1. Introduction
Current endeavors in the realm of connected and autonomous vehicles revolve around autonomous driving and futuristic intelligent transportation systems. These objectives are underpinned by a multifaceted agenda, including the prevention of vehicular collisions, the expedited dissemination of alerts and notifications, the reduction of traffic congestion, and the optimization of road-based services to improve automotive safety and law enforcement [
1]. The advent of motor vehicles has historically facilitated and continues to exert a pervasive influence on global economic and social progress. Nevertheless, motor vehicle-related accidents are a principal cause of mortality and physical harm. These accidents yield a staggering annual death toll of 1.35 million individuals worldwide [
2] and impose an estimated economic burden of approximately
$1.8 trillion on the global economy, spanning the years from 2015 to 2030 [
3]. It is noteworthy that traffic accidents also feature prominently among the leading causes of fatalities in the United States, with substantial economic ramifications. According to the United States National Highway Traffic Safety Administration (NHTSA), motor vehicle traffic crashes account for an annual tally of around 34,000 fatalities and 4 million injuries [
4], while simultaneously inflicting an economic toll exceeding
$836 billion each year [
5]. Furthermore, it is crucial to acknowledge that individuals fatally affected by traffic accidents represent more than statistical data or economic burdens, as their absence creates a significant societal impact. In this paper, we want to investigate the performance of connected vehicle communication using open-source simulators and latest communication standard under different network settings.
Connected and autonomous vehicle (CAV) technologies possess the capacity to substantially diminish the incidence of human fatalities and associated economic costs. A connected vehicle, at regular intervals, transmits safety messages, often referred to as “heartbeat messages,” such as the Cooperative Awareness Message (CAM) [
6] or the Basic Safety Message (BSM) [
5]. These messages convey real-time operational data about the vehicle, including information on its position, velocity, orientation, acceleration, deceleration, and more.
A vehicle with onboard devices relying on the Global Navigation Satellite System (GNSS), such as GPS, transmits and receives safety messages. Upon reception of these safety messages from adjacent vehicles and utilizing a cooperative vehicle safety system application [
7], a vehicle can generate a neighborhood map and instructions or guidelines for safe maneuvering. According to estimates by the United States Department of Transportation (USDOT), approximately 82% of accidents involving unimpaired drivers can be mitigated through the successful implementation of connected vehicle (CV) technology [
8]. Endowing vehicles with automated control features like emergency braking and preemptive warnings facilitated through connectivity significantly reduces the potential for human errors. Additionally, autonomous vehicles offer various other benefits, including the potential for improved air quality due to reduced traffic congestion and lower emissions. Furthermore, autonomous cars are poised to enhance fuel consumption efficiency and boost overall productivity, ultimately saving countless unproductive hours that drivers currently spend stuck in traffic each year [
9].
The IEEE 802.11p standard, based on Dedicated Short Range Communication (DSRC) technology, has historically been the go-to choice for on-road communication. A more recent standard, 802.11bd [
10], has also gained attention in this context. Many countries have invested substantially in test-bed deployment and platform development for DSRC.
However, recent research findings have brought to light some significant shortcomings of DSRC technology, including issues like low reliability, frequent collisions, hidden node problems, uncontrolled delays, and intermittent vehicle-to-infrastructure (V2I) connectivity [
11]. Recognizing these limitations, and with the rapid global expansion and commercialization of Long-Term Evolution (LTE), there has been a shift in focus toward exploring LTE-5G as a potential wireless access technology for supporting vehicular applications. This transition is exemplified by the 5G Automotive Association (5GAA), which has become a Market Representation Partner (MRP) within the Third Generation Partnership Project (3GPP) framework [
12]. The 5GAA brings the expertise of vehicle manufacturers and key players from the automotive industry into the 3GPP ecosystem.
According to the 3GPP Release 14 standards [
13], LTE-Vehicle (LTE-V)/C-V2X has demonstrated its ability to meet the low-latency requirements of vehicular networks. Cellular networks offer inherent advantages in delivering V2I communications due to their high data rates, widespread coverage, robust quality of service (QoS) support, and deep network penetration. The 3GPP-based cellular technology, endorsed by 5GAA, is believed to provide superior performance and a more future-proof radio access solution than DSRC. It can also leverage existing upper-layer standards and testing protocols from ETSI-ITS, ISO, SAE, and IEEE, refined and endorsed by the automotive industry and the Intelligent Transportation Systems (ITS) community over the past decade.
The study report from 5GAA [
14] shows a comparative performance analysis between LTE C-V2X and 802.11p and demonstrates that LTE C-V2X has better performance due to its superior physical layer than DSRC. However for the more advanced V2X applications (e.g., semi- or fully-automated driving), which have very low latency and very high bandwidth requirements, even LTE C-V2X falls short in such cases [
15]. Hence, 5G New Radio (NR) comes into play to become a complementary solution to the existing LTE C-V2X technology. For that, 3GPP formulates the technical specifications for NR V2X in TR 38.885 [
16]. Furthermore, 3GPP Release 16 standardizes the V2X on top of 5G NR, which is standardized in release 15 [
17]. However, so far, very little work has been done on 3GPP NR V2X system and specifications [
1,
18,
19].
Furthermore, there is a scarcity of open-source simulators designed for 5G V2X communications that align with the 3GPP NR V2X Release 16 specifications, which poses a significant limitation for the research community. Conversely, commercially available simulators are expensive and offer limited opportunities for source code modification to suit research needs. Additionally, the substantial costs associated with deploying vehicular test beds and conducting pre/post experimental validation underscore the ongoing demand for a realistic simulation platform [
20,
21,
22]. Therefore, it would be advantageous to allocate more resources to the development and enhancement of open-source simulators tailored to NR V2X communication in compliance with 3GPP Release 16, enabling a comprehensive performance analysis in diverse network scenarios
1[
23].
The remainder of the chapter is organized as follows.
Section 2 describes the system model. Problem statement is presented in
Section 3. A brief overview of DSRC, LTE C-V2X and 5G NR V2X is presented in
Section 4. Simulation setup and performance analysis are shown in
Section 5. Finally, we conclude the chapter in
Section 6.
2. System Architecture
Figure 1 shows the typical architecture of connected vehicular networks using the NR V2X sidelink communication. Each vehicle broadcasts the basic safety message periodically.
The message contains the instantaneous states of the vehicles which helps a vehicle to generate the neighbourhood map which eventually help the vehicle for its informed motion planning. In this work, we want to study the latency and reliability performance of V2X applications in a multi-lane highway scenario using 3GPP’s NR system without the support from a base station.
Figure 2 shows the summary of advanced V2X applications supported by 3GPP NR V2X system.
It also highlights important features that are added by 3GPP working group. These advanced applications requires high reliability (up to 99.99%), ultra low latency (as low as
), and high bandwidth (as high as an order of terabytes (TB)/hour) [
24]. A brief description of important features is given below.
Automated driving: A vehicle can sense the surrounding environment and can take the lateral and longitudinal decisions by itself or by taking a very little input from humans.
Cooperative perception: A vehicle can increase its field of view and perception about neighbors by exchanging the onboard sensory information through V2V and V2I communication.
Platooning: A platoon of vehicles will route from the source to the destination in close proximity following the leader vehicle. The follower vehicles should not need any input from humans.
Infotainment: Information (e.g., road maintenance updates, weather updates, etc.) and entertainment (e.g., live traffic updates, news, music, etc.) can be disseminated from one vehicle to other vehicles through V2V and/or I2V communication in very short duration (within the order of milliseconds).
NR V2X has a number of advanced features which enable it to provide higher reliability, lower latency, and higher bandwidth [
16]. These are supports for (a) flexible frame structure by providing flexible numerology, (b) multiple frequency ranges including sub 6 GHz and millimeter wave bands (24.25 - 52.6 GHz), (c) mini-slot scheduling for time-critical services, (d) sidelink network architecture for vehicles outside of the communication range of a base station, (e) resource allocation by base station (mode 1) and by the vehicles autonomously (mode 2), (d) sophisticated MAC and PHY layers.
Although NR V2X supports working in high frequency ranges, it imposes several challenges in terms of communication point of view, including high signal attenuation, communication range restriction, higher background noise, and difficulty in costly hardware implementation. Hence, it is imperative to implement and do a comprehensive study of the 3GPP NR V2X stack in a widely accepted simulator (such as ns-3 [
25] under different realistic traffic scenarios.
4. Overview of DSRC, LTE C-V2X, and 5G NR V2X
The purpose of this section is to present a brief overview of recent technologies, DSRC, LTE C-V2X, and 5G NR V2X, deployed in connected and autonomous vehicles.
Table 1 compares and contrasts the specifications of DSRC, C-V2X, and 5G NR.
4.1. DSRC
Over the past two decades, Dedicated Short Range Communications (DSRC) has been one of the primary radio access technologies (RATs) used for V2X communications. IEEE 802.11p [
34], the basis of DSRC, was the first V2X technology used worldwide. It operates in the 5.9 GHz frequency band and is an extension of the IEEE 802.11 standard (commonly known as Wi-Fi) tailored for fast and reliable communication between vehicles and roadside infrastructure. Its design prioritizes safety applications, such as collision avoidance and traffic management, by delivering low-latency communication, high data rates, and robust connectivity in dynamic vehicular environments.
While DSRC 802.11p has been a pioneering standard for vehicular communication, it exhibits certain limitations compared to Cellular Vehicle-to-Everything (C-V2X) technology. DSRC operates in a single dedicated frequency band (5.9 GHz), which may lead to potential congestion and interference in densely populated areas. Additionally, DSRC lacks the inherent support for diverse communication modes that C-V2X offers, encompassing both direct short-range communication (similar to DSRC) and wide-area communication facilitated by cellular networks. C-V2X also boasts improved non-line-of-sight (NLOS) performance, enabling communication around obstacles.
To narrow the performance gap, IEEE 802.11bd [
35,
36,
37,
36] has been developed as the next-generation DSRC protocol to improve V2X performance. It allows up to three repetitions per packet, aiming to increase time diversity and enable maximum ratio combining at the receiver, thus improving the probability of correct decoding [
37]. Additionally, it adds extra modes of operation to enhance throughput and extend communication ranges by reducing noise sensitivity level [
28,
38].
4.2. LTE C-V2X
C-V2X, or Cellular Vehicle-to-Everything, commonly referred to as LTE-V2X due to its foundation on Time Division Long-Term Evolution (TD-LTE) 4G cellular technology, is a prominent communication framework designed to enhance vehicular connectivity and road safety. It was developed by 3GPP as an alternative to DSRC.
C-V2X enables direct and reliable wireless communication among vehicles (V2V), between vehicles and infrastructure (V2I), and even with pedestrians and other vulnerable road users (V2P), thus forming a comprehensive V2X ecosystem. This technology leverages the existing cellular network infrastructure to provide low-latency, high-throughput data exchange, enabling vehicles to exchange critical information such as traffic conditions, emergency alerts, and real-time sensor data. C-V2X holds significant promise in advancing intelligent transportation systems, reducing traffic accidents, and enabling future autonomous driving applications, making it a focal point of research and development in the field of connected vehicles and smart transportation.
C-V2X is regarded as the most powerful competitor to 802.11p [
39], and consequently, many studies have investigated their comparative performances. Comparison at the link level showed that C-V2X could improve the link budget over IEEE 802.11p by around 7 dB [
39]. In addition, the study also found the coverage of LTE V2X to be larger than DSRC under the same velocity. Furthermore, owing to the modulation technique used by LTE V2X, it is less susceptible to noise, which allows it to guarantee a more reliable communication link than DSRC. A more detailed performance comparison between the two technologies are covered in the next section as below.
4.3. DSRC vs. C-V2X
There has been a heated debate regarding the performance of DSRC and C-V2X technologies, leading to extensive studies comparing their performance [
40,
41,
42,
43,
44,
45]. The commonly used key performance indicators (KPI) include: Packet Delivery Ratio (PDR) [
44,
46], Packet Reception Ratio (PRR) [
41,
42,
43], Block Error Ratio (BLER) [
39], Packet Inter-Reception time (PIR), and packet loss [
44].
In the literature, certain studies [
44,
46,
47,
48] have reported that DSRC outperforms C-V2X, while others have observed similar performance [
49] in the specific scenarios they examined. Molina-Masegosa et al. [
44] demonstrated that variations in message size and pattern significantly affect the Medium Access Control (MAC) operation and performance of both technologies and that DSRC handles variations in the message size and time interval more effectively than LTE-V2X’s sensing-based semi-persistent scheduling, except under very low channel loads. Petrow et al. [
46] showcased that only the DSRC technology is able to support the V2I communication scenarios without any major limitations and achieves an average end-to-end delay of less than 100 milliseconds and a PDR above
in all of their investigated simulation scenarios.
However, the majority of research suggests that C-V2X performs better [
42,
50]. Hafeez et al. [
40] proposed an analytical model for assessing the reliability of DSRC control channels in safety applications. The model considers factors such as vehicle follow-on safety rules, mobility, transmitter-receiver speeds, channel fading, hidden terminals, and collisions. Simulation results using realistic vehicular traces validate the modeling and analysis, showcasing that DSRC may lead to significant performance degradation in terms of delay and packet reception probability in dense and high-mobility conditions. Hu et al. [
39] conducted link-level simulations comparing LTE V2X and DSRC for urban and freeway scenarios with and without Line of Sight (LOS). Their study demonstrated that LTE V2X achieves the same Block Error Ratio (BLER) at a lower Signal-to-Noise Ratio (SNR) or with reduced receiving power compared to DSRC, resulting in a more reliable link. Wang et al. [
41] conducted simulations to compare the average PRR between LTE and DSRC at different vehicle densities in both urban and freeway scenarios with varying distances. Their study illustrates that LTE-based alternatives, including LTE multicast and LTE sidelink, outperform IEEE 802.11p in all the studied cases. This improved performance is credited to LTE’s more robust PHY layer signaling scheme and more efficient resource allocation mechanism. Nguyen et al. [
42] compared DSRC and Cellular V2X, with Cellular V2X consistently outperforming or matching DSRC in various aspects. Notably, Cellular V2X offers an extended communication range, crucial for vehicular safety. The evaluation favors DSRC despite using an advanced DSRC receiver and a standard Cellular V2X receiver with conservative AGC loop training assumptions. Zhao et al. [
43] compared LTE-V2X and DSRC in terms of average PRR at different distances in freeway and urban scenarios. Their findings indicate that LTE-V2X offers longer, more reliable communication ranges compared to DSRC. This advantage is attributed to LTE-V2X’s efficient sensing and SPS resource allocation scheme, confirming its superiority over DSRC.
It is worth noting that recent research also investigates solutions to leverage the advantages of DSRC and C-V2X [
28,
51,
52,
53,
54] and mitigate limitations for joint use of DSRC and C-V2X. Ansari et al. [
28] reviewed the background and technical aspects of both technologies and elaborated on V2X platform models allowing concurrent and simultaneous propagation of DSRC and C-V2X messages for a hybrid V2X environment. The study highlights the challenges arising from the nature of hybrid V2X and argues that the operation of the two technologies in the same channel for concurrent transmissions, without a mutual synchronization solution, would result in harmful co-channel interference. Both co-channel and adjacent-channel interferences remain open research problems in hybrid V2X systems. Mir et al. [
52] proposed an architecture and a suite of protocols to enable DSRC and C-V2X hybrid vehicular networks. The protocol suite comprises an enhanced network protocol stack, an adaptive RAT selection mechanism, a Vertical Handover (VHO) algorithm, and dynamic communication management (DCM) algorithms to address various challenges in the hybrid network. Simulation results using Matlab demonstrate the effectiveness of this architecture and protocol suite in terms of packet delivery ratio, latency, and throughput. Qi et al. [
51] introduced the Traffic Differentiated Clustering Routing (TDCR) mechanism within a Software Defined Network (SDN)-enabled hybrid vehicular network. This mechanism comprises a centralized one-hop clustering approach and an optimization method for data delivery. Specifically, the optimization aims to strike a balance between cellular bandwidth cost and end-to-end delay, allowing Cluster Heads (CHs) to deliver their aggregated data either through multi-hop Vehicle-to-Vehicle (V2V) transmissions or via cellular networks.
4.4. Challenges faced by DSRC and C-V2X
Scalability is a major challenge for DSRC as its performance degrades at higher vehicle densities due to packet collisions. However, due to frequency re-using by the C-V2X sidelink mode 4 algorithm over a given geographical area, higher traffic density also results in an increased interference level among C-V2X users. Hence, although C-V2X offers several performance benefits over DSRC, it suffers from the same traffic density issue [
55].
Another challenge for both DSRC and C-V2X is supporting advanced safety applications requiring faster response times. Both technologies can support a basic set of vehicular safety applications that generate driver alerts to indicate potentially dangerous conditions [
40]. These day-1 basic safety applications have lower periodicity and higher end-to-end latency requirements. However, advanced vehicular safety applications depend on transmitting messages for maneuver changes, trajectory alignments, platoon formations, sensor data exchange, as well as sharing information gathered from neighboring vehicles’ live camera feeds [
56]. This additional information is paramount to ensuring both safer autonomous cars and human-driven vehicles, better traffic management, and the increasing demand for in-vehicle infotainment.
4.5. 5G NR V2X
The development of the 3GPP sidelink transmission protocol played a crucial role in the long-term evolution-advanced (LTE-A) communication technology for public safety and V2X services [
57]. However, recent advancements in data-driven applications require more sidelink functions, such as feedback channels, grant-free access, and enhanced channel sensing procedures, which are not provided by the LTE-A technology. Advanced applications that deal with an overwhelming degree of data packets usually demand higher bandwidth, more frequent message passing, and lower latency, which C-V2X cannot provide [
33]. Moreover, these advanced applications also use larger and variable-sized packets transmitted aperiodically, unlike the basic safety applications. As a result, a new communication technology, 5G New Radio (NR) V2X, has been developed to supplement C-V2X in supporting those particular use cases. More specifically, 5G NR V2X provides advanced functionalities in addition to 5G NR air interface that meet stringent requirements of users and/or services in connected and autonomous vehicles.
Among the most significant parts of the 5G NR V2X communication technology is the sidelink transmission. A thorough analysis of 5G NR V2X communications is presented in [
58] and some references therein. It is important to note, however, that the 5G NR V2X communication technology is not meant to replace C-V2X communication; rather, it can operate in cooperation with the existing C-V2X technology to meet the requirements of complex services in recent data-driven connected and autonomous vehicles. It is also worth noting that an individual application may be operated and supported with either its corresponding communication technology or a combination of different communication technologies. For instance, advanced V2X services provided by the NR sidelink transmission may coexist with those provided by the LTE sidelink transmission in different channels [
59].
Sidelink communication is the key enabler of direct V2X communication without the support of a base station in 5G NR. According to [
60], the NR V2X supports four different physical channels for enabling unicast, groupcast, and broadcast communications among the vehicles: (1) the PSBCH (Physical Sidelink Broadcast Channel) is used to disseminate broadcast information; (2) the PSCCH (Physical Sidelink Control Channel) is communicating control information; (3) the PSSCH (Physical Sidelink Shared Channel) is used for exchanging control, data and CSI (Channel State Information) for unicast communication; and (4) the PSFCH (Physical Sidelink Feedback Channel) is used for exchanging HARQ (Hybrid Automatic Repeat Request) feedback for unicast and groupcast communication. For enabling these channels at sub 6 GHz frequency band, three numerologies (
) are used with different Sub-carrier Spacing (SCS): (1)
with SCS = 15 KHz; (2)
with SCS = 30 KHz; and (3)
with SCS = 60 KHz. One numerology differs from another by varying (1) the number of available subchannels/resource blocks in communication, and (2) the resource selection window length. For instance, a higher numerology has a lower number of sub-channel availability, but it also has a shorter slot length which results in a lower resource selection window in ms.
6. Conclusion and future work
In this work, firstly we have developed the 3GPP’s 5G NR V2X network simulation model using the ns-3 discrete event simulator and then studied the V2X performance under various network settings. We found that in 5G NR V2X, a higher sub-carrier spacing () has an overall better performance than a lower sub-carrier spacing ( or ) in terms of achieving a higher throughput and packet reception ratio and achieving a lower inter-packet reception delay. From the analysis, we also observed that in the highway setting, till communication range, there were no noticeable packet drops in NR V2X communication. Additionally, we found that increasing transmitting power is positively associated with improving performance. However, although increasing packet size boosts throughput but is negatively associated with achieving lower inter-packet reception delay. We have also found that increasing modulation and coding scheme (MCS) value, increases the network performance for different sub-carrier spacing. However, with a setting of communication range, for , we get the best result at the MCS value of 10, whereas for and we need to increase the MCS value up-to 15 to get the same better performance.
In our future research work, we want to extend our simulation by introducing communication from infrastructure to vehicle (I2V) and vehicle to pedestrian (V2P). Another interesting future research direction might be leveraging the developed NR V2X communication model to achieve enhanced and extended cooperative perception for improved vehicle motion planning.
Author Contributions
This work was completed with contributions from all the authors. “conceptualization, G.G.M.N.A., M.N.S., M.S.M. and S.A.S.; methodology, G.G.M.N.A., M.S.M., and S.A.S.; software, G.G.M.N.A., M.N.S., and S.A.S.; validation, G.G.M.N.A., M.N.S., M.S.M. and S.A.S.; formal analysis, G.G.M.N.A., and M.S.M.; data curation, G.G.M.N.A., M.N.S., and S.A.S.; writing—original draft preparation, G.G.M.N.A., M.N.S., M.S.M., S.A.S., and Y.W.; writing—review and editing, G.G.M.N.A., M.N.S., M.S.M., S.A.S., and Y.W.; visualization, G.G.M.N.A., M.S.M., S.A.S.; funding acquisition, G.G.M.N.A. and Y. W. All authors did edit, review and improve the manuscript. All authors have read and agreed to the published version of the manuscript. ”