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Augmenting Vehicle-to-Roadside connectivity in multi-channel vehicular

Ad Hoc Networks

C. Campolo a,n, H.A. Cozzetti b, A. Molinaro a, R. Scopigno b

a Universita’ Mediterranea di Reggio Calabria, DIMET (Dipartimento di Informatica, Matematica, Elettronica e Trasporti), Italyb BWA Lab (Broadband Wireless Access), Istituto Superiore Mario Boella, Turin, Italy

a r t i c l e i n f o

Article history:

Received 14 July 2011Received in revised form

20 December 2011

Accepted 3 April 2012

Keywords:

Connectivity

IEEE 802.11p

Multichannel organization

Urban environment

Network advertisements

Vehicular networks

WAVE

a b s t r a c t

Vehicle-to-Roadside (V2R) wireless communication is a cornerstone for providing a wide plethora of

intelligent transportation system (ITS) applications in the near future. Initial investment costs coulddiscourage the deployment of a ubiquitous roadside infrastructure to support on-the-road networks;

this would imply discontinuous coverage and short-lived connectivity.

The purpose of this paper is to design techniques that make the best of sparse road-side unit (RSU)

placement by supporting the spreading of network initialization advertisements from RSUs, when

considering the multichannel features of the recently published IEEE 802.11p/IEEE 1609.4 standards for

wireless access in vehicular environment (WAVE). The proposed techniques leverage time, space and

channel diversity to improve efficiency and robustness of the network advertisement procedure in a

urban scenario where obstructions to signal propagation due to buildings and traffic jam could hinder

successful message spreading. Simulation under different RSU density, vehicular networking technol-

ogy penetration rate, data rate, and packet size, aims at assessing effectiveness and efficiency of the

proposed solutions.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Several ongoing research projects supported by car manufac-

turers, electronic industries, governments and academia have

been underway to accelerate the deployment of short-range

wireless networks that exploit Vehicle-to-Vehicle (V2V) and

Vehicle-to-Roadside infrastructure (V2R) communications. These

networks, named Vehicular Ad Hoc Networks (VANETs), are

characterized by rapidly changing topologies and short connec-

tion lifetime.

Drivers and passengers on VANET-equipped vehicles may

benefit of safety-critical, transport efficiency and information/

entertainment (infotainment ) services (Hossain et al., 2010). The

latter two classes, referred to as non-safety services, aim atoffering traffic information and augmenting comfort/entertain-

ment for travelers on the road. So they have great potential as a

driver for VANET market penetration.

Beside multimedia interactive Internet-based applications (e.g.,

media streaming, voice over IP, Internet gaming, web browsing),

new applications, not necessarily IP-based, are expected to be

specifically tailored to the vehicular environment (Gerla and

Kleinrock, 2011). For instance, vehicles could collect data about

traffic density and average car speed, or pollution measurements

retrieved through on-the-road sensors; additionally, media-rich

streaming (e.g., videos recorded by on board cameras) could be

uploaded for monitoring purposes to remote Intelligent Trans-

portation Systems (ITS); moreover, identifications of detected

malicious vehicles could be uploaded to a Certification Authority

responsible for certificate revocation. On the other hand, Roadside

Units (RSUs) could broadcast location-based information, e.g.,

news items, proximity advertisements about nearby petrol sta-

tions, museums, theaters, points-of-interests, and traffic-related

information.

We can say that almost the totality of non-safety applications

targeted for the vehicular environment requires reliable connec-tions between On-board Units (OBUs) on vehicles and the road-

side communication infrastructure made up of Road-side Units

(RSUs) located at the strategic places, e.g., at city crossroads or at

gas stations along highways.

High costs required for the deployment of a ubiquitous road-

side communication infrastructure would result in only a few RSU

installations in the early stage of vehicular network develop-

ments, thus providing discontinuous connectivity along the road.

The RSU limited coverage plus OBU mobility (especially fast in

highways) would lead to short RSU-OBU connection lifetimes.

V2R connectivity is further challenged in urban scenarios, because

of obstructions due to buildings which limit the network radio

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/jnca

Journal of Network and Computer Applications

1084-8045/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.jnca.2012.04.001

n Corresponding author. Tel.: þ393491775654.

E-mail addresses: [email protected] (C. Campolo),

[email protected] (H.A. Cozzetti), [email protected] (A. Molinaro),

[email protected] (R. Scopigno).

Please cite this article as: Campolo C, et al. Augmenting Vehicle-to-Roadside connectivity in multi-channel vehicular AdHoc Networks. Journal of Network and Computer Applications (2012), http://dx.doi.org/10.1016/j.jnca.2012.04.001

Journal of Network and Computer Applications ] (]]]]) ]]]–]]]

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coverage. Therefore, it is critical to make vehicles aware of nearby

RSUs offering connectivity services in order to utilize the RSUs’

resources efficiently.

Recent studies have investigated the feasibility and perfor-

mance of V2R communications to support non-safety applications

(Ott and Kutscher, 2004; Bychkovsky et al., 2006; Zhao et al.,

2008; Yoo et al., 2010). Some of them (Zhao et al., 2008; Yoo et al.,

2010) have proposed to leverage V2V communications as a

complement to possible lack of connectivity with the roadinfrastructure. However, to the best of our knowledge, previous

works do not deal with the features and capabilities of the IEEE

802.11p/WAVE (Wireless Access in Vehicular Environments)

standard (IEEE Std, 2010), recently ratified to better cope with

the high dynamicity of vehicular environments.

The standard relies on a multi-channel architecture that supports

the delivery of safety and non-safety applications (IEEE 1609.4,

2011). Safety and control messages are conveyed on a fixed

frequency during a common control channel (CCH) interval; for

the rest of the time, vehicles switch on a service channel (SCH) for

non-safety data exchange within a Basic Service Set (BSS). RSUs (or

even special OBUs) aiming to initialize a BSS are called providers;

they advertise their presence during the CCH time interval by

broadcasting WAVE Service Advertisement (WSA) messages, which

specify the SCH frequency selected for BSS set-up, the offered

services, and other connection parameters. A user interested in the

services offered by the provider simply switches on the advertised

frequency during the next SCH interval and joins the BSS.

In the challenging vehicular environment, a BSS initialization

is determined by the success of the WSA-based network adver-

tisement procedure. Being broadcast messages, WSAs are never

acknowledged by receivers. Their delivery could be hindered by

channel impairments (e.g., fading, obstructions) or by collisions

with interfering traffic delivered over the same channel, i.e.,

event-based safety messages (like collision warning) and periodic

status messages called beacons. Beacons are one-hop broadcasted

short messages through which all vehicles send status informa-

tion about the vehicle type, position, speed, and direction. They

are very useful to cooperative vehicular applications, such as

collision avoidance, driver assistance, and cruise control,1 to

neighbor discovery-based routing protocols, and to smart safety

dissemination strategies.

Robustness and efficiency of the critical WSA-based 802.11p/

WAVE advertisement procedure have not been sufficiently inves-

tigated in the literature. Hence, this paper serves the twofold

purpose of

1. investigating limitations of the WAVE advertisement proce-

dure in V2R communication scenarios under harsh propaga-

tion and variable traffic load conditions;

2. designing standard-compliant techniques that augment the

WAVE advertisement procedure in order to increase the

RSU-awareness, i.e., the percentage of vehicles which detect aRSU and hence benefit from the BSS services offered on SCH.

The smart and lightweight set of proposed techniques for

strengthening information spreading about the RSU-advertised

BSSs leverage time, space, and channel diversity. Time diversity is

achieved by enabling multiple repetitions of WSAs per CCH

interval, as suggested by the IEEE 1609.3 standard in IEEE

P1609.3 SWG (2010). Space diversity is achieved by letting

vehicles which receive a WSA from a one-hop distant RSU to

piggyback partial or full information about the advertised BSS

onto their own beacons, as initially proposed in our previous

work in Campolo and Molinaro (2011a). Channel diversity

exploits an additional WSA transmission at the SCH interval

beginning to complement the advertisement phase carried out

on the CCH interval.

The rest of this paper is organized as follows. The 802.11p/

WAVE standard specifications are reported in Section 2; back-

ground and motivations for our research are provided in Section

3. The set of solutions proposed to improve robustness and

efficiency of the WAVE advertisement procedure are described

in Section 4. Simulation assumptions and performance results

under several propagation and traffic density conditions are

presented in Section 5. Discussion and conclusive remarks are

reported in Section 6.

2. IEEE 802.11p/WAVE networks

The IEEE 802.11p task group (IEEE Std, 2010) has recently

issued a set of physical (PHY) and medium access control (MAC)

layer specifications to permit communications in the rapidly

changing vehicular environment, which operates in the Dedicated

Short Range Communication (DSRC) frequency band of 5.85–

5.925 GHz. The IEEE 802.11p PHY layer is an amended version of

the 802.11a specifications, based on Orthogonal Frequency-Divi-

sion Multiplexing (OFDM), but with 10 MHz channels and data

rates ranging from 3 Mbps to 27 Mbps. The IEEE 802.11p MAC

layer has the same core mechanism of the Enhanced Distributed

Channel Access (EDCA) specified for 802.11e, which is based on a

prioritized Carrier Sense Multiple Access with Collision Avoidance

(CSMA/CA) scheme. It provides differentiated channel access by

assigning to traffic belonging to each of the four access categories

(ACs) a set of distinct EDCA parameters, including Arbitration Inter-

Frame Space (AIFS) and contention window (CW) size. The four

classes are referred to as background (AC_BK), best effort (AC_BE),video (AC_VI) and voice (AC_VO) with increasing priority order.

Higher-priority ACs have smaller CWs and shorter AIFSs.

The IEEE 802.11p working group cooperates with IEEE 1609 in

order to define a whole protocol stack for vehicular environments,

Fig. 1.

The overall WAVE stack relies on one CCH, which is reserved

for transporting control (e.g., WSAs, beacons) and safety data, and

a given number of SCHs (it is up to four in the ETSI context, or to

6 in DSRC context) used to exchange non-safety data. The MAC

layer is properly modified to work with the multi-channel

organization; two separate EDCA functions are implemented,

one for CCH and one for SCH, which handle different sets of

queues for packets destined to be transmitted in different channel

intervals with different EDCA parameter sets.

Fig. 1. 802.11p/WAVE stack.

1 Although not explicitly mentioned in 802.11p/WAVE standards, beacons are

cited as heartbeat messages in SAE International, DSRC Implementation Guide

(2010) and as Cooperative Awareness Messages (CAMs) in ETSI TS 102 637-2

V1.1.1 (2010).

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WAVE devices are expected to be developed either as single-

radio devices, which operate on one radio channel at a time, or as

multi-radio devices, which are capable of simultaneous operation

on multiple radio channels. To allow both kinds of devices

exchanging safety and non-safety data, the alternating channel

access scheme has been proposed by IEEE 1609.4 (IEEE 1609.4,

2011). According to it, the channel time is divided into synchro-

nization intervals with a fixed length of 100 ms, Fig. 2, consisting

of a CCH interval, during which all vehicular devices tune in the

CCH frequency, and an SCH interval, during which vehicles

(optionally) switch to one of the SCH frequencies. Channel

coordination exploits a global time reference, such as the Coordi-

nated Universal Time (UTC), which can be provided by a global

navigation satellite system.

The main straightforward drawback of this scheme is the

halved channel capacity: more than half of the SCH bandwidth

cannot be used during the CCH interval, and more than half of the

CCH bandwidth channel cannot be used during the SCH interval.

Mandatory alternating channel switching is also the cause of

another type of event that is unique to WAVE networks: a frame

cannot be transmitted if the residual time before the end of a

channel interval is shorter than its transmission delay. Such a

frame can be queued waiting for the next channel interval or

dropped in case its lifetime is exceeded; in the latter case frame

loss is due to channel expiry time.

The WAVE stack supports both IPv6 protocol and a new

lightweight WAVE-mode short message protocol (WSMP) speci-

fically designed for the exchange of short messages in vehicular

networks. WSMP packets can carry high-priority time-sensitive

safety messages, traffic and road messages, or also beacon frames.

WSMP traffic can be directly exchanged by WAVE devices without

the IP overhead both on CCH and SCH, while IP data can be

exchanged only on the SCH.

The 802.11p standard also introduced a new operational mode

to facilitate communications in the highly dynamic vehicular

environment; it is referred to as outside the context of a BSS

(OCB). It allows wireless stations that are not members of a BSS totransmit data without preliminary authentication and associa-

tion, by only relying on default parameter values. In OCB mode,

data can be sent to either an individual or a group destination

address with a wildcard BSS identifier (all 1s). Although the OCB

mode is mainly expected to be used for delivering locally-relevant

safety messages on CCH, its usage could be extended to allow non

IP-based data exchange also during the SCH interval.

3. Background and motivations

The WAVE multichannel organization allows a provider to

reserve a given SCH frequency for its BSS setup by broadcasting

WSA frames to one-hop distant nodes during the previous CCH

interval. The role of a WAVE provider can be played both by RSUs

or OBUs; but without loss of generality, in the remainder of the

paper we refer to RSUs as the unique type of WAVE providers,

since we are interested in V2R communications.

WSAs contain all the information identifying the offered WAVE

services and the network parameters necessary to join the BSS,

such as the unique identifier of the BSS (BSSID), the Provider

Service Identifier (PSID), the SCH this BSS will use, timing informa-

tion, the EDCA parameters set to be used on the SCH, and

information about how to connect to the Internet, e.g., default

gateway and domain name server address, as depicted in Fig. 3.

Some information carried in WSAs can change from a CCH interval

to the successive. This is the case, for example, of the EDCA

parameter set and of the advertised SCH; the SCH may be changed

if it is perceived as interfered because of the arrival of a new WAVE

provider that is using it under the same radio coverage.

There is no feedback on WSA successful reception by inter-

ested vehicles; so, the provider cannot know if its BSS advertise-

ment has failed due to either adverse channel conditions or WSA

collisions with interfering traffic simultaneously transmitted on

CCH, e.g., safety messages or beacons. While safety messages are

triggered by the occurrence of a hazardous event on the road, so

they are only sporadically transmitted, beacons can be especially

harmful due to the fact that they are routinely transmitted by all

vehicles and at every CCH interval.2

The consequences of a lack of received WSA can be negative

under many aspects. On one hand, WSA reception also serves the

purpose of providing link quality information through signal

strength measurements at the receiver (IEEE P1609.3 SWG,

2010), which can infer quality and stability of the connection to

a given provider. This information can be used by each receiver to

determine whether to attempt accessing an advertised BSS. On

the other hand, a failure in the WSA reception prevents non-

safety data exchange within the context of a BSS during the next

SCH interval, given that potential users would be unaware of the

BSS setup. The impossibility to exchange data on a given SCH

interval could be particularly detrimental for short-lived connec-tions expected in case of V2R communications. Things are made

more difficult in an urban setting, where the radio obstructions

caused by buildings could make BSS initialization and subsequent

data exchange even more critical, by further reducing the RSU

coverage area and consequently connection lifetimes.

This complex context constitutes the underlying motivation of

the proposed study.

To improve reliability of the WAVE advertisement procedure,

the 1609.3 standard only suggests providers to transmit multiple

Fig. 2. WAVE 1609.4 multichannel operation.

2 Although in literature there are many examples of adaptation of beacon

generation rate (Schmidt et al., 2010; Sommer et al., 2011a) to reduce the channel

load, in this study we refer to the worst case scenario.

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WSA copies per CCH interval. Intuitively, the higher the number

of WSAs the higher the probability that nearby vehicles detect the

RSU. Nevertheless, repeated WSA transmissions would contribute

to congestion on CCH. This is a serious concern because of the

already scarce CCH capacity due to channel switching. Therefore,

WSA repeats per CCH interval should be carefully tuned not to

penalize beacon and safety message transmissions on the same

channel.

The driving idea of the techniques proposed in this paper is to

augment efficiency and robustness of the WAVE advertisement

procedure by making the best of the two standard types of

messages, WSAs and beacons, transmitted on CCH in order to

spread the RSU awareness among vehicles. Specifically, they all

rely on cooperation among vehicles which use their routinely

transmitted beacons as additional ‘‘delivery vectors’’ of informa-

tion about advertised BSSs and their parameters and WSA repeti-

tions to exploit time diversity.

Early germs of such techniques to the purpose of improving

WAVE advertisement procedures can be found in Campolo et al.

(2009), where we proposed to modify the WSA messages to let

providers propagate SCH reservation information of nearby BSSs to

two-hop neighboring nodes in order to improve the service channel

selection procedures, and in Campolo and Molinaro (2011a), where

we demonstrated the effectiveness of piggybacking information

about the BSS’s SCH onto beacons in a highway scenario.

In this paper, steps forward are taken by designing additional

techniques that spread full information about the BSS’s services

and connection parameters useful for benefiting from Internet

applications, and also leverage the recently standardised OCBcommunication mode.

4. The proposed set of augmented WAVE advertisement

procedures

4.1. Exploiting space diversity: the SCH piggybacking scheme

In the first scheme, referred to as SCHp (that stands for SCH-

piggybacking ), SCH information is carried not only in RSU-generated

WSAs as envisioned by the standard, but it is also piggybacked onto

beacons generated by vehicles aware of the RSU presence. This is the

scheme we proposed in Campolo and Molinaro (2011a) that is

reported here for the sake of completeness, since it is used as a

benchmark for the newly introduced solutions.

Before over-the-air transmission of its own beacon, each

vehicle, that has already received a WSA from a nearby RSU,

piggybacks in its beacon a one-byte field of the received WSA.

Specifically, it only includes the ChannelNumber field (IEEE

P1609.3 SWG, 2010) that identifies the SCH where the advertised

BSS will be initialized (Fig. 4). Piggybacking can be done by

exploiting the WSMP header extension fields provided in Annex G

of IEEE 1609.3 (IEEE P1609.3 SWG, 2010), while being fully

compliant with the 802.11p/WAVE specifications.

Vehicles, detecting WSAs from different providers, only piggy-

back the information received by the nearest RSU. Distance from

RSUs can be easily computed since (i) each vehicle is equipped

with a Global Positioning System (GPS) receiver, hence knows

about its current location, and (ii) every provider includes its own

GPS position in the Latitude and Longitude fields of the WSA frame

(IEEE P1609.3 SWG, 2010).

Broadcasting of such extended beacons helps nearby vehicles

that missed transmitted WSAs during a given CCH interval to

become aware of the RSU presence and tune onto the advertised

SCH. Specifically, according to the SCHp scheme, the following

cases can occur:

– vehicles directly detect a nearby RSU by receiving a WSA frame

from it; hence, they can tune to the advertised SCH and

exchange data with the RSU;

– vehicles miss all WSAs from nearby RSUs during a given CCH

interval, but they receive at least one beacon frame piggyback-

ing SCH information; hence, they can tune on the SCH

advertised in the beacon;

– vehicles detect information about a nearby RSU neither

directly from WSAs nor through beacons, so they remain

tuned on the CCH also during the successive SCH interval.

The main advantage of the SCHp scheme is that it keeps

the additional overhead very low: only 1 byte per transmitted

Fig. 3. WSA frame format (adapted from (IEEE P1609.3 SWG, 2010)).

Fig. 4. Extended beacon frame with piggybacked WSA’s ChannelNumber.







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beacon. Its main weakness is that vehicles only become aware of

the advertised SCH where the BSS will be initialized, but they

miss all other BSS information, e.g., the offered services, the EDCA

parameters set, configuration parameters for connecting to the

Internet. Therefore, they cannot exchange IP-based data with

remote hosts in the Internet; however, it is reasonable to

assume that when switching to the SCH they can exchange WSMP

packets with the RSU3 by relying on the OCB communication

mode. The RSU can broadcast locally relevant information, as for

example public utility messages, like point-of-interest notifica-

tion, e-maps/traffic congestion updates, proximity advertising

and marketing segments (e.g., movie clips from the nearby

theaters, prices of nearby gasoline stations), which may be

exchanged outside the context of the BSS through WSMP packets.

4.2. Exploiting space diversity: the WSA piggybacking scheme and

the hybrid piggybacking scheme

In order to remove the limit of the SCHp scheme and let

vehicles exchange IP-based data with any node in the Internet

when there is a nearby RSU, we propose the WSAp scheme (that

stands for WSA piggybacking ) that makes vehicles fully aware

about the operational parameters of advertised BSSs. According to

it, vehicles that have received a WSA can piggyback the entire

WSA frame onto their transmitted beacons, Fig. 5. So vehicles

missing WSAs transmitted by nearby RSUs, but receiving at least

one extended beacon frame, can tune on the advertised SCH

and exchange with the RSU not only OCB packets but also IP-

based data.Certainly, this scheme helps in spreading full-awareness

among vehicles in the RSU neighborhood but it would suffer

from high overhead4 and cause congestion on CCH in the case

all vehicles decide to append the entire WSA in their beacons.

Therefore, we foresee a simple probability-based technique coupled

with the WSAp scheme, according to which only a subset of

vehicles piggyback WSAs into their beacons with a given prob-

ability. Each vehicle receiving a WSA from a nearby RSU and with

a pending beacon waiting for being transmitted in the current

CCH interval, extracts a random number uniformly distributed

between 0 and 1. If the number is greater than a given piggy-

backing probability p, the vehicle piggybacks the WSA frame,

otherwise it sends a legacy beacon. A probability p equal to

1 corresponds to the case in which all vehicles detecting a nearby

BSS piggyback the received WSA, a value of p equal to 0 means

that no vehicles enforce piggybacking.

Increasing the size of transmitted beacons can have another

side effect: because of the bounded duration of the CCH interval,

given the channel switching mechanism, some transmissions of

these longer beacon frames could not be accommodated, due to

CCH expiry time. Furthermore, longer beacon frames are more

prone to channel errors under the assumption of independent bit

errors. Failed beacon transmissions could be detrimental for

cooperative applications that rely on up-to-date information

conveyed by all vehicles into beacons.

In order to improve the number of SCH-aware vehicles, we

further enhance the WSAp scheme by combining it with SCHp.

The resulting hybrid scheme lets those vehicles detecting a nearby

RSU and not piggybacking the entire WSA (because the extracted

probability is greater than the defined p), piggyback only the

ChannelNumber field into their own beacons.

4.3. Exploiting channel diversity: the enhanced SCH piggybacking

scheme

In order to reduce probability of congestion on the bounded

CCH interval and increase full RSU-awareness, we propose a

further scheme, referred to as eSCHp (that stands for enhanced

SCH piggybacking ). It is intended to combine the low-overhead

SCHp scheme with the transmission of an additional delayed WSA

at the beginning of the SCH interval. Figure 6 shows the resulting

data transmission scheme on CCH and SCH interval.

Vehicles receiving a beacon with piggybacked SCH information

can switch on the advertised SCH and wait for the additional WSA

sent by the RSU. This WSA is transmitted at the highest priority

allowed by the 802.11p standard in order to reduce contention

Fig. 5. Enhanced beacon frame with piggybacked WSA.

Fig. 6. eSCHp scheme.

3 It should be noticed that among vehicles that have missed a WSA from the

RSU but have received an extended beacon from a nearby vehicle, there could be

some with a very bad link towards the RSU. These vehicles would be unable to

exchange data with RSU in the next SCH interval. In this case, relay-based

solutions would help to support V2R communication, as we have preliminarily

investigated in Campolo and Molinaro (2011b). Design and analysis of relaying

techniques are outside the scope of this work.4 The WSA size is variable; it depends on the type and number of offered

services. Typical lengths range between 100 and 400 bytes, if services are offered

over IPv6 (Kenney, 2009). The typical size of beacons is 100 bytes (Xu et al., 2004).







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with other competing traffic on the SCH. According to this solution,

the following cases can occur:

– vehicles directly detect a nearby RSU by receiving at least one

WSA from it during the CCH interval; hence, they can tune to

the advertised SCH and exchange both IP-based and OCB mode

packets with the RSU;

– vehicles miss all WSAs transmitted from nearby RSUs, both on

the CCH and the SCH, but they receive at least one beacon packet

piggybacking the SCH information. They can tune on the adver-

tised SCH, but, since they missed all other BSS information, they

are only allowed to exchange OCB mode packets;

– vehicles detect at least one beacon on the CCH and the delayed

WSA on the SCH; they become fully aware of the BSS operational

parameters and can exchange any kind of data with the RSU;

– vehicles detect information about a nearby RSU neither directly

nor through beacons; hence, they remain tuned into the CCH also

during the SCH interval.

Beside improving the RSU-awareness among vehicles missing

WSAs in the CCH interval, this scheme has a further benefit:

vehicles receiving a delayed WSA can better infer the quality of

the radio link towards the RSU. In fact, the WSAs received on CCH

may not give a reliable indication of the SCH link quality (Kenney,

2009); this is especially true when the CCH is particularly

congested because of several beacon and WSA transmissions.

5. Performance evaluation

We analyze the performance of the proposed schemes against

the legacy WAVE advertisement procedure through simulations

carried out in ns-2 (NS-2, Network Simulator tool, 2011). For the

sake of clarity, the main features of the compared schemes are

summarized in Table 1.

An urban area is modeled as a 750 m-wide grid including 5Â5

two-lane roads, spaced 150 m apart. A variable number of RSUs

(6, 10, 14) is positioned at crossroads, as illustrated in Fig. 7, and

provide connectivity to vehicles. Considering the hostile propagation

conditions in urban environments (due to signal fading, attenuation,and diffraction), the RSUs must be deployed at strategic places, such

as at crossroads, with the aim of maximizing the coverage, while

keeping the infrastructure cost as low as possible.

The mobility traces are generated by SUMO (2011) for 451

vehicles moving at a mean speed of 60 km/h. Mobility traces are

given as inputs to ns-2.

To improve realism of simulations, we use the ns-2 version in

Chen et al. (2007), which encompasses computation of the cumula-

tive Signal-to-Interference and Noise Ratio (SINR) and the modulation

scheme at the receiver. The received signal strength is computed by

considering a statistical component modeling fading, and a determi-

nistic one accounting for urban obstructions. The statistical compo-

nent follows the Nakagami distribution (Nakagami, 1960), which is

widely used in the vehicular environment (Taliwal et al., 2004). In

order to model medium fading conditions the fading intensity para-

meter, m, of the Nakagami distribution is set equal to 3.

Quite recently some models have been proposed in literature to

account for obstructions in urban scenarios (Giordano et al., 2010);Sommer et al., 2011b; Pilosu et al., 2011; Scopigno and Cozzetti,

2010). In CORNER (Giordano et al., 2010) an analytical model is

validated by ray-tracing and by measurements, but it applies only to

specific square-corner topologies. In Pilosu et al. (2011) RADII is

proposed as a methodology to automatically segment any complex

topology into regions where an analytical model describes propaga-

tion phenomena as deduced by ray-tracing. However, RADII is very

recent and lacks an integration with network simulators; addition-

ally, it requires pre-processing for any specific urban map.

For these reasons the model proposed in Scopigno and Cozzetti

(2010) is adopted in this paper. It considers an additional path-

loss component (extra-attenuation ai) accounting for obstruction

by buildings. As shown in Fig. 8, an intermediate layer is added

into the simulation architecture between the SUMO and the ns-2

Table 1

Main features of the compared protocols.

Feature SCHp WSAp Hybrid WSAp eSCHp

SCH-awareness Yes Yes Yes Yes

Full-awareness No Yes Yes Yes

Piggybacking scope All nodes A su bset of nodes with pro bability p All nodes All nodes

Overhead per beacon 1 byte (ChannelNumber) 1WSA with probability p 1WSA with probability p

1 byte (ChannelNumber) with probability 1-p

1 byte (ChannelNumber)

Overhead on SCH No No No Yes, 1 WSA per RSU

Fig. 7. RSUs positions in the Manhattan-like grid topology. The three simulatedcases are distinguished by colors: 6-RSUs displayed as red units; 10-RSUs:

redþblack; 14-RSUs: redþblackþgreen. (For interpretation of the references to

color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Logical simulative architecture.







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layer, to facilitate the off-line classification of possible mutual

node positions with respect to buildings, in a regular square grid.

More in details, coordinates of vehicles are regressed into a

discrete version. In this way, this information can be computed

off-line and appended in the file containing node positions. This

method makes easier and faster for the network simulator to

perform comparisons. A discrete math is then deployed on the

discrete coordinates to classify mutual node positions with

respect to the obstacles.The classification function is embedded in the network simulator

and can be used in real-time without major impacts on the

computational load of simulations (Scopigno and Cozzetti, 2010).

It is computational negligible and permits to identify, based on

coordinates, if two nodes are in Line-of-Sight (LoS), Near-Line-of-Sight

(NLoS), and non-Line-of-Sight (nLoS). Line-of-Sight (LoS) conditions

apply to vehicles in the same road (vehicles S and A in Fig. 9), Near-

Line-of-Sight (NLoS) to vehicles on the legs of a crossing not farther

than a building (vehicles S and B in Fig. 9), and non-Line-of-Sight

(nLoS) to all the other cases (vehicles S and C in Fig. 9).

The corresponding extra-attenuation ai assumes the following

different values: a0¼0 dB in case of LoS; a1¼À13 dB, in case of

NLoS; a2¼À30 dB, in case of nLoS.

While the proposed model is somehow simplistic and does not

completely reflect the analytical model of CORNER or RADII, it has

the advantage of adding an automatic mutual positions classifica-

tion that simplifies integration into ns-2. Additionally, it has been

widely justified, by measurements from literature, in Scopigno

and Cozzetti (2010) and further validated in Campolo et al.

(2011a) by ray-tracing simulations.

The WAVE multi-channel organization and the alternating chan-

nel switching, with 50-ms long CCH and SCH intervals, are built on

the top of the 802.11p PHY and MAC layers, whose parameters are

summarized in Table 2. Unless differently specified, the data rate is

6 Mbps, which is the most common value suggested in the literature

( Jiang et al., 2008). The transmission power is set to 7 dBm, among

the values allowed by the standard. With this power value, the

maximum distance at which packet reception is still possible is

150 m, by assuming deterministic path loss.

Since 802.11p/WAVE only mandates the use of the highest

priority for safety packets, we set the AC of beacons and WSAs to

be AC_BE according to suggestions in SAE International, DSRC

Implementation Guide (2010).

The number of WSA repeats is a variable number in our study.

The standard does not specify how providers schedule WSA

transmissions during the CCH interval; we assume that they

transmit at random time instants in order to increase time

diversity (Campolo et al., 2011b) compared to the case in which

they transmit WSAs at the CCH interval beginning. The same

generation policy applies for beacons.

Moreover, we consider the application layer aware of channel

switching, so that it passes beacons to the MAC layer only during

the CCH interval, as suggested in Qi et al. (2009).

The beacon packet size is set to 100 bytes ( Xu et al., 2004),

while the WSA packet size is set to 500 bytes, except in the case

where we investigate the influence of the WSA size. We added

178 bytes accounting for security overhead (e.g., digital signature

plus a certificate) to both packet types (IEEE 1609.2, 2006).

Beacon generation rate is set to 10 Hz (Schmidt et al., 2010), thismeans they are transmitted once per CCH interval.

5.1. Impact of obstructions on legacy BSS advertisement

The first set of results aims at evaluating the impact of

obstructions on the connectivity degree guaranteed by the legacy

BSS advertisement procedure with WSA repetitions (IEEE 1609.4,

2011) under a variable number of deployed RSUs. The metric used

for this purpose is the average percentage of connected users, i.e., the

percentage of vehicles that have received (at least) one WSA frame

from a nearby RSU during the CCH interval and can exchange both

IPv6 and WSMP data packets during the successive SCH interval.

The analysis is carried out when the CCH traffic load is

composed of (i) only WSAs transmitted by RSUs (Fig. 10), and(ii) WSAs and beacons respectively transmitted by RSUs and

vehicles (Fig. 11), for a different number of RSUs (6, 10, 14)—posi-

tioned according to the placement scheme in Fig. 7. The results

achieved when using the model with obstructions (dashed curves

labeled with w obstructions) are compared with the ones achieved

when considering the Nakagami model without extra attenuat-

ion due to obstructions (continuous curves labeled with w/o

obstructions).

The detrimental effect of obstructions is obvious in Fig. 10, and

also the positive effect of an increase in the number of WSA

Fig. 9. Obstructions in the considered urban topology and corresponding attenuation factors.

Table 2

802.11p PHY and MAC parameters (IEEE Std, 2010).

Category Parameter Value

PHY Frequency 5.9 GHz

Channel bandwidth 10 MHz

Transmission power 7 dBm

Power monitor threshold À120 dBm

Noise floor À99 dBm

Carrier sense threshold À95 dBm

MAC Slot time 16 ms

SIFS time 32 ms

Header length (T h) 40 ms

aCWmin 15

AIFS number (AIFSN) 6







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repeats. The most significant gain is achieved when passing

from 1 to 2 repeats, because of the achieved better time diversity

in transmission attempts. Under the Nakagami model and with

4 WSA repeats, 10 RSUs are able to guarantee full connectivity over

the topology, i.e., each vehicle is under the coverage of at least one

RSU. Conversely, under the model with obstructions, buildings

affect connectivity. This is especially true when considering the

scenario with the least redundant infrastructure coverage, 6 RSUs.

In this case, when RSUs transmit 5 WSA repeats, connectivity is

provided to less than 80% of vehicles.In Fig. 11, as expected, due to higher load on CCH—both WSAs

and beacons are transmitted—the connectivity performance sig-

nificantly decreases. Full connectivity is never guaranteed on the

simulated topology. However, in this case, obstructions surpris-

ingly have a positive effect on connectivity. They, in fact, keep

signal propagation and interference low. This is especially appre-

ciable under high traffic load.

Henceforth, simulations will consider the Nakagami model

with extra-attenuation due to obstructions.

5.2. Impact of VANET penetration rate: SCHp vs. legacy

The second set of results better analyzes the impact of

beaconing traffic on the legacy and the proposed piggybacking-

enhanced BSS advertisement procedure (SCHp) when varying the

penetration of VANET-equipped vehicles on the roads. Simulations

are carried out with a variable percentage of vehicles equipped

with 802.11p/WAVE transceivers, and consequently with a vari-

able number of vehicles transmitting beacons and being poten-

tially capable to detect the WAVE advertisements.

In Figs. 12, 13, and 14, the SCHp scheme is evaluated versus the

penetration rate, when varying the WSA repeats, and for 6, 10, and

14 RSUs, respectively. The two sets of curves, dashed and continuous,are respectively labeled as fully aware and SCH aware. Fully-aware

vehicles are made aware of all the BSS operational parameters,

included the SCH where a BSS will be initialized, by receiving at

least one WSA transmitted by a nearby RSU, as also foreseen by the

legacy WAVE advertisement scheme. SCH-aware vehicles are made

aware of the SCH where BSS services will be delivered by receiving a

WSA or a beacon piggybacking the ChannelNumber field; hence, this

percentage accounts for the improvement achieved thanks to the

enforcement of the SCHp scheme.

The results achieved by the legacy scheme are not explicitly

reported in Figs. 12–14 in order to reduce cluttering of plots, since

they are very close to the results reported for the SCHp fully

aware case.

The RSU detection capability, through the direct WSA recep-

tion of the legacy approach, decreases as the penetration rate

Fig. 10. Legacy: percentage of connected users vs. WSA repeats when varying the

number of RSUs in absence of interfering traffic on CCH.

Fig. 11. Legacy: percentage of connected users vs. WSA repeats when varying the

number of RSUs in presence of beaconing traffic on CCH.

Fig. 12. Legacy (fully aware) vs. SCHp: percentage of connected users vs. VANET-

equipped vehicles penetration rate when varying WSA repeats with 6 RSUs.









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increases, because of higher congestion on the CCH caused by

beacons, whatever the number of RSUs is.

The redundancy and the better spatial diversity achieved by

spreading the SCH information through beacons in SCHp helps in

counteracting the WSA losses due to the harsh propagation

conditions and collisions. Notwithstanding, it can be observed

that the number of SCH-aware vehicles either tends to saturate or

to decrease (e.g., for 1 WSA repeat, 6 RSUs) as the penetration rate

increases. As an example, for 6 and 10 RSUs, it can be clearly

noticed that the maximum number of SCH-aware vehicles is

achieved for a penetration rate equal to 50%, then the connectivitysaturates or slightly decreases. Such a trend witnesses the high

congestion experienced on CCH when the number of vehicles

transmitting beacons increases, which may reduce the benefits of

the proposed SCHp scheme. Nevertheless, whatever the penetra-

tion rate, the SCHp scheme increases the percentage of SCH-

aware users with respect to the legacy approach.

5.3. Impact of transmission data rate: SCHp vs. legacy

In Fig. 15 the SCHp performance is evaluated when considering

the two lowest data rates allowed in the 802.11p standard:

3 Mbps (continuous lines) and 6 Mbps (dashed lines) and when

only 6 RSUs are deployed on the topology. Lower data rates

require lower SINR thresholds for successful frame reception

(Table 3). On one hand, a lower data rate is less vulnerable to

interference—thus is expected to improve the connectivity

performance—but, on the other hand, the longer transmission

delay hinders the accommodation of all the scheduled WSA and

beacon transmissions during the CCH interval (the channel

capacity is halved).

Achieved results show that for the legacy scheme, when only

WSAs are transmitted on CCH, the lowest data rate guarantees

higher percentage of connected users. In fact, without interfering

beacons, all the scheduled WSAs can be accommodated during the

CCH interval. Moreover, in this case, results are not affected by the

WSA size. Conversely, when adding beaconing traffic, the lowest

data rate has a detrimental effect on the connectivity performance

of legacy and SCHp schemes; both percentages of fully aware (i.e.,

legacy) and SCH aware vehicles significantly decrease. The data rate

reduction implies, in fact, a lower number of transmitted packets

(both beacons and WSAs) on CCH due to the higher probability that

packets are dropped5 due to channel expiry time.

The reduction in the number of transmitted beacons not only

reduces the percentage of SCH-aware vehicles, but has also negative

effects on the neighborhood awareness requested by cooperative

cruise control applications. In fact, reducing the number of trans-

mitted beacons causes a reduction in the percentage of vehicles that

receive updated position and kinematics information from their

neighborhood. Results in Table 4 show the percentage reduction in

the number of vehicles reached by beacons at a data rate of 3 Mbps

when compared to 6 Mbps. Such a reduction becomes more severe

as the WSA size increases, because the longer packet transmission

delays—either due to a lower data rate or to longer packets—cannot

be accommodated during the CCH interval.

Additional tests performed with 12 Mbps, not shown in the paper,

demonstrate more significant performance worsening because of the

higher vulnerability to channel errors. In summary, the 6 Mbps data

rate suggested in Jiang et al. (2008) turns out to be the best choiceeven when considering the WAVE channel switching.

5.4. Legacy vs. beacon-enhanced schemes: connectivity analysis

In this subsection we present the comparison between the legacy

and the proposed solutions, SCHp, WSAp, hybrid and enhanced

piggybacking schemes in terms of achieved connectivity. The most

critical scenario is considered when only 6 RSUs are deployed on the

topology.



Fig. 15. Legacy vs. SCHp: percentage of connected users vs. WSA size with variable

data rate, 6 RSUs, and 4 WSA repeats.

Table 3

Data rate settings.

Data rate Modulation SINR Threshold (dB)

3 Mbps BPSK 5

6 Mbps QPSK 8

Table 4SCHp: reduction in the percentage of vehicles reached by transmitted beacons at

3 Mbps when compared to 6 Mbps, with 6 RSUs and 4 WSA repeats.

WSA size (bytes) R ¼3 Mbps w.r.t. R ¼6 Mbps (%)

100 21.27

300 20.92

500 20.53

800 19.94

5 Due to the short-relevance time of information conveyed in WSAs and

beacons, non transmitted packets at the end of the channel interval are dropped

and replaced with newly generated packets in the subsequent CCH interval.







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Figure 16 shows the percentage of vehicles which are made

fully aware of the BSS operational parameters by the WSAp

scheme when varying the piggybacking probability p according

to which vehicles piggyback (or not) over beacons the entire WSA

frame received from a nearby RSU.

The two sets of curves, continuous and dashed, respectively

refer to the percentage of fully aware vehicles that only receive at

least one WSA directly from a nearby RSU (curves labeled as

direct ), and to the case in which they either receive a direct WSA

or a piggybacked-over-beacon WSA (curves labeled as beacon-

aided) following the WSAp rules.WSAp performance increases with the number of WSA repeti-

tions and with the piggybacking probability p. With four WSA

repeats, also a small piggybacking probability ( p¼0.25) is capable

to achieve the same connectivity degree (about 70%) offered by

Legacy in Fig. 10 under the same conditions but without beacon-

ing traffic. Such a finding suggests that WSAp can improve the

RSU-awareness by counteracting the detrimental effects of colli-

sions between WSAs and beacons.

The hybrid scheme is analyzed in Fig. 17 for different piggy-

backing probabilities p and in comparison with legacy (same

results as in curves labeled with SCHp, fully aware, direct ), SCHp,

and WSAp (same results as in curves labeled with hybrid, fully

aware). By combining SCHp and WSAp features, the hybrid

scheme is able to outperform the other schemes.

The hybrid scheme lets all vehicles detecting an RSU—and not

appending the entire WSA to their beacons—to piggyback only

SCH information, thus increasing the percentage of SCH-aware

vehicles when decreasing p. The highest percentage of SCH-aware

vehicles is achieved by the low-overhead SCHp scheme. These

users cannot exploit IP-based services, but can benefit of services

offered by the RSU in OCB mode. On the other hand, hybrid and

WSAp show the same results as regards the fully awareness; the

higher p the higher the percentage of fully aware vehicles thatreceive extended beacons in the beacon-aided schemes.

The WSA size has also a significant impact on the perfor-

mance: whatever the deployed advertisement scheme, the higher

the WSA size the lower the connectivity performance is. The

increased WSA size, coupled with the bounded CCH interval

duration, may hinder the transmission of all scheduled WSAs

and extended beacons as the p probability increases, thus causing

degradation in the connectivity degree.

Results reported in Fig. 18 show the performance of the eSCHp

scheme that couples the simple SCHp scheme with the repetition

of a delayed WSA at the beginning of the SCH interval. Thanks to

channel diversity, the percentage of vehicles which are fully aware

of the operational parameters of a nearby BSS increases as

compared to the SCHp scheme. The same percentage of SCH-aware

vehicles is recorded since eSCHp relies on the same beacon-aided

Fig. 16. WSAp: percentage of fully aware users vs. WSA repeats when varying

piggybacking probability p with 6 RSUs, WSA size¼500 bytes.

Fig. 17. Hybrid vs. legacy and SCHp schemes: percentage of connected users vs.

WSA size with 6 RSUs and 4 WSA repeats.

Fig. 19. eSCHp vs. SCHp: percentage of connected users vs. WSA repeats with

variable WSA size and 6 RSUs (delayed WSA transmitted at 3 Mbps).

Fig. 18. eSCHp vs. SCHp: percentage of connected users vs. WSA repeats with

variable WSA size and 6 RSUs (delayed WSA transmitted at 6 Mbps).







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scheme used by SCHp. Moreover, results in Fig. 19 show that

slightly further improvements can be achieved by letting RSUs

transmit the delayed WSA at 3 Mbps, differently from the

6 Mbps rate used to transmit WSAs during the CCH interval. This

increases robustness and range of the WSA transmission on

the SCH.

It can be further noticed that the delayed WSA can be used to

reduce the number of WSA repeats transmitted during the CCH

interval, by guaranteeing the same percentage of fully aware

vehicles. As an example, the eSCHp scheme with 3 WSA repeats

on the CCH and a delayed WSA transmitted at 6 Mbps on the SCH

achieves the same performance achieved when the SCHp scheme

is deployed with 4 WSA repeats, and even higher when it is

transmitted at 3 Mbps.

Such a result suggests reducing the number of WSA repeats on

the CCH, to keep the CCH less congested. This could be particu-

larly useful under heavy traffic load on the CCH and in presence of

several WAVE providers, for example in the case of a platoon of

vehicles where several pairs of OBUs exchange data.

5.5. Overall comparison

Results reported in the previous subsections focused on the

effectiveness of the proposed schemes in terms of connectivity

when varying different parameters which could affect perfor-

mance: penetration rate, WSA packet size and number of repeats,and data rate. Here, we aim instead to analyze the efficiency of

the proposed schemes.

To this purpose, we define an additional metric, the overhead

on CCH , computed as the ratio between the number of bytes

added to beacons by the proposed piggybacking schemes and the

total number of bytes transmitted by vehicles in their beacons.

Results are shown in Table 5, for a data rate equal to 6 Mbps, WSA

size of 500 bytes and a number of WSA repeats equal to 4. In

order to better figure out the trade-off between effectiveness and

efficiency of the proposed schemes, the connectivity values, both

in terms of SCH-awareness and full-awareness, are also reported

in the same table.

Results show that the SCHp and eSCHp scheme incurs the

lowest, and almost negligible, overhead, by achieving the highestnumber of SCH-aware users. It has to be noticed that the eSCHp

scheme incurs an additional overhead on the SCH, because of the

transmission of the delayed WSA by each provider. Specifically,

for a WSA size of 500 bytes and a data rate of 3 and 6 Mbps, the

fraction of SCH bandwidth occupied by the delayed WSAs is

negligible and respectively equal to 1.5% and 3% of the useful SCH

interval duration.6

As already discussed in previous subsections, the most per-

forming schemes in terms of achieved number of fully aware

users are the WSAp and hybrid piggybacking schemes with

the highest value of probability p. More in detail, the hybrid

piggybacking scheme achieves a higher number of SCH aware

users as compared to WSAp, by experiencing almost the same

overhead.

Although it seems tempting to increase the piggybacking

probability to further improve full awareness of vehicles about

nearby RSUs, it should be noticed that increasing p could have a

negative impact on channel load. In fact, both the WSAp and

hybrid schemes make the beacon size to increase, because of the

piggybacking of the entire WSA. The resulting overhead increases

with p and it is not negligible, reaching more than 50% with p

equal to 1.

The main impact of the high overhead is on the delivery of

beacons, whose periodical reception is required by cooperative

cruise control applications. In order to evaluate this impact, for all

the proposed schemes we compute the following additional

metrics: (i) the percentage of beacons effectively transmitted by

vehicles over the number of scheduled beacons, i.e., one per CCH

interval per vehicle (labeled in Table 5 as transmitted beacons); (ii)

the reduction in the percentage of vehicles reached by beacons

w.r.t. to the legacy scheme (labeled in Table 5 as reached vehicles

reduction). It can be observed that the WSAp and hybrid schemes

lead to a reduction of the number of transmitted beacons as

compared to the legacy scheme, values pass from 96% to 86% with

p equal to 1. The SCHp and the eSCHp schemes, instead, exhibit

the same performance as the legacy approach. In addition, a

reduction in the number of successfully received beacons is

experienced with the probabilistic schemes. When p is equal to

1 a reduction of about 21% is achieved, this directly translates in

the reduced reliability of beacons, with a consequent potential

bad effect on cooperative cruise applications requiring accurate

neighborhood awareness.

Nonetheless the high benefits of the hybrid scheme both in

terms of fully aware and SCH aware vehicles, the additional over-

head incurred by this scheme, with the resulting reduced beacon

reliability, even when low piggybacking probabilities are consid-

ered, cannot be acceptable. Such a finding let us renounce to this

scheme and prefer the eSCHp, exploiting the simplicity of the

low-overhead SCHp scheme and further improving its connectiv-

ity performance through the delayed WSA transmitted on SCH.

6. Discussion and conclusions

In this work we have investigated the feasibility of V2R commu-

nications, when jointly considering features and constraints of

multi-channel operations envisioned by IEEE 802.11p/WAVE and

realistic settings for signal propagation and vehicle mobility in

challenging and crowded urban environments.

In order to increase the number of vehicles able to make the

best of a short-lived connectivity to nearby RSUs, we proposed a

set of solutions that exploits the repetition of BSS advertisements

during the CCH interval, the piggybacking of BSSs parameters

over beacons, and the introduction of BSS advertisement during

the SCH interval.

Table 5

Effectiveness and efficiency comparison between the proposed protocols and the legacy solution.

Scheme Legacy SCHp WSAp Hybrid WSAp eSCHp

Metric \ p 0.25 0.5 0.75 1 0.25 0.5 0.75 1

SCH-awareness (%) – 93.8 71.13 78.47 82.08 84.26 91.93 89.82 87.1 84.26 93.8

Full-awareness (%) 53.6 53.6 71.13 78.47 82.08 84.26 71.13 78.47 82.08 84.26 58.25

Overhead on CCH (%) – 0.23 19.96 34.59 45.24 53.79 20.03 34.63 45.25 53.8 0.23

Transmitted beacons (%) 96.01 96.01 93.86 91.47 89.06 86.46 93.88 91.47 89.06 86.46 96.01

Reached vehicles reduction (%) – 0 7.62 13.29 17.84 21.31 7.62 13.29 17.84 21.31 0

6 Transmissions are not allowed at the beginning of the CCH and SCH intervals

for a 4 ms-long guard period accounting for switching delays (IEEE 1609.4, 2011).







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The easy-to-deploy proposed solutions leverage packets already

available on the CCH interval, so they can be enforced by introdu-

cing slight modifications to the standard and by exploiting the low-

overhead and flexible WSMP packets.

Results show that the introduction of the WSA transmission at

the beginning of the SCH interval, coupled with the beacon-aided

spreading of BSS information among nearby vehicles, as foreseen

by the eSCHp scheme, successfully achieves the twofold purpose

of improving RSU-awareness, by overcoming effects of WSAlosses due to collisions and channel impairments, and reducing

channel load incurred instead by piggybacking the entire WSA. As

an additional benefit, it does not negatively affect the delivery of

beacons, crucial for cooperative cruise control applications.

Moreover, the beacon-aided mechanism allows to increase the

percentage of vehicles that could benefit of the widely expected

location-relevant and non IP-based services by relying on the OCB

mode provided in the 802.11p/WAVE standard.

The WSA transmission during the SCH interval has the further

great potential to allow vehicles to rely on more reliable link

quality assessment and to enforce channel- and rate-adaptive

transmissions, which will be subject matters for our future works.

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Please cite this article as: Campolo C et al Augmenting Vehicle-to-Roadside connectivity in multi-channel vehicular Ad

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