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IEEE 802.11bd & 5G NR V2X: Evolution of Radio Access Technologies for V2X Communications

Gaurang Naik, Biplav Choudhury, Jung-Min, Park

arXiv:1903.08391v2cs.IT

TL;DR

Current DSRC and C-V2X support basic safety applications but fall short for advanced V2X use cases demanding high reliability, low latency, and high throughput. The paper reviews IEEE 802.11bd and NR V2X, explaining their mechanisms, challenges, spectrum issues, and preliminary performance projections. These evolving RATs show promising improvements, although their standards and performance evaluations remain preliminary.

  • Problem

    Current DSRC and C-V2X fall short of the stringent QoS requirements of many advanced V2X applications.

  • Method

    The paper compares IEEE 802.11bd and NR V2X with their predecessors, describing objectives, PHY/MAC enhancements, design challenges, and spectrum-management issues.

  • Results

    NR V2X achieves approximately 99.7–99.8% packet delivery rate in highway scenarios using 60 kHz sub-carrier spacing and a 20 MHz channel.

  • Takeaways & Limitations

    IEEE 802.11bd and NR V2X are major overhauls expected to improve latency, reliability, and throughput relative to their predecessors.

Abstract

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With rising interest in autonomous vehicles, developing radio access technologies (RATs) that enable reliable and low latency vehicular communications has become of paramount importance. Dedicated Short Range Communications (DSRC) and Cellular V2X (C-V2X) are two present-day technologies that are capable of supporting day-1 vehicular applications. However, these RATs fall short of supporting communication requirements of many advanced vehicular applications, which are believed to be critical in enabling fully autonomous vehicles. Both DSRC and C-V2X are undergoing extensive enhancements in order to support advanced vehicular applications that are characterized by high reliability, low latency and high throughput requirements. These RAT evolutions---IEEE 802.11bd for DSRC and NR V2X for C-V2X---can supplement today's vehicular sensors in enabling autonomous driving. In this paper, we briefly describe the two present-day vehicular RATs. In doing so, we highlight their inability to guarantee quality of service requirements of many advanced vehicular applications. We then look at the two RAT evolutions, i.e., IEEE 802.11bd and NR V2X and outline their objectives, describe their salient features and provide an in-depth description of key mechanisms that enable these features. While both, IEEE 802.11bd and NR V2X, are in their initial stages of development, we shed light on their preliminary performance projections and compare and contrast the two evolutionary RATs with their respective predecessors.

I. INTRODUCTION

The paper examines how IEEE 802.11bd and NR V2X evolve DSRC and C-V2X to address advanced vehicular applications with stringent reliability, latency, and throughput requirements. It describes their objectives, enhancements, design challenges, performance projections, and spectrum-management issues.

  • Motivation: Advanced V2X applications impose QoS requirements that current DSRC and C-V2X technologies often cannot satisfy.Both technologies can support basic safety applications, but many advanced use cases require more stringent guarantees.
  • RAT evolution: 802.11bd and NR V2X are being developed to improve reliability, reduce end-to-end latency, and support higher-throughput applications.The two evolutions share these broad objectives despite using different design methodologies.
  • Design methodologies: 802.11bd must preserve interoperability and backward compatibility with 802.11p, whereas NR V2X is developed without an equivalent backward-compatibility constraint.This difference significantly shapes the two development methodologies.
  • Scope boundary: The paper limits discussion to key features and functionalities likely to appear in the final standards because both technologies remain under development.The paper also explicitly restricts its focus to PHY and MAC layers.
  • Paper scope: The paper describes key objectives, enhancements, design challenges, and performance projections for both evolutionary RATs.Its coverage focuses on PHY and MAC layers and includes spectrum-management hurdles to deployment and management.

A. Dedicated Short Range Communications (DSRC)

The section outlines the PHY and MAC foundations of DSRC and the LTE-based C-V2X architecture, including infrastructure-assisted and direct sidelink operation. It also describes the resource-allocation mechanisms used by C-V2X sidelink modes.

  • DSRC: DSRC uses an OFDM-based PHY derived from IEEE 802.11a and a CSMA MAC protocol designed for vehicular networks.Its channel bandwidth is 10 MHz, and its contention window remains fixed because broadcast transmissions do not use acknowledgements.
  • C-V2X: C-V2X, introduced in 3GPP Release 14, supports direct low-latency V2X communications through sidelink transmission modes over the PC5 interface.It can operate with or without cellular infrastructure.
  • C-V2X PHY: C-V2X uses an LTE-like resource structure with 1 ms subframes, 15 kHz sub-carriers, QPSK or 16-QAM, and turbo coding.The sidelink packet includes data and sidelink control information transmitted over PSCCH and PSSCH channels.
  • C-V2X sidelink: In sidelink mode 3, the eNodeB allocates resources using semi-persistent, UE-report-based, and cross-carrier scheduling.This mode is intended for scenarios with eNodeB coverage.
  • C-V2X sidelink: In sidelink mode 4, out-of-coverage UEs autonomously reserve resources after sensing the channel for 1 second.The resource-reservation procedure seeks orthogonal time-frequency resources for neighboring UEs.

III. NEED FOR EVOLUTION

Existing DSRC and C-V2X can support moderate-density, driver-alert safety applications, but advanced applications require more stringent and diverse communication behavior. These demands motivate evolution of both RATs.

  • Existing RAT performance: DSRC supports many safety applications near 100 ms end-to-end latency when vehicle density is moderate, but performance deteriorates at higher density.The main causes are packet collisions from simultaneous transmissions and hidden nodes.
  • Existing RAT performance: C-V2X generally outperforms DSRC in link budget and spectrum utilization, yet its performance also drops rapidly as traffic density increases.The degradation is particularly pronounced for C-V2X mode 4.
  • Basic safety applications: Basic safety applications typically send periodic messages at 1–10 Hz with 50–100 ms end-to-end latency requirements.These day-1 applications primarily issue driver alerts about potentially dangerous situations.
  • Advanced applications: Advanced V2X applications span vehicle platooning, advanced driving, extended sensors, and remote driving.They also support traffic management and passenger infotainment needs.
  • Need for evolution: Advanced applications require more stringent latency and reliability than basic safety applications and use large, variable-sized, aperiodic packets.This traffic diversity makes a major overhaul of existing V2X technologies necessary.

IV. IEEE 802.11BD: EVOLUTION OF IEEE 802.11P

802.11bd extends 802.11p with higher performance targets while preserving interoperability, coexistence, and fair channel access. Its PHY evolution addresses high-mobility channel conditions through narrower numerologies and midambles.

  • Objectives: 802.11bd targets twice the MAC throughput of 802.11p while supporting relative velocities up to 500 km/hr.
  • Objectives: 802.11bd targets twice the communication range of 802.11p and at least one vehicle-positioning mode affiliated with V2X communications.
  • Objectives: 802.11bd must interoperate with 802.11p, allowing each technology to decode at least one transmission mode of the other.
  • Objectives: 802.11bd and 802.11p must coexist by detecting each other’s transmissions and deferring channel access.
  • PHY evolution: Narrower OFDM numerologies and midambles are considered to improve efficiency and track rapidly varying vehicular channels.802.11p uses 156.25 kHz sub-carrier spacing; 802.11bd considers 2× down-clock designs and midambles between data symbols.

2) Re-transmissions:

802.11bd combines retransmissions and newer PHY/MAC mechanisms to improve reliability, efficiency, and throughput. Candidate techniques include DCM, LDPC coding, spatial diversity, narrower numerologies, and fair contention parameters.

  • Re-transmissions: Retransmitting packets can improve reliability for both 802.11p and 802.11bd devices, with transmissions sharing or separately using channel-access opportunities.For legacy devices, the original and retransmitted packets appear independent, and reception succeeds if either is received.
  • Alternate OFDM Numerologies: Narrower OFDM numerologies increase OFDM efficiency by lengthening useful symbol duration relative to the invariant cyclic prefix.Options include 2× down-clock with 64 sub-carriers and 4× down-clock with 128 sub-carriers.
  • Alternate OFDM Numerologies: Intercarrier interference can be difficult to mitigate when channel variations occur across an OFDM symbol.
  • Dual Carrier Modulation: Dual Carrier Modulation repeats symbols on sufficiently separated sub-carriers to obtain frequency diversity and improve BLER performance.The modulation order is doubled to maintain throughput when each symbol is transmitted twice.
  • Other mechanisms: 802.11bd considers LDPC coding, multiple antennas, and reuse of 802.11p contention parameters to improve reliability, throughput, and fairness.Multiple antennas can provide spatial diversity or spatial multiplexing, while legacy contention parameters support equal channel access.

C. Challenges

802.11bd must accommodate deployed 802.11p devices while supporting new capabilities, creating constraints across frame design and channel access. Simulations indicate substantial link-level gains, but system-wide improvement remains un demonstrated.

  • Interoperability & Backward Compatibility: 802.11bd interoperability and backward compatibility constrain its PHY and MAC design because 802.11p-equipped vehicles are already deployed.
  • Interoperability & Backward Compatibility: Appending 802.11bd data to legacy fields lets both device types decode shared content while newer devices exploit midambles, LDPC coding, and higher-order MCS.The packet structure supports transition without changing higher layers.
  • Interoperability & Backward Compatibility: Parity-based interoperability encodes 802.11p data blocks with an outer Reed-Solomon code before legacy BCC encoding and appending.
  • Coexistence: Coexistence allows 802.11p devices to detect 802.11bd transmissions and defer access without decoding 802.11bd-specific frames.The messages are intended only for 802.11bd devices.
  • Performance Projections: 3−8 dB gains at BLER 10^-1 are reported for 802.11bd retransmission combining, versus 0.5−1.7 dB for 802.11p devices.Parity-based interoperability provides a 1−3 dB gain at BLER 10^-1 and uses less airtime for a given reliability improvement.
  • Performance Projections: Throughput doubled with midambles and LDPC coding over a 20 MHz highway NLOS channel, but only at SNR values above 20 dB.
  • Performance Projections: Actual system-wide performance gains for 802.11bd have not yet been demonstrated.

E. Comparison with IEEE 802.11p

NR V2X is designed to supplement rather than replace C-V2X, targeting advanced applications with diverse latency, reliability, throughput, and transmission-pattern requirements. Its objectives span sidelink, Uu interfaces, resource allocation, interface selection, QoS, and coexistence.

  • Objectives: NR V2X supplements C-V2X by supporting use cases that C-V2X cannot support, rather than replacing the existing technology.
  • Objectives: NR V2X addresses applications requiring periodic or aperiodic traffic, broadcast or groupcast, and unicast delivery with varied latency, reliability, and throughput demands.
  • Objectives: NR V2X objectives include redesigned sidelink procedures and enhancements to the NR Uu interface for advanced V2X applications.
  • Objectives: NR V2X studies Uu-based sidelink resource allocation, selection among LTE and NR interfaces, and QoS solutions across radio interfaces.
  • Objectives: NR V2X includes feasibility and technical studies for in-device coexistence between C-V2X and NR V2X.

1) NR V2X sidelink modes:

NR V2X sidelink supports network-assisted and out-of-coverage direct communication, with flexible transmission structures and more responsive control/data multiplexing than C-V2X.

  • NR V2X sidelink mode 1 uses gNodeB-allocated resources for direct vehicular communication within coverage, while mode 2 supports out-of-coverage operation.
  • NR V2X supports unicast, groupcast, and broadcast communication, which can be active simultaneously for different message types.
  • 15, 30, and 60 kHz sub-carrier spacings are supported in FR1, while 60 and 120 kHz are supported in FR2, enabling shorter transmission times at higher spacings.
  • Mini-slot scheduling lets latency-critical transmissions begin at any of 14 OFDM symbols and occupy any number of symbols within a slot.
  • NR V2X multiplexes PSCCH and PSSCH in time, transmitting control information before data so receivers need not buffer the entire sub-frame.
  • NR V2X supports retransmissions informed by reception feedback, addressing the resource inefficiency of C-V2X blind retransmissions.

5) Other PHY layer enhancements:

NR V2X introduces PHY and sidelink resource-selection enhancements for diverse traffic, including flexible sensing, autonomous selection, assistance, and group scheduling.

  • 5) Other PHY layer enhancements:: NR V2X inherits PHY enhancements including LDPC coding, 64-QAM, and a flexible number of DMRS symbols per slot.
  • 6) Introduction of new sub-modes of NR sidelink mode 2:: NR V2X sidelink mode 2 includes autonomous resource selection, UE assistance, pre-configured grants, and selection of resources for other UEs.
  • 6) Introduction of new sub-modes of NR sidelink mode 2:: Autonomous resource selection is unlikely to gather assistance from all receivers for broadcast transmissions because the signaling overhead would be prohibitively high.
  • 6) Introduction of new sub-modes of NR sidelink mode 2:: Short-term sensing targets aperiodic traffic, while long-term sensing supports periodic traffic through channel-occupation analysis over sensing and selection windows.
  • 6) Introduction of new sub-modes of NR sidelink mode 2:: Flexible sensing-window duration can reflect vehicular mobility because sensing results become obsolete quickly in high-mobility scenarios.
  • 6) Introduction of new sub-modes of NR sidelink mode 2:: Mode 2(d) assigns resources for nearby UEs, supporting platooning and applications using groupcast or unicast transmissions.

D. Challenges

NR V2X must coexist with C-V2X despite incompatibility, requiring non-co-channel resource coordination while balancing interference, power, synchronization, and latency constraints.

  • NR V2X is not backward compatible with C-V2X, partly because C-V2X devices cannot decode NR transmissions using 30 or 60 kHz spacing.
  • The coexistence study considers non-co-channel operation using either frequency division multiplexing or time division multiplexing.
  • FDM avoids tight synchronization but can suffer adjacent-channel leakage and power splitting when both RATs operate in the same band.
  • TDM permits each technology to use maximum transmission power and avoids cross-channel leakage, but restricts synchronization and can delay latency-critical packets when NR V2X is off.
  • Pre-emption lets high-priority messages use resources reserved for lower-priority periodic traffic, but frequent pre-emptions may reduce periodic-message reliability.

E. Performance Projections

Preliminary studies project strong NR V2X performance, especially with 60 kHz spacing, while urban density, path loss, and demanding message rates remain open concerns.

  • 60 kHz sub-carrier spacing can produce large gains over the 15 kHz spacing used in C-V2X, especially at higher relative velocities.
  • Reducing DMRS symbols per slot from 4 to 2 at 60 kHz causes practically no loss with multiple antennas, even at 500 kmph.
  • 99.7–99.8% packet delivery rate is reported for all listed communication and message types in highway scenarios using 60 kHz spacing and a 20 MHz channel.
  • 93–97% packet delivery rate is reported in urban scenarios, indicating that further enhancements are required for reliable urban communications.
  • The reported results use relatively low message transmission rates of approximately 10 Hz, leaving performance for more demanding applications unresolved.

C. Interference from Adjacent Bands

Adjacent-band Wi-Fi can degrade V2X receiver performance by elevating its noise floor, with losses depending on frequency separation. The evolutionary RATs pursue improved latency, reliability, and throughput but retain different design constraints and face spectrum-management challenges.

  • Adjacent-band interference: 85 m and 65 m reductions in the 90% PDR range are observed for C-V2X channels 182 and 180, respectively, under adjacent-channel Wi-Fi interference.The scenario places a Wi-Fi access point 10 m from the C-V2X receiver and uses saturated downlink traffic.
  • Evolutionary RATs: 802.11bd and NR V2X are expected to improve latency, reliability, and throughput relative to their predecessors.The paper presents both as major overhauls and compares their features in Table V.
  • Evolutionary RATs: 802.11bd redesigns 802.11p while preserving backward compatibility, whereas NR V2X builds on 5G NR without the same backward-compatibility constraint.Backward compatibility is critical for 802.11bd because some countries already have DSRC-equipped vehicles.
  • Spectrum management: Parallel deployment of the evolutionary RATs can create spectrum-management and operational difficulties requiring proactive resolution.Regional regulators and manufacturers have two options, but simultaneous adoption can complicate operation within a geographical region.
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