Exploring NR-V2X Dynamic Grant Limitations for Aperiodic Trafﬁc

Brian McCarthy and Aisling O’Driscoll

School of Computer Science and Information Technology, University College Cork, Cork, Ireland

Keywords:

Cellular V2X (C-V2X), LTE-V, Sidelink, NR-V2X, Aperiodic, CAM, DENM, Autonomous Resource

Selection, 4G, 5G, SPS.

Abstract:

The recent 3GPP NR-V2X standard (Rel. 16) has largely built upon its precursor Cellular-V2X (Rels. 14 &

15) but has introduced new approaches for dealing with application trafﬁc exhibiting aperiodic arrival rates

in the sidelink. This is vital as safety services based on ETSI Cooperative Awareness Messages (CAMs) and

Decentralised Environmental Notiﬁcation Messages (DENM) exhibit such characteristics. It is further en-

visaged that future vehicular services will also exhibit high aperiodicity to support increased autonomy. In

this paper we quantitatively evaluate the reasons why the Sensing based Semi-Peristent Scheduling (SB-SPS)

mechanism performs poorly when scheduling aperiodic trafﬁc. We then provide the ﬁrst in-depth evaluation

of the NR-V2X Dynamic Grant mechanism in contrast to schemes that parameterise the existing C-V2X SPS

algorithm and evaluate the performance of alternative dedicated scheduling mechanisms speciﬁcally designed

for aperiodicity. This paper highlights that the level of aperiodicity exhibited by the application model greatly

impacts scheduling performance, both for the default SB-SPS and dedicated approaches. As such we conclude

that a novel aperiodic scheduling mechanism must be devised, or more promisingly, an approach to enable ap-

plication trafﬁc to mimic periodic characteristics allowing it to co-exist with the existing scheduling approach.

1 INTRODUCTION

The Third Generation Partnership Project (3GPP) has

speciﬁed mobile standards to support V2X (vehicle to

everything) communications as an alternative to the

existing wireless standards based on IEEE 802.11p.

These standards, known as Release 14 (3GPP, 2016)

and Release 15 (3GPP, 2019) support vehicle to in-

frastructure/network (V2I/V2N) communications via

the traditional Uu interface but also allow for direct

communication between vehicles (V2V) via the PC5

sidelink interface. The radio resources necessary to

facilitate V2V communications can be selected and

managed by the cellular network (Mode 3) or selected

autonomously by the vehicles (Mode 4) using the dis-

tributed scheduling algorithm, Sensing Based Semi-

Persistent Scheduling (SB-SPS). The speciﬁcation in

Rel. 14 and Rel. 15, known as Cellular-V2X (C-V2X)

has acted as the basis for the New Radio (NR) speci-

ﬁcation in 3GPP Release 16 (3GPP, 2020) with Mode

4 forming the basis for NR-V2X Mode 2.

The original focus of the 3GPP Rel. 14 standard

was on trafﬁc safety and efﬁciency applications with

the assumption that safety messages (CAMs) would

be shared periodically between vehicles. This as-

sumption does not typically hold true as ETSI mes-

sage generation rules (ETSI, 2019a) specify CAM

transmission based on vehicle dynamics. Further-

more, the 3GPP has also speciﬁed enhanced V2X

(e-V2X) applications for connected and automated

vehicles including platooning, extended sensors, ad-

vanced and remote driving that will need to support

aperiodic application trafﬁc patterns.

However, the transmission of aperiodic applica-

tion trafﬁc has a large impact on the operation of

the SB-SPS MAC scheduling mechanism. It was de-

signed to assume periodic arrival rates thereby en-

abling accurate prediction of free radio resources.

Aperiodic trafﬁc which by it’s nature does not have a

strict pattern introduces signiﬁcant inconsistencies to

the historical sensing inhibiting this prediction. This

may result in C-V2X and its evolution NR-V2X being

unable to adequately meet application requirements,

thus rendering them unreliable. NR-V2X has intro-

duced a dynamic grant mechanism to deal with this

and additionally includes some changes to the SB-

SPS historical sensing mechanism. A detailed quan-

titative study is however missing from literature to

evaluate if this can adequately address high levels

of aperiodicity. As such an open research challenge

McCarthy, B. and O’Driscoll, A.

Exploring NR-V2X Dynamic Grant Limitations for Aperiodic Trafﬁc.

DOI: 10.5220/0010937500003191

In Proceedings of the 8th International Conference on Vehicle Technology and Intelligent Transport Systems (VEHITS 2022), pages 17-25

ISBN: 978-989-758-573-9; ISSN: 2184-495X

is to fully understand the cause of SB-SPS and dy-

namic grant performance degradation when dealing

with aperiodic application trafﬁc and to redesign the

C-V2X MAC scheduling algorithm to more reliably

support such aperiodicity, ideally in co-existence with

the current scheduling algorithm.

This paper provides an in-depth study to deter-

mine the precise reasons and conditions under which

the C-V2X and NR-V2X SB-SPS algorithms exhibit

signiﬁcant performance degradation when supporting

aperiodic application trafﬁc across a variety of vehic-

ular densities. Next, the paper investigates the perfor-

mance limitations of NR-V2X dynamic grant, high-

lighting its shortcomings as well as the conditions

under which it can function adequately. To address

the shortcomings, either a dedicated scheduling ap-

proach should be devised or a means of enabling ape-

riodic trafﬁc to mimic periodic characteristics to ﬁt

with the existing scheduling approach. Finally this

paper evaluates the viability of some alternative dedi-

cated scheduling approaches.

The main contributions of this paper are:

• An indepth quantitative study of the behaviour

of the C-V2X and NR-V2X SB-SPS algorithms

when faced with aperiodic application trafﬁc char-

acteristics.

• An evaluation of the effectiveness of the NR-

V2X dynamic grant mechanism in order to gain

deeper insight into how aperiodic application traf-

ﬁc, characteristic of future e-V2X 5G NR applica-

tions, can be better supported.

• A study of how dedicated aperiodic scheduling

mechanisms could perform and co-exist with de-

fault SB-SPS for hybrid application models.

The remainder of the paper is organised as fol-

lows. Section 2 describes the related literature with

Section 3 describing the operation of the PHY &

MAC layers of C-V2X and NR-V2X. Subsection 4.1

provides a detailed simulation study of the challenges

faced by SB-SPS in supporting aperiodic application

trafﬁc for an ETSI CAM application model. Subsec-

tion 4.2 highlights the shortcomings of the NR-V2X

dynamic grant mechanism in supporting fully aperi-

odic services, with Subsection 4.3 highlighting the

impact of considering hybrid periodic vs aperiodic ap-

plication arrival rates. Finally Subsection 4.4 shows

that dedicated aperiodic scheduling schemes can par-

tially address the highlighted shortcomings but not

for high levels of aperiodicity which will be likely in

practice. Section 5 concludes the paper with a sum-

mary of key ﬁndings and a discussion on the most

beneﬁcial future solution.

2 RELATED LITERATURE

Existing literature has not quantitatively evaluated

whether Rel. 16 NR-V2X, including dynamic grant,

can adequately support aperiodic trafﬁc, with evalua-

tions largely focusing on the Rel. 14 C-V2X SB-SPS

standard. Such a study is vital as ETSI CAM and

DENM transmissions are fully aperiodic and these

will form the basis for future e-V2X services to sup-

port increased autonomy.

Existing studies have stated that SB-SPS has

been designed to better facilitate periodic trafﬁc

(Bazzi, 2019; Gonzalez-Mart

ın et al., 2019). Molina-

Masegosa et. al (Molina-Masegosa et al., 2020)

conducted a study contrasting C-V2X Mode 4 with

802.11p for periodic and aperiodic application mod-

els, as well as variable packet sizes. They further

study adapting the resource reservation interval by

balancing re-selections against occurrences of wasted

resources using three strategies. Notably, the authors

suggest that the SB-SPS mechanism is fundamentally

counter-productive for aperiodic trafﬁc and highlight

that further dedicated schemes are required. They do

not consider grant breaking or dynamic grant in their

evaluation.

Similarly, Bartoletti et. al (Bartoletti et al., 2021)

also investigated the performance of C-V2X Mode

4 with grant breaking for aperiodic trafﬁc patterns,

highlighting its poor performance. The authors con-

clude that while wasteful of resources, disabling grant

breaking performs adequately when dealing with ape-

riodic trafﬁc. The same authors go on to investigate

SB-SPS for the NR-V2X Mode 2 standard in (Todisco

et al., 2021). The ﬁndings were interesting in that

they showed that the removal of RSSI ﬁltering and

RSRP averaging in Mode 2 detrimentally impacts SB-

SPS performance for aperiodic trafﬁc. Only the rein-

troduction of these features results in similar perfor-

mance to C-V2X Mode 4. As such they conclude us-

ing a static RRI within SB-SPS scheduling is not ef-

fective. Both studies do not investigate the NR-V2X

dynamic grant mechanism nor are dedicated aperiodic

scheduling schemes considered.

Romeo et. al (Romeo et al., 2020a) consider ape-

riodic trafﬁc in the form of Decentralized Environ-

mental Notiﬁcation Messages (DENMs), sent to alert

vehicles of hazardous road conditions. The authors

examine the impact of tuning SB-SPS parameters to

support aperiodic packets e.g. by reducing sensing

windows, selection windows and selection probability

(RSel) when providing CSRs to the MAC layer. Peri-

odic CAM transmissions with single DENM packets

are considered as opposed to a trafﬁc pattern/model

with aperiodic arrival rates. In a later study (Romeo

VEHITS 2022 - 8th International Conference on Vehicle Technology and Intelligent Transport Systems

et al., 2020b), DENM re-transmissions are considered

including the likelihood of two or more vehicles ar-

bitrarily choosing the same resource, with the sub-

sequent collision being maintained for the duration

of the grant, thereby impacting successful receipt of

the DENM. However, the authors consider a one time

reservation to mimic the Dynamic Grant mechanism

but this operates using the C-V2X Mode 4 SB-SPS

standard i.e. includes RSSI ﬁltering and RSRP aver-

aging. As such it is does accurately model NR-V2X

Mode 2.

Finally, Lusvarghi et. al (Lusvarghi and Merani,

2020) investigate a hybrid application model using

aperiodic and periodic trafﬁc. The authors do not

consider NR-V2X Mode 2 but rather examine C-V2X

Mode 4 performance. Their ﬁndings show that pe-

riodic trafﬁc is prioritised, resulting in poor perfor-

mance for aperiodic trafﬁc. This disparity is worsened

when large aperiodic packet sizes are considered e.g.

1000 Bytes, due to high subchannel occupation.

3 OPERATION OF THE C-V2X &

NR-V2X SIDELINK

The following sections describe the most important

aspects of the PHY and MAC layers of C-V2X Mode

4 and NR-V2X Mode 2.

3.1 C-V2X Physical Layer

The PHY layer of 3GPP Rel. 14 C-V2X implements

Single-Carrier Frequency Division Multiple Access

(SC-FDMA). In the time domain, resources are or-

ganised into subframes of 1 ms, which are further

grouped into frames of 10 ms. In the frequency do-

main, the channel is divided into subcarriers of 15

kHz. These subcarriers are grouped into Resource

Blocks (RBs), with each RB containing 12 subcarri-

ers and spanning over 1 subframe. Unlike the conven-

tional resource structure of LTE, C-V2X groups RBs

into subchannels. The number of RBs per subchannel

and the number of subchannels are conﬁgurable but

limited by the allocated bandwidth, which can be of

10 or 20 MHz.

Two physical channels exist in C-V2X; the Phys-

ical Sidelink Shared Channel (PSSCH) and Physi-

cal Sidelink Control Channel (PSCCH). The PSSCH

carries the application data, also known as Transport

Blocks (TBs). The PSCCH carries the Sidelink Con-

trol Information (SCI), which is critical for schedul-

ing and decoding. The SCI contains information such

as the Modulation and Coding Scheme (MCS) used to

transmit the packet, the frequency resource location of

the transmission, and other scheduling information.

The PSCCH and PSSCH can be transmitted us-

ing adjacent or non-adjacent schemes. In the adja-

cent scheme, the PSCCH and PSSCH are transmit-

ted in contiguous RBs. In the non-adjacent scheme,

the PSCCH and PSSCH are transmitted in different

RB pools. In terms of occupancy, the PSCCH re-

quires 2 RBs, while the number of RBs required by

the PSSCH is variable and depends on the size of the

TB. It is worth noting that the PSSCH and PSCCH

are always transmitted in the same subframe indepen-

dently of the transmission scheme.

3.2 C-V2X Medium Access Control

Layer

At the MAC layer, C-V2X implements SB-SPS

(Mode 4) to allow vehicles to select resources au-

tonomously. The process starts with the reception of

a packet from the upper layers. Upon reception, the

MAC layer creates a scheduling grant containing the

number of subchannels, the number of recurrent slots

for which the subchannels will be reserved, and the

periodicity between transmissions. If a grant has al-

ready been created at the time a packet is received

from the upper layers, the transmission is scheduled

using the existing grant. The number of subchannels

is pre-conﬁgured and depends on the application re-

quirements. The number of recurrent transmissions is

deﬁned by the Resource Reselection Counter (RRC),

which is set by randomly selecting an integer value

between 5 and 15. Finally, the periodicity between

transmissions is deﬁned by the Resource Reservation

Interval (RRI), whose value is set by upper layers.

Once the grant is created, it is then passed to the

PHY layer, which generates a list of all the subchan-

nels meeting the grant speciﬁcations. These subchan-

nels are known as Candidate Single-Subframe Re-

sources (CSRs) and consist of one or multiple sub-

channels in the same subframe. The list contains all

the CSRs within a selection window comprising the

period between the time the packet is received from

the upper layers and the maximum allowed latency.

The list is then ﬁltered using the information in

the SCIs received during a sensing window comprised

of the last 1000 subframes. Based on this informa-

tion, CSRs are excluded if the SCI indicates that it

will be reserved during the upcoming selection win-

dow and if the average PSSCH Reference Signal Re-

ceived Power (RSRP) of the CSR exceeds a prede-

ﬁned threshold. After excluding all CSRs that meet

these two conditions, at least 20% of all the CSRs

should remain available. Finally, the PHY selects the

Exploring NR-V2X Dynamic Grant Limitations for Aperiodic Trafﬁc

20% of CSRs with the lowest Sidelink Reference Sig-

nal Strength Indicator (RSSI) averaged over the sens-

ing window. This ensures the CSRs with the low-

est levels of interference are considered for selection.

The remaining CSRs are passed to the MAC layer,

where a single CSR is selected at random to reduce

the probability of multiple vehicles choosing the same

CSR. The CSR is selected for a number of recurrent

slots deﬁned by the RRC, whose value is decreased

by one after each transmission.

3.3 NR-V2X - Key Differences

Generally, 3GPP Rel. 16 NR-V2X Mode 2 main-

tains much the same approach as described for Mode

4. However some changes have been introduced at

the PHY layer. This includes ﬂexible numerology to

allow for reduced sub-carrier spacing enabling low

latency transmission. Additional channels have also

been speciﬁed to enable groupcast and unicast com-

munications which were not available for Mode 4.

This introduces a multi-stage SCI format to enable

these communication patterns. More information re-

lating to NR-V2X PHY changes can be found in (Gar-

cia et al., 2021).

At the MAC layer, we highlight only the changes

that are signiﬁcant with respect to impacting SB-SPS

scheduling performance. The ﬁrst relates to the RSRP

ﬁltering stage; rather than using an average RSRP

value for the CSR in the sensing history as is the case

in Mode 4, Mode 2 instead uses only the RSRP of the

most recent transmission when determining reserved

resources. The second key change is the outright re-

moval of the RSSI ﬁltering stage. As such, all CSRs

not removed at the RSRP ﬁltering stage will be re-

ported to the MAC layer for selection. These mod-

iﬁcations are important as shown in (Todisco et al.,

2021), as they signiﬁcantly impact the performance of

SB-SPS when handling aperiodic trafﬁc. Importantly,

a dynamic grant scheduling mechanism is deﬁned in

5G NR-V2X to deal with aperiodic trafﬁc patterns. It

operates by using the same underlying SB-SPS algo-

rithm but assuming a single transmission. The SCI

message for this transmission will highlight that the

resource will not be maintained in the future. This

does not apply when retransmissions are enabled, in

which case a grant is maintained for said retransmis-

sions. We do not consider retransmissions in this

study.

4 RESULTS

In this section, the causative effects of declined SB-

SPS performance when faced with aperiodic applica-

tion trafﬁc are quantitatively explored. We next in-

vestigate the limitations of the latest standardised ap-

proach to address this i.e. NR-V2X dynamic grants,

when application trafﬁc is highly aperiodic. Finally,

we show that application trafﬁc with hybrid periodic

and aperiodic characteristics can perform adequately,

motivating the need for a dedicated scheduling mech-

anism for highly aperiodic application trafﬁc or a

mechanism to allow aperiodic trafﬁc to mimic peri-

odic characteristics in order to minimise disruption to

SB-SPS scheduling.

Table 1: Simulation Parameters.

Parameter Value

Vehicular scenario

Vehicular density (β) 0.12 veh/m & 0.3

veh/m

Highway length 2000 m

Number of lanes 3/6 in each direction

(6/12 total)

Vehicle Speed 70km/h

Vehicle Mobility SUMO (step-length

= 1ms)

Channel settings

Carrier frequency 5.9 GHz

Channel bandwidth 10 MHz

No. subchannels 3

Subchannel size 16 Resource Blocks

MAC & PHY layer

Resource keep probability 0

RSRP threshold -126 dBm

RSSI threshold -90 dB

Propagation model Winner+ B1

MCS 6 (QPSK 0.5)

Transmission power (P

T x

) 23 dBm

Noise ﬁgure 9 dB

Shadowing variance 3 dB

Results are based on the OpenCV2X

simulator

(McCarthy and O’Driscoll, 2019) in conjunction with

SUMO

(Lopez et al., 2018), and Artery

(Riebl

et al., 2015). Table 1 describes the simulation pa-

rameters used. The vehicular scenario was chosen

to comply with recommended 3GPP C-V2X simula-

tion guidelines, speciﬁcally the ”Highway Slow” sce-

nario which has a density of 0.12 veh/m (3GPP, 2016)

http://www.cs.ucc.ie/cv2x/

https://sumo.dlr.de/docs/index.html

http://artery.v2x-research.eu/

VEHITS 2022 - 8th International Conference on Vehicle Technology and Intelligent Transport Systems

and adapted to the increased density of 0.3 veh/m.

Packet sizes of 190B and bounded latency require-

ments within 100ms remain constant throughout all

experiments. Application trafﬁc is modelled as fol-

lows:

• Periodic: Vehicles transmit at a mean rate of 4Hz.

This rate is deliberately chosen to allow for com-

parable analysis with the ETSI model with respect

to channel load. Half of all vehicles transmit ev-

ery 200ms with the remainder transmitting every

300ms, selection of 200ms or 300ms rate is done

at random for each simulation.

• Aperiodic (ETSI): Literature often assumes a pe-

riodic CAM transmission rate. This does not

align with the aperiodic transmission of CAMs ac-

cording to the ETSI speciﬁcation (ETSI, 2019a)

where CAMs are triggered based on a vehicle’s

dynamics i.e. deviation in heading (>4°), posi-

tion (>4m) and speed (>0.5m/s) or at 1s intervals

if these conditions are not satisﬁed.

4.1 SB-SPS Performance for Aperiodic

Trafﬁc

We now quantitatively explore why SB-SPS exhibits

declined performance when application trafﬁc follows

aperiodic arrival rates. Both C-V2X (Rel. 14) and

NR-V2X (Rel. 16) are considered. The distinction

between these, as described fully in Section 3.2, is

that NR-V2X removes the RSSI ﬁltering stage and

only uses the RSRP of the most recent transmission

when ﬁltering candidate resources.

Ultimately, the results show that both SB-SPS

standards exhibit declined performance when han-

dling aperiodic trafﬁc. This is due to inefﬁcient re-

source use as a consequence of inaccurate knowledge,

with the reasons differing depending on whether grant

breaking is enabled or not. This leads to a rise in colli-

sions and decreased packet delivery rates. Subsequent

results prove this to be the case.

Table 2: Absolute number of colliding grants.

Scheduling Mechanism γ

TSim

C-V2X (Periodic) 64

NR-V2X (Periodic) 20

C-V2X (ETSI GB) 4234

NR-V2X (ETSI GB) 1789

C-V2X (ETSI No-GB) 320

NR-V2X (ETSI No-GB) 86

Fig. 1 shows that SB-SPS with grant breaking (de-

noted as GB) performs worst for both NR-V2X and

C-V2X. A grant break occurs as no TB exists to send

Figure 1: SB-SPS performance of NR-V2X (Mode 2)

and C-V2X (Mode 4) for periodic and ETSI application

models.β = 0.12 veh/m. With and without grant breaking.

in a scheduled slot, thus the resource goes unused as

SB-SPS ends the grant when a missed transmission

occurs. When vehicles frequently break their grants,

they must schedule new resources. Each time re-

sources are reserved there is a probability that a neigh-

bouring vehicle will choose the same set of resources,

particularly if inaccuracies are introduced to the sens-

ing history. Furthermore, frequent grant breaking

leads to under-utilisation of reserved resources which

causes other vehicles to incorrectly consider increas-

ingly small and similar resource pools. This in-

creases the probability of vehicles choosing the same

resources, which can be exacerbated further as vehi-

cle density increases. This has a severe impact on C-

V2X Mode 4 due to it’s CSR selection mechanism

incorporating RSSI ﬁltering. In contrast, this stage is

removed for NR-V2X which results in a larger CSR

pool. This is shown by the γTSim metric in Table

2 which represents the number of instances where

grants collide i.e. where neighbouring vehicles select

the same subchannel in the same subframe for trans-

missions. It can be seen that C-V2X incurs a high

number of colliding grants. While NR-V2X incurs

less, for the reasons just outlined, it is still evident that

grant breaking is incompatible with aperiodic trafﬁc

patterns.

Fig. 1 also shows NR-V2X and CV2X SB-SPS

performance when grant breaking is disabled (de-

noted as No-GB). The grant is maintained irrespective

of a missed transmission. It can be seen that while

this greatly improves performance for C-V2X when

compared to grant breaking, it still comes at a cost.

Resources are reserved that may not be used. Further-

more these reserved but unused resources introduce

inconsistencies in the CSR selection process where a

vehicle does not send an SCI due to a missed trans-

mission and neighbouring vehicles mistakenly believe

Exploring NR-V2X Dynamic Grant Limitations for Aperiodic Trafﬁc

the resource to be free for use. This can lead to col-

lisions on future transmissions. What is particularly

noteworthy is that NR-V2X performance when grant

breaking is disabled is worse than the performance of

C-V2X. This indicates that when the grant is main-

tained, RSSI ﬁltering can be a positive feature by dis-

couraging the selection of resources that exhibit high

levels of interference. Furthermore, RSRP averaging

can result in more accurate sensing history.

Irrespective of whether grant breaking is enabled

or not, it has been shown that the performance of SB-

SPS for NR-V2X and C-V2X becomes compromised

due to inefﬁcient resource scheduling. This leads to

increased packet collisions as shown in Fig. 2.

Figure 2: Packet collisions for periodic and ETSI applica-

tion models. Vehicle density of 0.12 veh/m is considered.

4.2 NR-V2X Scheduling for Aperiodic

Trafﬁc

Having established why SB-SPS does not perform

well for aperiodic ETSI trafﬁc, we next investigate

whether the NR-V2X Dynamic Grant mechanism

proposed in Release 16 adequately addresses these

limitations.

Fig. 3 shows the performance of dynamic grant

versus NR-V2X SB-SPS for the ETSI aperiodic

model. Disabled grant breaking is assumed. It is ev-

ident that at the lower density of 0.12 veh/m, the dy-

namic grant mechanism can marginally out-perform

SB-SPS. However when the density increases to 0.3

veh/m, dynamic grant under performs SB-SPS. This

is due to reliance on the SB-SPS sensing window. In

the case of fully aperiodic application trafﬁc such as

ETSI, all TBs are scheduled using dynamic grant. As

such all sensing history is irrelevant as no ﬁltering will

be done in the selection window. Ultimately this re-

sults in dynamic grant operating as a simple random

selection mechanism where any resource in the selec-

tion window is equally likely to be selected. As such

Figure 3: NR-V2X dynamic grant performance for fully

aperiodic applications trafﬁc. Vehicle densities of β=0.12

veh/m and β=0.3 veh/m are considered.

dynamic grant performs ineffectively in a fully aperi-

odic scenario.

This is signiﬁcant as ETSI and DENM messages

which are fully aperiodic (ignoring retransmissions)

are the basis for cooperative vehicular services, and it

is widely envisaged that the messages associated with

increased vehicle autonomy and cooperative sensing

will also demonstrate highly aperiodic arrival rates.

Given this, it highlights the need to either develop a

dedicated scheduling mechanism to better handle traf-

ﬁc with aperiodic arrival rates or alternatively to en-

sure that application trafﬁc mimics periodic arrivals in

order for dynamic grant to function more effectively.

4.3 NR-V2X Scheduling for Hybrid

Trafﬁc

In the previous section, it was shown that the perfor-

mance of NR-V2X dynamic grants is poor for fully

aperiodic applications, particularly at higher densi-

ties. In this section, it is investigated whether dy-

namic grant can demonstrate improved performance

if a proportion of the application trafﬁc exhibits peri-

odic characteristics, thereby leading to a more accu-

rate SB-SPS sensing window.

Fig. 4a considers 3 hybrid application models:

• (90:10): 90% of the trafﬁc follows an aperi-

odic arrival rate in line with ETSI vehicle dynam-

ics with the remaining 10% representing periodic

trafﬁc with a 10Hz transmission rate.

• (50:50): This application model generates a 50:50

aperiodic/periodic arrival pattern.

• (10:90): 10% of trafﬁc follows an aperiodic ar-

rival rate as per ETSI, with 90% arriving periodi-

cally.

VEHITS 2022 - 8th International Conference on Vehicle Technology and Intelligent Transport Systems

(a) NR-V2X PDR for SB-SPS & dynamic grant.

(b) NR-V2X CBR for SB-SPS & dynamic grant.

Figure 4: NR-V2X performance when considering hybrid

aperiodic:periodic application models.

It is clear in Fig. 4a that the performance of dy-

namic grant for hybrid application models is actu-

ally worse than the performance of NR-V2X SB-SPS

when assuming disabled grant breaking. There is a

5% decline in performance at almost all ranges for

ratios up to 50%. Fig. 4b highlights a noteworthy

aspect regarding performance; while CBR increases

by over 10% as the models incorporate more periodic

arrival rates, PDR performance doesn’t change signif-

icantly. Indeed, the PDR performance is best for the

highest CBR when the ratio is 90% periodic. Thus

increasing the amount of periodic application trafﬁc

improves the performance of both these scheduling

mechanisms as it reduces the overall number of col-

lisions and more effectively utilises the SB-SPS sens-

ing history. For the hybrid model with the highest

level of periodic characteristics (10:90), the difference

in PDR between the NR-V2X SB-SPS and Dynamic

Grant is negligible.

Ultimately it can be concluded that NR-V2X dy-

namic grant is not adequate when scheduling highly

aperiodic trafﬁc patterns. This motivates the study

in the next sections, where alternative dedicated ape-

riodic schemes are implemented and quantitatively

evaluated.

4.4 Alternative Dedicated Scheduling

Mechanisms for Aperiodic Trafﬁc

The results thus far have highlighted the limitations of

NR-V2X dynamic grant in effectively accommodat-

ing fully aperiodic application trafﬁc. One approach

to address this would be to devise an entirely separate

scheduling mechanism, independent of SB-SPS. Two

mechanisms are next considered that were proposed

as part of 3GPP working groups prior to Rel. 16.

These have been incorporated into OpenCV2X but to

the best of our knowledge have not been thoroughly

quantitatively evaluated in literature. Their operation

is brieﬂy described:

• Short-term Reservations (Labeled STR) (Eric-

sson, 2019): Proposed by Ericsson, short-term

reservations also referred to as short term sens-

ing, makes use of two selection windows. It works

by sending a reservation signal in the ﬁrst selec-

tion window, which reserves resources in the sec-

ond selection window on which to transmit the

SCI and TB pair. In this mechanism, the resource

on which to send the reservation signal is chosen

randomly, discounting resources reserved for SCI

and TB pairs. Until the reservation signal is sent,

the vehicle continues to listen in case the reserva-

tion slot becomes reserved by another vehicle. At

the time of sending the reservation signal, a future

free resource is selected if it has not already been

reserved in the second sensing window. This re-

duces contention for the transmission of SCI and

TB pairs.

• Counter based Mechanism (LG-Electronics,

2019): This approach makes use of a simple

counter to increase the randomness of the resource

selection process. Upon receiving an application

layer packet, a counter is randomly selected be-

tween {1, 40}. In each subframe, the counter is

decremented by the number of free subchannels.

Once the counter reaches 0, a free subchannel is

chosen at random in the next available subframe

where the SCI and TB pair will be transmitted.

It is clear from Fig. 5 that counter demonstrates

negligible improvement over NR-V2X dynamic grant

as it is essentially a random scheduling mechanism.

STR demonstrates improved PDR but comes at the

cost of signiﬁcantly higher CBR as shown in Fig.

5b. This is attributable to the extra control over-

head generated by reservation signals before SCIs

and TBs are transmitted. This limitation would re-

sult in even worse performance at higher densities as

the reservation signals would become a cause of col-

lisions. While STR has limited for applicability for

Exploring NR-V2X Dynamic Grant Limitations for Aperiodic Trafﬁc

(a) NR-V2X PDR when for SB-SPS, dynamic grant & ded-

icated aperiodic scheduling.

(b) NR-V2X CBR for SB-SPS, dynamic grant & dedicated

aperiodic scheduling.

Figure 5: Dedicated aperiodic scheduling mechanisms.

ETSI CAMs, this approach could be applicable for

high priority aperiodic trafﬁc or trafﬁc only generated

sporadically such as DENMs.

5 CONCLUSIONS & DISCUSSION

This paper provides an in-depth study on the limi-

tations of both C-V2X and NR-V2X SB-SPS algo-

rithms in reliably supporting application trafﬁc with

aperiodic characteristics. It further provides im-

portant insights into the limitations of the NR-V2X

dynamic grant scheduling mechanism, and is the

ﬁrst paper to do so while considering the Rel. 16

MAC with removed RSSI ﬁltering and RSRP aver-

aging. Ultimately it ﬁnds that neither approach is ad-

equate for dealing with aperiodic trafﬁc, either per-

forming poorly or introducing inconsistencies into

their scheduling decisions. Signiﬁcantly, the perfor-

mance of the NR-V2X standard under-performs C-

V2X when dealing with fully aperiodic arrival rates

due to the removal of RSSI ﬁltering and RSRP aver-

aging. Furthermore, dynamic grant performance has

also been shown to be inadequate resembling random

scheduling. It is shown that to perform adequately,

dynamic grant must have a hybrid of periodic and ape-

riodic trafﬁc to allow for accurate resource ﬁltering.

It should be noted that dynamic grant performance

may improve with retransmissions although overhead

may be prohibitive. Alternative dedicated aperiodic

scheduling mechanisms were also explored but have

their own drawbacks such as increased channel usage,

which can limit their applicability.

Aperiodic trafﬁc patterns are of vital importance

as current and future communication patterns are and

will continue to be aperiodic in nature e.g. cooper-

ative awareness services based on CAMs, DENMs

(ETSI, 2019b) and Cooperative Perception (ETSI,

2019c). These services will underpin vehicular com-

munications and the current standards are not ade-

quate to effectively manage these patterns. It is our

conclusion that new approaches will need to be im-

plemented to address this. This can be through the

design of more effective dedicated aperiodic schedul-

ing mechanisms which either work in conjunction

with existing SB-SPS scheduling or as a standalone

approach. Alternatively, and more promisingly, an

approach that provides higher layer trafﬁc shaping

or more intelligent dynamic SB-SPS RRI selection

would ensure that aperiodic trafﬁc is scheduled sim-

ilarly to periodic trafﬁc, thus eliminating wasted re-

sources and inconsistent sensing history. The authors

in (Yoon and Kim, 2021) sought to address this, how-

ever the solution is based on an empirical data-set

and cannot be generalised to predict application layer

packet arrival. Such an approach would be effective

for services such as cooperative awareness and per-

ception which are frequently transmitted and based on

vehicle/object dynamics that can be predicted, thus al-

lowing SB-SPS to dynamically match this rate. Such

an approach would allow frequently aperiodic traf-

ﬁc that forms the backbone of vehicular communica-

tions to mimic periodic arrival rates. Adopting such a

mechanism would allow the NR-V2X dynamic grant

to only be used for low frequency aperiodic trafﬁc

such as DENMs. This was shown in Fig. 4a to per-

form reliably. An investigation into the feasibility and

performance of such a mechanism will form part of

future work.

VEHITS 2022 - 8th International Conference on Vehicle Technology and Intelligent Transport Systems

ACKNOWLEDGEMENTS

This publication has emanated from research [ con-

ducted with the ﬁnancial support of Science Foun-

dation Ireland under Grant number 17/RC-PhD/3479.

For the purpose of Open Access, the author has ap-

plied a CC BY public copyright licence to any Author

Accepted Manuscript version arising from this sub-

mission.

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