With the 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 (DSRCs) 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 the 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-the IEEE 802.11bd for the DSRC and NR V2X for C-V2X-can supplement today's vehicular sensors in enabling autonomous driving. In this paper, we survey the latest developments in the standardization of 802.11bd and NR V2X. We begin with a brief description of the two present-day vehicular RATs. In doing so, we highlight their inability to guarantee the quality of service requirements of many advanced vehicular applications. We then look at the two RAT evolutions, i.e., the IEEE 802.11bd and NR V2X, outline their objectives, describe their salient features, and provide an in-depth description of key mechanisms that enable these features. While both, the 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.

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

The Third Generation Partnership Project (3GPP) has recently published its Release 16 that includes the first Vehicle-to-Everything (V2X) standard based on the 5G New Radio (NR) air interface 5G NR V2X introduces advanced functionalities on top of the 5G NR air interface to support connected and automated driving use cases with stringent requirements This article presents an in-depth tutorial of the 3GPP Release 16 5G NR V2X standard for V2X communications, with a particular focus on the sidelink, since it is the most significant part of 5G NR V2X The main part of the paper is an in-depth treatment of the key aspects of 5G NR V2X: the physical layer, the resource allocation, the quality of service management, the enhancements introduced to the Uu interface and the mobility management for V2N (Vehicle to Network) communications, as well as the co-existence mechanisms between 5G NR V2X and LTE V2X We also review the use cases, the system architecture, and describe the evaluation methodology and simulation assumptions for 5G NR V2X Finally, we provide an outlook on possible 5G NR V2X enhancements, including those identified within Release 17

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A Tutorial on 5G NR V2X Communications

The development of light detection and ranging, Radar, camera, and other advanced sensor technologies inaugurated a new era in autonomous driving. However, due to the intrinsic limitations of these sensors, autonomous vehicles are prone to making erroneous decisions and causing serious disasters. At this point, networking and communication technologies can greatly make up for sensor deficiencies, and are more reliable, feasible and efficient to promote the information interaction, thereby improving autonomous vehicle’s perception and planning capabilities as well as realizing better vehicle control. This paper surveys the networking and communication technologies in autonomous driving from two aspects: intra- and inter-vehicle. The intra-vehicle network as the basis of realizing autonomous driving connects the on-board electronic parts. The inter-vehicle network is the medium for interaction between vehicles and outside information. In addition, we present the new trends of communication technologies in autonomous driving, as well as investigate the current mainstream verification methods and emphasize the challenges and open issues of networking and communications in autonomous driving.

Networking and Communications in Autonomous Driving: A Survey

This paper investigates the spectrum sharing problem in vehicular networks based on multi-agent reinforcement learning, where multiple vehicle-to-vehicle (V2V) links reuse the frequency spectrum preoccupied by vehicle-to-infrastructure (V2I) links. Fast channel variations in high mobility vehicular environments preclude the possibility of collecting accurate instantaneous channel state information at the base station for centralized resource management. In response, we model the resource sharing as a multi-agent reinforcement learning problem, which is then solved using a fingerprint-based deep Q-network method that is amenable to a distributed implementation. The V2V links, each acting as an agent, collectively interact with the communication environment, receive distinctive observations yet a common reward, and learn to improve spectrum and power allocation through updating Q-networks using the gained experiences. We demonstrate that with a proper reward design and training mechanism, the multiple V2V agents successfully learn to cooperate in a distributed way to simultaneously improve the sum capacity of V2I links and payload delivery rate of V2V links.

Spectrum Sharing in Vehicular Networks Based on Multi-Agent Reinforcement Learning

The C-V2X or LTE-V standard has been designed to support vehicle to everything (V2X) communications. The standard is an evolution of LTE, and it has been published by the 3GPP in Release 14. This new standard introduces the C-V2X or LTE-V Mode 4 that is specifically designed for V2V communications using the PC5 sidelink interface without any cellular infrastructure support. In Mode 4, vehicles autonomously select and manage their radio resources. Mode 4 is highly relevant since V2V safety applications cannot depend on the availability of infrastructure-based cellular coverage. This paper presents the first analytical models of the communication performance of C-V2X or LTE-V Mode 4. In particular, the paper presents analytical models for the average packet delivery ratio (PDR) as a function of the distance between transmitter and receiver, and for the four different types of transmission errors that can be encountered in C-V2X Mode 4. The models are validated for a wide range of transmission parameters and traffic densities. To this aim, this study compares the results obtained with the analytical models to those obtained with a C-V2X Mode 4 simulator implemented over Veins.

Analytical Models of the Performance of C-V2X Mode 4 Vehicular Communications

This article provides an overview of the long-term evolution-vehicle (LTE-V) standard supporting sidelink or vehicle-to-vehicle (V2V) communications using LTE's direct interface named PC5 in LTE. We review the physical layer changes introduced under Release 14 for LTE-V, its communication modes 3 and 4, and the LTE-V evolutions under discussion in Release 15 to support fifth-generation (5G) vehicle-to-everything (V2X) communications and autonomous vehicles' applications. Modes 3 and 4 support direct V2V communications but differ on how they allocate the radio resources. Resources are allocated by the cellular network under mode 3. Mode 4 does not require cellular coverage, and vehicles autonomously select their radio resources using a distributed scheduling scheme supported by congestion control mechanisms. Mode 4 is considered the baseline mode and represents an alternative to 802.11p or dedicated shortrange communications (DSRC). In this context, this article also presents a detailed analysis of the performance of LTE-V sidelink mode 4, and proposes a modification to its distributed scheduling.

LTE-V for Sidelink 5G V2X Vehicular Communications: A New 5G Technology for Short-Range Vehicle-to-Everything Communications

V2X (Vehicle to everything) communications can be currently supported by standards based on IEEE 80211p (eg DSRC or ITS-G5) or LTE-V2X (also known as Cellular V2X or C-V2X) technologies There has been an intense debate in the community on which technology achieves best performance However, existing studies do not take into account the variability present in the generation and size of V2X messages This variability can significantly impact the operation and performance of the Medium Access Control (MAC) This study progresses the state of the art by conducting an in-depth evaluation of both technologies under different message traffic patterns In particular, we consider aperiodic and periodic messages of constant or variable size based on the standardized ETSI Cooperative Awareness Messages (CAMs) This study considers different scenarios and possible configurations of IEEE 80211p and LTE-V2X We demonstrate that IEEE 80211p can better cope with variations in the size and time interval between messages We also demonstrate (and characterize) that the LTE-V2X sensing-based semi-persistent scheduling faces certain inefficiencies when transmitting aperiodic messages of variable size These inefficiencies result in that IEEE 80211p generally outperforms LTE-V2X when transmitting aperiodic messages of variable size except when the channel load is very low

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Comparison of IEEE 802.11p and LTE-V2X: An Evaluation With Periodic and Aperiodic Messages of Constant and Variable Size

The 3GPP has recently published the first version of the Release 14 standard that includes support for V2V communications using LTE sidelink communications (referred to as LTE-V, LTE-V2X, LTE-V2V or Cellular V2X). The standard includes a mode (mode 4) where vehicles autonomously select and manage the radio resources without any cellular infrastructure support. This is highly relevant since V2V safety applications cannot depend on the availability of infrastructure-based cellular coverage, and transforms LTE-V into a possible (or complimentary) alternative to 802.11p. The performance of LTE-V in mode 4 is highly dependent on its distributed scheduling protocol (sensing-based Semi-Persistent Scheduling) that is used by vehicles to reserve resources for their transmissions. This paper presents the first evaluation of the performance and operation of this protocol under realistic traffic conditions in urban scenarios. The evaluation demonstrates that further enhancements should be investigated to reduce packet collisions.

System Level Evaluation of LTE-V2V Mode 4 Communications and Its Distributed Scheduling

The 3GPP has released the C-V2X standard to support V2X (Vehicle-to-Everything) communications using the LTE sidelink PC5 interface. This standard includes two modes of operation, and this study focuses on the Mode 4. This mode does not require the support of the cellular infrastructure, and vehicles can autonomously select their sub-channels for their V2V transmissions. The adequate operation of C-V2X Mode 4 requires a careful configuration of its main parameters. This study analyzes the optimum configuration of the parameters that mostly influence the operation and performance of C-V2X or LTE-V Mode 4. This analysis is conducted for different channel loads and traffic conditions. The conclusions obtained are compared with existing studies taking into account the importance of using accurate models for adequately configuring the C-V2X Mode 4 interface.

Rafael Molina-Masegosa

Papers

LTE-V for Sidelink 5G V2X Vehicular Communications: A New 5G Technology for Short-Range Vehicle-to-Everything Communications

Analytical Models of the Performance of C-V2X Mode 4 Vehicular Communications

Comparison of IEEE 802.11p and LTE-V2X: An Evaluation With Periodic and Aperiodic Messages of Constant and Variable Size

System Level Evaluation of LTE-V2V Mode 4 Communications and Its Distributed Scheduling

Configuration of the C-V2X Mode 4 Sidelink PC5 Interface for Vehicular Communication