1 IEEE 802.11bd & 5G NR V2X: Evolution of Radio Access Technologies for V2X Communications Gaurang Naik, Biplav Choudhury, Jung-Min (Jerry) Park Abstract—With rising interest in autonomous vehicles, devel- based RAT—that can enable C-V2X capable vehicles to oping radio access technologies (RATs) that enable reliable and operate in a distributed manner in the absence of cellular low latency vehicular communications has become of paramount infrastructure, while leveraging the infrastructure for efficient importance. Dedicated Short Range Communications (DSRC) and Cellular V2X (C-V2X) are two present-day technologies that resource allocations when vehicles operate within coverage. are capable of supporting day-1 vehicular applications. However, Existing literature shows that C-V2X offers performance these RATs fall short of supporting communication requirements advantages over DSRC in terms of its additional link budget, of many advanced vehicular applications, which are believed to higher resilience to interference and better non line-of-sight be critical in enabling fully autonomous vehicles. Both DSRC (NLOS) capabilities [3]. Further, studies indicate that both and C-V2X are undergoing extensive enhancements in order to support advanced vehicular applications that are characterized DSRC and C-V2X can reliably support safety applications by high reliability, low latency and high throughput requirements. that demand an end-to-end latency of around 100 milliseconds These RAT evolutions—IEEE 802.11bd for DSRC and NR V2X (msec) as long as the vehicular density is not very high [4]. for C-V2X—can supplement today’s vehicular sensors in enabling However, as the quality of service (QoS) requirements of V2X autonomous driving. In this paper, we briefly describe the two use-cases become more stringent, which is the case in many present-day vehicular RATs. In doing so, we highlight their inability to guarantee quality of service requirements of many advanced V2X applications [5], the two current V2X RATs advanced vehicular applications. We then look at the two RAT fall short of providing the desired performance. evolutions, i.e., IEEE 802.11bd and NR V2X and outline their In order to diminish the performance gap between DSRC objectives, describe their salient features and provide an in-depth and C-V2X and to support additional modes of operations and description of key mechanisms that enable these features. While increase the offered throughput, a new Study Group called both, IEEE 802.11bd and NR V2X, are in their initial stages of development, we shed light on their preliminary performance the IEEE 802.11 Next Generation V2X was formed in March projections and compare and contrast the two evolutionary RATs 2018 [6]. This resulted in the formation of IEEE Task Group with their respective predecessors. 802.11bd (TGbd) in Jan. 2019. On the other hand, 3GPP is Index Terms—V2X, DSRC, C-V2X, IEEE 802.11bd, NR V2X working toward the development of New Radio (NR) V2X for its Rel. 16, building atop of 5G NR that was standardized in 3GPP Rel.15. NR V2X is expected to support advanced V2X I. INTRODUCTION applications that require much more stringent QoS guarantees Vehicle-to-Everything (V2X) communications has the po- compared to applications that can be supported by C-V2X [5]. tential to significantly bring down the number of vehicle Some of these use-cases require the end-to-end latency to be crashes, thereby reducing the number of associated fatali- as low as 3 msec with a reliability of 99:999%! Coupled with ties [1]. However, the benefits of V2X are not limited to safety the existing challenges offered by high-mobility environments, applications alone. V2X-capable vehicles can assist in better these additional constraints make the design of 802.11bd and traffic management leading to greener vehicles and lower fuel NR V2X extremely challenging. costs [2]. Intelligent Transportation Systems (ITS) constitute In terms of their design objectives, 802.11bd and NR such vehicular safety and non-safety applications. Today, the V2X have certain similarities. For example, both evolutionary two key radio access technologies (RATs) that enable V2X RATs are being designed to improve the reliability of offered arXiv:1903.08391v2 [cs.IT] 26 Mar 2019 communications are Dedicated Short Range Communications services, lower the end-to-end latency and support applications (DSRC) and Cellular-V2X (C-V2X). DSRC is designed to that require high throughput. However, their design method- primarily operate in the 5.9 GHz band, which has been ologies significantly differ. TGbd requires the new standard, earmarked in many countries for ITS applications. On the other i.e., 802.11bd to be backward compatible with 802.11p. This hand, C-V2X can operate in the 5.9 GHz band as well as in implies that 802.11bd and 802.11p devices must be able to the cellular operators’ licensed carrier [2]. communicate with each other while operating on the same DSRC relies on the IEEE 802.11p standard for its physical channel. On the other hand, 3GPP does not impose a similar (PHY) and medium access control (MAC) layers. DSRC uses constraint on NR V2X. Vehicles equipped with NR V2X can a MAC protocol that is simple, well-characterized and capable still communicate with C-V2X devices. However, this will of distributed operations. However, the adoption of DSRC be achieved through a dual-radio system — one radio for C- in vehicles has been delayed due to its poor scalability and V2X and another for NR V2X. The backward compatibility communication challenges imposed by high-mobility envi- requirement for 802.11bd has implications on its design and ronments. Meanwhile, the 3rd Generation Partnership Project performance, as we will discuss later in this paper. (3GPP) has developed C-V2X—a Long Term Evolution (LTE) IEEE 802.11bd and NR V2X are technologies that are The authors are with the Bradley Department of Electrical and Computer currently under development, and hence, in this paper, we Engineering, Virginia Tech. (Email:fgaurang, biplavc, [email protected]) limit our discussions to a set of key features and functionalities 2 that are likely to be included in the final standard. The main bandwidth of 10 MHz. Thus, in comparison to Wi-Fi, DSRC contributions of this paper are as follows: sub-carrier spacing is reduced by a factor of two. The MAC • We outline the key objectives of 802.11bd and NR V2X, protocol used in DSRC is Carrier Sense Multiple Access [7]. followed by a detailed description of important enhance- However, there is no exponential back-off in DSRC, i.e. the ments being made to DSRC and C-V2X in the process parameter Contention Window used in contention-based MAC of development of 802.11bd and NR V2X, respectively. protocols remains fixed in DSRC [7] due to two main reasons, • We elaborate on critical challenges encountered in the i) because DSRC is designed mainly for broadcast-based design of the two RATs and their potential solutions. systems, there is no acknowledgement frame sent back to 1 • We look at performance projections of 802.11bd and NR the transmitter , and ii) exponential back-off can lead to large V2X in light of their respective design objectives. Contention Window sizes, thereby leading to high latencies. • Finally, we discuss a number of key spectrum manage- ment issues that represent hurdles to the deployment and B. Cellular V2X (C-V2X) management of V2X technologies. While there are several interesting and challenging research C-V2X is a V2X RAT developed by 3GPP in its Rel. 14. C- problems in the design of these evolving RATs (e.g., V2X V2X users can benefit from leveraging the existing widespread security), we restrict our focus in this paper to the design cellular infrastructure. However, since the presence of cellular of the PHY and MAC layers. To the best of our knowledge, infrastructure cannot always be relied upon, C-V2X defines ours is the first work that provides an in-depth look at the transmission modes that enable direct V2X communications design considerations and development process of these two using the sidelink channel over the PC5 interface. 3GPP Rel. evolutionary RATs. Throughout this paper, we use the terms 14 introduced two new sidelink transmission modes (modes 3 DSRC and 802.11p interchangeably. A summary of acronyms and 4) to support low latency V2X communications [8]. used in this paper is outlined in Table I. The basic time-frequency resource structure of C-V2X is similar to that of LTE, i.e. the smallest unit of allocation TABLE I: Summary of Acronyms in time is one sub-frame (1 msec comprising of 14 OFDM Acronym Full-form symbols) and the smallest frequency-granularity is 12 sub- 3GPP 3rd Generation Partnership Project carriers of 15 kHz each (i.e. 180 kHz). In each OFDM sub- BCC Binary Convolutional Coding C-V2X Cellular Vehicle-to-Everything carrier, C-V2X devices can transmit using Quadrature Phase DMRS Demodulation Reference Signals Shift Keying (QPSK) or 16-Quadrature Amplitude Modula- DSRC Dedicated Short Range Communications tion (QAM) schemes with turbo coding [4]. In addition to FCC Federal Communications Commission FDM Frequency Division Multiplexing data symbols, however, C-V2X users also transmit control ITS Intelligent Transporartion Systems information and reference signals. The demodulation reference LDPC Low Density Parity Check signal (DMRS) is one such signal, which is used for channel LTE Long Term Evolution MAC Medium Access Contol (layer) estimation. In LTE, DMRS symbols are inserted in two of the MCS Modulation and Coding Scheme fourteen OFDM symbols. However, since C-V2X is designed NLOS Non line-of-sight for high-mobility environments, four DMRS symbols are OFDM Orthogonal Frequency Division Multiplexing PDR Packet Delivery Ratio inserted in a C-V2X sub-frame [9].