Safety services of Vehicular Ad Hoc Network (VANET) require reliable broadcasts. We propose a reliable broadcast mechanism for urban roads called VANET Broadcasting for Urban areas based on Road Layout (VBURL), which tries to minimize the dependency on information that may become inaccurate to maximize the efficiency of broadcast. Specifically, the proposed mechanism takes into account the road layout information accessible from the digital map and only the real-time information obtained from the broadcast messages or beacons. VBURL basically makes the vehicle that is farthest from the current forwarding vehicle take the role of next forwarding vehicle and, if possible, makes an additional broadcast happen at the intersections where the effect of signal attenuation caused by the road side obstacles is low in order to have prompt and reliable dissemination of safety messages towards all roads connected to the intersections. The simulation results verified that VBURL achieves the same high performance as that of the compared legacy schemes in terms of reliability with much higher efficiency. Even though the message reception delay of VBURL is slightly longer than those of compared schemes, it is far less significant to impair the original purpose of safety message.
The Vehicular Ad Hoc Network (VANET) consists of vehicles that use mobile communications and enables communication between vehicles or between vehicles and fixed infrastructure along the road. The road condition and traffic information collected through various sensors installed in vehicles and on road side units are transmitted in real time through VANET. The IEEE 802.11p standard and the IEEE 1609 working group defined technologies for Wireless Access in Vehicular Environments (WAVE) [
VANET enables the provision of services to improve passenger safety and traffic flow through the propagation of information on dangerous situations and the detection of traffic congestion, as well as various services for passenger conveniences and entertainment, such as games, chatting, and data sharing between vehicles [
Various mechanisms have been proposed to make up for 802.11p for the provision of reliable broadcasts [
Differing from [
However, the effect of traffic concentration due to the red light of traffic signals, as shown in Figure
Characteristics of urban roads, formation of concentrated vehicle groups due to red light of traffic signals.
Similar to [
Intersection with no vehicle and with significant signal attenuation due to obstacles.
As the diverse applications of the Intelligent Transportation System (ITS) are expected along with the development of VANETs, maximizing the usage efficiency of limited wireless resources is emerging as a critical issue. To this end, studies are conducted on dynamic and flexible utilization of wireless channels based on the real-time usage information [
In this paper, hence, we propose a safety message broadcasting scheme named “VANET Broadcasting for Urban areas based on Road Layout (VBURL)” that enhances the efficiency of safety message broadcast by minimizing the possibility of falsely selecting a forwarding vehicle based on the incorrect information while having little dependency on the beacon interval as well. VBURL is the extension of the work proposed by the authors in [
Note that the urban roads consist of intersections and the roads interconnecting them. In VBURL, different forwarding strategies are applied depending on whether there is an intersection within the transmission range of a vehicle that receives the broadcast message and is supposed to determine whether to take the role of message forwarding. It is assumed that the signal attenuation caused by obstacles along the road sides is not significant on the straight roads between intersections, and thus, VBURL simply makes the vehicle that is farthest from the current forwarding vehicle perform the rebroadcast on the straight roads for rapid and efficient broadcasting. Meanwhile, for the message dissemination around the intersections, VBURL makes an additional broadcast happen at the intersections in addition to the one made by the farthest vehicle as long as there exists a vehicle hearing the broadcast at the intersection or else at least there exist one or more vehicles that are moving toward the intersection with that intersection within their transmission range. Since the effect of signal attenuation caused by the road side obstacles is low inside of the intersections, it enables the prompt and reliable dissemination of safety message toward all roads connected to the intersections.
This paper is organized as follows. After the introduction in Section
In this section, existing studies that implement the efficiency of reliable broadcasting on urban roads on top of the plain 802.11p, similar to VBURL, are examined [
ABSM uses the Neighbor Elimination Scheme (NES) as proposed in [
Furthermore, message reception is indicated in the beacon so that, when a new vehicle with no message appears as a neighboring vehicle, it can be detected and the message can be sent to the new neighbor. ABSM updates the lists
In ABSM, not only the CDS virtual backbone network set up but also the broadcast timer setting is determined based on the holding information conveyed by the beacons. In order for ABSM to operate efficiently, the real-time locations of neighboring vehicles and positional relationships must be accurately identified. On urban roads, which have frequent intersections and high vehicle densities, however, the beacon may not be received properly due to signal attenuation or a collision, and thus, the locations of neighboring vehicles may not be accurately identified or actual neighbors may not be recognized as neighbors (or vice versa), lowering the broadcast performance of ABSM.
Especially, with the traffic concentration at the red traffic light, it is more likely that there exist multiple vehicles with the same number of neighboring vehicles that have not received the broadcast message, resulting in a broadcast collision. Broadcast collision degrades the performance of ABSM by causing additional broadcasts. In particular, many such broadcast collisions can occur in a series of attempted broadcasts when vehicles with no message are discovered by a beacon. Figure
Example of a general situation where vehicles with no message are discovered through a beacon.
Furthermore, ABSM may incorrectly judge the message reception status of neighboring vehicles. ABSM implicitly determines the message reception status of neighbors by considering the distance between a neighbor and a message-forwarding vehicle without considering the signal attenuation by obstacles. This may, however, turn out to be a false guess. In particular, as shown in Figure
Example of a broadcast near an intersection.
ReC is proposed to improve the speed of message delivery as well as the broadcast efficiency [
ReC manages neighbors with three lists by adding list
As with ABSM, however, the inaccuracy of beacon information due to the signal attenuation around the intersection or beacon collisions as well as non-real-time characteristics of holding information may result in suboptimal selection of forwarding vehicle.
Differing from ABSM and ReC, BRNT configures a CDS consisting of the road segments connecting the intersections instead of the individual vehicles [
If there exist one or more vehicles at every intersection or the signal attenuation at intersections is not significant so that all CDS road segments are connected by the intersections, BRNT could work optimally in terms of both delay and efficiency since the message is disseminated via the CDS road segments which have the shortest waiting time for forwarding, with no duplicate transmissions. As the ratio of intersections with no vehicle or with nonnegligible signal attenuation gets higher, though, the broadcast dissemination delay could get very long since not only some of the road segments connected to those intersection can only receive the message via a detouring path but also the broadcast message may have to be disseminated via the non-CDS road segments, which requires longer forwarding delay than the CDS road segments, due to the incomplete connectivity of CDS road segments. Note that chance of having broadcast at the intersection is higher in the proposed VBURL than in BRNT since intersection broadcast cannot happen in BRNT if there is no vehicle at the intersection at the point of broadcast message reception, whereas VBURL may have the vehicle moving toward the intersection perform the intersection broadcast even in the case where no vehicle exists at the intersection at the point of broadcast message reception. Furthermore, no mechanism is provided to ensure the delivery of broadcast message to a vehicle that newly appears or missed the broadcast due to some signal interference on the road segment where the message broadcast has been completed once.
Safety service aims to alert the occurrence of a traffic accident or the falling of obstacles on the road through broadcasts to all vehicles near the point of accident so that they can decelerate or reroute to a bypass. The purpose of the proposed VBURL is to improve the efficiency while ensuring the reliability of safety message broadcasting by considering the environmental characteristics of urban roads such as a high vehicle density, as well as frequent intersections and traffic signals. VBURL attempts to take appropriate broadcast dissemination in consideration of the road layout without depending on neighboring vehicle information which is identified through a beacon. On straight roads between intersections, where the effect of signal attenuation by obstacles is low, it allows the vehicle located farthest from the current forwarding vehicle perform the next broadcast forwarding in order to deliver the message to the largest possible number of vehicles similar to the Contention Based Forwarding (CBF) [
When a vehicle receives a new safety message that is required to be broadcast, it determines the forwarding method depending on the existence of an intersection within its transmission range. First, the forwarding method of a vehicle that has no intersection within its transmission range is described. The forwarding method when there is an intersection within the transmission range is then described.
The vehicle that has received a new safety message saves it in the broadcast message buffer and maintains it until the expiration of message life time. In addition, it uses the Forwarding Timer (FT) to determine the next forwarding vehicle for continuous dissemination of the message. Similar to CBF, every vehicle receiving the safety message sets its FT in inverse proportion to its distance from the current forwarding vehicle using the location information of forwarding vehicle conveyed in the message, and the vehicle whose timer expires before hearing others’ broadcast performs the broadcast. As a result, the farthest vehicle from the current forwarding vehicle takes the role of message-forwarding vehicle. On a straight road where the effect of obstacles is relatively little, broadcasting by the vehicle that is farthest from the message-forwarding vehicle potentially disseminates the message to the largest possible number of vehicles that have not received the message.
If the received safety message is already stored in the buffer, it will be labeled as a duplicate message. When a vehicle receives a duplicate of the message for which the FT is running, it means that another vehicle already has taken the role of the next forwarding vehicle. In this case, the FT is released to avoid unnecessary duplicate broadcasts.
Near an intersection, due to the effect of obstacles such as buildings around the intersection, there can be roads where message dissemination is impossible through the simple broadcasting by a vehicle whose FT expires first. For instance, the vehicles on roads R3 and R4 in Figure
Broadcasting near an intersection.
Intersection area.
In order to do this, the vehicles that have the intersection in their transmission range (groups
Target vehicles of IFT setting for broadcasting in the intersection area.
The goal of IFT is to facilitate the dissemination of broadcast message to all roads adjacent to the intersection. Thus, it is released only when a duplicate broadcast from the intersection area is heard. The FT and IFT may be running simultaneously at a vehicle for a specific safety message, and whichever that expires first makes the broadcast be performed. If the FT expires first and the vehicle is in the intersection area, it means that broadcast is already occurring in the intersection area due to the FT expiration, and the IFT is released to prevent an unnecessary duplicate broadcast. On the other hand, if the vehicle is still on its way to the intersection upon the broadcasting caused by the FT expiration, broadcast within the intersection area is still necessary and the running IFT is maintained. If the FT is running in the vehicle when the IFT expires, the FT is always released to prevent unnecessary duplicate broadcasts: MaxWT is maximum waiting time;
For reliable broadcasting, the safety message must be rebroadcast in case the safety message fails to be disseminated to the next forwarding vehicle. For this purpose, it must be possible for the current forwarding vehicle to confirm the reception of a safety message by the next forwarding vehicle. There are largely two methods of confirming the receipt of a safety message: explicit acknowledgment, which requires the exchange of a control message, and implicit acknowledgment, which regards the broadcast of safety message by the next forwarding vehicle as the reception confirmation message and considers that all vehicles located between itself and the next forwarding vehicle have received the safety message [
When the vehicle that has broadcast a safety message receives it redundantly from another vehicle, it assumes that successful progress of continuous message dissemination is made by next forwarding vehicles. Otherwise, the vehicle determines that the dissemination of broadcast has stopped and performs a rebroadcast. For this purpose, every forwarding vehicle sets the Rebroadcast Timer (RT) after broadcasting a message. In the study [
To avoid early expiration, RT and IRT are defined according to ( MaxWT is maximum waiting time; MaxPT is maximum dissemination delay of the broadcast message, which is the time until the message is disseminated to the transmission range.
Neighboring vehicles that have not received a broadcast of the safety message may appear for various reasons, such as collision, radio interference, and the entrance of a new vehicle. In this case, an additional broadcast must be performed to deliver the safety message to the vehicle with no message. The Neighbor Forwarding Timer (NFT) is used for this purpose. The NFT is set when none of FT, IFT, RT, and IRT is running for a valid safety message, and the message is not specified in the received message list in the beacon of a neighbor. The purpose of NFT is to avoid the collision of broadcasts or unnecessary broadcasts by deciding the optimum vehicle that will take the broadcasting role when multiple vehicles with the message discover a vehicle with no message simultaneously in a geographically close area. For this purpose, among the vehicles that have the message and have received a beacon from a vehicle with no message, VBURL makes the vehicle closest to the vehicle with no message perform the broadcast of safety message. Considering that usually the vehicles with no message and the vehicles with the message encounter each other in groups on urban roads due to the traffic signals as shown in Figure MaxWT is maximum waiting time; TR is transmission range.
The broadcasting and rebroadcasting mechanisms of VBURL are illustrated in pseudocodes in Algorithm
Save the message Set FT Set IFT = Set IFT =
Cancel FT Cancel IFT Keep waiting implicit ACK Cancel RT Cancel IRT Keep waiting implicit ACK Cancel NFT
Broadcast the message Set IRT = Set RT = Wait for implicit ACK
Broadcast the message Set IRT = Cancel FT
Broadcast the message Num. of rebroadcast by RT or IRT ++ Set RT or IRT = Wait for implicit ACK
Reset IFT =
Set NFT
A simulation is conducted using OPNET to compare the performance of VBURL with ABSM and ReC, which are the legacy schemes implementing the reliable broadcast for urban roads on top of IEEE 802.11p. The simulation environments are first explained in Section
The urban roads used in the experiment are bidirectional one-lane roads where 25 intersections are located at intervals of 350 m as shown in Figure
Urban road layout for simulation.
At the beginning of the simulation, vehicles running at random speeds start one by one at fixed intervals at each of the 20 entrances in the simulation network roads and are directed to run straight along the road. The vehicle arriving at the end of the road exits the simulation network, and a new vehicle instead starts on the lane of the opposite direction to maintain a constant vehicle density on the simulation network roads. Each intersection has signal lights, and the vehicles repetitively drive/wait according to the signals. All of the signal lights on the east-west roads turn to driving (waiting) signals simultaneously, and all of the signal lights on the south-north roads turn to waiting (driving) signal simultaneously. The vehicles are set to stop at 4 m intervals at the waiting signal of intersections considering the average body length of vehicles.
As shown in Figure
Both ABSM and ReC use the location information of neighboring vehicles transmitted through beacons, and when the beacon cycle increases, the accuracy of neighboring vehicles’ location information decreases. Furthermore, all of the three compared schemes discover the neighboring vehicles with no message by exchanging the received safety message identifier through beacons. Thus, when the beacon cycle is short, the broadcast dissemination attempts for the vehicles with no message are made frequently. In this experiment, therefore, beacon exchange cycle, which is an important parameter that can affect the performance, is increased to 0.5, 1.0, 3.0, 5.0, 7.0, and 10.0 seconds.
The driving speed on urban roads is generally slower than on the express or freeways due to a high vehicle density and frequent intersections. Furthermore, it is usual to have higher traffic density during the commuting hours due to a large number of inflowing vehicles. In this experiment, two scenarios of traffic flow are assumed: a case of rush hours, such as commuting hours, during which vehicles are congested (hereinafter referred to as the “congestion scenario”), and a case of noncommuting hours, during which the traffic flow is relatively smooth (hereinafter referred to as the “smooth scenario”). For the congestion scenario, longer traffic signal cycles are set to reflect the reality. Specifically, for the congestion scenario, 200 vehicles running at random speeds between 20 and 30 km/h start at 1.5 sec intervals at each road entrance (a total of 4000 vehicles, i.e., traffic density of 86.4 vehicles/km), and the waiting/driving signal duration is set to 25 seconds. For the smooth scenario, 100 vehicles running at random speeds between 40 and 50 km/h start at 2 sec intervals at each road entrance (a total of 2000 vehicles, i.e., traffic density of 43.2 vehicles/km), and the signal changes at intervals of 15 seconds.
In this experiment, the MaxWT value is set to 0.1 sec, which is used to set the FT (
Simulation Parameters.
Simulation parameters | Values |
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Transmit power | 0.005 W |
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Min. frequency | 5885 MHz |
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Data rate | 6 Mbps |
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Channel bandwidth | 10 MHz |
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Transmission range | 300 m |
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Network dimensions | Target area, 2,135 m × 2,135 m, 1 lane in 2 directions |
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Vehicle velocity | 20–30 km/h (congestion scenario) |
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Vehicle density | 4000 vehicles, 86.4 vehicles/km (congestion scenario) |
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Beacon interval | (0.5, 1.0, 3.0, 5.0, 7.0, 10.0) seconds |
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Beacon hold time | (1.5, 3.0, 9.0, 15.0, 21.0, 30.0) seconds |
The performance measurement values collected for performance evaluation are as follows. All the measurement values are averages of the measurements for 10 broadcasts. Message reception ratio: this is the ratio of vehicles that have received the message before expiration of the life time of the message among all the target vehicles. It is used to evaluate the reliability of a broadcast scheme. Number of receiving vehicles per broadcast: this is the number of receiving vehicles per broadcast forwarding. It is used to evaluate the efficiency of a broadcast scheme. End to end delay: this is the time elapsed until a target vehicle receives a message after the message is first generated. It is used to evaluate the rapidity of a broadcast scheme.
The performances of three schemes are compared in terms of reliability, efficiency, and rapidity, respectively.
All the three compared schemes show a 99% or higher reception rate in all beacon cycles of the congestion and smooth scenarios. VBURL, ABSM, and ReC ensure broadcast reliability by performing rebroadcast whenever a vehicle that has not received the message is found. The reception rate does not reach 100%, though, because there are vehicles moving to exit the road on each edge of the target area and exit the simulation network before receiving the message. Because these vehicles are moving farther away from the accident point, there is no safety problem even if they do not receive the message. It is only that they could not take the helpful role to forward safety messages to vehicles beyond the target area.
Figure
Broadcast efficiency.
Congestion scenario
Smooth scenario
The efficiency of VBURL decreases slightly as the beacon exchange cycle is lengthened in both of the scenarios. It is because, for the intersection broadcast, VBURL also utilizes holding information from the beacons to determine the roads on which neighboring vehicles exist. As the beacon cycle increases, the beacon information holding time also increases, and, as a result, a road with no neighboring vehicle anymore actually tends not to be recognized early enough, causing unnecessary rebroadcasts. Furthermore, for higher vehicle speed, the information accuracy becomes more sensitive to the beacon cycle. Thus, the broadcast efficiency decreases slightly in the 7 sec or longer beacon cycles in the congestion scenario and in 3 sec or longer beacon cycles in the smooth scenario in VBURL.
ABSM has the lowest efficiency among the compared schemes due to its great inefficiency in the broadcast when a new neighbor with no message is found as shown in Table
Number of broadcasts due to the reception of a safety message (A) and the number of broadcasts due to the discovery of a vehicle with no message by a beacon (B).
Congestion scenario
Beacon cycle | VBURL | ABSM | ReC | |||
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(A) | (B) | (A) | (B) | (A) | (B) | |
0.5 | 175.6 | 9.7 | 192.1 | 21148.1 | 573.9 | 9321.7 |
1.0 | 190.5 | 8.9 | 219.5 | 21727.2 | 371.8 | 1854.8 |
3.0 | 181.8 | 7.3 | 267.4 | 24039.6 | 796.7 | 1581.6 |
5.0 | 182.2 | 7.9 | 271.7 | 16707.0 | 751.6 | 875.3 |
7.0 | 193.7 | 8.3 | 305.4 | 11911.1 | 757.0 | 674.0 |
10.0 | 206.3 | 10.5 | 376.6 | 10384.6 | 754.3 | 346.8 |
Smooth scenario
Beacon cycle | VBURL | ABSM | ReC | |||
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(A) | (B) | (A) | (B) | (A) | (B) | |
0.5 | 155.1 | 3.6 | 174.8 | 2434.8 | 334.5 | 1692.7 |
1.0 | 157.7 | 3.1 | 190.2 | 2979.2 | 253.9 | 299.2 |
3.0 | 166.8 | 4 | 217.1 | 1831.2 | 227.5 | 135.4 |
5.0 | 175.2 | 4 | 249.3 | 1544.4 | 244.8 | 119.5 |
7.0 | 166.6 | 5.1 | 260.4 | 1318.0 | 287.1 | 97.3 |
10.0 | 194.8 | 5.4 | 271.5 | 1344.3 | 451.3 | 95.0 |
Specifically, the message broadcast is attempted when
The difference between VBURL and the two compared legacy schemes is particularly prominent in
The size of a vehicle group tends to be larger in an environment with high vehicle density and worsens the situation. Thus, in VBURL, ABSM, and ReC, the number of broadcasts due to the discovery of a vehicle with no message in the congestion scenario is approximately 2 times, 9.2 times, and 2.1 times larger than in the smooth scenario, respectively. Both ABSM and ReC have a decreased chance of performing broadcast by discovering the vehicles with no message as the beacon cycle is lengthened. As a result, the number of broadcasts owing to the discovery of vehicles with no message decreases as the beacon cycle gets longer in Table
VBURL incurs the smallest number of broadcasts resulting from the reception of a safety message even though the difference among the schemes is not as large as the difference in terms of the number of broadcasts resulting from the discovery of vehicles with no message. In all three schemes, the number of broadcasts resulting from the reception of a safety message is around 1.1–1.5 times larger in congestion scenario than in the smooth scenario. It is because the loss of a beacon due to a beacon collision causes the inaccuracy of beacon information in the environment with high vehicle density resulting in decision of suboptimal forwarding vehicle and/or rebroadcast. The performance difference depending on the traffic density scenario is the least in VBURL though since it is the least dependent on the holding information obtained from beacons; that is, the utilization of holding information is limited to the rebroadcast decision at the intersections.
In VBURL, the number of broadcast attempts upon receiving a new safety message slightly increases as the beacon cycle is lengthened. It is because, as the beacon cycle lengthens, a road with no neighboring vehicle anymore actually tends not to be recognized early enough, causing unnecessary rebroadcasts.
The number of broadcasts by the reception of a new safety message is the largest in ReC, which is the most dependent on the accuracy of location information provided by beacons. In ReC, if the beacon cycle is too short, not only the loss of the beacons due to collisions increases, but also more frequent broadcasts occur because the broadcast waiting time becomes shorter (note that the maximum waiting time is set as the beacon cycle in ReC). Note that difference in waiting time among the vehicles as well as the length of waiting time becomes smaller as the maximum waiting time becomes shorter and vehicle density becomes higher, leading to higher chance of unnecessary broadcasts. On the other hand, when the beacon cycle becomes too long, not only the inaccuracy of location information obtained from beacon increases, but also the number of vehicles considered as neighbors increases, both of which lead to the degradation in efficiency. As a result, in ReC, the number of broadcasts initiated by the reception of a new safety message first decreases as the beacon cycle became longer, then it starts to increase when the beacon cycle rises above a certain value.
In the case of ABSM, the number of broadcast attempts upon receiving a new safety message increases as the beacon cycle is lengthened. For ABSM, not only the beacon inaccuracy increases as the beacon cycle is lengthened but also the average number of neighboring vehicles tends to increase due to the increase in beacon hold time. If the number of vehicles regarded as neighbors increases, the broadcast waiting time becomes shorter and tends to be similar among vehicles in ABSM, thus increasing the chance of unnecessary broadcasts or collisions.
Figure
Delay of message reception.
Congestion scenarios: VBURL (
Congestion scenarios: ABSM (
Congestion scenarios: ReC (
Smooth scenarios: VBURL (
Smooth scenarios: ABSM (
Smooth scenarios: ReC (
The percentage of vehicles receiving a message within one second after the first broadcast in the smooth scenario (Figures
Meanwhile, ABSM has a slightly higher percentage of message reception within one second compared to VBURL and ReC. This is not only because waiting time of ABSM tends to be shorter than the other two schemes, but also at the cost of very low efficiency and large number of broadcasts as shown in Figure
In the congestion scenario (Figures
Safety message delivery QoS for vehicles receiving messages 30 seconds after the first broadcast.
Figure
Average message reception delay.
Congestion scenario
Smooth scenario
VBURL and ReC show a trend of slightly increasing average reception delay as the beacon cycle is lengthened, because it takes longer to discover a vehicle with no message. According to the way how the broadcast waiting time is set, VBURL tends to have a longer broadcast waiting time than ABSM and ReC. As a result, in most of the cases, the average reception delay of VBURL is slightly longer than those of the compared schemes. In the case of ReC, the message reception delay tends to be higher when the vehicle density is high and the beacon cycle is long. It is because the information inaccuracy increases as the vehicle density and/or the beacon cycle increase, and as a result, multiple forwarding vehicles with the highest priority may exist resulting in collisions or broadcast delay increases due to the lack of a vehicle with the highest priority. In the smooth scenario, where the vehicle speed is high and the inaccuracy of location information tends to be higher, there are cases where the average reception delay of ReC is longer than that of VBURL. In the case of ABSM, the average reception delay is the shortest among the compared schemes for all beacon cycles. The waiting time of ABSM tends to be very short due to too many rebroadcasts meaning extremely low efficiency, as shown in Figure
A broadcast message dissemination scheme with minimal beacon dependency is proposed considering the characteristics of urban roads such as frequent intersections, high vehicle density, and traffic concentration due to frequent traffic signals. The proposed VBURL selects the forwarding vehicle only using its location and the location of the current forwarding vehicle which is conveyed in the received message, that is, real-time information, and by leveraging the road layout information which is available from the digital map obtained through the DMB network. On urban roads, the intersections and the roads connecting the intersections are repeated. On a straight road, the vehicle that is farthest from the current forwarding vehicle simply performs broadcast forwarding for rapid and efficient broadcast dissemination. In intersections, however, where signal attenuation is more serious due to surrounding obstacles such as buildings, the vehicle located in the intersection area that is free from such influence performs additional broadcasts in order to facilitate broadcast dissemination to all the roads adjacent to the intersection. The beacon only serves a supplementary role to deliver a safety message to the vehicles that have not received the message, which are newly entering an area where the broadcast has been completed. For performance evaluation, the percentage of vehicles receiving messages, the number of vehicles receiving the message per broadcast, and the message reception delay are measured for various beacon cycles in the congestion/smooth scenarios. All of the three compared schemes, VBURL, ABSM, and ReC, show reliabilities close to 100%, but the broadcast efficiency of VBURL is 2–100 times higher than that of ABSM and ReC for all beacon cycles in both of the congestion and smooth scenarios. Difference in efficiency between VBURL and the compared schemes is greater in congestion scenario for which the traffic concentration as well as the loss of beacons is more serious. In particular, the broadcasts inefficiency of ABSM and ReC is significant when neighboring vehicles with no message are detected. In terms of message propagation speed, the average message reception delay of VBURL is slightly longer than that of ABSM and ReC. The difference in the average reception delay among the three schemes is, though, less than 1.5 seconds, which would not have a significant effect on quality of service. Furthermore, in VBURL, the vehicles whose message reception delay is long are those located far from the accident point at the time of the first broadcast, and all the vehicles receive the safety message early enough to have sufficient chance to reroute to a bypass before reaching the accident point.
The authors declare that they have no conflicts of interest.
This work was supported by Institute for Information & Communications Technology Promotion (IITP) Grant funded by the Korea government (MSIT) (no. 2015-0-00183, “A Study on Hyper Connected Self-Organizing Network Infrastructure Technologies for IoT Service”).