Integration Challenges of Facilities-Layer DCC for Heterogeneous V2X Services.pdf

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Integration Challenges of Facilities-Layer DCC for

Heterogeneous V2X Services

I. Khan

†

, J. Härri

†

EURECOM, 450 route des Chappes, 06904 Sophia-Antipolis, France

E-mails:{khanm, haerri}@eurecom.fr

Abstract—Decentralized Congestion Control (DCC) for

802.11p based inter-vehicular communication (V2X) is a critical

mechanism for distributed wireless resource allocation of future

connected intelligent vehicles. Studies so far mostly focused on

optimizing resources for a single Cooperative Awareness service,

whereas future connected intelligent vehicles will be based on

multiple heterogeneous new V2X services. In this paper, we

present a Facilities-layer DCC, currently being standardized in

Europe, capable of handling heterogeneous V2X services and

evaluate its integration impact with legacy DCC mechanisms.

We ﬁrst emphasize signiﬁcant wireless resource under-utilizations

and application performance degradations stemming from con-

ﬂicting decisions between legacy and Facilities-layer DCC. We

then show the capability of the DCC mechanism purely based at

service layer and illustrate its ﬂexibility for agile wireless resource

allocations between V2X services for intelligent vehicles.

I. I

NTRODUCTION

Automated Driving and Platooning are paramount examples

of beneﬁts of future cooperative intelligent vehicles to safe

and smart transportation. To reach that goal, intelligent vehi-

cles must not only acquire environmental awareness through

their advance embedded sensors, but also share it with other

vehicles via V2X communication to increase their awareness

horizon. Depending on the scope and scale, the required

data will be exchanged via heterogeneous V2X services, such

as Cooperative Awareness (CA), Collective Perception (CP),

Position & Time (POTI), or Local Dynamic Map (LDM). The

reliability of these services will depend on the dependability

of the underlying V2X communication technologies.

Whether cellular or WiFi, V2X communication technologies

require vehicles to take autonomous communication decision

and as such operate in ad-hoc mode. In particular, spatio-

temporal channel resources must be regulated among various

V2X services in a fully decentralized way to guarantee reliable

and fair V2X communication conditions. This process is called

Decentralized Congestion Control (DCC) in Europe, and many

DCC mechanisms have been proposed [1]–[4], while some

have been standardized [5]–[7].

In Europe, ETSI is in charge of V2X communication

standards for Cooperative Intelligent Transport System (C-

ITS). In 2016, it completed a full set of speciﬁcation for

the ﬁrst generation of C-ITS applications. The vast majority

is based on a single service called Cooperative Awareness

and a single message called Cooperative Awareness Message

(CAM). Accordingly, DCC has been optimized mostly for

this message only. While ETSI currently moves towards the

second generation of C-ITS applications, such as Cooperative

Adaptive Cruise Control (CACC), Platooning, or Safety of

Vulnerable Road Users (VRU), DCC will need to deal with a

larger set of V2X services and messages.

In this paper, we introduce a recent ETSI DCC proposal

[7] designed to support heterogeneous V2X Services. Located

at the ETSI Facilities layer [8], it allows V2X services to

directly request wireless resources, whereas the legacy ETSI

Access Layer DCC only blocks trafﬁc based on WiFi trafﬁc

priority. Although providing more agile tuning of the required

resources per V2X service, the interaction of Facilities DCC

with Access DCC can be problematic, creating potential

conﬂicts between legacy and Facilities DCC. Our contribu-

tions are three folds: (i) we introduce the new ETSI cross-

layer DCC architecture; (ii) we describe the Facilities-DCC

mechanism, notably its key mechanisms to share wireless

resources among V2X services; (iii) we ﬁnally integrate and

evaluate the Facilities DCC with and without the Legacy DCC.

We show via simulations on the iTETRIS platform [9] that

Legacy and Facilities DCC mechanisms strongly interfere,

severely degrading V2X services while under-utilizing the

available wireless resources. We also illustrate that DCC at

Facilities layer alone performs better and is sufﬁcient by itself

to efﬁciently regulate V2X communications.

The rest of the paper is organized as follows: Section II

gives a brief overview of the DCC mechanism, followed by

Section III outlining the evolution of V2X services for C-

ITS. Section IV introduces the Facilities DCC, illustrating the

integration issues with Access DCC, followed by Section V

providing performance evaluation results. Finally, Section VI

concludes the paper.

II. D

ECENTRALIZED CONGESTION CONTROL - OVERVIEW

Operating in ad-hoc mode, V2X communication technolo-

gies leave each node autonomously contend for channel ac-

cess. Uncoordinated, such contention-based channel access

may lead to severe packet collisions or channel resource

exhaustions by potential selﬁsh nodes. Moreover, considering

that the CA service relies on broadcast transmissions only,

collisions cannot be corrected. Therefore if individual trans-

missions are not regulated, collisions rapidly increase with the

number of neighbors, creating scalability concerns.

Cyber

Physical

measuringchannel

adjustingTXpolicies

Fig. 1: DCC as Cyber-Physical System

2018 IEEE Intelligent Vehicles Symposium (IV)

Changshu, Suzhou, China, June 26-30, 2018

V2V

Cooperative

Awareness (Day 1)

Collective

Perce ption

Local Dynamic Map

Exchange

Trajectory

Inten t

Accurate Position

and Time

Platoon Control

TCU

OBU

LTE

TCU

OBU

LTE

Fig. 2: Heterogeneous Services for Day 2

In order to solve this problem, DCC protocols have been

developed to limit the transmit parameters, mainly transmit

rate and power, of each vehicle based on channel condition.

Rate control sets the maximum number of transmissions

allowed in a given period to limit the temporal utilization of

channel, while power control sets the maximum power to limit

the spatial channel utilization and optimize spatial reuse of

wireless resources.

Conceptually speaking, DCC may be seen as a Cyber-

Physical System (CPS), as shown on Fig 1, where transmit

decisions are optimized based on a feedback loop from mea-

sured channel conditions. The physical block in each node

continuously senses the Channel Load (CL) conditions, via

metrics such as Channel Busy Ratio (CBR). Based on the

sensing metric from the physical block, the control algorithm

in the cyber block adjusts its control parameters, i.e. trans-

mit rate, transmit power, modulation or any other parameter

inﬂuencing the metrics from the physical block.

DCC has been extensively studied in the literature and

many protocols have been proposed for congestion control

and resource allocations. Several proposals focused on trans-

mission rate optimizations [1], [2] while keeping the transmit

power constant. Other strategies such as [3], [4] have proposed

adapting the transport power for better spatial channel usage.

Yet other works [10] have proposed hybrid adaptations of both

rate and power. There are other approaches using different

control parameters such as optimizing the data rate [11]

or the physical carrier sense threshold [12] based on the

channel quality. Several studies [13], [14] have questioned

the effectiveness of combining multiple control parameters

for congestion control, as made available by the standards.

A survey of DCC mechanisms is presented in [15].

Nevertheless, almost all existing works, except a few (e.g.

[16]), deal with a single type of message i.e. CAM, when

analyzing DCC strategies. The work in [16] highlights the

problem of Access DCC when dealing with multiple types of

packets, without considering Facilities DCC. In this paper, we

implement Facilities DCC for managing resource allocation of

multiple services and illustrate how Access DCC may hinder

the functioning of Facilities DCC.

Several DCC protocols have been standardized for Day 1, by

the Car2Car Consortium and ETSI in Europe and by Society of

Automotive Engineers (SAE) [17] in the USA. The approach

to the US DCC is cross-layer, which considers multiple

sensing parameters, such as vehicular trafﬁc density, packet

error rate, neighbor tracking error. Until recently, the EU DCC

has been mainly limited to the Access Layer. Although Access

DCC would sufﬁce for Day 1 considering a single message,

unable to differentiate between services, it is not suitable for

multiple messages and V2X services for Day 2.

III. V2X S

ERVICES FOR CONNECTED COOPERATIVE

AUTOMATED VEHICLES

Safety and trafﬁc efﬁciency Day 1 applications in Europe

are based on periodic CAM and occasionally event trig-

gered messages called Decentralized Environment Notiﬁcation

Message (DENM). However as shown on Fig. 2, there will

be multiple heterogeneous messages for Day 2 applications,

realizing a concept called ‘extended horizon’, where vehicles

gather information outside the range of their built-in sensors

through cooperative V2X communications. The conjunction of

the various V2X services and messages are critical for creating

such ‘extended horizon’ and allows future automated vehicles

to take optimal control decisions.

Accordingly, several new services are currently being de-

veloped in Europe, which require new messages such as:

• Collective Perception Message (CPM) - ETSI TS 103

324: shares a vehicle’s various sensor information with

other ITS stations.

• Position and Time Message (POTI) - ETSI TS 102

890–2: obtains precise position and time from other ITS

stations.

• Local Dynamic Map (LDM) messages - exchanges of

of the LDM [18] with other ITS stations.

Further down the road, communication capabilities will be

used for cooperative driving and navigation, and it is expected

that further messages will be developed to exchange a vehicle’s

‘trajectory intent’ (i.e. for vehicles to negotiate and coordinate

their actions).

Accordingly, plethora of V2X services will have hetero-

geneous packet size, periodicity, urgency, or relevance area.

Although existing congestion control mechanisms at Access

layer may regulate cooperative services, without considering

the heterogeneous message characteristics, new services will

be penalized. Access DCC strategies can only drop or delay

packets via queuing and ﬂow control (more details in the

next section). However Day 2 scenarios will require smarter

strategies to distribute the sparse network resources or transmit

opportunities among multiple applications, such as optimiz-

ing modulation, packet size, or prioritizing information as a

function of the application’s needs and context. Therefore,

there is a need to regulate wireless channel resources for

heterogeneous V2X services, which we analyze in the rest

of this paper.

IV. A

NALYSIS OF ACCESS AND FACILITIES DCC FOR

HETEROGENEOUS SERVICES

In European DCC standards, Transmit Rate Control (TRC)

has been the most signiﬁcant control mechanism. TRC can

either limit the number of packets released into the medium

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