Research Article Reliable and Swift Message Broadcast Method in Vehicular …downloads.hindawi.com/journals/ijdsn/2015/219689.pdf · 2015-11-24 · random as the relay node. Black

Research ArticleReliable and Swift Message Broadcast Method inVehicular Ad Hoc Networks

Jaesung Park and Yujin Lim

Department of Information Security and Department of Information Media, University of Suwon, San 2-2, Wau-ri, Bongdam-eup,Hwaseong, Gyeonggi-do 445-743, Republic of Korea

Correspondence should be addressed to Yujin Lim; [email protected]

Received 26 September 2014; Accepted 18 December 2014

Academic Editor: Kun Hua

Copyright © 2015 J. Park and Y. Lim. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

A vehicular ad hoc network (VANET) could deliver safety-related messages reliably within a short time to increase road safety.Since safety-related messages should be sent to a set of unspecified receivers, they are delivered by broadcast method. However, thebroadcast method specified in the IEEE 802.11p does not have a collision avoidance procedure and receivers do not acknowledgewhen they receive a broadcast frame. In addition, frames could be lost and corrupted.Therefore, as the portion of nodes that do notreceive a broadcast frame increases, the effectiveness of a safety application decreases. To tackle the problem, we propose a reliableand swift message broadcast method (RSMB). In RSMB, to expedite message dissemination process, a relay node is selected in adistributed manner considering the progress made to a frame and the delay requirements of an application. In addition, a relaynode broadcasts a message multiple times to assure that the probability that the other nodes successfully receive the message atleast once is larger than a given threshold value. Since the number of rebroadcasts is regulated based on the successful messagereception probability, the additional bandwidth needed to increase the reliability of broadcast is reasonably small.

1. Introduction

Vehicular ad hoc network (VANET) has drawn attentionas a key technology to provide road safety, comfort, andcommercial applications. VANET is also expected to leveragethe intelligent transport system.

Recognizing the potential of VANET, FCC licensed5.9GHz frequency band for the dedicated short range com-munication (DSRC), which is divided into seven 10MHzchannels and 5MHz guard bands. On the other hand, IEEE802.11p task group is formed to develop a standard forVANET, which is called the wireless access in vehicularenvironment (WAVE). The group has been specifying theMAC protocol by modifying the enhanced distributed chan-nel access (EDCA) of IEEE 802.11e. In the specification, onecontrol channel (CCH) and six service channels (SCHs) aredefined. For each type of channel, four access classes aredefined. Safety applications use the CCH while the otherapplications use one of the SCHs. To provide both thesafety applications and other applications cost effectively,the CCH and the SCHs are switched alternately at every

synchronization interval of 100ms. Since all nodes must tuneto the CCH during the CCH interval, safety-related andsystem control data could be exchanged among them. After5ms of guard band, the nodes switch to one of the SCHs theyare subscribed to.

Safety application is one of the most important targetapplications of VANET. Safety applications require thatsafety-related messages should be disseminated through acertain area within a very short time and received by all thevehicles vulnerable to traffic accidents [1]. Since the applica-tion requirement is tight, if the communicationmethod is notengineered carefully, the effectiveness of safety applicationsmay decrease. Broadcasting is used to deliver safety-relatedmessages to many unspecified vehicles. In addition, thedistance from the message originator to the point wheresafety-relatedmessages should be delivered is longer than thetransmission radius of a wireless communication interface,they should be delivered among the vehicles in a multihopfashion.

However, in IEEE 802.11p, receivers do not send acknowl-edgement for the broadcasted packets. Therefore, it is not

Hindawi Publishing CorporationInternational Journal of Distributed Sensor NetworksVolume 2015, Article ID 219689, 8 pageshttp://dx.doi.org/10.1155/2015/219689

2 International Journal of Distributed Sensor Networks

guaranteed that safety-related messages are received by therelevant vehicles. Moreover since a message is sent by mul-tiple relays, the message dissemination delay could increase.Therefore, to support the service requirements of a safetyapplication, an efficient relay selection method is requiredthat considers the required message delivery time and themessage reception reliability.

In [2], a geocast is proposed for message routing inVANET. Geocast is composed of a forwarding phase anda diffusion phase. In the forwarding phase, a message isdelivered to a certain area. In the diffusion phase, a messageis sent to all the nodes in the area. However, geocast is mainlyfocused on the relay selection method in the forwardingphase while simple flooding is used in the diffusion phase.Since each vehicle rebroadcasts once when it receives amessage, so-called broadcast storm problem may occur towaste communication bandwidth and fail a safety application[3].

A fast relay method is proposed in [4–6] to avoid thebroadcast storm problem. Among the vehicles receiving amessage from the current relay, the fast relay selects thevehicle that is farthest away from the current relay as the nextrelay. The fast relay could reduce the number of broadcastmessages by forcing the vehicles residing between the currentrelay and the next relay not to rebroadcast the message. Inaddition, since the farthest vehicle from the current relay isselected as the next relay, the message could be delivered fast.

However, they do not consider the message receptionprobability when the current relay broadcasts the message.In IEEE 802.11p, a sender broadcasting a message couldnot be convinced whether or not all of its neighbor nodesreceived themessage because there is neither RTS/CTS frameexchange nor ACK issued by a receiver. On the other hand,the measurement studies for the VANET channels show thatthe received signal strength varies widely at all distances[7, 8].Therefore, if the portion of vehicles that reside betweenthe current relay and the next relay and do not receive asafety-related message increases, the effectiveness of a safetyapplication deteriorates seriously.

In this paper, we propose a reliable and swift messagebroadcasting (RSMB) method for safety applications inVANET. RSMB is designed based on the successful messagereception probability and is composed of two stages. In thefirst stage, to expedite a message delivery, the next relayof a message is selected in a way that could make thefarthest progress to the message. The relay selection processoperates in a distributed manner considering the location ofvehicles and the requirement on the dissemination delay ofthe message from the message originator to a given effectivedistance. The selected relay broadcasts the received messagemultiple times to increase the probability that the vehicleslocated between the previous relay and itself successfullyreceive the message at least once to a given threshold level.RSMB avoids the broadcast storm problem by preventing thevehicles that are not selected as a relay from rebroadcastinga message. However, since RSMB makes a relay to broad-cast a message multiple times, network bandwidth may bewasted if the number of broadcast messages increases exces-sively large with the number of vehicles. Since RSMB uses

the successful message reception probability, the number ofmessage broadcasts by a relay does not increase with thedensity of vehicles. In addition, since RSMB operates in adistributed manner, RSMB is scalable and adaptive to rapidnetwork topology change.

The rest of the paper is organized as follows. In Section 2,we review the related works. We describe our reliable andswift broadcast method for VANET in Section 3 wherewe also provide an analytical framework for the messagereception probability. In Section 4, we discuss the simulationresults. Section 5 concludes the paper.

2. Related Works

In the broadcastingmethods of the IEEE 802.11p standard, allnodes have the samepriority to relaymessages. It leads to seri-ous redundancy, contention, and collision to a large numberof nodes trying to resend the packet to their neighbors at thesame time. To solve the problem, many broadcast methodsfor message dissemination in VANETs have been proposedin the literature.

In the modified MAC approach, the backoff mechanismof IEEE 802.11p standard is improved to adjust the backoffwindow sizes. In [9], it is addressed that the backoff windowsizes are not adaptive to the dynamics of the numbers ofnodes. Thus, two algorithms are proposed which need exactinformation about the number of concurrent communicatingnodes to calculate the optimal window size. In [10], authorsalso address the backoff mechanism of the standard. In thestandard, each node uniformly chooses its backoff time inevery backoff stage. In this way, fairness among all the nodesis guaranteed. However, this backoff mechanism ignoresthe propagation distance. It means that the farthest nodedoes not have the higher priority to relay. In the proposedalgorithm, the lengths of the backoff times are generatedfrom a nonuniform distribution. They are related with thedistances between the sender and the receivers.

Even though the modified MAC approach is efficientto relay messages, it needs to improve the standard MACprotocol. Thus, another approach is proposed to disseminatemessages without the modification of the standard. In thedistance-based approach, the farthest node is chosen as therelay node. The farthest node offers maximum coverage asa result of which the number of hops is reduced. Thus, itreduces the end-to-end delay.

In normal distance-based approach, the sender transmitsthe broadcasting message, and the receivers then wait topropagate it within each waiting time. The waiting time isshorter for more distant receivers. When one of the receiverstransmits the receivedmessage, other nodes do not propagatethe same message and discard it [11]. In the urban multihopbroadcast (UMB) [12], to select the farthest node as the relaynode, the area inside the transmission range is divided intoa certain number of segments of equal width. The nodesin all segments choose black-burst lengths proportional tothe distance of their segment from the sender with thefarthest segment having the longest black-burst duration. Oncompletion of the black burst, the node replies to the sender.

International Journal of Distributed Sensor Networks 3

O

Relay nodesMessage trajectory

R

Message originatorO

(xo, yo)

dE

lW

r

Figure 1: Safety message dissemination in VANET.

Then, the sender transmits the broadcast packet. In thesmart broadcast (SB) [5], the similar segment-based approachis used. It differs from UMB in a way that each segmentis assigned a fixed-size contention window. On receivingrequest from the sender, nodes randomly choose a backofftime from thewindow allocated to their segment.The backofftimes in a contentionwindow increase as approaching towardthe sender. Thus, a node in the farthest segment times outfirst and is chosen as the relay node. In [13], it addressesthe latency issue in the segment-based approach. It uses abinary-partition-based approach to iteratively partition thearea inside the transmission range to produce the farthestnarrow segment. Then, a node in that segment is chosen atrandom as the relay node. Black burst is used to select apotential segment and eliminate the nonpotential segmentfrom further consideration.

3. Data Dissemination Method

In this section, we describe a reliable and swift messagebroadcast method in a VANET. The proposed method iscomposed of two distinct steps. First, a relay node is selectedin a distributed manner. Then, the elected relay node broad-casts a message repeatedly to guarantee that the probabilitythat nodes located between the previous relay node and thecurrent relay node successfully receive amessage at least oncebecomes higher than a given threshold value.

3.1. System Model. We consider a scenario that a safety-related message originated from a node O is disseminated aneffective distance 𝑑

𝐸fromO in amultihopmanner. Broadcast

is an efficient method to deliver a message to unspecifiednodes. However, we need to specify a set of nodes that shouldreceive a safety-related message. Noting that a safety messagebecomes less valuable as the distance from O increases, weintroduce the concept of the effective distance to specifythe target receivers. In other words, the goal of RSMB is toachieve that the probability that nodes located in the circlecentered at O and having a radius 𝑑

𝐸receive successfully a

safety-relatedmessage at least once becomes a threshold value𝑝th (Figure 1). We assume that all the nodes have the sametransmission radius of 𝑟 and the road width 𝑙

𝑊is less than 𝑟.

We also assume 𝑑𝐸> 𝑟. We denote the 𝑥, 𝑦 coordinates of

a node 𝑖 as (𝑥𝑖, 𝑦𝑖) and the Euclidean distance between node 𝑖

and 𝑗 as 𝑑𝑖,𝑗.

3.2. Relay Node Selection. A relay node is selected in adistributed manner. When a node 𝑖 receives a message, 𝑖checks whether it receives the message for the first time ornot by checking the identifier of themessage originatorO andthe sequence number contained in the message. If 𝑖 receivedthe message for the first time, 𝑖 examines the location of thecurrent relay node C. If the current relay node is farther awayfromO than the node 𝑖, 𝑖does not participate in the relay nodeselection process. On the other hand, if 𝑑O,C < 𝑑O,𝑖, 𝑖 becomesa potential relay node. Then, 𝑖 joins in the competition forelecting the next relay node by setting up a wait timer as𝑊𝑡= 𝑇𝑤𝑟/𝑑C,𝑖.

To expedite the message dissemination process, the nextrelay node should be the one that makes the farthest progressto the message. Thus, the wait time is inversely proportionalto the distance between C and 𝑖. The parameter 𝑇

𝑤is the

maximum time that can be tolerated to deliver a messagefrom the current relay node to the next relay node. Since 𝑑

𝐸

is longer than the road width, the minimum number of hopsto deliver a message from O to the point 𝑥 where 𝑑O,𝑥 = 𝑑𝐸becomes ℎ

𝑚= 𝑑𝐸/𝑟.

The message transmission delay from a node to itsneighbor is the sum of the message propagation delay (𝑡pro),random delay caused by MAC layer contention (𝑡mac), andthe waiting time involved in the relay selection process (𝑊

𝑡).

Since the timescale of𝑊𝑡is much larger than that of 𝑡pro and

𝑡mac, we approximate the message transmission delay as𝑊𝑡.

Then, if the message should be sent to 𝑥 within 𝑡R, each hopcould spend 𝑇

𝑤= 𝑡R/ℎ𝑚 on average to forward a message

from the current relay node to the next one. We assumethat the message originator determines 𝑇

𝑤and writes it in a

message header so that the rest of the nodes can read it whenthey receive the message.

When a node 𝑖 receives the same message before its waittimer expires, it checks the location of themessage sender 𝑗. If𝑖 is closer to O than 𝑗, 𝑖 abandons the competition because themessage passed it. On the other hand, if 𝑑O,𝑗 < 𝑑O,𝑖, 𝑖 remainsin the competition to expedite the message transfer. If 𝑖 doesnot receive the same message until its𝑊

𝑡expires, it becomes

the next relay node by rebroadcasting themessage it received.Since the broadcast in IEEE 802.11 MAC does not have

collision avoidance procedure, a message broadcasted by 𝑖may not be received by its neighbor nodes. In addition, sincethe wireless channel fluctuates widely at all distances, a nodewithin a transmission range of 𝑖may not receive the messageeven if it is close to 𝑖. To increase the message receptionprobability to 𝑝th, a relay node sends the message multipletimes according to the following reliable broadcast procedure.

3.3. Reliable Broadcast. Each node manages its messagereception probability. Since a message should travel fast, weassume that a network topology does not change during thetime between when O sends the message and the time ittraveled the effective distance. For example, if a node moves120 km/h, it could move 3.3m approximately during 100ms.


In a road, the radio propagation environment of a relaynode is usually similar to those of its neighbors. In addition,the number of neighboring nodes of a relay node 𝑖 is similarto that of 𝑖’s neighbors. Thus, we approximate the messagereception probabilities of 𝑖’s neighboring nodes as that of arelay node 𝑖. We denote the message reception probabilityof a relay node by 𝑝

𝑟. Then, when a relay node broadcasts

a message 𝑛𝑟times, the probability that a neighboring node

receives the message at least once becomes 𝑝𝑟(𝑛𝑟) = 1 −

(1 − 𝑝𝑟)𝑛𝑟 . Therefore, to ensure 𝑝

𝑟(𝑛𝑟) = 𝑝th, the number of

times in which a relay node must send a message is obtainedas

𝑛𝑟= ⌊

log (1 − 𝑝th)2 log (1 − 𝑝

𝑟)⌋ , (1)

where, ⌊𝑥⌋ represents theminimum integer that is not smallerthan 𝑥.

3.4. Message Reception Probability. A node may not receivea message sent by a relay node if a collision occurs at aMAC layer or the received signal strength of the message isbelow a given threshold value. Since these two factors areindependent of each other, we derive the message receptionprobability at each layer independently.

(1) MAC Layer. There can be two types of collision whena message is broadcasted. A synchronous collision occurswhen at least one of the neighboring nodes of a relay nodesends a frame while the relay node broadcasts a message.On the other hand, an asynchronous collision occurs whena hidden node of a relay node transmits while the relay nodeis transmitting a message.

Let us denote 𝑛 as the number of neighboring nodes of arelay node R. We also denote 𝑝

𝑡as the message transmission

probability of a node at the beginning of each time slot.Following the assumptions in [14], we assume that the densityof nodes in a network (𝜌) follows Poisson distribution.Then,the probability that a synchronous collision does not occurbecomes

𝑝𝑠=

∞

∑

𝑛=0

(1 − 𝑝𝑡)𝑛(𝜌𝜋𝑟2)𝑛

𝑛!𝑒−𝜌𝜋𝑟2

= 𝑒−𝜌𝜋𝑟2

𝑝𝑡 = 𝑒−𝑁𝑝𝑡 ,

(2)

where𝑁 is the average number of nodes within a circle witha radius 𝑟.

If the average message size is 𝑀 and the average trans-mission rate of a node is C, the frame transmission timebecomes 𝑇

𝑎= 𝑀/𝐶. If we denote the size of time slot as 𝑡

𝑠,

the frame transmission time normalized by 𝑡𝑠is𝑇𝑚= ⌊𝑇𝑎/𝑡𝑠⌋.

Therefore, an asynchronous collision occurs if at least one ofthe hidden nodes transmits during 2𝑇

𝑚.

We take a geographic approach to calculate the asyn-chronous collision probability. In Figure 2, the relay node Ris interfered if nodes residing in the interference area of anode B which is surrounded by the arc ACD multiplied by2 transmit while it sends a message to a node B that is 𝑑 away

r

r

Interference region by

hidden nodes of R.

d

Relay node

Receiving node

R

A

B C D

𝜃1 𝜃2

Figure 2: Interference region of the current relay node.

from R. Let 𝜃1and 𝜃2be ∡ARB and ∡ABC, respectively. If we

denote the size of a triangle RAB as 𝐴1, the size of an area

surrounded by the arc RAC as 𝐴2, and the size of a region

surrounded by the arc BAD as 𝐴3, they can be obtained as

𝐴1=1

2𝑑√𝑟2 −

𝑑2

4,

𝐴2=𝜃1

2𝑟2,

𝐴3=𝜃2

2𝑟2.

(3)

Since cos(𝜃1) = 𝑑/2𝑟, cos(𝜋 − 𝜃

1) = − cos(𝜃

2), the size of

the region surrounded by ACD becomes

𝐴4= 𝐴3− (𝐴2− 𝐴1)

=𝑟2

2acos(− 𝑑

2𝑟) +

𝑑

2

√𝑟2 −𝑑2

4−𝑟2

2acos( 𝑑

2𝑟) .

(4)

We denote the number of nodes in the interference areaof the node B as 𝑁

𝑐. Then, 𝑁

𝑐should be zero during 2𝑇

𝑚

or all the nodes in 2𝐴4should not send a frame during 2𝑇

𝑚

for B to receive a message from R without an asynchronouscollision.Therefore, the probability that a node 𝑑 away from arelay node receives a framewithout an asynchronous collisionbecomes

𝑝𝑎 (𝑑) = (Pr (𝑁𝑐 = 0) +

∞

∑

𝑛=1

(1 − 𝑝𝑡)𝑛 (2𝜌𝐴4)

𝑛

𝑛!𝑒−2𝜌𝐴

4)

2𝑇𝑚

= 𝑒−8𝑇𝑚𝜌𝐴4𝑝𝑡 .

(5)

To receive a message from a relay node successfully, boththe synchronous collision and the asynchronous collisionshould not occur. Therefore, if the distance between a relaynode and its neighboring node is 𝑑, the probability ofreceiving a message from a relay node without MAC layercollision becomes

𝑝mac (𝑑) = 𝑝𝑠𝑝𝑎 (𝑑) = 𝑒−(𝑁+8𝑇

𝑚𝜌𝐴4)𝑝𝑡 . (6)


(2) PHY Layer. The frame reception probability at a physicallayer depends on the channel characteristics. We adoptthe channel model proposed by WINNER (Wireless WorldInitiative New Radio) project to analyze the frame receptionprobability at a physical layer. The WINNER channel modelcan be applied to 2∼6GHz frequency bands. If the distancebetween a sender and a receiver is 𝑑, the path loss is modeledas [15]

Ψ (𝑑, 𝑓𝑐) = 𝜒log

10(𝑑) + 𝜓 + 𝜔log

10(𝑓𝑐

5) + 𝑋, (7)

where 𝑓𝑐is a system frequency in GHz. The parameter 𝜒 is

the path loss exponent determined by the radio propagationenvironments, 𝜓 is an intercept, and 𝜔 is the path lossexponent determined by a carrier frequency. The influenceof the shadow fading is represented by a random number Xthat follows a log-normal distribution with mean zero andstandard deviation 𝜎

𝑥. In the WINNDER project, seven dif-

ferent radio propagation environments are specified and therelevant parameters are defined for each radio propagationenvironment.

According to the WINNER model, the signal power at areceiver that is 𝑑 away from a sender is obtained as

𝑆𝑑B (𝑑, 𝑓𝑐) = 𝛼 − Ψ (𝑑, 𝑓𝑐) , (8)

where the parameter 𝛼 is the frame reception power at areceiver when the distance between a sender and a receiver isthe reference distance.𝛼 is determined by the transmit power,the antenna heights of a sender and a receiver, antenna gain,and the reference distance which depends on the size of a cell[16]. If the threshold value in which a node receives a framewithout error at a physical layer is 𝑆th, then, the successfulframe reception probability at a physical layer is obtained as𝑝phy(𝑑) = Pr(𝑆

𝑑B(𝑑, 𝑓𝑐) ≥ 𝑆th).Denote

Φ(𝑑, 𝑓𝑐) = 𝜒log

10(𝑑) + 𝜓 + 𝜔log

10(𝑓𝑐

5) ,

𝑝phy (𝑑) = Pr (𝑋 ≥ 𝑆th + Φ (𝑑, 𝑓𝑐) − 𝛼)

= 1 − ∫

−𝛼+𝑆th+Φ(𝑑,𝑓𝑐)

𝑥=−∞

1

𝜎𝑥√2𝜋

𝑒−𝑥2

/2𝜎2

𝑥𝑑𝑥.

(9)

From (6) and (9), when the distance between a sender anda receiver is 𝑑, the probability that a receiver successfullyreceives a message from a sender becomes

𝑝𝑟 (𝑑) = 𝑝max (𝑑) 𝑝phy (𝑑) . (10)

4. Performance Analysis

4.1. Numerical Results. We evaluate the broadcast messagereception probability according to IEEE 802.11p systemparameters (such as transmission range, data rate, minimumcontention window, and time slot) and the network operat-ing environments (such as radio propagation environmentand node density). IEEE 802.11p standard uses channels of

0.0

0.2

0.4

0.6

0.8

1.0

0 50 100 150 200 250 300Distance from a sender (r)

Broa

dcas

t mes

sage

rece

ptio

n pr

obab

ility

(urb

an m

acro

cell

LOS)

(𝜌, pt) = (0.1, 0.1)

(𝜌, pt) = (0.9, 0.1)(𝜌, pt) = (0.1, 0.05)

(𝜌, pt) = (0.1, 0.01)

(𝜌, pt) = (0.5, 0.1)

(𝜌, pt) = (0.5, 0.05)

(𝜌, pt) = (0.5, 0.01)

(𝜌, pt) = (0.9, 0.05)

(𝜌, pt) = (0.9, 0.01)

Figure 3: Broadcast message reception probability according to thedistance from a sender in the good radio propagation environment.

10MHzbandwidth in the 5.9GHz band.We assume a 10MHzcontrol channel (CCH) operating at a data rate of 6Mbps.Theslot time is 16 𝜇s and the frame size is set to 200 bytes. Thetransmission power is set to 20mW such that the receivingpower at the communication range, that is, 300m, is the sameor greater than the received power threshold. The numberof nodes in the transmission range of a sender is set to100. The message transmission probability of a node at thebeginning of each time slot 𝑝

𝑡is set to 0.1, 0.05, and 0.01,

respectively, which indicates the instantaneous data rate of10Mbps, 5Mbps, and 1Mbps.

Figures 3 and 4 show the broadcast message receptionprobability according to the distance between a sender anda receiver. In Figure 3, we assume the urban macrocell lineof sight (LOS) with(A, B, C,𝜎

𝑥) = (26, 25, 20, 4) in WINNER

channel model [15]. In Figure 4, the urban macrocell nonlineof sight (NLOS) with (A, B, C,𝜎

𝑥) = (35.7, 26.5, 23, 8). The

reception probability in the poor propagation environmentdecreases more than that in the good propagation environ-ment. Besides, when the distance from a sender is small,𝑝𝑡could impact more than the other parameters. When the

distance is long, the distance could impact more than theothers.

In Figures 5 and 6, the effect of the broadcast messagereception probability at the MAC and PHY layer on thetotal broadcast message reception probability is analyzed,respectively. In Figure 5, we assume the urban macrocellLOS. As shown in the figure, at the MAC layer, 𝑝

𝑡could

impact more than the number of nodes and the distancefrom a sender. In Figure 6, we analyze the effect in varyingpropagation environments. At the PHY layer, the distancecould impact more than the other parameters, when thepropagation environment is poor (i.e., path loss exponent 𝐴is large).


0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

Broa

dcas

t mes

sage

rece

ptio

n pr

obab

ility

(urb

an m

acro

cell

NLO

S)

Distance from a sender (r)

(𝜌, pt) = (0.1, 0.1)

(𝜌, pt) = (0.9, 0.1)(𝜌, pt) = (0.1, 0.05)

(𝜌, pt) = (0.1, 0.01)

(𝜌, pt) = (0.5, 0.1)

(𝜌, pt) = (0.5, 0.05)

(𝜌, pt) = (0.5, 0.01)

(𝜌, pt) = (0.9, 0.05)

(𝜌, pt) = (0.9, 0.01)

Figure 4: Broadcast message reception probability according to thedistance from a sender in the poor radio propagation environment.

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

Broa

dcas

t mes

sage

rece

ptio

n pr

obab

ility

at M

AC la

yer


(𝜌, pt) = (0.1, 0.1)

(𝜌, pt) = (0.1, 0.05)

(𝜌, pt) = (0.1, 0.01)

(𝜌, pt) = (0.5, 0.1)

(𝜌, pt) = (0.5, 0.05)

(𝜌, pt) = (0.5, 0.01)

Figure 5: Broadcast message reception probability at theMAC layeraccording to the distance from a sender.

As a result, the broadcast message reception probabilityof nodes in the transmission range of a sender depends on 𝑝

𝑡

and the distance from a sender. In other words, nodes to belocated between a sender and a relay node may fail to receivethe broadcast frame. Thus, in order to improve the broadcastmessage reception probability, we need a rebroadcastingmethod, as well as a relay selection method.

4.2. Experimental Results. We evaluate RSMB based onthe message delivery ratio and the number of transmittedmessages. The message delivery ratio is defined as the ratioof the number of receiving nodes to the total number ofnodes in the effective area. The safety message propagatesto the effective distance (𝑑

𝐸= 3000m). We consider traffic

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0


Broa

dcas

t mes

sage

rece

ptio

n pr

obab

ility

at P

HY

laye

r

(A, B,C, 𝜎x) = (23.8, 27.2, 20, 4): suburban macrocell LOS(A, B,C, 𝜎x) = (35.7, 23.5, 23, 8): suburban macrocell NLOS(A, B,C, 𝜎x) = (26, 25, 20, 4): urban macrocell LOS(A, B,C, 𝜎x) = (35.7, 26.5, 23, 8): urban macrocell NLOS

Figure 6: Broadcast message reception probability at the PHY layeraccording to the distance from a sender.

in a straight one-way, four-lane highway which consists ofvarious numbers of vehicles. The vehicles’ speed ranges from80 to 120 km/h. The message originator is located at themiddle in the left end of the effective area. Safety applicationsrequire at most a 10msmean delay and a 99.9% probability ofsuccessful message transmission in order to be effective [1].Based on the requirements, we set 𝑝th to 0.999. We comparethe performance of RSMB with the conventional distance-based scheme [11]. In both of the conventional scheme andRSMB, the farthest node from a sender is selected as a relaynode. However, the relay node broadcasts the safety messageonce in the conventional scheme. In RSMB, the broadcastis repeated 𝑛

𝑟times according to (1) to increase the safety

message reception probability by 𝑝th.Figures 7 and 8 show the message delivery ratio in two

propagation environments. We assume the urban macrocellLOS and NLOS in Figures 7 and 8, respectively. In thefigures, “farthest node” indicates the conventional distance-based scheme. We vary the message transmission probabilityof a node at the beginning of each time slot, 𝑝

𝑡. The node

density 𝜌 represents the average number of nodes located ina unit rectangular area approximating the effective area. Invarious node densities, the message delivery ratio of RSMBis over 93% due to the multiple rebroadcasts. On the otherhand, the performance of the conventional scheme dependson the node density. When the density increases to a certainpoint, the performance also increases. When the densityincreases over a certain point and the collision probabilitysharply increases, the performance decreases instead, dueto the decrease of message reception probability. For theconventional distance-based scheme, the message deliveryratio is better when the radio propagation environment ispoor compared to when it is good. As the radio propa-gation environment becomes poorer, the distance betweenthe current relay node and the next relay node becomes


0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.6

0.7

0.8

0.9

1.0

Node density (𝜌)

Mes

sage

del

iver

y ra

tio

RSMB (pt = 0.1)RSMB (pt = 0.05)

RSMB (pt = 0.01)Farthest node

Figure 7: Message delivery ratio in the good environment.

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.90

0.92

0.94

0.96

0.98

1.00

Node density (𝜌)

Mes

sage

del

iver

y ra

tio

RSMB (pt = 0.1)RSMB (pt = 0.05)


Figure 8: Message delivery ratio in the poor environment.

shorter because it ismore probable that themessage receptionprobability of a node decreases with the distance. Thus, thenodes that are not selected as a relay node have more chanceto receive the message, which increase the message deliveryratio.

Figures 9 and 10 show the number of the transmittedmessages which is divided by the number of nodes inthe effective area. Because each relay node broadcasts themessage once, the number of the transmitted messages of theconventional scheme is lower than that of RSMB. In RSMB,when 𝑝

𝑡increases and the collision probability increases, the

number of the transmitted messages increases. Besides, eventhough the node density and the collision probability increasein RSMB, the growth rate of the number of transmittedmessages due to the collision is less than the growth rate of thenumber of nodes in the effective area.Thus, the number of themessages decreases instead when the node density increases.

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

1

2

3

4

Num

ber o

f tra

nsm

itted

mes

sage

s/nu

mbe

r of n

odes

Node density (𝜌)

RSMB (pt = 0.1)RSMB (pt = 0.05)


Figure 9: The number of the transmitted messages in the goodenvironment.

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

2

4

6

8

Num

ber o

f tra

nsm

itted

mes

sage

s/nu

mbe

r of n

odes

Node density (𝜌)

RSMB (pt = 0.1)RSMB (pt = 0.05)


Figure 10: The number of the transmitted messages in the poorenvironment.

5. Conclusion

In this paper, we propose a reliable and swift message broad-casting (RSMB) method for safety applications in VANET.RSMB is designed based on the successful message receptionprobability. To expedite a message delivery, the next relay ofa message is selected in a way that could make the farthestprogress to the message. The selected relay broadcasts thereceived message multiple times to increase the probabilitythat the vehicles located between the previous relay anditself successfully receive the message at least once. Throughperformance analysis, we show that the message deliveryratio of RSMB is over 93% in various node densities. Inaddition, the increase in the number of broadcast messages ismarginal with the number of vehicles in the network becauseRSMB determines the number of message broadcast with theprobability of successful message reception.

For the future work, we plan to evaluate the performanceRSMB in various real scenarios.


Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

This work was supported by the GRRC program of Gyeonggiprovince ((GRRCSUWON2014-B3), Development of CloudComputing-based Intelligent Video Security SurveillanceSystem with Active Tracking Technology). This work issupported by Basic Science Research Program through theNational Research Foundation of Korea funded by theMinistry of Education, Science and Technology (NRF-2011-0007076 (2014010024)).

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Research Article Reliable and Swift Message Broadcast Method in Vehicular …downloads.hindawi.com/journals/ijdsn/2015/219689.pdf · 2015-11-24 · random as the relay node. Black