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

Gaurang Naik, Biplav Choudhury, Jung-Min (Jerry) Park

Abstract—With rising interest in autonomous vehicles, devel-oping radio access technologies (RATs) that enable reliable andlow latency vehicular communications has become of paramountimportance. Dedicated Short Range Communications (DSRC)and Cellular V2X (C-V2X) are two present-day technologies thatare capable of supporting day-1 vehicular applications. However,these RATs fall short of supporting communication requirementsof many advanced vehicular applications, which are believed tobe critical in enabling fully autonomous vehicles. Both DSRCand C-V2X are undergoing extensive enhancements in order tosupport advanced vehicular applications that are characterizedby high reliability, low latency and high throughput requirements.These RAT evolutions—IEEE 802.11bd for DSRC and NR V2Xfor C-V2X—can supplement today’s vehicular sensors in enablingautonomous driving. In this paper, we briefly describe the twopresent-day vehicular RATs. In doing so, we highlight theirinability to guarantee quality of service requirements of manyadvanced vehicular applications. We then look at the two RATevolutions, i.e., IEEE 802.11bd and NR V2X and outline theirobjectives, describe their salient features and provide an in-depthdescription of key mechanisms that enable these features. Whileboth, IEEE 802.11bd and NR V2X, are in their initial stagesof development, we shed light on their preliminary performanceprojections and compare and contrast the two evolutionary RATswith their respective predecessors.

Index Terms—V2X, DSRC, C-V2X, IEEE 802.11bd, NR V2X

I. INTRODUCTION

Vehicle-to-Everything (V2X) communications has the po-tential to significantly bring down the number of vehiclecrashes, thereby reducing the number of associated fatali-ties [1]. However, the benefits of V2X are not limited to safetyapplications alone. V2X-capable vehicles can assist in bettertraffic management leading to greener vehicles and lower fuelcosts [2]. Intelligent Transportation Systems (ITS) constitutesuch vehicular safety and non-safety applications. Today, thetwo key radio access technologies (RATs) that enable V2Xcommunications are Dedicated Short Range Communications(DSRC) and Cellular-V2X (C-V2X). DSRC is designed toprimarily operate in the 5.9 GHz band, which has beenearmarked in many countries for ITS applications. On the otherhand, C-V2X can operate in the 5.9 GHz band as well as inthe cellular operators’ licensed carrier [2].

DSRC relies on the IEEE 802.11p standard for its physical(PHY) and medium access control (MAC) layers. DSRC usesa MAC protocol that is simple, well-characterized and capableof distributed operations. However, the adoption of DSRCin vehicles has been delayed due to its poor scalability andcommunication challenges imposed by high-mobility envi-ronments. Meanwhile, the 3rd Generation Partnership Project(3GPP) has developed C-V2X—a Long Term Evolution (LTE)

The authors are with the Bradley Department of Electrical and ComputerEngineering, Virginia Tech. (Email:{gaurang, biplavc, jungmin}@vt.edu)

based RAT—that can enable C-V2X capable vehicles tooperate in a distributed manner in the absence of cellularinfrastructure, while leveraging the infrastructure for efficientresource allocations when vehicles operate within coverage.

Existing literature shows that C-V2X offers performanceadvantages over DSRC in terms of its additional link budget,higher resilience to interference and better non line-of-sight(NLOS) capabilities [3]. Further, studies indicate that bothDSRC and C-V2X can reliably support safety applicationsthat demand an end-to-end latency of around 100 milliseconds(msec) as long as the vehicular density is not very high [4].However, as the quality of service (QoS) requirements of V2Xuse-cases become more stringent, which is the case in manyadvanced V2X applications [5], the two current V2X RATsfall short of providing the desired performance.

In order to diminish the performance gap between DSRCand C-V2X and to support additional modes of operations andincrease the offered throughput, a new Study Group calledthe IEEE 802.11 Next Generation V2X was formed in March2018 [6]. This resulted in the formation of IEEE Task Group802.11bd (TGbd) in Jan. 2019. On the other hand, 3GPP isworking toward the development of New Radio (NR) V2X forits Rel. 16, building atop of 5G NR that was standardized in3GPP Rel.15. NR V2X is expected to support advanced V2Xapplications that require much more stringent QoS guaranteescompared to applications that can be supported by C-V2X [5].Some of these use-cases require the end-to-end latency to beas low as 3 msec with a reliability of 99.999%! Coupled withthe existing challenges offered by high-mobility environments,these additional constraints make the design of 802.11bd andNR V2X extremely challenging.

In terms of their design objectives, 802.11bd and NRV2X have certain similarities. For example, both evolutionaryRATs are being designed to improve the reliability of offeredservices, lower the end-to-end latency and support applicationsthat require high throughput. However, their design method-ologies significantly differ. TGbd requires the new standard,i.e., 802.11bd to be backward compatible with 802.11p. Thisimplies that 802.11bd and 802.11p devices must be able tocommunicate with each other while operating on the samechannel. On the other hand, 3GPP does not impose a similarconstraint on NR V2X. Vehicles equipped with NR V2X canstill communicate with C-V2X devices. However, this willbe achieved through a dual-radio system — one radio for C-V2X and another for NR V2X. The backward compatibilityrequirement for 802.11bd has implications on its design andperformance, as we will discuss later in this paper.

IEEE 802.11bd and NR V2X are technologies that arecurrently under development, and hence, in this paper, welimit our discussions to a set of key features and functionalities

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that are likely to be included in the final standard. The maincontributions of this paper are as follows:

• We outline the key objectives of 802.11bd and NR V2X,followed by a detailed description of important enhance-ments being made to DSRC and C-V2X in the processof development of 802.11bd and NR V2X, respectively.

• We elaborate on critical challenges encountered in thedesign of the two RATs and their potential solutions.

• We look at performance projections of 802.11bd and NRV2X in light of their respective design objectives.

• Finally, we discuss a number of key spectrum manage-ment issues that represent hurdles to the deployment andmanagement of V2X technologies.

While there are several interesting and challenging researchproblems in the design of these evolving RATs (e.g., V2Xsecurity), we restrict our focus in this paper to the designof the PHY and MAC layers. To the best of our knowledge,ours is the first work that provides an in-depth look at thedesign considerations and development process of these twoevolutionary RATs. Throughout this paper, we use the termsDSRC and 802.11p interchangeably. A summary of acronymsused in this paper is outlined in Table I.

TABLE I: Summary of AcronymsAcronym Full-form

3GPP 3rd Generation Partnership ProjectBCC Binary Convolutional Coding

C-V2X Cellular Vehicle-to-EverythingDMRS Demodulation Reference SignalsDSRC Dedicated Short Range CommunicationsFCC Federal Communications CommissionFDM Frequency Division MultiplexingITS Intelligent Transporartion Systems

LDPC Low Density Parity CheckLTE Long Term EvolutionMAC Medium Access Contol (layer)MCS Modulation and Coding SchemeNLOS Non line-of-sightOFDM Orthogonal Frequency Division MultiplexingPDR Packet Delivery RatioPHY Physical (layer)

PSCCH Physical Sidelink Control ChannelPSFCH Physical Sidelink Feedback ChannelPSSCH Physical Sidelink Shared ChannelQAM Quadrature Amplitude ModulationQoS Quality of Service

QPSK Quadrature Phase Shift KeyingRAT Radio Access Technology

SC-FDMA Single Carrier Frequency Division Multiple AccessTDM Time Division MultiplexingUE User Equipment

U-NII Unlicensed National Information InfrastructureV2X Vehicle-to-Everything

II. STATE-OF-THE-ART

A. Dedicated Short Range Communications (DSRC)

The PHY and MAC layers of DSRC are defined in theIEEE 802.11p standard, which is largely derived from IEEE802.11a. Traditionally, Wi-Fi standards have been developedfor low mobility applications. However, since DSRC was de-signed for vehicular networks characterized by high-mobility,enhancements were introduced to make DSRC suitable forsuch environments. DSRC uses an Orthogonal FrequencyDivision Multiplexing (OFDM)-based PHY with a channel

bandwidth of 10 MHz. Thus, in comparison to Wi-Fi, DSRCsub-carrier spacing is reduced by a factor of two. The MACprotocol used in DSRC is Carrier Sense Multiple Access [7].However, there is no exponential back-off in DSRC, i.e. theparameter Contention Window used in contention-based MACprotocols remains fixed in DSRC [7] due to two main reasons,i) because DSRC is designed mainly for broadcast-basedsystems, there is no acknowledgement frame sent back tothe transmitter1, and ii) exponential back-off can lead to largeContention Window sizes, thereby leading to high latencies.

B. Cellular V2X (C-V2X)

C-V2X is a V2X RAT developed by 3GPP in its Rel. 14. C-V2X users can benefit from leveraging the existing widespreadcellular infrastructure. However, since the presence of cellularinfrastructure cannot always be relied upon, C-V2X definestransmission modes that enable direct V2X communicationsusing the sidelink channel over the PC5 interface. 3GPP Rel.14 introduced two new sidelink transmission modes (modes 3and 4) to support low latency V2X communications [8].

The basic time-frequency resource structure of C-V2X issimilar to that of LTE, i.e. the smallest unit of allocationin time is one sub-frame (1 msec comprising of 14 OFDMsymbols) and the smallest frequency-granularity is 12 sub-carriers of 15 kHz each (i.e. 180 kHz). In each OFDM sub-carrier, C-V2X devices can transmit using Quadrature PhaseShift Keying (QPSK) or 16-Quadrature Amplitude Modula-tion (QAM) schemes with turbo coding [4]. In addition todata symbols, however, C-V2X users also transmit controlinformation and reference signals. The demodulation referencesignal (DMRS) is one such signal, which is used for channelestimation. In LTE, DMRS symbols are inserted in two of thefourteen OFDM symbols. However, since C-V2X is designedfor high-mobility environments, four DMRS symbols areinserted in a C-V2X sub-frame [9].

Because C-V2X can operate in in-coverage as well as out-of-coverage scenarios, C-V2X can operate using the traditionalLTE air interface as well as the sidelink air interface.

1) V2X using LTE-Uu Air Interface: LTE-Uu is the tradi-tional air interface between an eNodeB and a User Equipment(UE). Any UE using the LTE-Uu interface must transmitits message to the eNodeB in the uplink, which is sentby the eNodeB to the destination UE in the downlink. Toreduce the scheduling overhead associated with V2X uplinktransmissions, the eNodeB can use semi-persistent scheduling,whereby the eNodeB assigns resources to a UE not only forthe very next transmission, but also for a number of subse-quent transmissions. Semi-persistent scheduling is beneficialfor V2X applications because a majority of such traffic isperiodic and have similarly-sized packets [10].

2) V2X using PC5 Air Interface: The PC5 air interfaceenables direct communications between UEs without requiringevery packet to pass through the eNodeB. UEs can use the PC5interface both in the presence and absence of the eNodeB.

1In traditional Wi-Fi networks, the contention window size is doubled whenthe transmitter does not receive an acknowledgement.

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A transmitted packet on the PC5 interface comprises of thedata component and the sidelink control information (SCI).The SCI carries important information required to decode thecorresponding data transmission, such as the modulation andcoding scheme (MCS) used, resources occupied by currentand future transmissions, etc. The physical channel used totransmit the SCI is called the Physical Sidelink Control Chan-nel (PSCCH), while the Physical Sidelink Shared Channel(PSSCH) carries the data component. In C-V2X, PSCCHand PSSCH are multiplexed in frequency, i.e. transmitted ondifferent frequency resources in the same sub-frame.

C-V2X sidelink Mode 3: In C-V2X sidelink mode 3, alloca-tion of resources for sidelink transmissions is handled by theeNodeB. Naturally, this mode is defined for scenarios whereeNodeB coverage is available. The C-V2X sidelink mode 3uses the following notable mechanisms.

• Semi-persistent scheduling: Like in LTE-Uu, eNodeBsupports semi-persistent scheduling for C-V2X mode 3.

• UE-report based scheduling: UEs can report their obser-vations on their radio environments to assist the eNodeBin sidelink resource allocation.

• Cross-carrier scheduling: If an operator has two or morecarriers at its disposal, the eNodeB can schedule re-sources on one of the carriers for sidelink transmissionsover the other carrier(s).

C-V2X sidelink Mode 4: UEs outside cellular coverage canuse C-V2X sidelink mode 4, whereby UEs reserve resourcesautonomously using the resource reservation algorithm. Thisresource reservation algorithm requires each UE to sense thechannel for 1 second and process the sensing results in orderto ensure that neighboring UEs pick and reserve orthogonal (intime, frequency or both) resources semi-persistently, therebyminimizing packet collisions. We refer the interested readerto [4] for more details on the resource reservation algorithm.

III. NEED FOR EVOLUTION

A. Performance of Existing Technologies

1) DSRC: Over the years, DSRC has been extensivelystudied using analytical models [11], extensive simulationstudies [12] and field trials [13]. It has been shown in [7] thatDSRC performance is satisfactory for most vehicular safetyapplications that require the end-to-end latency to be around100 msec as long as the density of vehicles is moderate. If,however, the vehicular density exceeds a certain limit, DSRCperformance rapidly deteriorates due to two major factors, (i)packet collisions due to simultaneous transmissions, and (ii)packet collisions due to hidden nodes. The poor scalabilityof DSRC is partially addressed by using congestion controlmechanisms such as those standardized in [14]. Such mecha-nisms typically involve the control of transmission parameterssuch as the transmission power or the message transmissionrate (in number of packets/second) or both.

2) C-V2X: Compared to DSRC, C-V2X is a newer andless-studied technology. Most studies that characterize C-V2Xperformance derive their results from simulation platforms.Reference [4] shows that the performance of C-V2X sidelinkmode 4 is superior to that of DSRC in terms of a higher

link budget, which is corroborated through experimental find-ings in [3]. Further, centralized control of resources in C-V2X sidelink mode 3 leads to an efficient utilization of thespectrum, thereby leading to better performance guaranteesas demonstrated in [15]. However, despite improvements overDSRC, when the traffic density increases, the performance ofC-V2X, too, drops rapidly [4], particularly for C-V2X mode4. The C-V2X mode 4 algorithm allows for frequency re-use over a given geographical area. When the traffic densityincreases, the re-use distance is reduced, resulting in anincreased interference level among C-V2X users.

B. Nature of Supported Applications

According to the results from past studies [4], [11] and theQoS requirements set for safety applications [16], DSRC andC-V2X are capable of supporting a basic set of vehicular safetyapplications that are based on issuing driver-alerts to indicatepotentially dangerous situations. Most of these applicationsrequire the delivery of periodic messages and have require-ments ranging from 1− 10 Hz peroidicity and 50− 100 msecend-to-end latency. Such applications are designed to aid thedriver in driving safely and efficiently. These applications arereferred to as day-1 applications due to the fact that V2X-capable vehicles are likely to support them before any of theadvanced use-cases discussed in the next sub-section.

C. Requirements of Advanced Vehicular Applications

One obvious need for evolution of both RATs is to improvethe reliability of existing use-cases while delivering packetsunder their latency budget. In addition to this, however, provi-sioning basic safety applications alone is unlikely to meet therequirements of self-driving autonomous cars. For example,while existing applications such as left turn assist and emer-gency electronic brake lights [17] are beneficial for vehiclesafety, autonomous vehicles will require vehicles to be capableof transmitting messages indicative of maneuver changes, tra-jectory alignments, platoon formations, sensor data exchange,etc. [5]. Besides, even for human-driven vehicles, processingof data received from sensors of surrounding vehicles—forexample, where one vehicle shares its live camera feed with avehicle behind it—is expected to increase the safety benefitswell beyond what can be achieved by basic safety applications.

Requirements of some advanced vehicular applications havebeen studied by the 3GPP in [5]. These advanced V2X use-cases not only improve road safety, but also assist in bettertraffic management and cater to the infotainment needs of pas-sengers. These applications fall under four broad categories:(i) vehicle platooning, (ii) advanced driving, (iii) extendedsensors, and (iv) remote driving. The QoS requirements ofthese applications are summarized in Table II.

As shown in Table II, the latency and reliability require-ments of these advanced V2X applications are much morestringent than those of basic safety applications. Furthermore,these advanced applications are characterized by the use oflarge and variable sized packets, and rely on messages thatare transmitted aperiodically. This is in stark contrast tothe applications that are based on the transmission of basic

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TABLE II: QoS requirements of advanced V2X applicationsUse Case Max. Payload Reliability Data Min.Group Latency Size (%) Rate Range

(msec) (Bytes) (Mbps) (meters)VehiclePlatooning

10 - 500 50 -6000

90 -99.99

50 - 65 80 - 350

AdvancedDriving

3 - 100 300 -12000

90 -99.999

10 - 50 360 -500

ExtendedSensors

3 - 100 1600 90 -99.999

10 -1000

50 -1000

RemoteDriving

5 - 99.999 UL: 25DL: 1

-

safety messages, which are transmitted periodically (typicallyonce every 100 msec). It is, therefore, clear that in orderto support such diverse and challenging V2X applications, amajor overhaul to the existing V2X technologies is necessary.

IV. IEEE 802.11BD: EVOLUTION OF IEEE 802.11P

A. Objectives

During the development of IEEE 802.11p the focus wasto develop a vehicular communication standard that assistedin (i) vehicular safety, (ii) better traffic management, and (iii)other applications that add value, such as parking and vehiculardiagnostics. The requirements set for 802.11p were to support:

• relative velocities up to 200 km/hr,• response times of around 100 msec, and• communication range of up to 1000 m.The 802.11p standard derived its PHY and MAC layers from

802.11a. Since then, however, 802.11a has given way to itssuccessors i.e., 802.11n and 802.11ac, while 802.11ax is inits final stages of standardization. Considering that 802.11pwas developed nearly two decades ago, advanced PHY andMAC techniques introduced in 802.11n/ac/ax can be leveragedto enhance 802.11p. With this objective, the IEEE 802.11Next Generation V2X Study Group was formed in March2018. After an initial feasibility study, the IEEE 802.11bdTask Group was created in January 2019. The primary designobjectives of 802.11bd include supporting the following [18]:

• at least one mode that achieves twice the MAC through-put of 802.11p with relative velocities up to 500 km/hr;

• at least one mode that achieves twice the communicationrange of 802.11p;

• at least one form of vehicle positioning in affiliation withV2X communications.

Additionally, 802.11bd must support the following [18]:• Interoperability: 802.11p devices must be able to decode

(at least one mode of) transmissions from 802.11bddevices, and vice-versa.

• Coexistence: 802.11bd must be able to detect 802.11ptransmissions and defer channel access, and vice-versa.

• Backward compatibility: At least one mode of 802.11bdmust be interoperable with 802.11p.

• Fairness: In co-channel scenarios, 802.11bd and 802.11pmust get equal channel access opportunities.

B. Mechanisms

1) Midambles: The 802.11 PHY layer is OFDM-basedwith 64 sub-carriers, typically with a sub-carrier spacing of

312.5 kHz. The PHY layer of 802.11p was derived directlyfrom that of 802.11a by reducing the sub-carrier spacing bya factor of two. For typical vehicle speeds, the 156.25 kHzsub-carrier spacing provided a trade-off between multi-pathfading and relative Doppler spread [19]. Thus, one approachto designing the PHY of 802.11bd is to use the 802.11ac PHYas a base-line and half the sub-carrier spacing (denoted as“2×down-clock”) so that the 64 802.11bd sub-carriers can fitin a 10 MHz channel. However, it has been shown in [20] thatthe 802.11ac PHY using 2 × down-clock with Low DensityParity Check (LDPC) coding, in fact, performs inferior to thatof 802.11p. This sub-par performance of the 802.11ac PHYis attributed to channel variations within the frame duration,resulting in the receiver’s inability to decode the frame.

To address the above issue, 802.11bd proposes to usemidambles, which are similar in form and function to thepreamble except their location within the frame. The preamble,which is at the beginning of the frame, is used for initialchannel estimation. However, for fast-varying channels, theinitial estimate may quickly become obsolete. Midambles,which will be introduced in between the OFDM data symbolswith appropriate frequency, will serve in channel trackingso that an accurate channel estimate is obtained for all datasymbols. Note that in C-V2X and NR V2X, a similar role isplayed by DMRS symbols (see Sec. II-B).

2) Re-transmissions: A mechanism to increase the relia-bility is to have one or more re-transmissions of a packet.Using the frame structure shown in Fig. 1, reliability gainscan be achieved for both 802.11p and 802.11bd devices. Notethat for 802.11p devices, the original transmission and its re-transmission(s) appear as independent packets, and the packetis received successfully as long as one of the packet receptionis successful. The initial transmission and its re-transmission(s)can either be sent within the same channel access opportunityor by using separate contention processes [21]. The TGbdproposes an adaptive re-transmission scheme, where decisionsto re-transmit a frame and the number of re-transmissions arebased on the congestion level [22]. A similar re-transmissionmechanism is used in C-V2X to boost its reliability.

Fig. 1: Frame format used for re-transmissions in 802.11bd.

3) Alternate OFDM Numerologies: An OFDM symbolcomprises of a cyclic prefix and the actual data symbol. TheOFDM efficiency, i.e. the ratio of useful symbol durationto the total symbol duration, increases as the sub-carrierspacing decreases since the cyclic prefix duration is invariantof the symbol duration. To increase the OFDM efficiency,TGbd members are exploring the use of narrower OFDMnumerologies (i.e. sub-carrier spacing) such that the numberof sub-carriers is increased while still occupying a 10 MHzchannel [23]. These options include 2 × down-clock with64 sub-carriers, 4 × down-clock with 128 sub-carriers, and8× down-clock with 256 sub-carriers. The design of alternateOFDM numerologies must, however, take the maximum rela-tive velocities into consideration. Channel variations across theframe duration can be estimated using midambles. However, if

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variations occur across an OFDM symbol, the resulting inter-carrier interference can be difficult to mitigate [23].

4) Dual Carrier Modulation: Dual Carrier Modulation(DCM) is a technique introduced in 802.11ax. DCM includestransmitting the same symbols twice over sufficiently far-apartsub-carriers such that frequency diversity is achieved [24].Because each symbol transmission is repeated over two differ-ent sub-carriers, the modulation order must be doubled (e.g.from BPSK to QPSK, or QPSK to 16-QAM) to maintain thethroughput. Despite the increase in modulation order, DCMcan help improve the block-error-rate (BLER) performance.

5) Other PHY & MAC Features: Other PHY layer featuresunder consideration for inclusion in 802.11bd include theuse of LDPC codes and multiple transmit/receive antennas toincrease the reliability using spatial diversity or increase thethroughput using spatial multiplexing [20], [25].

At the MAC layer, to ensure equal and fair channel accessopportunities for 802.11bd and 802.11p devices, 802.11bd willre-use 802.11p’s contention parameters for different EnhancedDistributed Channel Access categories [26].

6) mmWave Frequencies: mmWave frequency bands (i.e.,60 GHz and above) have enormous potential in catering touse-cases that require communication over small distances,but with a very high throughput (e.g., video streaming, down-loading high-resolution 3D maps, etc). mmWave bands areparticularly lucrative due to the abundance of spectrum therein,allowing very high throughputs even at lower order MCS [27].The basis for design of mmWave 802.11bd can be existing802.11 standards like 802.11ad, or its enhancement 802.11ay,which already operate in the mmWave bands [28]. However,one drawback of this frequency band is that its utility is limitedto use-cases that do not require a large communication range.

C. Challenges

1) Interoperability & Backward Compatibility: As de-scribed in Sec. IV-A, interoperability and backward com-patibility are two critical requirements that 802.11bd mustsatisfy. Given that 802.11p equipped vehicles are already onthe roads [9], without interoperability, 802.11bd (802.11p)devices will be able to communicate only with other 802.11bd(802.11p) devices, which is clearly undesirable. Interoper-ability and backward compatibility requirements place cer-tain constraints on the design of PHY and MAC layers of802.11bd. For example, multiple antenna schemes like spacetime block coding or the use of alternate waveforms violatesthe interoperability requirement [22].

Fig. 2 shows a frame format that can be used by 802.11bddevices to achieve interoperability between 802.11bd and802.11p devices [29]. The three legacy fields in Fig. 2, i.e.Legacy Short Training Field (L-STF), Legacy Long Train-ing Field (L-LTF) and Legacy Signal (SIG), along with the802.11p data field can be decoded by both 802.11p and802.11bd. The same payload, but now using 802.11bd featuressuch midambles, LDPC coding and higher order MCS (alongwith 802.11bd headers), is appended to the 802.11p data.During this extended duration, 802.11p devices will sense thechannel as busy and defer channel access, while 802.11bddevices can achieve a higher reliability owing to the use

of 802.11bd features and combining the two versions (i.e.802.11p and 802.11bd) of the transmitted payload. The ad-vantage of this packet structure is that its benefits are accruedwithout a need to change the higher layers – a fundamentalrequirement for a smooth transition from 802.11p to 802.11bd.

Fig. 2: Interoperability through appending 802.11bd data

Another frame format that achieves interoperability, whilealso increasing the reliability of 802.11bd transmissions isshown in Fig. 3a. The process of appending the parity bitsto and extracting them from the transmitted data is shownin Fig. 3b. The parity bits are generated by splitting the802.11p data into blocks and encoding them using errorcontrol schemes such as the Reed Solomon (RS) code [30](referred to as the outer code). The block comprising of802.11p data and the RS parity bits are then encoded usingthe legacy Binary Convolutional Coding (BCC) scheme andappended at the end of the block. These parity bits can beleveraged at 802.11bd devices to increase the probability ofsuccessful decoding, while the contention state machine oflegacy, i.e. 802.11p, devices ignore these parity bits [30]. Toensure that parity bits do not pass as valid data sequences, asingle byte is added to the parity bits to make sure that theframe check sequence mechanism fails [31].

(a) Concept

Legacy 802.11p

data

Legacy 802.11p

dataParity

802.11p Packet

Encoding using outer code (RS)

Parity

Legacy 802.11p

data

Legacy 802.11p

data

Parity802.11p PacketDecoding

for outer code (RS)

Parity

Encoding using inner code (BCC)

Decoding for inner

code (BCC)

CH

AN

NEL

(b) Implementation

Fig. 3: Interoperability through appending parity bits

2) Coexistence: 802.11bd also considers scenarios wherecoexistence between 802.11p and 802.11bd devices is desired.Coexistence differs from interoperability and backward com-patibility in that the former does not require 802.11p devicesto decode 802.11bd frames, but only to detect 802.11bdtransmissions as valid 802.11 frames and defer channel ac-cess. Coexistence is desirable when the transmitted messagescorrespond to 802.11bd-specific use cases. Using the frameformat shown in Fig. 4, 802.11bd devices transmit messagesthat are intended only for other 802.11bd (and not 802.11p)devices [24], while legacy devices will identify the channelas Busy and defer channel access after decoding the legacyfields, i.e. L-STF, L-LTF and SIG.

Fig. 4: Frame format for 802.11p–802.11bd coexistence.

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In a scenario where 802.11bd and 802.11p devices operateco-channel in the same geographical region, 802.11bd devicescan certainly transmit all frames using the 802.11p frameformat. However, there must be a mechanism for an 802.11bddevice to notify other 802.11bd devices about its capabilities.Otherwise, it is possible that even after there are no 802.11pdevices in the vicinity, 802.11bd devices continue to transmitusing the 802.11p frame format. Several options exist for an802.11bd to indicate its advanced capabilities. For example,the PHY or the MAC header can be used, with potentialinteroperability issues with the former option [32].

D. Performance Projections

Link-level simulation results for 802.11bd have been re-ported in [20], [21], [30], [33]. Using the re-transmissionscheme described in Sec. IV-B2 (see Fig. 1), 802.11bd devicescan experience a gain of 3 − 8 dB (at BLER of 10−1) bycombining the original transmission and its re-transmission(s),while the corresponding gain for 802.11p devices is 0.5 −1.7 dB (at BLER of 10−1) depending on the number of re-transmissions used [21]. On the other hand, using the parity-based interoperability mechanism described in Sec. IV-C1 (seeFig. 3), 802.11bd can benefit from a 1− 3 dB gain (at BLERof 10−1). Note, however, that for a given improvement inreliability of 802.11bd devices, the parity-based mechanismis more efficient because of the lower air-time utilization.

The DCM mechanism has been shown to provide gains of4, 0.6 and 2 dB at MCS 0, MCS 1 and MCS 2, respectively in802.11ax [34]. DCM can be used in 802.11bd in coexistencescenarios where 802.11bd devices need to communicate onlywith other 802.11bd devices. In terms of throughput improve-ments, insertion of midambles in between data symbols (seeSec. IV-B1) makes the use of higher order MCS feasible.This is illustrated in [33], where the throughput is shownto be doubled using midambles and LDPC coding over a20 MHz channel in the highway NLOS scenario. The doublingof throughput is observed only at large signal-to-noise ratio(SNR) values (> 20 dB), which can be readily achieved inscenarios where the inter-vehicle distance is small—a typicalscenario for high-throughput applications.

The promising performance gains obtained at a singletransmitter-receiver link, as discussed above, is expected totranslate into a better system level performance. However, withthe development of 802.11bd ongoing, the actual system-wideperformance gain has not been demonstrated.

E. Comparison with IEEE 802.11p

Table III summarizes the key differences between fea-tures/mechanisms of 802.11p and 802.11bd.

V. NEW RADIO (NR) V2X: EVOLUTION OF C-V2X

A. Objectives

The NR V2X Study Item [35] indicates that the designobjective of NR V2X is not to replace C-V2X, but to sup-plement C-V2X in supporting those use cases that cannot besupported by C-V2X. Because C-V2X is already standardized

TABLE III: Comparison of 802.11p and 802.11bdFeature 802.11p 802.11bdRadio bands of operation 5.9 GHz 5.9 GHz & 60 GHzChannel coding BCC LDPCRe-transmissions None Congestion dependentCountermeasures against None MidamblesDoppler shiftSub-carrier spacing 156.25 kHz 312.5 kHz, 156.25 kHz,

78.125 kHzSupported relative speeds 252 kmph 500 kmphSpatial Streams 1 Multiple

and commercial deployments are underway [36], it is likelythat C-V2X and NR V2X might coexist in the same geograph-ical region, where newer vehicles will have both C-V2X andNR V2X capabilities. Under such circumstances, use casesthat can be supported reliably by using C-V2X can use C-V2X procedures, while the remaining use cases can use NRV2X procedures [35]. However, to ensure that NR V2X canprovide a unified support for all V2X applications in the future,NR V2X must be capable of supporting not only advancedV2X applications, but also basic safety applications that aresupported by present-day C-V2X.

NR V2X is being designed to support V2X applicationsthat have varying degrees of latency, reliability and throughputrequirements. While some of these use cases require thetransmission of periodic traffic, a large number of NR V2Xuse cases are based on reliable delivery of aperiodic mes-sages. Furthermore, while some use cases require broadcasttransmissions, others such as vehicle platooning are efficientlysupported by transmission of messages only to a specificsub-set of vehicles (UEs). In some cases, in fact, 3GPPsees benefits in transmitting packets to only a single vehicle(UE) [5]. To support such use cases, two new communicationtypes, viz., unicast and groupcast, will be introduced in NRV2X. Like IEEE 802.11bd, NR V2X also considers the useof mmWave bands for V2X applications, particularly forapplications that require a short range and high to very highthroughputs. However, considering the limited timeline of Rel.16 (expected to be standardized by Dec. 2019), NR V2XmmWave operations are de-prioritized in its Study Item.

The NR V2X Study Item outlines its following objectives.• Enhanced sidelink design: Re-design sidelink procedures

in order to support advanced V2X applications.• Uu interface enhancements: Identify enhancements to the

NR Uu interface to support advanced V2X applications.• Uu interface based sidelink allocation/configuration:

Identify enhancements for configuration/allocation ofsidelink resources using the NR Uu interface.

• RAT/Interface selection: Study mechanisms to identifythe best interface (among LTE sidelink, NR sidelink, LTEUu and NR Uu) for a given V2X message transmission.

• QoS Management: Study solutions that meet the QoSrequirements of different radio interfaces.

• Coexistence: Feasibility study and technical solutions forcoexistence of C-V2X and NR V2X within a singledevice, also referred to as in-device coexistence.

B. Terminology1) NR V2X sidelink modes: Like C-V2X, NR V2X defines

two sidelink modes. The NR V2X sidelink mode 1 defines

7

Fig. 5: Communication types in NR V2X.

mechanisms that allow direct vehicular communications withingNodeB coverage. In this mode, the gNodeB allocates re-sources to the UEs. The NR V2X sidelink mode 2, on the otherhand, supports direct vehicular communications in the out-of-coverage scenario. For direct comparison with 802.11bd, in theremainder of this section, we keep our discussions restrictedto NR V2X sidelink mode 2.

2) Unicast, Groupcast and Broadcast: In NR V2X unicasttransmissions, the transmitting UE has a single receiver UEassociated with it. On the other hand, the groupcast mode isused when the transmitting UE wishes to communicate withmore than one, but only a specific sub-set of UEs in its vicinity.Finally, broadcast transmissions enable a UE to communicatewith all UEs within its transmission range. These communi-cation types are illustrated in Fig. 5. Note that C-V2X onlyprovisions support for broadcast transmissions. A single UEcan have multiple communication types active simultaneously.For example, a platoon leader UE can communicate with itsplatoon member UEs using the groupcast mode, while usingthe broadcast mode to transmit other periodic messages to UEsthat are not part of the platoon as shown in Fig. 5.C. Mechanisms

1) Use of NR Numerologies: Support for flexible numerolo-gies is a key feature introduced in 3GPP Rel. 15. As opposedto a fixed sub-carrier spacing used in LTE, NR supportsdifferent sub-carrier spacings, which are multiples of theLTE sub-carrier spacing, i.e. 15 kHz. Sub-carrier spacing of15, 30 and 60 kHz will be supported for sub-6 GHz NRV2X (i.e. Frequency Range 1, FR1), while 60 and 120 kHzwill be supported for frequency bands above 6 GHz (i.e.FR2) [37]. The use of higher sub-carrier spacings facilitateslatency reduction. Assuming each UE requires one slot for itstransmission, the transmission time of a UE decreases as thesub-carrier spacing increases. Note that the term “slot” and“sub-frame” hold different meanings in NR V2X. NR definesthe duration corresponding to 14 OFDM symbols as one slot,while a sub-frame has a fixed duration of 1 msec. Furthermore,due to smaller slot durations at higher sub-carrier spacings,variations within the slot will be smaller, thereby needingfewer DMRS symbols per slot for channel estimation.

2) Slot, Mini-slot and Multi-slot Scheduling: In LTE andC-V2X, the transmission time is tightly coupled to the sub-frame duration, i.e. all UEs always transmit for a duration of1 sub-frame (1 msec). However, if a UE has only a smallamount of data to send, which can be accommodated in lessthan 14 OFDM symbols, allocating the entire slot for itstransmission is resource inefficient. Second, whenever a packetarrives at a UE for transmission, the UE has to wait until

(a) Multiplexing in C-V2X (b) Multiplexing in NR V2X

Fig. 6: Multiplexing of PSCCH and PSSCH in C-V2X and NR V2X.

the beginning of the next slot to begin transmitting. Such aslot-based scheduling is supported by default in NR V2X.However, NR V2X will also support mini-slot scheduling,where UEs that have latency-critical messages to send canstart their transmissions at any of the 14 OFDM symbolsand can occupy any number of OFDM symbols within theslot. Furthermore, slot-aggregation, i.e. combining two or moreslots to form a multi-slot, will also be supported in NR V2X tocater to use-cases that require exchange of large-sized packets.

3) Multiplexing of PSCCH and PSSCH: In C-V2X, PSCCHand PSSCH are multiplexed in frequency (see Fig. 6a). Thedrawback of this approach is that a receiver must buffer themessage for the entire sub-frame and can decode the messageonly at the end of the sub-frame. This may prove to be inef-ficient in NR V2X due to tight latency constraints of certainmessages. To address this problem, PSCCH and PSSCH willbe multiplexed in time in NR V2X, i.e. the PSCCH will betransmitted first, followed by the transmission of PSSCH. Thisis illustrated in Fig. 6b, where the use of resources marked as“Idle/PSSCH” is still under consideration and can be left idleor used for the transmission of PSSCH [37].

4) Introduction of sidelink feedback channel: For unicastand groupcast communications, reliability can be improvedif the source UE can re-transmit the packet if the receptionfails at UE(s) during the initial transmission. Although C-V2Xprovides support for re-transmissions, these re-transmissionsare blind, i.e. the source UE, if configured, re-transmitswithout knowing if the initial transmission has been receivedby surrounding UEs. Such blind re-transmissions are resourceinefficient if the initial transmission is successful. Blind re-transmissions are also ineffective if more than two trans-missions are required for a given reliability requirement.Furthermore, if the source UE has access to the channel stateinformation at its destination UE, this can be leveraged toadapt transmission parameters such as the MCS. To facil-itate these two enhancements, i.e. enabling feedback-basedre-transmissions and channel state information acquisition,NR V2X will introduce a new feedback channel—PhysicalSidelink Feedback Channel (PSFCH) [38]. Selection of re-sources to use for PSFCH is still under study. However,in order to reduce the complexity associated with resourceselection, preliminary studies in 3GPP recommend that thetransmitter UE must notify the receiver UE about whichresource to use for transmission on PSFCH [39].

5) Other PHY layer enhancements: In addition to the abovefeatures, NR V2X will include many other enhancements at

8

the PHY layer, most of which are inherited from NR. Theseinclude the use of LDPC coding, higher order MCS including64-QAM, and a flexible number of DMRS symbols per slot.

6) Introduction of new sub-modes of NR sidelink mode 2:Unlike in C-V2X sidelink mode 4, where there were no sub-modes, 3GPP began with evaluating four sub-modes of NRV2X sidelink mode 2 [40]. These sub-modes are as follows:

• Mode 2 (a): Each UE autonomously selects its resources.This mode is similar to C-V2X sidelink mode 4.

• Mode 2 (b): UEs assist other UEs in performing resourceselection. The UE providing assistance can be the receiverUE, which can potentially notify the transmitting UE ofits preferred resources using the PSFCH.

• Mode 2 (c): In this sub-mode, UEs use pre-configuredsidelink grants to transmit their messages. This sub-modewill be facilitated through the design of two-dimensionaltime-frequency patterns such as those described in [41].

• Mode 2 (d): UEs select resources for other UEs.In subsequent 3GPP meetings, it has been agreed to

no longer support modes 2(b) and 2(c) as separate sub-modes [42]. UE assistance (i.e., mode 2(b)) can be used inmodes 2(a)/(d) improve the performance of resource selection.On the other hand, even though mode 2 (c) will not besupported as a separate sub-mode, the use of time-frequencyresource patterns in mode 2(a) is not precluded [42]. The de-sign of modes 2(a) and 2(d) present unprecedented challenges,which are discussed through the rest of this sub-section.

Design of mode 2 (a): In this sub-mode, the transmittingUE must select its resources in an autonomous fashion. TheUE can use any assistance information facilitated by sub-mode 2 (b). However, this is unlikely at least for broadcasttransmissions because gathering information from all receivingUEs will lead to a prohibitively high overhead.

The C-V2X sidelink mode 4 resource reservation algorithmleverages the periodicity and fixed-size assumption of basicsafety messages. Since this assumption is no longer validfor NR V2X use-cases in general, the resource selectionmechanism must be re-engineered. For aperiodic traffic, sincethe arrival of future packets cannot be inferred from sensingprevious transmissions from surrounding UEs, several 3GPPmembers propose the use of short-term sensing [43]. Aclassic example of short-term sensing is the Listen BeforeTalk protocol, such as what is used in Wi-Fi or in LicenseAssisted Access2. On the other hand, periodic traffic can besupported using long-term sensing similar to that used in C-V2X. Long term sensing involves sensing and analyzing thechannel occupation during the sensing window, and using thisinformation to select resources from the selection window.Some suggested enhancements to the C-V2X sidelink mode4 algorithm are as follows [43]:

• Make the duration of the sensing window flexible basedon vehicular mobility in contrast to the fixed 1 sec win-dow used in C-V2X mode 4. This is beneficial in high-mobility scenarios, where sensing results can quicklybecome obsolete.

2License Assisted Access is the unlicensed flavor of LTE developed by3GPP in its Rel. 14 for operations in the unlicensed frequency bands.

• It has been shown in [43] that by eliminating the RSSIaveraging procedure used in the C-V2X mode 4 resourcereservation algorithm (see [4] for details), the perfor-mance of long-term sensing can be improved.

The short-term and long-term sensing procedures can beused as stand-alone mechanisms in sub-mode 2 (a), or thetwo can be used as supplementary mechanisms. Reference [44]describes how long-term and short-term sensing can be usedin conjunction. If both long-term and short-term sensing areconfigured, then UEs perform sensing and resource exclusionover the sensing window and select a transmission resourcewithin the selection window. However, before transmitting, theUE must perform short-term sensing to detect the presence ofother signals on its selected resource. This is beneficial forscenarios where periodic and aperiodic traffic will share thesame resource pool(s). The use of short-term, long-term andcombined sensing is illustrated in Fig. 7.

Fig. 7: Short-term and long-term sensing in NR V2X.

Design of mode 2 (d): In this sub-mode, a UE performsresource allocation for a group of UEs in its vicinity. Thissub-mode is especially useful in platooning applications wherevehicles move along the same direction with small relativevelocities [45]. Furthermore, this sub-mode is likely to be usedin applications that require groupcast or unicast transmissions.The UE performing resource allocation for other UEs withinthe group is referred to as the scheduling UE (S-UE) [46].Because the S-UE can overlook the resource allocation ofUEs within its group, this mode is beneficial in significantlyreducing the number of collisions between group member UEs.

The mechanism to select a UE as the S-UE is still understudy. Some possible options include selection based on geo-location or pre-configuration. Geo-location based selectionof S-UE is beneficial in platooning applications, where thelength of platoon can be as long as 250 m [47]. In suchconditions, a vehicle at the center of the platoon is morelikely to have an accurate estimate of radio environments ofall vehicles in the platoon than vehicles at the front or backof the platoon. Pre-configuration based S-UE selection, onthe other hand, implies that certain vehicles with additionalhardware/processing capabilities can take on the responsibilityof resource allocation for surrounding vehicles. Regardless of

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the eventual mechanism used to select S-UE, the selectionprocedure will be performed at the application layer [42].

Despite its benefits, this sub-mode’s success will depend onthe efficiency of solutions to the following sub-problems:

• Which resources can the S-UE use while performingresource allocation for its member UEs? In particular,can the S-UE coexist in a resource pool where other UEsselect resources using mode 2 (a)?

• How to mitigate interference between neighboring UEsthat are assigned resources by different S-UEs?

• Once the S-UE performs resource allocation for itsmember UEs, how does it convey these allocations tothe member UEs, without prohibitively increasing thesignaling overhead?

D. Challenges

1) Coexistence of C-V2X and NR V2X: Vehicles equippedwith C-V2X are expected to hit the roads soon [36]. Consid-ering that vehicles typically have a life-span of one or moredecades [48], NR V2X is likely to have to coexist with C-V2X. However, NR V2X is not backward compatible withC-V2X [35]. This incompatibility stems from, among otherfactors, the use of multiple numerologies in NR V2X. A C-V2X device operating at 15 kHz sub-carrier spacing, cannotdecode messages transmitted using the 30 or 60 kHz spacing.Newer vehicles will, thus, be equipped with modules of bothtechnologies, i.e. C-V2X and NR V2X, making it imperativeto design effective coexistence mechanisms [35].

For C-V2X and NR V2X coexistence, the NR V2X studyitem [35] considers only the “not co-channel” scenario, i.e.a scenario where C-V2X and NR V2X operate in differentchannels. Two approaches can be used for such non co-channelcoexistence [49]: i) frequency division multiplexing (FDM), orii) time division multiplexing (TDM). Note that the term TDMis somewhat misleading in this context because not only areC-V2X and NR V2X resources orthogonal in time, but theyare also orthogonal in frequency.

FDM approach for coexistence: In this approach, transmis-sions on the two RAT can overlap in time. This approachis advantageous because there is no need for tight timesynchronization between the C-V2X and NR V2X modules.However, despite the use of two different radios (one for C-V2X and another for NR V2X), if the assigned channels areadjacent or are not sufficiently far apart, leakage due to out-of-band emissions from one radio terminal will impair thereception at the other radio terminal. Furthermore, if the twoRATs operate in the same band (e.g. 5.9 GHz ITS band),the total power radiated by the vehicle may be restricted byregulatory limits and may have to be split across the two RATs,affecting the QoS requirements of V2X applications.

TDM approach for coexistence: In this approach, transmis-sions on the two RATs occur in different channels and atdifferent time instants. The advantage of this approach is thatthe maximum permissible transmission power can be usedby both technologies because only one interface transmitsat any given time. Furthermore, there is no leakage acrosschannels. However, the TDM approach is disadvantageous for

latency critical use-cases as the NR V2X interface may be offwhile a latency sensitive packet is generated at the vehicle.Additionally, the TDM approach puts severe restrictions ontime synchronization between C-V2X and NR V2X [50].

2) Coexistence across communication types & periodicities:Different messages transmitted by the same UE using NR V2Xmay have very different QoS requirements. For example, theplatoon leader UE in Fig. 5 must transmit broadcast, groupcastand unicast messages. Furthermore, some of these messagesmay be periodic, while others aperiodic. Periodic broadcasttraffic can use the C-V2X sidelink mode 4 resource reservationalgorithm in the out-of-coverage scenario. However, other cat-egories of traffic (such as aperiodic unicast) may use differenttransmission mechanisms as discussed in Sec. V-C.

One approach to address the coexistence issue between di-verse traffic patterns is to use the pre-emption mechanism [45].Consider a scenario where periodic and aperiodic traffic sharethe same resource pool(s) while a high priority message arrivesat a UE and all resources within the packet’s latency budgetare reserved by low-priority periodic traffic. Using the pre-emption mechanism, the UE can use one of the resourcesthat was originally reserved by another UE for lower-prioritytraffic as illustrated in Fig. 8. The UE’s intent to pre-empt suchtraffic can be communicated through the use of a pre-emptionindication (PI) message—a message that can be sent on thesame resource pool or on a dedicated resource pool reservedfor transmitting PI messages.

Fig. 8: Pre-emption mechanism in NR V2X

Design of the pre-emption mechanism is, however, notdevoid of challenges. For example, frequent pre-emptions ofperiodic traffic may significantly affect the reliability of appli-cations that rely on such messages. A resource inefficient, yetsimple, alternative to pre-emption is to use separate resourcepools, one for each class of communication requirements [51].

E. Performance Projections

Although the development of mechanisms that will con-stitute NR V2X is ongoing, preliminary performance studieshave been conducted by some 3GPP members [52]–[54].Results in [54] indicate that large performance gains can beachieved using the 60 kHz sub-carrier spacing over the 15 kHzspacing used in C-V2X. These gains are more pronounced athigher relative velocities (280, 500 kmph). For example, at 500kmph, using 4 DMRS symbols/slot and QPSK modulation, acoding gain of 7 dB is achieved for a BLER of 10−2. Inorder to cover large distances, use of the 60 kHz sub-carrierspacing requires the extended cyclic prefix, adding to the

10

communication overhead. However, this can be compensatedfor by using fewer DMRS symbols/slot at 60 kHz sub-carrierspacing. As shown in [53], with the use of multiple-antennatechniques, reduction in the number of DMRS symbols/slotfrom 4 to 2 at 60 kHz sub-carrier spacing leads to practicallyno loss even at 500 kmph! Thus, by leveraging larger sub-carrier spacings made possible by the use of flexible NRnumerologies, NR V2X can significantly outperform C-V2X.

The superior link-level performance of NR V2X translatesto a superior system-level performance as shown in [52].Under highway scenarios, using the 60 kHz sub-carrierspacing and a 20 MHz channel, the packet delivery rate(PDR) is ∼ 99.7 − 99.8% for all communication types(i.e. broadcast, groupcast and broadcast) and message types(i.e. periodic and aperiodic). The packet generation ratefor periodic traffic is 10 Hz, while for aperiodic traffica packet is generated once every X msec, where X =(50 + an exponential random variable with mean 50) msec.This indicates that, at least in the highway scenario, NR V2Xis close to meeting some of the performance requirementsoutlined in Table II. However, in the urban scenario, whichis typically characterized by a higher density of vehiclesand large path losses, the performance of NR V2X variesin ∼ 93 − 97% range, thereby indicating that further en-hancements are required for reliable communications in ur-ban environments. Further, results presented in [52] are forrelatively low message transmission rates (∼ 10 Hz). Theperformance of NR V2X for more demanding applicationsremains to be seen. It must be noted that results shown in [52]–[54] do not account for all features described in Sec. V-C andwith the development and refinement of several mechanismsin NR V2X still underway, the gap between performancerequirements and achieved performance may become smallerby the time NR V2X is finalized.

F. Comparison with C-V2X

Table IV summarizes the key differences between fea-tures/mechanisms of C-V2X and NR V2X.

TABLE IV: Comparison of C-V2X and NR V2XFeature C-V2X NR V2XComm. types Broadcast Broadcast, Groupcast, UnicastMCS QPSK, 16-QAM QPSK, 16-QAM, 64-QAMWaveform SC-FDMA OFDMRe-transmissions Blind HARQPHY channels PSCCH, PSSCH PSCCH, PSSCH, PSFCHControl & datamultiplexing

FDM TDM

DMRS Four/sub-frame FlexibleSub-carrier spacing 15 kHz sub-6 GHz: 15, 30, 60 kHz

mmWave: 60, 120 kHzScheduling interval one sub-frame slot, mini-slot or multi-slotSidelink modes Modes 3 & 4 Modes 1 & 2Sidelink sub-modes N/A Modes 2(a), 2(d)

VI. SPECTRUM MANAGEMENT ISSUES

A. Interoperability of multiple V2X RATs

With DSRC and C-V2X as the two major developed RATs,and their corresponding evolutions—IEEE 802.11bd and NRV2X—underway, regional regulators and automakers have dif-ferent choices to provision V2X communications in vehicles.

While some automakers have committed to the use of DSRCin their vehicles [55], others prefer C-V2X [36]. At present,only DSRC is permitted to operate in the ITS band in US,while the 5G Automotive Association has requested a waiverto the US Federal Communications Commission (FCC) toallow C-V2X operations in the 5905-5925 MHz [56]. The USDepartment of Transportation subsequently released a requestfor comments [57] on questions regarding the choice of V2XRAT, their interoperability, and ability to support advancedV2X applications. Considering these factors, at least in theinitial years, it is plausible that multiple V2X technologies mayoperate simultaneously within a given geographical region.

C-V2X and DSRC are not compatible with each other. Thus,if some vehicles use DSRC and others use C-V2X, thesevehicles will be unable to communicate with each other—a scenario where the true potential of V2X communicationscannot be attained. In order to address this, some proposalssuch as [58] suggest that, at least within a given geograph-ical region, regulatory agencies must permit only one V2Xtechnology (either DSRC or C-V2X) to operate in a vehicle.

B. Coexistence with Wi-Fi

In the US and Europe, the 5.9 GHz band has been exploredfor Wi-Fi-like secondary operations. Because V2X applica-tions demand high reliability, unlicensed Wi-Fi operations canbe permitted only if they do not cause interference to theprimary V2X technologies. At the time regional regulatorsstarted studying the possible coexistence of Wi-Fi and V2Xtechnologies, DSRC was the only developed V2X technology.Therefore, solutions for secondary Wi-Fi operations in US andEurope [59], [60] have been developed considering DSRCas the default V2X technology. Besides such standardizedsolutions, the study on coexistence between DSRC and Wi-Fihas received considerable attention in the literature [61]–[63].A large number of such studies leverage the similarities in theMAC protocols of Wi-Fi and DSRC. Thus, by increasing theContention Window size and/or Inter-frame space of Wi-Fi,the priority of Wi-Fi transmissions can be reduced so that ifpackets are available at both DSRC and Wi-Fi transmitters,DSRC transmitters will gain access to the channel with ahigher probability. Since the MAC protocol of 802.11bd isexpected to be similar to that of 802.11p [26], it is possible thatcoexistence mechanisms developed for DSRC–Wi-Fi coexis-tence may also be suitable for 802.11bd–Wi-Fi coexistence.

In contrast to DSRC–Wi-Fi coexistence, the coexistencebetween C-V2X and Wi-Fi has not been well investigated. Thissubject has only been touched upon in [64]. It must be notedthat mechanisms developed for DSRC and Wi-Fi coexistencecannot be re-used for C-V2X and Wi-Fi coexistence becauseC-V2X uses a considerably different MAC protocol from Wi-Fi. If C-V2X is used to provision V2X applications, solutionsfor secondary Wi-Fi operations need to be re-designed withthe MAC protocol of C-V2X taken into consideration. Ideally,coexistence mechanisms must be agnostic to the choice ofV2X technology. However, considering the differences inMAC protocols of DSRC and C-V2X, a unified coexistencemechanism may be improbable. Nevertheless, mechanismsmust be at least forward compatible, i.e. any coexistence

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mechanism developed for C-V2X–Wi-Fi coexistence must alsobe compatible with NR V2X–Wi-Fi coexistence.

C. Interference from Adjacent Bands

The performance of the V2X RAT can be affected by out-of-band emissions from adjacent bands. In the US, at thelower end of the 5.9 GHz band is the Unlicensed NationalInformation Infrastructure (U-NII) 3 band, which is used byWi-Fi devices for provisioning wireless local area networks.Additionally, the FCC has released a Notice for ProposedRulemaking to consider unlicensed Wi-Fi operations in the6 GHz band [65], which lies at the upper end of the 5.9 GHzband. Furthermore, one of the two mechanisms consideredin the US for DSRC–Wi-Fi coexistence [59] proposes todivide the 5.9 GHz band into two sub-bands. The lower 40MHz will be used for vehicular non-safety applications, andcan, therefore, tolerate a slightly higher level of interference.Spectrum sharing between DSRC and Wi-Fi can be permittedin this sub-band. The upper 30 MHz, on the other hand, willbe used exclusively for vehicular safety applications and nospectrum sharing will be permitted in this sub-band. Thisproposal requires DSRC and Wi-Fi systems to operate inadjacent channels, without any guard band.

In each of the aforementioned scenarios, regardless of theV2X RAT used, if a Wi-Fi device operating in the adjacentchannel is located very close to the V2X receiver, the noisefloor of that receiver will be elevated and will inevitably leadto a loss in its performance. The performance loss dependson the frequency separation between the Wi-Fi and the ITSchannel. Consider for example a Wi-Fi network operating inchannel 1773. Fig. 9 shows the impact of adjacent channel Wi-Fi transmissions on the system-wide performance of C-V2Xmode 4 (operating in channels 180/182), where the PDR ofC-V2X is plotted against the distance between the C-V2Xtransmitter and receiver. The frequency separation betweenWi-Fi and C-V2X is 0 and 10 MHz for channels 180 and182, respectively. The simulated scenario is the Urban Fastscenario [10], where the Wi-Fi access point is located 10 maway from the C-V2X receiver at which the performance isobserved. C-V2X devices transmit basic safety messages at10 Hz, while the Wi-Fi access point has saturated downlinktraffic for its ten associated clients. In the simulated scenario,the 90% PDR range is reduced by 85 m and 65 m for channels182 and 180, respectively! Allocations of frequency bandsadjacent to the ITS band must take such negative effects ofadjacent channel operations into consideration.

VII. SUMMARY & CONCLUSIONS

The two evolutionary RATs—802.11bd and NR V2X—are a major overhaul of their respective predecessors andare expected to significantly improve on performance metricslike latency, reliability and the throughput. Some noteworthyfeatures of 802.11bd and NR V2X are outlined in Table V.

Even though the two RATs share some design objectives,their design principles are largely different. The TGbd is set

3The use of channel 177 will be permitted if the U-NII-4 re-channelizationproposal [59] is approved.

0 50 100 150

Distance between C-V2X Tx and Rx (m)

0

0.2

0.4

0.6

0.8

0.9

1

PD

R

Without Wi-Fi

Wi-Fi ch. 177, C-V2X ch. 180

Wi-Fi ch. 177, C-V2X ch. 182

Fig. 9: Impact of adjacent channel Wi-Fi on C-V2X mode 4 perfor-mance.

TABLE V: Comparison of IEEE 802.11bd & NR V2XFeature IEEE 802.11bd NR V2XBase Technology IEEE 802.11n/ac 5G NRPHY layer OFDM SC-FDMA, OFDMMAC layer CSMA Mode 1: gNodeB scheduling

Mode 2: Flexible sub-modesInteroperability Yes Non co-channelBackward compat-ibility

Co-channel Not backward compatible

mmWave support Yes Yes

out to re-design a two decade old technology—802.11p—while including all those enhancements that have made recentWi-Fi standards, i.e. 802.11n/ac, extremely popular in today’snetworks. While doing so, care is being taken to retainbackward compatibility with 802.11p and to make the newstandard suitable for extremely high mobility environments.The backward compatibility requirement for 802.11bd is crit-ical because, at least in a few countries, initial deploymentsof DSRC-equipped vehicles have already taken place.

On the other hand, building atop 5G NR, NR V2X canleverage many of the newer and advanced PHY and MAClayer techniques and features. NR V2X shows promising signsin terms of relative improvements over its predecessor technol-ogy as evident in discussions in Sec. IV-D and Sec. V-E. Oneof the reasons for the relatively high gains of NR V2X overC-V2X is that NR V2X starts out with a clean slate, withoutimposing any backward compatibility constraints on the newRAT despite the fact that C-V2X was standardized only in2017. However, lack of backward compatibility may not beas critical for NR V2X as it is for 802.11bd considering theaggressive time-line of 3GPP Rel. 16 standardization and thetime-line of adoption of C-V2X among auto-manufacturers.

Design of two parallel evolutionary RATs presents re-gional regulators and auto-manufacturers with two optionsto choose from, based on regional requirements. However,simultaneous adoption of the two evolutionary RATs (ortheir predecessors) within a geographical region can resultin challenging spectrum management issues and operationaldifficulties. Such spectrum management concerns need to bepro-actively resolved to exploit the maximum benefits of V2Xcommunication capabilities within vehicles.

REFERENCES

[1] US Department of Transportation, “Vehicle-to-vehicle communicationtechnology.” https://www.nhtsa.gov/sites/nhtsa.dot.gov/files/documents/v2v fact sheet 101414 v2a.pdf, July 2017.

[2] X. Wang, S. Mao, and M. X. Gong, “An Overview of 3GPP CellularVehicle-to-Everything Standards,” GetMobile: Mobile Computing andCommunications, vol. 21, no. 3, pp. 19–25, 2017.

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[3] 5GAA, “V2X Technology Benchmark Testing .”https://www.fcc.gov/ecfs/filing/109271050222769, Sept. 2018.

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