efcient and reliable broadcast in inter-vehicle
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Efficient and Reliable Broadcast inInter-Vehicle Communications
Networks: A Cross Layer Approach
Yuanguo Bi, Lin X. Cai, Xuemin(Sherman) Shen, Hai ZhaoDate Submitted: 7 November 2009Date Published: 16 November 2009
Updated information and services can be found at:https://engine.lib.uwaterloo.ca/ojs-
2.2/index.php/pptvt/article/view/587
These include:
Subject Classification Vehicular Technology
Keywords Inter-vehicle communications; Cross layer design;Relayingmetric; Broadcast protocol;
Submitting Author'sComments
This paper has been submitted to IEEE TVT
Comments You can respond to this article at:https://engine.lib.uwaterloo.ca/ojs-2.2/index.php/pptvt/comment/add/587/0
Copyright Copyright © Date Submitted: 7 November 2009 Yuanguo Bi etal. This is an open access article distributed under the CreativeCommons Attribution 2.5 Canada License , which permitsunrestricted use, distribution, and reproduction in any medium,provided the original work is properly cited.
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Effi cient and Reliable Broadcast in Inter-VehicleCommunications Networks: A Cross Layer Approach
Yuanguo Bi †,‡, Lin X. Cai ‡, Student Member, IEEE
Xuemin (Sherman) Shen ‡, Fellow, IEEE , and Hai Zhao †
Department of Computer Science and Technology †,
Northeastern University, Shenyang, 110004, China
[email protected], [email protected]
Department of Electrical and Computer Engineering ‡,
University of Waterloo, Waterloo, ON N2L 3G1, Canada
{ygbi, lcai, xshen }@bbcr.uwaterloo.ca
Correspondence: Professor Xuemin (Sherman) ShenDepartment of Electrical and Computer Engineering
University of Waterloo
Waterloo, Ontario, Canada N2L 3G1
Phone: + 1 (519) 888-4567 ext. 32691
Fax: + 1 (519) 746-3077
Email: [email protected]
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Efficient and Reliable Broadcast in Inter-Vehicle
Communications Networks: A Cross Layer Approach
Abstract
Broadcast transmission is an e ff ective way to disseminate safety-related information for cooperative
driving in inter-vehicle communications (IVC). However, it is fraught with fundamental challenges such
as message redundancy, link unreliability, hidden terminal, and broadcast storm, which greatly degrade
the network performance. In this paper, we introduce a cross layer approach to design an e fficient
and reliable broadcast protocol for emergency message dissemination in inter-vehicle communication
systems. We rst propose a novel composite relaying metric for relay selection, by jointly considering
geographical locations, physical layer channel conditions, and moving velocities of vehicles. Based on
the relaying metric, a distributed relay selection scheme is proposed to assure a unique relay is selected
to reliably forward the emergency message in the desired propagation direction. We further apply IEEE
802.11e EDCA MAC to guarantee QoS provisioning to safety related services. In addition, an analytical
model is developed to study the performance of the proposed CLBP in terms of relay selection delay
and emergency message access delay. NS-2 simulation results are given to validate the analysis. It
is shown that CLBP can not only minimize the broadcast message redundancy, but also quickly and
reliably deliver emergency messages in IVC.
Index Terms
Inter-vehicle communications, Cross layer design, Relaying metric, Broadcast protocol
I. I
Cooperative driving can improve safety and e fficiency by enabling vehicles to exchange
emergency messages to each other in the neighborhood. In vehicular ad hoc network (VANET),
vehicles transmit tra ffic and safety related information including tra ffic congestion avoidance
message, accident warning, and accident report, etc., which assists drivers to make proper
decisions to avoid vehicle collisions and congestions. Compared to vehicle-to-infrastructure
communications, inter-vehicle communications is more exible for deployment with low cost [2],
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and research on IVC systems has attracted great attention from academia, industry, and govern-
ments recently. The U.S. Federal Communications Commission (FCC) has approved 75 MHz
(5 .850
−5.925 GHz) bandwidth for Dedicated Short-Range Communications (DSRC) systems
in support of Intelligent Transportation Systems (ITS) applications [3]. Industry manufacturers
have launched several projects to study cooperative driving in an IVC, such as Advanced
Driver Assistance Systems (ADASE2) [4], Crash Avoidance Metrics Partnership (CAMP) [5],
CarTALK2000 [6], FleetNet [7], Partners for Advanced Transit and Highways (PATH) [8], etc.
Broadcast transmission is a frequently used approach to advertise information in VANET.
Nevertheless, e ff ectively broadcasting emergency messages to other vehicles in an IVC system
is extremely challenging especially due to the high mobility and hostile wireless environment.
First, as no acknowledgment (ACK) mechanism is applied for broadcast messages in the medium
access control (MAC) layer, message loss due to packet collisions or poor channel conditions
cannot be easily detected. Since life critical emergency messages have to be delivered to other
vehicles as fast and reliable as possible [9], the traditional broadcasting scheme without an ACK
mechanism is not suitable for emergency message delivery in an IVC system. Second, due to
the limited transmission range, message relaying from intermediate nodes 1 is required to reach
remote vehicles. However, without an e ff ective broadcast control mechanism, multiple copies of
the broadcast messages may be delivered among nodes, which could result in broadcast storm
problem [10] and degrade the network resource utilization. Some research works propose to
reduce message redundancy and prevent broadcast storm by selecting a subset of neighboring
nodes to forward the broadcast message. However, it is a non-trivial task to determine a proper
subset of nodes that can guarantee the message reliability and achieve e fficient resource utilization
simultaneously.
To address the aforementioned issues, several broadcasting protocols have been proposed in the
1 the terms “node” and “vehicle” are used interchangeably throughout the paper.
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literature. Some protocols use network layer broadcast control algorithms to reduce the message
redundancy [12]–[16], but they cannot guarantee the MAC layer reliability. Other protocols aim at
improving the the transmission reliability by repeatedly broadcasting messages [17] or selecting
the farthest node to relay messages [18]–[20]. However, repeated broadcast cannot completely
guarantee the transmission reliability but degrade the resource utilization. The farthest node may
suff er from high packet error rate (PER) and is not an ideal relay candidate, especially in high
speed vehicle networks. In this paper, we propose a cross layer broadcast protocol (CLBP) for
emergency message dissemination in a multihop IVC system, aiming to improve the transmission
reliability and minimize the message redundancy. Considering the particular characteristics of
VANETs, we design a novel relaying metric which is composed of geographical locations, phys-
ical layer channel conditions, and moving velocities of vehicles. Based on the metric, we apply
a revised request-to-send / clear-to-send (RTS / CTS) scheme to select an appropriate relaying node
distributedly. In specic, after receiving the broadcast RTS (BRTS), each relay candidate starts
its backo ff timer to reply a broadcast CTS (BCTS) based on the calculated relaying metric in a
distributed manner. After a successful BRTS / BCTS handshake, one node is successfully selected
as the next hop relay to forward the broadcast message in the desired propagation direction.
Furthermore, to support di ff erent services with various quality of service (QoS) requirements in
the IVC system, we adopt the priority based enhanced distributed coordination access (EDCA)
of IEEE 802.11e MAC to support safety services. The emergency messages are served with the
highest priority and thus the minimum channel access delay can be achieved.
The main contributions of this paper are three-fold. First, we design a novel metric for selecting
a proper relaying node to forward the emergency message. Second, based on the derived metric,
we propose a cross layer approach to e fficiently broadcast emergency messages in the desired
propagation direction. MAC layer service di ff erentiation is applied for emergency messages.
Third, we analytically study the network performance in terms of the packet error rate (PER) of
the emergency message, relay selection delay, and emergency message access delay. Analytical
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and simulation results with NS-2 demonstrate that the proposed cross layer approach can quickly
and reliably deliver emergency messages while minimizing the broadcast message redundancy.
The remainder of this paper is organized as follows. We briey review the related work
in Sec. II. The proposed CLBP is described in Sec. III. An analytical model is developed to
study the performance of CLBP in Sec. IV. The simulation results are given to demonstrate the
efficiency of CLBP in Sec. V, followed by conclusions in Sec. VI.
II. R W
Broadcast protocols in mobile ad hoc networks (MANET) can be classied into four cate-
gories [11]: i) simple ooding [12], [13], in which a node rebroadcasts a new message until it
reaches all connected nodes in the network; ii) probability based methods [14], in which protocols
can be further divided into two sub-classes: (a) a node rebroadcasts a message according to a
predened probability, and this scheme becomes simple ooding one if the probability is set
to 1; and (b) a node decides whether to rebroadcast a message based on the number of the
received copies during a certain interval; iii) area based method [14], in which a node that
can cover more additional area is selected to forward the received message; and iv) neighborknowledge method [15], [16], in which a node makes a forwarding decision according to the
knowledge of its one-hop or two-hop neighbors. All these aforementioned broadcast protocols
aim to reduce the number of redundant messages at the network layer, without considering
the MAC layer hidden terminal problem, collisions, and link reliability, etc. It is well known
that broadcast transmission is not reliable due to the lack of ACK scheme in the MAC layer.
However, some emergency messages are life critical, the delivery of which should be guaranteed.
Therefore, previous works on network layer broadcast protocol design can not be directly applied
to IVC. Recently, some protocols have been proposed for emergency message delivery in IVC.
In [25], a distributed MAC scheme for emergency message dissemination is presented. A node
reserves the data channel for emergency message broadcast by sending a pulse signal through
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the control channel. A MAC designed for emergency message broadcasting is studied in [17],
where a node broadcasts emergency messages for several times to increase the transmission
reliability. However, repeatedly broadcasting messages cannot guarantee the successful reception
of broadcast messages but may increase the contention level in a distributed vehicle network and
waste the scarce wireless network resources. A black burst based ad hoc multi-hop broadcast
(AMB) protocol is proposed for emergency message dissemination in [18]. A neighboring node
sends channel jamming signal (black-burst) with the time duration proportional to its distance.
Thus, the farthest neighboring node sending the longest jamming signal wins the contention and
becomes the next hop relaying node. Nevertheless, the largest jamming duration used by the
relay candidate causes a long delay for emergency messages. In the position based multi-hop
broadcast protocol (PMBP) [19], the farthest neighboring node waits the shortest time duration
to reply the broadcast node. However, the farthest node usually su ff ers from a large path loss
and a high packet error rate (PER) which may cause MAC layer retransmissions and thus a
longer link delay. In [20], each node maintains a list of neighboring nodes and always selects
the farthest neighboring node as the next hop relay. However, network topology of IVC changes
dynamically due to the high mobility of vehicles, to e ff ectively update the list of neighbors is
not a trivial task.
In order to reduce link delay and improve throughput, many channel condition based relaying
and routing metrics have been proposed in cooperative relaying schemes and routing protocols.
In [21], the expected transmission count (ETX) is proposed to measure the expected number of
transmissions that a node attempts until a packet is successfully delivered to the next relaying
node. The routing scheme based on ETX assures that the selected path achieves the minimum link
delay. Similar routing metrics such as expected transmission time (ETT) and weighted cumulative
ETT (WCETT) [22] also consider channel conditions and link reliability in the metric design.
In [23], the enhanced interior gateway routing protocol (EIGRP) adopts a composite metric,
which uses weight factors to decide the impacts of minimum link bandwidth, tra ffic load, link
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delay, and reliability on path selection. The cooperative MAC (CoopMAC) is proposed in [24],
in which each node maintains a table of relays that can improve the link throughput, and selects
the relay with a better channel condition and higher data rate. All these schemes select paths or
relays based on channel conditions. However, they do not consider the specic characteristics
of VANET, i.e., high mobility. In this paper, we propose to jointly consider the geographical
locations, the channel conditions, and the relative velocities of vehicles to make a relay decision
in IVC.
III. P C L B P
Consider a highway with M lanes. Half of the lanes are for vehicles driving to one direction,
while the other half for vehicles driving to the opposite direction. A vehicle’s velocity is randomly
distributed among a discrete set V = {V i | V i−1 < V i, i∈
(1 , P ]}, and the velocity is directional
since vehicles may move to two di ff erent directions. Each vehicle is equipped with a half-
duplex transceiver and a Globe Positioning System (GPS) by which it can acquire its position
information, moving velocity, and moving direction. The transmission range of a vehicle Rt is
divided into a number of blocks, and the length of each block is φ which should be at leastthe minimum safety distance for two adjacent moving vehicles. Therefore, there are Q = Rt /φ
blocks within Rt , and their distances to the broadcast vehicle are represented as {Bi | Bi = i·φ, i∈
[1 , Q]}. We use carrier sense multiple access with collision avoidance (CSMA / CA) based 802.11e
MAC for channel access and service di ff erentiation among multiple nodes. To provide reliable
transmissions of broadcast messages, broadcast request to send (BRTS) / broadcast clear to send
(BCTS) frames are exchanged before emergency messages. In addition, in the proposed CLBP,
one relaying node is selected to forward the emergency message in the desired propagation
direction, based on a novel relaying metric designed for the IVC system.
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A. BRTS/ BCTS handshake
The structure of a broadcast RTS (BRTS) frame is shown in Fig. 1. Compared to the traditional
RTS frame, ve elds are added in BRTS: em inf o , t direction , t velocity , r x, r y. The eld
em inf o takes the information of the source node which initially transmits the emergency
message, and it contains: i) the source node address init addr ; ii) the position information of the
source node init x , and init y ; iii) the sequence number of the emergency message em seq ; and
iv) the weight factors α 1, α 2, α 3 that are used for relaying metric calculation and relaying node
selection. t direction is the message propagation direction, t velocity is the moving velocity of
the current broadcast node, and r x and r y indicate the position of the broadcast node.
duration em_info t_yt_x fcst_addrr_addr
init_addr init_yinit_x em_seq
t_direction
1 2 3
frame_control t_velocity
Fig. 1. Format of BRTS.
When a node has an emergency message for transmission, it rst broadcasts BRTS based on
the CSMA / CA mechanism and starts a timer, t brts r = t brts + t d i f s + t bcts , where t brts and t bcts are the
transmission times of BRTS and BCTS, respectively, and t d i f s is the time duration of a DIFS.
If there is no BCTS response within t brts r , the node contends for channel access to rebroadcast
BRTS immediately until BCTS is successfully received. The broadcast node sets its duration
eld in BRTS such that any node that hears the BRTS but is not eligible for replying BCTS
will set its NAV and defer its own transmissions accordingly.
After receiving BRTS, a node decides whether it is eligible for replying BCTS based on
direction information or the position information of the received BRTS. If init addr in the
received BRTS is the same as the address of the current broadcast node, it implies that this is
the rst hop emergency message dissemination, and the node will decide whether to reply BCTS
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based on propagation direction t direction . Otherwise, if its own position is between the original
source node and the current broadcast node, it will not reply BCTS since there is no distance
gain along the propagation direction. In this case, the node updates its NAV according to the
duration eld in the received BRTS. Otherwise, it starts a backo ff timer for replying BCTS and
keep sensing the channel in the mean time. As shown in Fig. 2, A is the source node that initiates
an emergency message, B is the current broadcast node. Node C will not reply BCTS since it
locates between A and B, while D is eligible for relaying the message and starts a backo ff timer
upon receiving BRTS. This guarantees that the emergency message will be e fficiently forwarded
along the desired propagation direction.
t R
A
B
C
D
A block
Fig. 2. Relaying node selection.
Each eligible relaying node which locates at ( x, y) and moves at velocity v will start a timer
for replying BCTS according to the following metrics: i) the distance from the current broadcast
node to itself; ii) the received SNR and PER which can be estimated from the received BRTS;
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and iii) the velocity di ff erence from the current broadcast node. Based on the three metrics, the
relay candidate evaluates the composite relaying metric F used for relay selection, which is
given by
F = α 1 ·(1 −d
BQ) + α 2 ·
eE max
+ α 3 ·v
2V P, (1)
where
d = (x −r x)2 + (y −r y)2
φ ·φ,
and
v=
| −→v −−−−−−−−−→t velocity |,d is the transmission distance, e is the PER of the emergency message that is calculated based
on the measured SNR, v is the relative velocity, BQ is the distance of the farthest block in
the transmission range, E max is maximum tolerable PER dened in [26], V P is the maximum
velocity, α 1 , α 2 , α 3 (α 1 0, α 2 0, α 3 0) are weight factors and usually congured by users.
For instance, if a user wants the messages to be delivered over a fewer number of hops or with
a reduced PER, he can set a larger α 1 or α 2 accordingly. Moreover, if the topology is relatively
steady, a small α 3 can be set.
The main objective of the proposed CLBP is to deliver the emergency message to other
vehicles as fast and reliable as possible. d is a metric to determine the number of hops, i.e.,
the message will be forwarded over a fewer number of hops with a larger d . In addition,
MAC layer delay of the message highly depends on the PER e. A higher PER may result in
retransmissions that lead to a longer link delay. Finally, a small relative speed v is usually
desirable in high speed vehicle networks to guarantee the channel between two moving nodes
is relatively stationary. The proof in [27] veries that if two routing metrics are bounded, their
additive composite metric is also bounded. As d ∈
[B1 , BQ], v∈
[0 , 2 ·V P ], and e∈
[0 , 1],
the composite metric F is consequently bounded. The maximum and minimum values of F are
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BRTS / BCTS frames, it will update its NAV accordingly.
It is possible that multiple relay candidates may choose the same mini-slot to reply BCTS,
which causes collisions. When a collision occurs, the relay candidates that have started their
backo ff timers but have not replied BCTS will sense the channel busy, and they will stop their
own timers accordingly. If a relay candidate which has replied BCTS receives a rebroadcast
BRTS, it will enter the backo ff stage again and divide 0 into W n segments, each of which is
1 = 0/ W n , i.e., the relay candidate will wait i (i∈
[1 , W n]) mini-slots to reply BCTS again if
F min + (F − F min )/ 0 · 0 + (i −1) 1 F , (4)
F < F min + (F − F min )/ 0 · 0 + i · 1 . (5)
The procedure continues until retransmissions due to BCTS collisions reach r max times, which
implies some nodes have very close F values. Then from the r max round, the relay candidates
that were collided in the last round will randomly select a mini-slot to reply BCTS until a relay is
successfully selected. In CLBP, the relaying metric consists of three variables and it is less likely
that two nodes have exactly the same
F . Therefore, the proposed collision resolution scheme
is very efficient for selecting a unique relaying node. The psuedo code of the relay selection
process is presented in Algorithm 1.
B. Emergency message broadcast
After a successful BRTS / BCTS handshake, the current broadcast node that successfully re-
ceives BCTS will broadcast the emergency message after one SIFS interval. The selected relay
will acknowledge the reception of the emergency message if the transmission is successful.
To avoid message redundancy, each node in the system maintains a list to keep records of all
received emergency messages. Each entry in the list records the address of the source node and
the sequence number of the emergency message, and entries with out-of-date messages will be
deleted. A node which receives an emergency message will check the list and drop this message
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Algorithm 1 Relay Selection Algorithm1: A node j received BRTS.2: if t addr = init addr then3: if j receives BRTS at the rst time then4: Check t direction .5: if j is in the propagation direction then
6: Go to line 24.7: else8: Set the NAV.9: end if
10: else11: Go to line 27.12: end if 13: else14: if j receives BRTS at the rst time then15: if j has distance gain in the propagation direction then16: Go to line 24.17: else18: Set the NAV.19: end if 20: else21: Go to line 27;22: end if 23: end if 24: Compute F min , F max , 0 , distance, relative velocity, and PER.25: Map F of j to mini-slots.26: Start the backo ff timer, and go to line 34.27: if 0 < t retry < r max then28: Compute t retry = 0 / (W n)t retry , map F of j to mini-slots.29: Start the backo ff timer, and go to line 34.30: else31: Randomly select a mini-slot from W n .32: Start the backo ff timer, and go to line 34.33: end if 34: while the backo ff timer 0 do35: if j receives BCTS replying the same BRTS then36: Stop the timer and set the NAV.37: break.38: end if 39: end while40: if the backo ff timer = 0 then41: Reply BCTS, and t retry ++ .42: end if 43: return.
if it has already been recorded. Otherwise, it will receive the message and update the list. After
successfully replying an ACK, the selected relay becomes the next relaying node and repeats
BRTS / BCTS handshake again at the MAC layer.
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C. Priority
To provide safety related services with satisfactory delay guarantee in IVC system, we use
priority based IEEE802.11e enhanced distributed channel access (EDCA) for service di ff erenti-
ation. We include the safety services and divide all services into ve classes. Di ff erent classes
of services have di ff erent priorities to access the channel based on the access categories (AC)
as shown in Table I. The setting of arbitration inter-frame space (AIFS) and contention window
(CW) are the same as those specied in IEEE 802.11e [26],
AIFS [AC ] = t s i f s + AIFSN [AC ] ·σ, (6)
CW [AC ] = min(( CW [AC ] + 1)PF [AC ], CWmax [AC ]), (7)
where σ is a time slot, PF is the persistence factor which is set to 1 for safety services and 2 for
other services. In other words, a node always selects a backo ff counter from the minimum CW
for emergency message delivery while it doubles its CW after each collision for other services.
In this way, emergency messages have the highest service priority.
TABLE IP
AC CWmin CWmax AIFSN PF0 CW MIN CW MAX 7 21 CW MIN CW MAX 3 22 (CW MIN + 1) / 2-1 CW MIN 2 23 (CW MIN + 1) / 4-1 (CW MIN + 1) / 2-1 2 24 (CW MIN + 1) / 4-1 (CW MIN + 1) / 4-1 2 1
IV. PERFORMANCE ANALYSIS
In this section, we develop an analytical model to analyze the performance of the proposed
CLBP. To make the proposed scheme tractable, we make the following assumptions: i) nodes
are randomly distributed, and the node density is λ per Rt ; ii) all nodes are saturated, i.e., the
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nodes always have data packets in their bu ff ers for transmissions, and data packets of the same
access category AC[i] have the same transmission probability p i and collision probability q i that
can be obtained by Eqs. (1), (2), (3), (4), and (7) in [30]; iii) all data packets of the same access
category AC[i] have the same payload size D i that is larger than rts threshold ; iv) PERs of
BRTS, BCTS, and ACK are negligible due to small packet size; and v) the retransmission time
due to BCTS collisions is no larger than r max .
In our proposed protocol, a node starts a timer (in terms of mini-slots) and contends to send
BCTS based on the composite relaying metric F in Eq. (1). d and v can be evaluated from the
received BRTS, and PER e is dependent on the channel conditions. To the best of our knowledge,
there is no consensus on fading and shadowing models for VANET so far [31]. In our analytical
model, we adopt the Friis free-space model [32] to determine the received signal power. Over
an additive white Gaussian noise (AWGN) channel, the bit error rate (BER) of the emergency
message with BPSK modulation is Q 2ε bN 0
= Q 2P r r b N 0
[33], where Q(x) = 1√ 2π ∞x e−t 2 / 2dt ,
ε b is the received energy per bit, N 0 is noise power spectral density, P r is the received power,
r b is the basic rate. Since e = 1 − 1 −Q 2P r r b N 0
L
= 1 − 1 −Q I d
L[34], Eq. (1) can be
rewritten as
F = α 1 ·(1 −d
BQ) +
α 2
E max · 1 − 1 −QI d
L
+ α 3 ·v
2V P(8)
where I = 2P t G t G r (c/ f c)2
r b N 0 (4π)2 , P t is the transmitted power, G t and G r are the transmitter and receiver
antenna gains, respectively, c is the speed of light, and f c is the carrier frequency. F is a function
of d , and v, given the parameters α 1, α 2, α 3, BQ , V P , P t , G t , G r , c, f c , r b , N 0, L. The derivations
of F min , F max are given in Appendix A. Therefore, the selection of mini-slots is dependent on
the distance and the relative velocity to the broadcast node.
Emergency message access delay T is dened as the time interval from an emergency
message arriving at the head of the queue until it is successfully acknowledged, which includes: i)
an AIFS; ii) T b consisting of the backo ff time, the backo ff frozen time due to other transmissions,
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retransmission time due to BRTS collisions, and a successful BRTS transmission time; iii) T c
consisting of retransmission time caused by BCTS collisions, a successful BCTS transmission
time, and iv) T d the sum of delay due to retransmissions caused by the emergency message
errors, a successful emergency message transmission and its acknowledgement. Thus, we have
T = AIFS [4] + T b + T c + T d . (9)
Relay selection delay T rs is dened as the time duration from a broadcast node attempting
to transmit BRTS until a relay is successfully selected, and we have T rs = AIFS [4] + T b + T c .
Denote w as the average time that a backo ff timer of a broadcast node reaches 0, and we have
T b =∞
m= 0
qm4 (1 −q4) [(w + t brts ) + m(w + t brts r )] , (10)
where qm4 (1 −q4) is the probability that the broadcast node successfully delivers BRTS at backo ff
stage m, and (w + t brts ) + m(w + t brts r ) is the corresponding delay. Denote w|j ( j∈
[0 , CW [4]]) as
the value of w given that the j’th time slot is selected. Since the broadcast node selects a time
slot uniformly from [0 , CW [4]], we have
w =CW [4]
j= 0
(w|j) ·1
CW [4] + 1, (11)
where
w|j =jk = 1 Y k , j
∈[1 , CW [4]]
0, j = 0,(12)
and Y k is the mean of Y k which denotes time delay in the k ’th slot of CW[4]. Y k can be one
idle time slot, or the frozen time due to a successful data transmission or collisions. Let E ks ,
E kc , and E ki denote the events that a node transmits a message successfully in slot k , a collision
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occurs in slot k , and no node transmits in slot k , respectively, we have
pr (E ki) =4
i= 0
(1 −p i)ni·xi,k
pr (E ks ) =4
i= 0
xi,k ·n i
1 ·p i ·(1 −p i)ni−1 ·i
∈[0 ,4] , i i
(1 −pi
)ni
pr (E kc) = 1 −pr (E ik ) −pr (E ks ), (13)
where
xi,k =1, i f AI FS [i] AIFS [4] + k ,
0, otherwise ,(14)
n i is the number of contending neighboring nodes belonging to AC[i], and xi,k denotes whether
neighboring nodes of AC[i] will contend for channel access with the broadcast node in the k ’th
slot of CW[4]. As shown in Fig. 3, CW[4] is divided into three sub-windows cw0 , cw1, cw2. If
the broadcast node selects time slot 0, it only contends with neighboring nodes of AC[3], AC[2],
and x3,0 = 1, x2,0 = 1, x1,0 = 0, x0,0 = 0. Similarly, if the node chooses a time slot k from cw1, it
must contend with neighboring nodes of AC[3], AC[2], AC[1], and x3,k = 1, x2,k = 1, x1,k = 1,
x0,k = 0.
AIFS[4]
AIFS[3]
AIFS[2]
AIFS[1]
AIFS[0]
…...
…...
…...
…...
0cw 1cw 2cw
CW[4]
Fig. 3. Contention windows.
We denote S as the average frozen time the broadcast node experiences for one successful
packet transmission, and S i as one successful transmission time of AC[i]. For AC[i] ( i∈
[0 , 3]),
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S i = 3 · t s i f s + t rt s + t cts + D i/ r d + t ack + AIFS [4], while for safety services, S 4 approximately
equals to 2 · t s i f s + t d i f s + t brts + t bcts + L/ r b + t ack + AIFS [4]. Since the successful transmission
probability of AC[i] is n i1
·p i
·(1
−p i)ni−1
·m∈
[0 ,4] ,m i (1
−pm)nm , we obtain S = 4
i= 0ni1
·p i
·(1 −p i)n i−1 · m∈
[0 ,4] ,m i (1 −pm)nm ·S i. Let C represent the average frozen time the broadcast
node experiences owing to one packet collision, and C approximately equals to t rts + AIFS [4].
Finally, we have
Y k = σ ·pr (E ki) + S · pr (E ks ) + C ·pr (E kc). (15)
With Eqs. (11), (12), (13), and (15), T b can be obtained.
T c , which denotes the time interval from the successful reception of BRTS to the successful
reception of BCTS, is a variable and depends on how long the broadcast node can successfully
receive BCTS from its relay candidates. In CLBP, a relay candidate starts its backo ff timer to
reply BCTS after receiving BRTS from the broadcast node. In order to capture the activities of
the backo ff timer of a relay candidate, the backo ff process is illustrated in a three dimension
diagram with the state space ( m, n, l), as shown in Fig. 4, where m (m∈
[0 , r max )) is the backo ff
stage, n (n∈
[1 , W n]) is the initial value of the backo ff timer, and l (l∈
[0 , W n]) is the number
of mini-slots elapsed since the start of the timer.
The state transitions of a backo ff timer are given in Appendix B-A, and therefore we have
T c = S 0 ·t 0 +r max
m= 1
m−1
j= 0
C m ·S m ·t m, (16)
where S m, C m, t m are successful transmission probability of BCTS, collision probability of BCTS,
and the average time taken for a relay candidate successfully replying a BCTS at backo ff stage
m, respectively, the derivations of which are given in Appendix B-B. Finally, T d which denotes
the time spent on emergency message transmission, can be represented as
T d =∞
m= 0
em ·(1 −e) · t s i f s + L/ r b + t s i f s + t ack + m ·(T b + T c + t s i f s + L/ r b + t s i f s + t ack ) , (17)
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0, ,0
0,2,0
nW 0, ,1
0, -1,0nW
0, ,2
0, -1,1nW
0, -2,0nW
0, , -2
0,3,1
nW
. . .
0,1,0
0, , -1
0,3,2
nW
. . .
0,2,1
0,1,1
0,3,3. . .
0,2,2
0,0,0
0, ,
, ,0
,2,0
nW , ,1nW
, -1,0nW
, ,2nW
, -1,1nW
, -2,0nW
, , -2
,3,1
nW
. . .
,1,0
,3,2. . .
,2,1
,1,1
,3,3
nW
. . .
,2,2
, ,
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
maxr
nW nW nW nW nW
nW , , -1nW maxr
nW nW
nW
Fig. 4. State transitions of the backo ff timer.
where em · (1 −e) is the successful transmission probability of the emergency message after
m retransmissions, and t s i f s + L/ r b + t s i f s + t ack + m · (T b + T c + t s i f s + L/ r b + t s i f s + t ack ) is the
corresponding time taken in the retransmission process.
V. SIMULATION RESULTS
In this section, we evaluate the performance of the proposed CLBP, in terms of the packet error
rate (PER), relay selection delay, and the emergency message access delay, via NS-2 simulations.
Vehicles are randomly distributed over a two-lane highway, and one is selected as the broadcast
node. The velocity of a vehicle takes a value among {20 , 25 , 30 , 35 , 40 , 45 , 50}m / s. In the default
setting, ve data ows are set up with the rate 100 packets / s. Other simulation parameters are
listed in TABLE II.
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TABLE II
P
Parameter Value Parameter Valuet s i f s 10 µs PLCP&preamble 192 µsσ 20 µs RTS 20byte
E max 8% CTS 14byter b 1M BRTS 37byter d 2M BCTS 17byteRt 250m L 1024byteφ 25m Data packet 512 byte
CW MIN 31 CW MAX 1023f c 2.4G P t 15dBm
G t 1 G r 1B1 25m BQ 250mV 1 20m / s V P 50m / sρ 1 µs t swith 1 µs
α 1 1 α 2 1
α 3 1 r max 7
A. PER of the emergency message
We rst compare the PER performance of CLBP with that of AMB proposed in [18] under
various N 0. For a smaller N 0, both CLBP and AMB achieve a low PER. When N 0 increases, the
PER of AMB increases while that of CLBP does not change much. In CLBP, the broadcast node
jointly considers the distance, channel condition, and the relative velocity to select the next hop
relaying node. Under an ideal channel condition, the farthest relay candidate has the lowest F ,and is selected as the relaying node; while under a poor channel condition, the received SNR at
the farthest relay candidate decreases and accordingly the achieved PER increases, in which case
a closer relay candidate with a lower PER may be selected with CLBP. As show in Fig. 6, the
PER of CLBP decreases slightly when N 0 increases from −174 .6 dBw / Hz to −173 .88 dBw / Hz .
Therefore, CLBP assures the PER performance of the emergency message and thus is more
suitable for IVC system with variant channel conditions.
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- 1 7 6 . 0 - 1 7 5 . 5 - 1 7 5 . 0 - 1 7 4 . 5 - 1 7 4 . 0 - 1 7 3 . 5
- 0 . 0 1
0 . 0 0
0 . 0 1
0 . 0 2
0 . 0 3
0 . 0 4
0 . 0 5
0 . 0 6
P
E
R
N 0 ( d B w / H z )
A M B ( )
C L B P ( )
Fig. 5. PER of the emergency message.
B. Relay selection delay
Relay selection delay is dened as the interval from the time the broadcast node attempts
to deliver BRTS to the time it successfully receives BCTS. In Fig. 7(a), we compare the relay
selection delays of CLBP and AMB. By applying service di ff erentiation in CLBP, the emergency
messages are served with the highest priority. Therefore, AMB which adopts basic CSMA / CA
achieves longer access delay compared with CLBP. In addition, the node sending the longest
channel jamming signal becomes the relaying node in AMB, while a node waiting the shortest
time to reply a BCTS becomes the relaying node in CLBP. As shown in Fig. 7(a), the relay
selection delay of CLBP is much shorter than that of AMB. However, both relay selection delays
of CLBP and AMB increase with the increase of node density due to retransmissions caused by
collisions.
C. Emergency message access delay
Finally, we show the emergency message access delay under various node densities and
background noise levels in Fig. 7(a)-7(d). It can be seen that the emergency message access
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5 1 0 1 5 2 0
1
2
3
4
5
R
e
l
a
y
s
e
l
e
c
t
i
o
n
d
e
l
a
y
(
m
s
)
( N 0 = - 1 7 5 . 8 1 d B w / H z )
A M B
C L B P
Fig. 6. Relay selection delay.
delays of AMB are higher than those of CLBP, and their di ff erences increase with the node
densities and background noise levels. This is because, rst, CLBP gives the highest priority for
safety services by adjusting AIFSN, PF, CWmin, and CWmax, which results in a smaller access
delay, whereas in AMB, emergency messages have to contend with other services with the same
priority. Second, in CLBP, the selected relaying node waits the minimum number of mini-slots
to reply BCTS, while in AMB, the selected relaying node sends the longest black burst signal
to win the opportunity to reply clear-to-broadcast (CTB). Third, under a poor channel condition,
the broadcast node in CLBP chooses an appropriate node with a reasonable PER performance
to relay the emergency message. In AMB, the broadcast node always selects the farthest relay
candidate, which incurs retransmissions with a high PER.
VI. C
In this paper, we have developed a composite relaying metric to select an appropriate re-
laying node, considering the special characteristics of vehicle networks. Based on the relaying
metric, we have proposed a cross layer broadcast protocol to e fficiently disseminate emergency
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A A
D F min , F max , 0
Consider continuous function z(x, y) = α 1
·(1
−x
BQ) + α 2
E max
·1
−1
−Q I
x
L+ α 3
·
y2V P
, where
x∈
[B1 , BQ], y∈
[0 , 2V P ], and its partial di ff erential coe fficient z x(x) and z y(y) can be expressed
as
z x(x) = −α 1
BQ+
α 2 ·L ·I · 1 −Q I x
L−1
√ 2π ·E max ·x2 ·eI 2 / 2x2(A.1)
z y(y) =α 3
2V P. (A.2)
Thus, z is a monotonic increasing function of x if z x(x) > 0, and a monotonic decreasing
function of variable x if z x(x) < 0, where x∈
[B1 , BQ]. Let X ∗
= {xi | z x(xi) = 0, z x(xi) > 0, xi∈[B1 , BQ]}, and X ∗= {xi |z x(xi) = 0, z x(xi) < 0, xi∈
[B1 , BQ]}. Let Z ∗
= z( xiφ ·φ, V 1) | xi∈
X ∗
z( xiφ
and Z ∗= z( xiφ ·φ, V 1) | xi∈
X ∗
z( xiφ ·φ, V 1) | xi∈
X ∗. Similarly, z is a monotonic increasing function of y since z y = α 32V P
0.
Therefore, the minimum value F min and maximum value F max of the discrete function F can be
expressed as
F min =
z(BQ , V 1), z x(x) 0, x∈
[B1 , BQ]
min (Z ∗), Z
∗ ∅z(B1 , V 1), z x(x) 0, x
∈[B1 , BQ],
F max =
z(B1 , V P ), z x(x) 0, x∈
[B1 , BQ]
max (Z ∗), Z ∗ ∅z(BQ , V P ), z x(x) 0, x
∈
[B1 , BQ],
(A.3)
and 0 = (F max − F min )/ W n is obtained.
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A B
D t c
Let random variables
{F 1 ,
F 2 ,
· · ·,
F N
}denote the relaying metrics of N relay candidates,
and F t = α 1 · (1 −d t
BQ) + α 2
E max · 1 − 1 −Q I d t
L+ α 3 ·
vt 2V P
is the relaying metric of node t .
Notice that the distances between the broadcast vehicle and other vehicles are not independent
variables because two vehicles can not locate in the same position. As a result, as the functions
of distances, the routing metrics {F 1 , F 2 , · · ·, F N }are not independently distributed either. In
a highway consisting of M lanes, at most M vehicles can choose the same block. Let events
A1 = { d 1 = B1 ∩ d 2 = B1 ∩ · · · ∩ d M = B1 ∩ d M + 1 = B2 , · · ·}, A2 = { d 1 = B1 ∩ d 2 =
B1 ∩·· ·∩d M = B2 ∩ d M + 1 = B2 , · · ·}, · · ·, A(M ·QN ) = { d 1 = BQ ∩ d 2 = BQ ∩·· ·∩d M = BQ ∩d M + 1 = BQ −1, · · ·}. Denote vt ,i(y) as the relative velocity between node t and the broadcasting
node, when F t = y and event Ai occurs. For example, d 1 = B1 in event A1, and therefore
v1,1(y) = 2V Pα 3 · y −α 1 ·(1 −
B1BQ
) −α 2
E max · 1 − 1 −Q I B1
L. Let ψ t ,m represent the number of
mini-slots that t backo ff s, and we have
ψ t ,m =
(
F t
− F min )/ 0 + 1, m = 0
[F t − F min −m−1k = 0 (ψ t ,k −1) · k ]/ m + 1, m
∈[1 , r max −1] , (B.1)
where m = 0/ (W n)m.
A. State transition probabilities of the backo ff timer
We denote N m as the set of contending relay candidates at the backo ff stage m, and initially
|N 0| = N . After receiving BRTS, relay candidate t starts its backo ff timer and prepares to reply
BCTS. The transition probability Pr [(0 , n, 0)|(0 , 0, 0)] , which denotes that t starts a backo ff timer
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with initial value n, can be expressed as
Pr [(0 , n, 0)|(0 , 0, 0)] = Pr (ψ t ,0 = n)
= Pr [F min + (n −1) · 0 F t < F min + n · 0] (B.2)
=(M ·QN )
i= 1
Pr [F min + (n −1) · 0 F t < F min + n · 0 | Ai] ·Pr (Ai) (B.3)
=1
M ·QN ·
(M ·QN )
i= 1
Pr [F min + (n −1) · 0 F t < F min + n · 0 | Ai]
=1
M ·QN ·
(M ·QN )
i= 1
Pr vt ,i(F min + (n −1) · 0) vt < vt ,i(F min + n · 0) . (B.4)
Because the velocities of the broadcast node and relay candidate t are directional and ran-
domly distributed, the relative velocity vt is also randomly distributed among the (2 P −1)!
relative velocities {V 1 , V 2 , · · ·, V (2P−1)!}. In addition, since vt does not depend on events
A1 , A2 , · · ·, A(M ·QN ), for specic values vt ,i(F min + (n −1) · 0) and vt ,i(F min + n · 0), we can acquire
Pr [vt ,i(F min + (n −1) · 0) vt < vt ,i(F min + n · 0)], and therefore probability Pr [(0 , n, 0)|(0 , 0, 0)]
is obtained.
In the proposed scheme, when node t has started the backo ff timer and successfully backo ff
one more mini-slot, it means the initial values of all other nodes’ backo ff timers are larger than
the mini-slots that node t has elapsed. Therefore, the transition probability Pr [(0 , n, l + 1)|(0 , n, l)],
which represents l mini-slots has elapsed and t ’s backo ff timer can backo ff one more mini-slot,
can be expressed as
Pr [(0 , n, l + 1)|(0 , n, l)] =1, l = 0,
Pr j∈
N 0 ψ j,0 l + 1 , l∈
[1 , W n −1] ,(B.5)
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where
Pr j∈
N 0
ψ j,0 l + 1
=(M ·QN )
i= 1
Pr j∈
N 0F j F min + l · 0 | Ai ·Pr (Ai)
=(M ·QN )
i= 1
Pr j∈
N 0
v j v j,i(F min + l · 0) ·Pr (Ai)
=1
M ·QN ·
(M ·QN )
i= 1 j∈
N 0
Pr v j v j,i(F min + l · 0) . (B.6)
Node t stops its backo ff timer and returns to the initial state (0 , 0, 0) when it or any other
relay candidate successfully transmits BCTS. In the former case, the number of elapsed mini-
slots equals to the initial value of t ’s backo ff timer, and less than the initial value of any other
timer. Whereas, in the latter case, at least one other timer’s initial value equals to the mini-slots
elapsed, and the initial value of t ’s timer is larger than the mini-slots elapsed. Therefore, the
transition probability Pr [(0 , 0, 0) | (0 , n, l)] is expressed as
Pr [(0 , 0, 0) | (0 , n, l)] =
0, l = 0
Pr j∈
N 0 , j t ψ j,0 l + 1 ψ t ,0 = l , l = n
Pr j∈
N 0 , j t ψ j,0 l ψ t ,0 l + 1 −Pr j∈
N 0 ψ j,0 l + 1 , l n,
(B.7)
where
Pr
j∈
N 0 , j t
ψ j,0 l + 1 ψ t ,0 = l =1
M
·Q
N ·
(M ·QN )
i= 1 j∈
N 0 , j t
Pr v j v j,i(F min + l · 0)
·Pr vt ,i(F min + (l −1) · 0) vt < vt ,i(F min + l · 0) ,
(B.8)
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and
Pr j∈
N 0 , j t
ψ j,0 l ψ t ,0 l + 1 −Pr j∈
N 0
ψ j,0 l + 1
=1
M ·QN ·
(M ·QN )
i= 1 j∈
N 0 , j t
Pr v j v j,i(F min + (l −1) · 0) ·Pr vt vt ,i(F min + l · 0)
−1
M ·QN ·
(M ·QN )
i= 1 j∈
N 0
Pr v j v j,i(F min + l · 0) , (B.9)
If t ’s backo ff timer decreases to 0, it means the elapsed mini-slots equal to the initial value
of the timer. t can either successfully deliver BCTS and returns to the initial state (0 , 0, 0) as
expressed by Eq. (B.8), or transmit BCTS simultaneously with other relay candidates. In the
latter case, given that t ’s BCTS collides with those from other relay candidates, the probability
that t sets the its backo ff timer to be n in the next backo ff stage is given by
Pr [(1 , n, 0) | (0 , l, l)]
= Pr j∈
N 0 , j t
ψ j,0 l F t − F min −(l −1) · 0 1
= n −1
−Pr j∈
N 0 , j t
ψ j,0 l + 1 F t − F min −(l −1) · 0 1
= n −1
=1
M ·QN ·
(M ·QN )
i= 1
N
j= 1, j t
Pr v j v j,i(F min + (l −1) · 0) −N
j= 1, j t
Pr v j v j,i(F min + l · 0)
·Pr vt ,i(F min + (l −1) · 0 + (n −1) · 1) vt vt ,i(F min + (l −1) · 0 + n · 1) . (B.10)
For m 1, we have
Pr [(m, n, l+ 1)|(m, n, l)]
= 1, l = 0,
= N x1 = 2 · · ·
xm−1xm= 2 Pr j
∈N m ψ j,m > l | |N m| = xm
·Pr (|N m| = xm | |N m−1| = xm−1) · · · ·Pr (|N 1| = x1), l∈
[1 , W n −1] ,
(B.11)
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where Pr j∈
N m ψ j,m l + 1 | |N m| = xm can be obtained similarly as Eq. (B.6), and Pr (|N m| =
xm | |N m−1| = xm−1) is given by Eq. (B.18). Conditioning on |N m|, |N m−1|, · · ·,|N 1|, we can acquire
Pr [(0 , 0, 0)
|(m, n, l)] and Pr [(m + 1, n, 0)
|(m, l, l)], respectively.
B. Calculation of T c
Let random variable Y = min (F 1 , F 2 , · · ·, F N ). Its probability mass function (PMF) is
F Y (y) = Pr (Y y) = 1 −Pr (Y > y)
= 1 −Pr (F 1 > y, F 2 > y, · · ·, F N > y)
= 1 −(M ·QN )
i= 1Pr (F 1 > y, F 2 > y, · · ·, F N > y | Ai) ·Pr (Ai)
= 1 −(M ·QN )
i= 1
Pr ( v1 > v1,i(y), · · ·, vN > vN ,i(y) | Ai) ·Pr (Ai)
= 1 −1
M ·QN ·
(M ·QN )
i= 1
N
j= 1
Pr ( v j > v j,i(y) | Ai)
= 1 −1
M
·Q
N ·
(M ·QN )
i= 1
N
j= 1
Pr ( v j > v j,i(y)). (B.12)
We represent S m and C m as successful transmission probability of BCTS and collision proba-
bility of BCTS at backo ff stage m, respectively. Let ψ 0 = (Y −F min )/ 0 and ψ m = (Y −F min −m−1i= 0 ψ i · i)/ m , where m
∈[1 , r max ]. Without loss of generality, we consider F t = Y as the
minimum value among {F 1 , F 2 , · · ·, F N }, and events K j,t ,m, H j,t ,m denote
K j,t ,m =(F j − F min −
m−1i= 0 ψ i · i)/ m > (F t − F min −
m−1i= 0 ψ i · i)/ m , m
∈[1 , r max −1]
(F j − F min )/ 0 > (F t − F min )/ 0 , m = 0,(B.13)
H j,t ,m =(F j − F min −
m−1i= 0 ψ i · i)/ m = (F t − F min −
m−1i= 0 ψ i · i)/ m , m
∈[1 , r max −1]
(F j − F min )/ 0 = (F t − F min )/ 0 , m = 0,(B.14)
and K j,t ,m(x, y, z), H j,t ,m(x, y, z) denote the events K j,t ,m and H j,t ,m under the conditions d j =
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x, d t = y, vt = z. Therefore, for m = 0, we have
S 0 = |N 0|1 ·Pr
j∈
N 0 , j t
K j,t ,0
= N ·(M ·QN )
i= 1
Pr j∈
N 0 , j t
K j,t ,0 | Ai ·Pr (Ai)
=N
M ·QN ·
(M ·QN )
i= 1 j∈
N 0 , j t
Pr (K j,t ,0 | Ai)
=N
M ·QN ·
(M ·QN )
i= 1 j∈
N 0 , j t
Pr (K j,t ,0 | d j = d j,i, d t = d t ,i)
= N M ·QN
·(M ·QN )
i= 1 j∈
N 0 , j t
(2P
−1)!
l= 1
Pr (K j,t ,0 | d j = d j,i, d t = d t ,i, vt = V l) ·Pr ( vt = V l)
=N
M ·QN ·(2P −1)! ·(M ·QN )
i= 1 j∈
N 0 , j t
(2P−1)!
l= 1
Pr K j,t ,m(d j,i, d t ,i, V l) (B.15)
where d j,i, d t ,i denote the values of d j and d t in event Ai, respectively. For m∈
[1 , r max −1],
we have
S m =
N
x1 = 2
x1
x2 = 2 · · ·xm−1
xm= 2
xm
1 Pr j∈
N m, j t K j,t ,m | |N m| = xm ·Pr (|N m| = xm | |N m−1| = xm−1) · · ·
·Pr (|N 2| = x2 | |N 1| = x1) ·Pr (|N 1| = x1), (B.16)
where
Pr j∈
N m , j t
K j,t ,m | |N m| = xm =1
M ·QN ·(2P −1)! ·(M ·QN )
i= 1 j∈
N m , j t
(2P−1)!
l= 1
Pr K j,t ,m(d j,i, d t ,i, V l) ,
(B.17)
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and
Pr (|N m| = xm | |N m−1| = xm−1)
= xm−1
xmPr
j∈
N m , j t
H j,t ,mj N m , j
∈N m−1
K j,t ,m−1
=xm−1xm
M ·QN ·
(M ·QN )
i= 1
Pr j∈
N m, j t
H j,t ,mj N m, j
∈N m−1
K j,t ,m−1 | Ai
=xm−1xm
M ·QN ·
(M ·QN )
i= 1 j∈
N m, j t
Pr (H j,t ,m | Ai) ·j N m, j
∈N m−1
Pr (K j,t ,m | Ai)
=xm−1xm
M ·QN ·(2P −1)! ·
(M ·QN )
i= 1 j∈
N m, j t
(2P−1)!
l= 1
Pr H j,t ,m
(d j,i
, d t ,i
, V l)
·j N m, j
∈N m−1
(2P−1)!
l= 1
Pr K j,t ,m(d j,i, d t ,i, V l) . (B.18)
whereas, for m = r max , we have
S m =N
x1= 2
x1
x2 = 2· · ·
xm−1
xm= 2
xm
1 ·1
W n
W n
k = 1
(1
W n −k )xm−1 ·Pr (|N m| = xm | |N m−1| = xm−1) · · ·
·Pr (|N 2| = x2 | |N 1| = x1) ·Pr (|N 1| = x1) , (B.19)
In the proposed scheme, if a relay candidate transmits BCTS simultaneously with other relay
candidates and introduces BCTS collisions, it will reply BCTS again after receiving a rebroadcast
BRTS until the retransmission times reach r max . Then, it will randomly select a mini-slot to reply
the BCTS, and any mini-slot has the same probability 1 / W n to be chosen. If a relay candidate
selects mini-slot k and successfully replies BCTS, other relay candidates should randomly select
mini-slots between k + 1 and W n , and Eq. (B.19 is obtained. Finally, we can acquire S m and
C m = 1 −S m. Let t m denote the average time taken for a relay candidate successfully replying
BCTS at backo ff stage m, which contains the backo ff time, delay of retransmissions caused by
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BCTS collisions, and BCTS successful transmission time. Therefore, it can be represented as
t m =ψ m ·τ + t bcts + m(T b + t d i f s + t bcts ), m
∈[0 , r max −1]
W n
2 ·τ + t
bcts+ m(T
b+ t
d i f s+ t
bcts), m = r
max,
where ψ m = (Y −F min −m−1i= 0 ψ i · i)/ m is the mean of ψ m. Given the PMF of Y in Eq. (B.12), Y
and ψ m can be obtained. At the backo ff stage r max , a relay candidate uniformly selects a mini-slot
to reply BCTS, and the average number of mini-slots it backo ff s is W n/ 2. Given S m, C m, and
t m, we can obtain T c by Eq. (16).
R
[1] C. L. Robinson, L. Caminiti, D. Caveney, and K. Laberteaux, “E fficient Coordination and Transmission of Data for
Cooperative Vehicular Safety Applications,” In Proc. of VANET’06 , pp. 10-19, Sep. 2006.
[2] J. Luo and J. P. Hubaux, “A Survey of Inter-Vehicular Communication,” Technical Report IC / 2004 / 24, School of Computer
and Communication Sciences, EPFL, 2004.
[3] Dedicated Short Range Communications Working Group, [Online], Available: http: // grouper.ieee.org / groups / scc32 / dsrc / .
[4] M. Heddebaut, J. Rioult, J. P. Ghys, C. Gransart, and S. Ambellouis, “Broadband Vehicle-to-Vehicle Communication Using
an Extended Autonomous Cruise Control Sensor,” Meas. Sci. Technol. , vol. 16, no. 6, pp. 1363-1373, Jun. 2005.
[5] M. Shulman and R. K. Deering, “Third Annual Report of the Crash Avoidance Metrics Partnership April 2003-March
2004,” Nat. Highw. Tra ffic Safety Admin., DOT HS 809 837, Jan. 2005.
[6] D. Reichardt, M. Miglietta, L. Moretti, P. Morsink, and W. Schulz, “CarTALK 2000: Safe and Comfortable Driving Based
upon Inter-Vehicle-Communication,” In Proc. of Intelligent Vehicle Symposium , PP. 545-550, Jun. 2002.
[7] R. Kruger, H. Fuler, M. Torrent-Moreno, M. Transier, H. Hartenstein, and W. E ff elsberg, “Statistical Analysis of the
FleetNet Highway Movement Patterns,” University of Mannheim, Mannheim, Germany, Tech. Rep. TR-2005-004, Jul.
2005.
[8] M. Ergen, D. Lee, R. Sengupta, P. Varaiya, “WTRP-Wireless Token Ring Protocol,” IEEE Trans. Vehi. Technol. , vol. 53,
no. 6, pp. 1863-1881, Nov. 2004.
[9] Y. G. Bi, K. H. Liu, L. X. Cai, X. Shen, and H. Zhao, “A Multi-Channel Token Ring Protocol for QoS Provisioning in
Inter-Vehicle Communications,” IEEE Trans. Wirel. Commun. , to appear.
[10] Y.-C. Tseng, S.-Y. Ni, Y.-S. Chen, and J.-P. Sheu, “The Broadcast Storm Problem in a Mobile Ad Hoc Network,” Wireless
Networks , vol. 8, no. 2 / 3, pp. 153-167, 2002.[11] B. Williams and T. Camp, “Comparison of Broadcasting Techniques for Mobile Ad Hoc Networks,” In Proc. of
MobiHoc’02 , pp. 194-205, Jun. 2002.
[12] C. Ho, K. Obraczka, G. Tsudik, and K. Viswanath, “Flooding for Reliable Multicast in Multi-hop Ad Hoc Networks,” In
Proc. of DIALM’99 , pp, 64-71, Aug. 1999.
[13] J. Jetcheva, Y. Hu, D. Maltz, and D. Johnson, “A Simple Protocol for Multicast and Broadcast in Mobile Ad Hoc Networks,”
Internet Draft: draft-ietf-manetsimple- mbcast-01.txt, Jul. 2001.
Downloaded from engine.lib.uwaterloo.ca on 14 April 2010 Page 32 of 33
8/7/2019 Efcient and Reliable Broadcast in Inter-Vehicle
http://slidepdf.com/reader/full/efcient-and-reliable-broadcast-in-inter-vehicle 34/34
32
[14] S. Ni, Y. Tseng, Y. Chen, and J. Sheu, “The broadcast Storm Problem in a Mobile Ad Hoc Network,” In Proc. of
MobiCom’99 , pp. 151-162, Aug. 1999.
[15] H. Lim and C. Kim, “Multicast Tree Construction and Flooding in Wireless Ad Hoc Networks,” In Proc. of MSWIM’99 ,
pp. 61-68, Aug. 2000.
[16] W. Peng and X. Lu, “On the Reduction of Broadcast Redundancy in Mobile Ad Hoc Networks,” In Proc. of Mobihoc’00 ,pp. 129-130, Aug. 2000.
[17] Q. Xu, T. Mak, J. Ko, and R. Sengupta, “Medium Access Control Protocol Design for VehicleCVehicle Safety Messages,”
IEEE Trans. Veh. Technol. , vol. 56, no. 2, pp. 499-518, Mar. 2007.
[18] G. Korkmaz, E. Ekici, and F. Ozguner, “Black-Burst-Based Multihop Broadcast Protocols for Vehicular Networks,” IEEE
Trans. Veh. Technol. , vol. 56, no. 5, pp. 3159-3167, Sept. 2007.
[19] Y. Bi, H. Zhao, and X. Shen, “A Directional Broadcast Protocol for Emergency Messages Exchange in Inter-Vehicle
Communications,” In Proc. of ICC’09 , pp. 1-5, Jun. 2009.
[20] L. Briesemeister and G. Hommel, “Role-based Multicast in Highly Mobile but Sparsely Connected Ad Hoc Networks,”
In Proc. IEEE / ACM Workshop MobiHOC , pp. 45-50, Aug. 2000.
[21] D. S. J. D. Couto, D. Aguayo, J. Bicket, and R. Morris, “A High-Throughput Path Metric for Multi-hop Wireless Networks,”
In Proc. MobiCom’03 , pp. 134-146, Sept. 2003.
[22] R. Draves, J. Padhye, and B. Zill, “Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks,” In Proc. MobiCom’04 ,
pp. 114-128, Sept. 2004.
[23] A. Zinin, “Cisco IP Routing,” Boston, MA: Addison-Wesley, 2002.
[24] P. Liu, Z. Tao, S. Narayanan, T. Korakis, and S. S. Panwar, “CoopMAC: A Cooperative MAC for Wireless LANs,” IEEE
J. Selected Areas Commun. , vol. 25, no. 2, pp. 340-354, Feb. 2007.
[25] J. Peng and L. Cheng,“A Distributed MAC Scheme for Emergency Message Dissemination in Vehicular Ad Hoc Networks,”
IEEE Trans. Veh. Technol. , vol. 56, no. 6, pp. 3300-3308, Nov. 2007.
[26] IEEE Standard for Information Technology–Telecommunications and Information Exchange between Systems–Local and
Metropolitan Area NetworksłSpecic Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specications.
[27] G. Mohammad and S. Marco, “Maximizable Routing Metrics,” IEEE / ACM Trans. Netw. , vol. 11, no. 4, pp. 663-675, Aug.
2003
[28] A. Bletsas, A. Khisti, D.P. Reed, and A. Lippman, “A Simple Cooperative Diversity Method Based on Network Path
Selection,” IEEE J. Selected Areas Commun. , vol. 24, no. 3, pp. 659-672, Mar. 2006.
[29] H. Shan, W. Zhuang, and Z. Wang, “Distributed Cooperative MAC for Multihop Wireless Networks,” IEEE Commun.
Mag. , vol. 47, no. 2, pp. 126-133, Feb. 2009.
[30] D. X. Xu, T. Sakurai, H. L. Vu, “An Access Delay Model for IEEE 802.11e EDCA,” IEEE. Trans. Mob. Comput. , vol. 8,
no. 2, pp. 261-275, Feb. 2009.[31] T. K. Mak, K. P. Laberteaux, R. Sengupta, M. Ergen, “An Access Delay Model for IEEE 802.11e EDCA,” IEEE Trans.
Veh. Technol. , vol. 58, no. 1, pp. 349-366, Jan. 2009.
[32] J. W. Mark and W. Zhuang, Wireless Communications and Networking , Prentice Hall, Inc., 2003.
[33] J. G. Proakis, Digital Communications: Fourth Edition , McGraw-Hill Companies, Inc., 2000.
[34] J. A. Roberts, “Packet Error Rates for DPSK and Di ff erentially Encoded Coherent BPSK,” IEEE Trans. Veh. Technol. , vol.
42, no. 2 / 3/ 4, pp. 1441-1444, Feb. / Mar. / Apr. 1994.