Reliability Analysis of IEEE 802.11p Wireless Communication

and Vehicle Safety Applications

Unai Hern´andez-Jayo and Idoia De-la-Iglesia

DeustoTech-Mobility Lab, Deusto Institute of Technology, Av. Universidaddes 24, 48007 Bilbao, Spain

Keywords:

Cooperative Vehicular Systems, Communications Reliability, Active Safety Applications Performance.

Abstract:

In this paper we analyze the performance of IEEE 802.11p vehicular networks used as communication media

to warn drivers about hazardous situations on the road. In particular, this paper is focused in three typical

active safety applications which are based on the cooperation among vehicles and intelligent elements placed

on the road. The reliability of these applications relies on the performance of the radio link among the agents

involved in the cooperative scenario, so ﬁrst we analyze the goodput, Package Delivery Rate (PDR) and delay

of this link. Once the behavior of the media is characterized, the kindness of the safety applications is studied.

These measures were carried out in real V2V and I2V scenarios using a compliant IEEE 802.11p prototype

developed by NEC.

1 INTRODUCTION

Vehicular Ad-Hoc Networks or VANETs have been

of particular interest to the communication research

area in order to develop a set of applications that

could help the driver to avoid or prevent from risky

situations. These services are based on the coopera-

tion among vehicles and offer great potential in reduc-

ing road accidents and therefore in improving drivers’

comfort and efﬁciency of highways from the trafﬁc

management point of view.

But ﬁrst, to provide these cooperative services a

stable and reliable wireless communication system

must be deployed on the road infrastructure. For

this propose, different technologies have been ana-

lyzed in the scenario of infrastructure-to-vehicle(I2V-

V2I) and vehicle-to-vehicle (V2V) communications

(Wewetzer et al., 2007) (Bazzi and Masini, 2011)

(Chou et al., 2009). Table I shows selected charac-

teristics and attributes of a few wireless systems that

could be used in a vehicle cooperative scenario (US-

DOT, 2004).

Once the communication link is stable then coop-

erative and warning services could be deployed with

the security that all agents involved in a VANET will

have the chance of sending and receiving information

regarding road events.

According to ETSI TC on ITS a set of applica-

tions can be used as a reference for developing co-

operative vehicular systems. In the same way, the U.

S. Department of Transportation (USDOT) has identi-

ﬁed similar applications to be deployed thanks to the

potential of DSRC to support wireless data commu-

nications between vehicles, and between vehicles and

infrastructure.

In this framework, this paper aims to determine if

a set of applications based on the description provided

by ETSI TC on TC can be deployed in a real VANET

using two IEEE 802.11p compliant devices, namely

LinkBird-MX communication modules provided by

NEC Technologies. In order to determine the reliabil-

ity of both the communication link and the applica-

tions, ﬁrst the main characteristics of tested coopera-

tive vehicle applications are described at Sections II

and III respectively, whereas the scenarios used dur-

ing this validation process are described at Section

IV.The results of this experiments and the analysis

about how some safety applications can work in these

scenarios are shown in Section V. Finally the conclu-

sions of the paper are presented at Section VI.

2 DSRC AND IEEE 802.11P

STANDARD OVERVIEW

Although all systems included at Table 1 provide spe-

ciﬁc solutions to different connectivity problems, in

a mobile environment in which all the nodes must

be able to send and receive reliable messages in real-

175

Hernández-Jayo U. and De-la-Iglesia I..

Reliability Analysis of IEEE 802.11p Wireless Communication and Vehicle Safety Applications.

DOI: 10.5220/0004401201750182

In Proceedings of the 10th International Conference on Signal Processing and Multimedia Applications and 10th International Conference on Wireless

Information Networks and Systems (WINSYS-2013), pages 175-182

ISBN: 978-989-8565-74-7

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

Table 1: Candidate Wireless Technologies and Atributes.

DSRC 3G WLAN

Range 1 km 4-6km 1km

One-way

to vehicle

One-way

from

vehicle

Two-way X X X

Point-to-

point

X X X

Point-to-

multipoing

Latency 200us 1.5-3.5 s 3-4 s

time, DSRC is the only one system that:

• Is dedicated to wireless access in vehicular Ad-

Hoc networks 1-hop and multihop communica-

tions.

• Provides active vehicle services with Line-Of-

Sight (LOS) and Non-LOS (NLOS) link scenar-

ios.

• Is ready to operate in a rapidly varying environ-

ment and to exchange messages without having to

join a Basic Service Set (BSS), that is, without the

management overhead.

• Makes possible low latency in communications

among vehicles and infrastructure allowing shar-

ing real-time information.

• Provides unicast, broadcast, real-time and bidirec-

tional communications.

IEEE 802.11p WAVE is only a part of a group of

standards related to all layers of protocols of Dedi-

cated Short Range Communications(DSRC) standard

(Uzcategui and Acosta-Marum, 2009). In the DSCR

5.9GHz band, FCC (Federal Communications Com-

mission) reserved seven channels (1 Control Channel

-CCH- , 4 Service Channel -SCH- , 1 Critical Safety

of life Channel and 1 Hi-Power Public Channel) of

10 MHz in a bandwidth of 75MHz for ITS applica-

tions while in Europe, ETSI reserved ﬁve channel, 1

Control Channel and 4 Service Channel (ETSI, 0 01).

The IEEE 802.11p standard is limited by the scope

of IEEE 802.11 which is the deﬁnition of MAC and

PHY layers, as it is shown at Figure 1 (Jiang and Del-

grossi, 2008). At PHY layer, IEEE 802.11p operates

in the band of 5GHz, reusing IEEE 802.11a OFDM

(Orthogonal Frequency Division Multiplexing) mod-

ulation considering 52 subcarriers that can be modu-

lated using BPSK, QPSK, 64-QAM or 16-QAM mod-

ulation schemes. Besides, IEEE 802.11preduces inter

symbol interference and the channel throughput (from

3Mbps to 27 Mbps, instead of 6Mbps to 54Mbps

commonly used in IEEE 802.11a). This setup allows

theoretically a communication range over 1000m (de-

pending on the antennas conﬁguration) and the estab-

lishment of communication among vehicles driving

up to 200km/h.

At MAC level, in order to speed up the ex-

change of messages among the vehicles (referenced

as On-Board Units) and Road-Side-Units (RSU),

IEEE 802.11p standard simpliﬁes initial connection

setup used in common IEEE 802.11 networks. The

basic MAC layer is based on CSMA/CA improved

with IEEE 802.11e EDCA protocol to provide ser-

vices with priority levels.

Anyway, MAC layer of IEEE 802.11p is a trend

research topic in which many alternatives and proto-

cols are being developed (Saeed et al., 0 11)(Bhm and

Jonsson, )(Bilstrup et al., 2008)(Han et al., 2012)(Bil-

strup et al., 2009).

IEEE 1609.4 deﬁnes a time-division scheme for

DSRC radios to alternately switch within these chan-

nels to support different applications concurrently,

that is, it supplements IEEE 802.11 features providing

frequency band coordination and management within

the MAC layer (Chen et al., 2009). This is possible

thanks to the coordinated operation on CCH (using it

only for broadcast, high priority and single-use mes-

sages) and SCH (for ongoing applications).

Meanwhile, IEEE 1609.3 speciﬁes operation and

management of the communications stack, deﬁning

the use of UDP transport protocol, coordinating the

IPv6 conﬁgurationand Logical Link Control (LLC) in

VANETs. This standard also manages WAVE Basic

Service Set (WBSS), which is required to handle the

SCH transmission.

WAVE Short Message Protocol (WSMP) is used

by IEEE 1609.3 networking services in CCH and

Figure 1: IEEE WAVE/802.11p protocol stack.

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SCH to enable communicationswith a maximumpay-

load of 1400 bytes. WSMP allows WAVE-aware

devices to directly control physical characteristics

(channel number and transmitter power).

Finally, IEEE 1609.2 standard deﬁnes secure mes-

sage formats and the processing of those secure mes-

sages in the WAVE system. It covers methods for se-

curing WAVE management and application messages,

with the exception of vehicle-originating safety mes-

sages.

To sum up, IEEE 802.11p WAVE is only a part

of a group of standards related to all layers of pro-

tocols for DSRC based operations which concerns to

physical and MAC layers. Therefore all the charac-

teristics of the links V2X and the performance of the

others DSRC layers depend on the efﬁciency of IEEE

802.11p standard. For this reason in this paper we will

evaluate ﬁrst the throughput of the IEEE 802.11p link

and then the reliability of the applications based on it.

3 COOPERATIVE BASED

APPLICATIONS

In the ﬁeld of cooperative ITS services, a huge va-

riety of applications and use cases can be described.

Taking into account strategic, economical and organi-

zational requirements, system capabilities and perfor-

mances as well as legal and standardization require-

ments, the ETSI TC on ITS has deﬁned a Basic Set

of Applications to be used as a reference for devel-

opers. These applications are close similar to those

described by the US DOT (USDOT, 2004).

In these applications, four types of communicat-

ing agents are considered: two mobile entities -OBUs

and people- and two stationary entities -the RSUs and

the central systems (it could be referenced as a Trafﬁc

Management System). These entities are able to run

four classes of applications: active road safety, co-

operative trafﬁc efﬁciency, cooperative local services

and global internet services. In each class, different

applications and uses cases are deﬁned.

Depending on the application and timing restric-

tions, data exchange among referenced entities can be

categorized as:

• Warning messages: these are deﬁned as Decen-

tralized environmental Notiﬁcation Messages and

they can be sent out to each vehicle or RSU.

• Heartbeat messages or beacons: these messages

are used by OBUs to report their position, speed

and ID to the RSUs. Moreover, these messages

are also used to keep updated information about

trafﬁc situation. For this, WAVE deﬁnes Coop-

Figure 2: Hardware Setup.

erative Awareness Messages (CAMs), which are

broadcasting periodically by each vehicle.

• Non-safety messages: which are used to enhance

driver information and comfort: tourism informa-

tion, Internet access, navigation aid, and so on.

In this paper due to this relevance and likelihood of

being deployed in a near future, only three active road

safety use cases have been analyzed : intersection col-

lision warning, emergencyvehicle warning and Road-

work warning.

4 SCENARIO DEFINITION

The three applications under test can be deployed both

in urban or highway scenario. Moreover to be opera-

tive in these environments, cooperative wireless com-

munications must be tested in Line-Of-Sight (LOS)

and Non-Line-Of-Sight (NLOS) conditions. For this

reason to test the performance of I2V IEEE 802.11p

link in an urban LOS and NLOS condition, an indus-

trial area has been chosen. In this location a straight

road and intersections with different height buildings

are available. The communication zone covered by

each RSU is limited to a maximum radius of 1km di-

ameter.

To test the V2V link in a highway under LOS and

NLOS conditions two vehicles have been equipped as

it is shown in Figure 2. In a real highway both ve-

hicles have been driven at different speeds and with

other vehicles (trucks/bus, cars) in the middle of the

communication link deployed between them.

Therefore, in both urban and highway scenario the

challenges of the described tests are:

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• Measuring the throughput, PDR and latency of a

V2X link according to different package size and

distance between the entities involved in each sce-

nario.

• According with the results obtained in previous

tests, determinate if the three proposed applica-

tions are reliable in this context.

4.1 Hardware Setup

In all tests described in this paper, a single omnidi-

rectional radio link is evaluated, either between two

OBUs or between a RSU and an OBU. In both enti-

ties, the same hardware has been deployed running as

OBU or as RSU. The selected hardware is LinkBird-

MX which embedded Linux machines based on a 64

bits MIPS processor working at 266MHz. Besides an

IEEE 802.11p interface, these modules are equipped

with an Ethernet connector that is used to commu-

nicate with the Application Unit (the one that runs

the applications in a regular PC), a GPS interface and

other interfaces as CAN or RS-232. Figure 2 shows

the hardware setup used in the tests that have been

carried out in the described scenarios.

Although LinkBird allows to select two channel

bandwidth, in these tests 10MHz bandwidth has been

selected instead of the 20 MHz one usually used by

802.11a devices, in order to minimize multipath de-

lay spread and Doppler effect that appears in mobility

and highway scenarios. Moreover, in order to main-

tain sufﬁcient reliability of the data transfer in a 1-

hop scenario, the lowest bit rate has been used, that

is 3Mbps (bit rates from 3 to 27Mbps are available

at IEEE 802.11p standard), so also the lowest cod-

ing rate (1/2) with BPSK modulation has been used

to transmit data packets.

Along with the communication modules, two an-

tennas whose characteristics ﬁt well with vehicular

applications are provided. One antenna is tuned to the

178 CCH frequency (5.890GHz) and the other one to

the 180 SCH frequency (5.9GHz) Technical charac-

teristics of hardware setup are shown at Table 2.

4.2 Applications Setup

As it has been mentioned in the introduction, one of

the objectives of this paper is to check the reliability

of three active road safety applications listed by the

ETSI TC on ITS. The goal is to test if the links V2X

that are deployed using previous hardware setup sat-

isfy the requirement of these applications. According

to (ETSI, 9 06), these requirements are summarized

in Table 3.

Table 2: IEEE 802.11P Hardware Setup.

LinkBird-MX

Parameter Values

Frequency 5725-5925 MHz

Bandwidth 10MHz

Tx Power 21dBm

Bitrates 3Mbps

Antenna

Parameter Values

Model ECO6-5500

Frequency 5.0-6.0 GHz

Gain 6dBi

Radation Omni-directional

The SDK provided by NEC with the LinkBird-

MX modules includes a set of Java API to

interact with the IEEE 802.11p protocol stack.

Thanks to these facilities, it is simple to de-

velop applications that send Geographical Unicast,

Topologically-Scoped Broadcast, Single-Hop Broad-

cast and Geographically-ScopedBroadcast or Unicast

messages. In this way and according to the applica-

tions that we want to test (Table 3), all the packets that

will be transmitted in our scenarios will be Single-

Hop Broadcast.

To carry out these experiments we have developed

a single application in Java in which UDP packages

are generated and sent. In this application the payload

and transmitted package rate parameters are modiﬁ-

able as well as the duration of transmission phase.

The IEEE 1609.3 standard species that the maximum

size of a WSMP message is 1400 bytes. For this

reason we have measured the throughput of the V2X

links using different payload size, from 100 bytes to

1400 bytes.

In order to measure the latency in the commu-

nications between OBUs (V2V) and between RSU

and OBU (I2V), we have developed a simple program

that sends Internet Control Message Protocol (ICMP)

echo request every a conﬁgurable time. In this test

also the ICMP payload is also conﬁgurable in order

to know if the latency depends on the package pay-

load.

5 EXPERIMENTAL RESULTS

In this section, the results of measurement campaigns

are shown. According with these results, the reliabil-

ity of the applications under test is analyzed.

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Table 3: Requirements of applications under test.

Intersection

collision

warning

Emergency

vehicle

warning

Roadwork

warning

Application

Driving

assistance

Co-

operative

awareness

Driving

assistance

Co-

operative

awareness

Driving

assistance

Road Haz-

ard Warning

Latency

Less than

100ms

Less than

100ms

Less than

100ms

Message

Fre-

quency

10Hz 10Hz 2Hz

Special

Needs

Accurate

position of

OBU

Triggered

by vehicle

RSU broad-

casts peri-

odic messg.

Link

V2V V2V I2V

Figure 3: Intersection crossing warning scenario.

5.1 Intersection Crossing Warning

Chosen scenario is shown at Figure 3, where there

are two vehicles approaching to the intersection. Al-

though tested scenario is placed at an industrial area,

this is a typical situation in urban scenarios where

buildings create closed intersections with Non Line-

Of-Sight between vehicles. Therefore intersection

crossing warning messages is created in order to warn

drivers of potential impact when entering an intersec-

tion.

The tested scenario recreated the situation where

both vehicles approach to the intersection at 50km/h

(13.88m/s). While in this scenario a stop bar is in

the lane of red vehicle, we consider the worst sce-

nario where this vehicle enters the intersection with-

Figure 4: Intersection crossing warning test results.

out stopping and could collision with green vehicle.

In order to determinatehow the IEEE 802.11psys-

tem can be used to avoid the collision, ﬁrst we work

backwards from the worst scenario, that is, the colli-

sion occurs. Applying kinematic equations to a ve-

hicle in movement, the breaking distance can be cal-

culated using equation 1, where V is the velocity ex-

pressed in Km/h, f represents the friction coefﬁcient

and i the slope of the road in %. At 50km/h a vehicles

requires at least 24m to stop in a ﬂat road. According

to (Triggs et al., 1982) (of State Highway and Ofﬁ-

cials, 2001), drive’s reaction time can be from 1.26s

to 3s. If we consider an average value of 2.5s, in this

time car travels 34m, so in total driver needs 57m to

stop the car.

254( f + i)

(1)

Hence, in order to avoid the collision and warn

the drivers about the presence of each other, the com-

munication link must be reliable and make possible

the messages interchange among vehicles before they

enter within 57m range, referenced as ’unavoidable

collision area’ (in red) at Figure 3.

In this scenario, the results obtained using

LinkBirds communication modules are shown at Fig-

ure 4. It exposes that in this NLOS scenario, these

modules can provide a reliable communication link

between both vehicles (speed@50km/h) with a Pack-

age Delivery Rate of 95% at a distance of 60 me-

ters from the collision point. Hence the vehicles will

be able to interchange awareness messages to inform

about their presence near the intersection in order to

avoid a collision. Moreover, the measure delay at 60m

is 5ms, that is less than the 100ms speciﬁed at (ETSI,

9 06) for this application. The links Goodput in this

scenario is shown at Table 4.

5.2 Emergency Vehicle Warning

In this test, PDR measurements were collected dur-

ing a trip from Mungia to Portugalete. The distance

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Table 4: Intersection Crossing Warning Test Goodput.

Payload Size (Bytes) Goodput (kbps) PDR (%)

100 244 93

500 1221 94

1000 2468 96

1400 2809 100

Figure 5: Emergency vehicle warning scenario.

between both cities is 34kms and the speed limit of

the motorway is 100km/h. The goal in this scenario is

to validate the communication link from one vehicle

to other vehicle where both are in movement at high

speed and there are other vehicles like cars or trucks

in the line of sight of both vehicles.

During the trip one OBU plays the role of vehi-

cle in emergency sending continuously warning mes-

sages to the surrounding vehicles with a period of

1Hz. The other OBU is the receiver and it ana-

lyzes the received message. Different situations were

recreated (Figure 5): V2V link with direct and re-

ﬂected paths (ground reﬂections) and different dis-

tances and relative speed between transmitter and re-

ceiver and V2V link with vehicle blockage (other cars

and trucks) between the OBUs, so it is a NLOS and

that it includes a diffracted signal path to the receiver,

creating the worst-case performance in this scenario.

Figure 6 gives the results of motorway experiment

to measure the PDR for V2V communication link test

using packages with a payload of 100bytes.

The results can be analyzed in sections according

to the events recorder during the test:

• Events ’A’: in both time slots a LOS situation is

shown. Both vehicles speed was the same, ﬁrst

80km/h and then 100km/h and their relative dis-

tance was 45 and 60m respectively. In both sit-

uations PDR is close to 100% being 98-96% the

lowest obtained values.

• Event ’B’: in this time slot, vehicles speed was

90km/h and the relative distance was 177m be-

cause 3 cars were located between OBUs under

test. In this NLOS situation the worst PDR is

78%.

• Event ’C’: it reﬂects the same previous NLOS link

but the distance between OBUs was bigger due

to the 5 vehicles that were in the middle of the

communication link. In this case, the number of

low PDR measures is bigger due to the number of

obstacles and distance between the OBUs.

• Events ’D’ and ’E’: these events represent the

scenario when a truck blocked the link between

OBUs. Here the worst PDR values were obtained

being the event ’E’ the time slot in which the low-

est PDR (29%) was measured. It happened when

the distance between OBUs where close to 200m.

and truck blocked the communication link.

• Event ’F’: during this time slot, only 4 vehicles

were between OBUs but it happened at the mo-

torway output ramp, where there is u-shaped bend

so it was NLOS link. In this case values of PDR

lower than 80% were measured.

5.3 Roadwork Warning

To test this safety application, we have recreated

a vehicle-to-infrastructure communication between a

mobile OBU and a static OBU that plays the role

of Road Side Unit (RSU). In this scenario mobile

OBU travels at 40km/h (11,12m/s) along a road that

has a straight line of 860m (A to B distance) while

RSU send warning messages (100bytes of payload) .

First we have measured the distance range covered by

the RSU using LinkBirds units. The measurements

showed at Figure 7 display a two-way trip from A to

B when RSU is located at ’A’ location and mobile

OBU starts the trip close to the RSU, travels until ’B’

and comes back to the original position. The obtained

PDR is shown in Figure 7, where dash line represents

point ’B’ where OBU starts come back trip.

Figure 6: Emergency vehicle warning test results.

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Figure 7: I2V scenario PDR measurements.

Figure 8: I2V goodput measurements.

During the test link PDR was closed to 0% in

some speciﬁc locations. It was because the orogra-

phy of the road due to some ’valleys’ in which mobile

OBU has not LOS to the RSU. The others low PDR

values (around 50%) were caused by some vehicles

that obstacle the link between the OBU and the RSU.

The goodput was measured at different RSU-OBU

distances and the results are shown at Figure 8. The

PHY layer of the IEEE 802.11p transceiver was set

up to 3Mbps. The maximum goodput (2.8Mbps) was

obtained when the payload is setup to the maximum

value of 1400bytes. In this conﬁguration, at the maxi-

mum RSU-OBU distance, the PDR was 100% and the

average delay was 7ms.

According to these results and considering that

RSU is equipped with an isotropic antenna, RSU’s

radio link cover area can be close to 1700m. In this

situation the success of a Roadwork Working safety

application depends on two factors: vehicle speed and

distance from the end of the covered area by the RSU

and the location of roadwork. The limiting cases hap-

pen when the OBU receives the warning message at

the end of the RSUs covered area and the distance to

the roadwork is equal to the distance that the drivers

needs to stop the vehicle. Both situations are shown at

Figure 9 with the calculations of the security distance

according to equation 1.

Figure 9: Roadwork warning scenario.

In case that the roadwork is located in a distance

lower than Ds, the driver will not manage to stop the

car and a hazard situation could happen. To avoid this

situation, two alternatives are suggested:

• Deploy a net of RSUs interconnected by a back-

bone in order to keep updated all of them with in-

formation about the trafﬁc events. Then the whole

road (or speciﬁc section that could concentrate

blackspots) could be covered by the IEEE 802.11p

net.

• Combine I2V with V2V communications. In this

way OBUs that are inside the RSU covered area

could inform about hazardous situations to OBUs

that come near to them.

6 CONCLUSIONS

The results obtained in these scenarios disclosed use-

ful ideas which can help vehicular networks and ac-

tive safety applications developers. Regarding the

goodput, measured values show that increasing the

payload of the messages, high values of goodput are

obtained. This is an obvious conclusion, but we have

tested at the same time that at distances bigger than

800m (RSU-OBU), the PDR of big payload messages

(1400bytes) is 100%. Likelyhood warning messages

do not need more than 400bits of payload if we con-

sider Basic Safety Message (BSM) as a reference

(SAEJ2735, 2009), so it can be settled that in the pro-

posed scenarios, the analyzed applications can work

in a correct way. Furthermore, the delay obtained in

all the measures is behind the threshold delay speci-

ﬁed by the ETSI TC on ITS (ETSI, 9 06).

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It also can be concluded that using the proposed

hardware conﬁguration, there are not problems in

LOS scenarios because high values of PDR and good-

put are obtained. However,this conﬁgurationpresents

poor IEEE 802.11p performance in NLOS conditions,

so in order to provide full coverage of a givenarea, the

orography and building distribution must be studied

and maybe a ﬁxed RSUs network should be deployed.

Before concluding this paper, we want to express

that more measurement campaigns should be per-

formed in a near future to complete this study, but

it could be considered as a starting point towards bet-

ter design of active safety applications. Higher dis-

tances among vehicles and RSUs should be tested and

in these situations problems as handover, beacons de-

lay or channel congestions issued will be tackled.

ACKNOWLEDGEMENTS

The authors would like to thank the EU Intelligent

Cooperative Sensing for Improved trafﬁc efﬁciency

(ICSI) project (FP7-ICT-2011-8) for its support in the

development of this work.

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