactive strategy:: Proactive coexistence includes robustness improvement through extra parity or outer coding, and resource sharing to support ongoing and URLLC data channels.
  • Proactive strategy:: 32% gain in throughput is achieved by selected-codeblock retransmission compared with LTE HARQ, while robustness improvement incurs about 17% throughput loss over the reactive strategy.

V. CONCLUSION

The paper surveys downlink physical-layer technologies for integrating URLLC into 5G NR, where stringent latency and reliability requirements motivate substantial redesign. It presents current techniques as a starting point while identifying unresolved directions and uplink scope boundaries.

  • URLLC supports emerging 5G applications including automated controls, tactile internet, remote operations, and intelligent transportation systems.
  • Physical-layer technologies for seamlessly integrating URLLC into 5G NR remain in their infancy.
  • The paper discusses URLLC requirements and physical-layer enabling technologies, noting that stringent latency and reliability require redesigning many physical-layer components.
  • The presented techniques are characterized as a starting point, with beamforming strategies, reconfigurable URLLC protocols, and advanced transceiver architectures identified for further study.
  • The article focuses on downlink URLLC, while uplink issues include one-shot access and active-user detection.
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