Proceedings of the 2008 IEEE International Conference on Vehicular Electronics and Safety Columbus, OH, USA. September 22-24, 2008

An Integrated Wireless Intersection Simulator for Collision Warning Systems in Vehicular Networks ¨ ¨ ¨ Boangoat Jarupan, Yalcin Balcioglu, Eylem Ekici, Fusun Ozguner, and Umit Ozguner Department of Electrical and Computer Engineering, The Ohio State University (bea,balciogy,ekici,ozguner,umit)@ece.osu.edu


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Abstract— In this paper, we present the architecture of an Integrated Wireless Intersection Simulator (IWIS) that is used to study the effect of different communication-based solutions on vehicle traffic and vehicle behavior in Intelligent Transportation Systems (ITS). IWIS consists of two components, a Vehicle Intersection Traffic Simulator (VITS) that simulates vehicle traffic, and a Wireless Simulator (WS) that simulates wireless packet transmission. The user can provide a variety of traffic configurations such as buildings, traffic lights, and vehicle density through a friendly graphical user interface. The modular nature of our simulator gives the ability to compare different communication protocols easily. The output from WS (packet collision rate and transmission delay) is fed to a collision warning system in VITS. We evaluate IWIS under different traffic conditions and different MAC protocols.


  

  




   

I. I NTRODUCTION

Fig. 1.

An intersection collision warning system uses ITS technologies to prevent collisions and reduce their severity. A warning system deployed on a vehicle monitors the movement of neighboring vehicles to send out alert signals to drivers on possible collisions. To investigate the performance of such systems in a cost efficient manner, development of integrated simulators is necessary. An integrated simulator is a complete software architecture consisting of both a traffic simulator and a wireless network simulator. In this paper, we present the complete architecture of our warning system simulator, Integrated Wireless Intersection Simulator (IWIS) being developed since 2003 [1], [2]. Recently many researchers have proposed integrated simulators such as VanetMobiSim [3], ISP [4], HiTSim [5], and the simulator in [6]. VanetMobiSim uses the Advanced Intelligent Driver Model (AIDM) for traffic generation and can use different network simulators such as ns-2 [7], GloMoSim [8], or QualNet [9]. This simulator studies different type of traffic behaviors. However, it does not focus on a collision warning system model. Similar work is being reported by Toyota Information Technologies (ISP) [4] aiming to develop a simulator platform with the ability to integrate vehicular traffic simulations and wireless network simulations. ISP studies V2V and V2I communications, and it can simulate multiple streets and intersections. ISP only models 802.11 in its MAC layer whereas IWIS has the capability to study a number of different MAC protocols. Young and Chang introduced HiTsim [5] which incorporates a collision avoidance system. It is similar to the work done in this paper

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Architecture of Integrated Wireless Intersection Simulator

in the sense that the traffic simulator is not collision free and the way it incorporates the levels for generated collision avoidance warning messages. In addition, it uses the wellknown ns-2 network simulator in a similar manner. HiTSim does not only generate random traffic but can also load input traffic scenario files. However, HiTSim can simulate only highways and it has a simple propagation model which does not model communication channel related issues such as shadowing, blocking and buildings. Kerner, Klenov, and Brakemeier [6] also try to overcome the simulation time scale problem by doing offline simulations. However this new test-bed only implements a simple two-ray ground radio propagation model. IWIS is built upon our previous research [1], [10], [11], [2] and it consists of two main components, Vehicle Intersection Traffic Simulator (VITS) and Wireless Simulator (WS) (Figure 1). In this paper, we adopt the architecture of VITS from our previous work which is built using a Visual C++ environment and provides a friendly graphical user interface (GUI). The VITS traffic simulator includes a driver model which takes action upon receiving a warning message. VITS is based on a realistic traffic scenario which includes multiple lanes and different type and size of vehicles. IWIS relies on detailed propagation models and accurately accounts for noise and interference. The model includes building blockage and vehicle shadowing considering different size vehicles. Performance of IWIS is investigated on an urban road

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network with an intersection. Unlike our previous work where WS was built using CSIM [12], WS in this work is built using the network simulator ns-2 [7]. Ns-2 brings in the flexibility and modularity to our system architecture where individual components can be changed transparently without affecting the rest of the system. In comparison with our previous work and existing simulators, we found that IWIS not only evaluates the collision warning system but it also can be used in evaluating and comparing different MAC protocols such as 802.11p [13], UMB [14], and DOLPHIN [15]. These MAC protocols were simulated in various traffic conditions using IWIS. The evaluation results are presented in Section V. Both VITS and WS are stand-alone simulators, and they can be run independently. To seamlessly integrate the simulation traffic flow and communication among vehicles, we introduce the automated interface model in this paper (Figure 2). The VITS-WS interface reads the traffic flow input files from VITS and translates them into the WS environment. The WS-VITS interface collects statistical results from the WS and automatically places them into the VITS environment. IWIS runs VITS and WS simulations interlinked by the interface models, and generates the results based on user defined inputs.

Fig. 2.

Flow of IWIS

II. IWIS OVERVIEW IWIS is a collision warning system simulator which consists of VITS and WS. VITS simulates vehicular behavior and produces traffic flow information, which is fed to WS. Based on traffic flow information from VITS, WS simulates the wireless communication among vehicles. WS produces several packet transmission statistics which are fed back into VITS, and then they are used in generating traffic alerts on important events (such as possible vehicle collisions). VITS and WS operate in different time scales. VITS simulates ITS events which are fewer in number and in milliseconds timescale level compared to WS which generates exhaustive communication events and operates at the microsecond level. The WS-VITS interface is used to avoid the

Fig. 3.

IWIS Graphical User Interface

timing conflicts between the two simulators by performing a statistical approximation of wireless transmission behavior. A. IWIS intersection environment and input variables IWIS is developed in a Visual C++ environment. It provides a GUI based interface which allows users to configure the following list of parameters. MAC protocols: IWIS supports three different MAC protocols: 802.11p, UMB, and DOLPHIN. Vehicle type and dimension: IWIS supports four vehicle types, passenger vehicles (cars), busses, trucks, and motorcycles. The vehicle type not only determines the physical dimensions but also properties such as maximum acceleration/deceleration rates, turning radius etc. IWIS uses these properties in accurately simulating the communication among vehicles. For example, the vehicle height contributes to a shadowing effect in the propagation model (Section IVB.3). Intersection vehicle throughput: This parameter controls the average number of vehicles at an intersection. It helps in studying the performance of IWIS in different traffic conditions. However, in the actual simulation, the number of vehicles and the time at which they enter and leave the intersection area are modeled as probabilistic distributions. Transmission interval: The road is divided into segments based on the user specified range. A transmission interval is the length of segments within a given range. Vehicles generate data packets in each transmission interval. Therefore, a smaller interval causes a higher packet generation rate. Building presence: Buildings can block signals, and therefore greatly hinder communication among vehicles. Section IV-B.3 describes the effect of signal path loss in detail. Repeater presence: A repeater placed on a high structure above the road can be optionally present at the intersection. The repeater re-broadcasts all received packets. Hence, it greatly improves the packet transmission success rate. Traffic signal presence: Traffic lights control the flow of traffic at intersections. A screen shot of the GUI used by IWIS is shown in Figure 3. The left side of the window displays the animation of vehicle movement, whereas the right hand side demonstrates vehicle collision warning messages generated by the simulator.

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III. VITS VITS simulates the movements of different vehicles according to the user-defined parameters described in Section II. It produces traffic flow information such as the position and speed of vehicles at different times. VITS consists of the collision warning system, the message generator, and the driver behavior model (Figure 1) as described below. A. Collision Warning System and Message Generator The Message Generator generates warning messages of three different levels: elevated; high; and severe. The data packets generated by a given vehicle contain the vehicle’s current position, velocity, and acceleration. These messages are generated based on the collision probability for a given vehicle that is computed from received data packets (from neighboring vehicles) and expected destination of the vehicle. The simulator detects collisions when two vehicles have overlapping bodies. Once the drivers receive a warning message, they take different actions to avoid the collision based on the driver behavior model. B. Driver Behavior Model This model determines driver’s response to different warning messages. We define three types of driver characteristics: aggressive, normal, and conservative. Based on the driver type and the level of the warning message, the driver responds by accelerating, decelerating, or breaking. For instance, aggressive drivers may ignore low level messages while conservative drivers may decide to decelerate the vehicle. We are in the process of developing a new version of VITS which includes more realistic traffic environments and additional user input parameters. IV. W IRELESS S IMULATOR The Wireless Simulator (WS) performs the detailed simulation of wireless packet transmission among vehicles and repeaters. WS incorporates fundamental protocol properties, such as carrier sensing, random backoff, transmission interval, and packet retransmission which are developed using the well-known wireless Network Simulator (ns-2). This simulator also models the functioning of repeaters which are optionally placed at the intersection. Due to the modularity of the ns-2 architecture, the packet transmission behavior of different MAC protocols are evaluated. The physical layer and channel propagation in ns-2 are enhanced to include the effect of signal path loss and frame error rate. Figure 4 shows the architecture of the WS. WS has three main functionalities: VITS-WS interface, WS-VITS interface, and wireless communication. The interfaces are responsible for transferring traffic flow information from VITS to WS (Section IV-A) and statistics from WS to VITS (Section IVC). The wireless communication module (Section IV-B) simulates the actual packet exchange among vehicles and the repeater.

Fig. 4.

Architecture of Wireless Simulator

A. VITS-WS interface The interface reads input files with the traffic flow information from VITS, and performs two actions: vehicle position update and packet generation. The VITS-WS interface reads the vehicle information and positions from VITS input files and periodically updates the vehicle configurations in the WS environment. The VITS-WS interface has a packet generation agent that monitors each vehicle position and its transmission border. Whenever the vehicle crosses the border, it broadcasts a packet with its current position, velocity, and acceleration. The data packet information helps other neighboring vehicles in tracking the position of the vehicle that sent the packet. Each vehicle employs a distance-based protocol for generating packets where it determines the transmission border by using its current position (given by input files from VITS) and transmission interval (a user-defined parameter). By employing such a distance-based protocol that relies on transmission borders, the load on the wireless channel is reduced when compared to a time-based strategy that sends packets at constant time intervals. For example, slow or stationary vehicles do not generate any redundant messages. Therefore in our protocol, the vehicle speed determines the frequency of packets and the number of packets depends on the distance traveled. B. Wireless communication The communication module of the Wireless Simulator is responsible for transmitting packets among vehicles and repeaters. The user can select the desired MAC protocol and other input parameters using the IWIS GUI window. All vehicles share a common wireless channel, and employ the propagation model. 1) MAC layer: Once a packet is enqueued into the queue, the vehicle determines the method of transmission based on the MAC protocol that the user has selected. Currently, IWIS supports three different MAC protocols: DOLPHIN [15], UMB [14], and 802.11p [13]. • DOLPHIN: DOLPHIN [15] is a p-persistent CSMA MAC protocol. The time is divided into slots, and in any given slot, a single packet is allowed to transmit with

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2) Routing layer: Since IWIS is based on broadcasts, we implemented a simple routing layer that is applicable to all MAC protocols. Unlike most existing routing protocols for vehicular networks, the routing layer neither performs neighbor discovery nor neighbor management through explicit packet exchange. In our protocol, vehicles directly forward the received packets to the application layer, and they do not rebroadcast the packets. The repeater nodes installed at intersections however rebroadcast the packets to different road directions. 3) Physical layer: The physical layer in IWIS is designed to model a wireless modem operating at the DSRC band. It computes the received power and bit error probability, which was proposed in [1]. Here, we briefly describe the ways in which signal path loss is computed. •

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Line-of-sight: In this case, there is no obstruction between the transmitting vehicle and the receiving vehicle. Therefore, the direct signal and the reflected signal (from ground) are the dominant contributors to the received power. The received signal path loss is computed using a Two-Ray model [17]. Building blockage: Buildings present between the sender and the receiver may block the communication signal, and therefore they act as major obstacles in vehicular networks. Here, the signals have no line-of-

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probability p. Before sending the packet, the vehicle senses the channel and waits for it to be idle. Once the channel becomes idle, it transmits the packet with probability p, a parameter that the user can specify. In case of packet collisions, the sender waits for a random amount of time and restarts the contention process. The maximum number of retransmissions is set to 5 times, and the slot size is set to 20 ms. UMB: UMB proposed in [14] is an IEEE 802.11based directional broadcasting protocol. In UMB, each successful packet transmission involves the RTB/BB/CTB/DATA/ACK handshake, where RTB is a request-to-broadcast packet, BB is a black-burst packet, and CTB is a clear-to-broadcast packet. Unlike in 802.11, the sender selects the farthest vehicle along the road in the direction of packet dissemination through the RTB/BB/CTB handshake. UMB relies on vehicles or a repeater at intersections to broadcast the packets in all directions. In other words, a selected vehicle or repeater issues new directional broadcasts to all road directions to cover an intersection area. IEEE 802.11p: IEEE 802.11p [13] is an extension to the well-known IEEE 802.11 standard [16] to support Intelligent Transportation Systems (ITS). The packets are transmitted using the flooding mechanism available in 802.11 that does not use the network topology or the neighborhood information. Vehicles employ a random backoff strategy to reduce the number of packet collisions. We use the latest version of ns-2 (ns-2.33) that has extensions to MAC and physical layers to support wireless access in vehicular environments.

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sight (between sender and receiver) and are received as a result of reflection and diffraction. In such scenarios, we employ a simple virtual source model [18] to compute the received signal path loss. Shadowing effect: In addition to buildings, large vehicles such as trucks and busses can potentially block the signals destined to nearby smaller vehicles. Such an effect is called the shadowing effect where the signal has no line-of-sight. We use the knife-edge diffraction model proposed by Rappaport to compute the path loss under shadowing effects [17].

C. WS-VITS interface WS collects the following statistics during simulation. Packet collision rate is the total number of packet collisions experienced by all vehicles (or repeater if it is present) during the simulation period. Packet latency is the packet delay from the source to the destination including contention and transmission delay. WS statistical results are fed into the IWIS database. Before starting the VITS simulation, IWIS checks the database availability. If the database is not available from previous simulations, IWIS automatically runs WS and generates the database for that simulation configuration. If the WS database is ready, IWIS can directly access it during VITS collision warning analysis. V. E VALUATION We study an urban environment with a four-way intersection. The intersection area is 215 x 215 m. Each road has two lanes running in opposite directions. The speed of vehicles is set to 45 mph (20 m/s). Additional IWIS userdefined parameters are set as per the discussion in Section II. WS simulation is run for 100 seconds. The data packet size is 500 bytes. Further, the maximum transmission power, data rate, and radio frequency is set 10 dBm, 3 Mbps, and 5.9 GHz (DSRC) respectively. First, we evaluate performance of the WS. In Figure 5, we compare average packet collision rate and vehicle throughput of 802.11p and DOLPHIN protocols. For this experiment, we set up the simulation with buildings at all four corners, a

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Transmission Delay Histogram (Traffic Signal, All buildings, Repeater)

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UMB and CSMA Packet Delay Distribution

traffic signal, and a repeater. We also considered other simulation scenarios. Due to space limitations, we omit the results from those experiments – however, we note that the trends are similar to the ones in Figure 5. We simulated with the transmission interval equal to 5 m (as 802.11p5 , Dol5 ) and 10 m (as 802.11p10 , Dol10 ). Overall, the results demonstrate that the packet collision rate is increased with the vehicle throughput. Furthermore, the transmission interval of 5 has a higher packet collision rate than the transmission interval of 10. This is because a smaller transmission interval causes higher number of packets to be generated. Hence, the packet collision rate is increased with smaller transmission interval and higher vehicle throughput. Between 802.11p and DOLPHIN, 802.11p has a higher collision rate because DOLPHIN uses p-persistent random access (p = 0.1) whereas 802.11p uses CSMA (broadcast mechanism). Since p (the probability of sending the packet at each time slot) is low, the vehicles could potentially avoid some of the collisions. Note that when vehicle throughput is from 5800 to 7200 vehicles/hour, 802.11p packet collision rate drops. This may be a result of channel overload where the sender drops the packets when it finds the channel busy (even after random backoff and retransmission). Although the results of UMB are not shown in the Figure 5, we found that the packet collision rate of UMB coincided with the X-axis and it is significantly lower than 802.11p and DOLPHIN. The

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low collision rate is mainly due to the handshake mechanism and also due to the presence of repeaters at the intersection. Figure 6 and Figure 7 show the delay distribution of DOLPHIN, UMB, and 802.11p. Out of all three protocols, DOLPHIN has a very high delay (up to 200ms), which is caused by the slot based p-persistent contention mechanism. When compared to 802.11p which has a delay up to 12ms, UMB incurs marginally higher delays (up to 19ms). This may be due to the extra packet overhead introduced by the handshaking mechanism. It is important to note that the increase in delay is much smaller than the decrease in collision rate achieved by UMB. In another experiment, we evaluated the performance of IWIS by measuring the vehicle collision percentage of MAC (802.11p and UMB) protocols and vehicle throughput (Figure 8). Overall, we find that vehicle collision rate increases with increased traffic flow. When packet transmission interval is reduced (packet generation rate is increased), the vehicle collision rate is reduced. Furthermore, we find that 802.11p has a higher vehicle collision rate than UMB. Although we do not show DOLPHIN results due to space limitation, we observed similar trends in DOLPHIN. Additional DOLPHIN simulation results can be found in [1]. VI. C ONCLUSION We present an Integrated Wireless Intersection Simulator (IWIS) in an urban environment that contains two standalone simulators: VITS and WS. VITS simulates traffic behavior while WS simulates communication events. The two simulators are connected through interfaces that transfer vehicle and statistical information from one simulator to the other. We propose a distance-based protocol to generate the data packets in our system. We evaluate our simulator under different traffic scenarios, and use the simulator to compare three different MAC protocols (DOLPHIN, 802.11p, UMB). Our WS results show that the packet collision percentage is increased with the packet generation rate. On the other hand, IWIS results show that vehicle collision rate is reduced when vehicles generate more messages. Furthermore, our simulations also show that UMB outperforms 802.11p and DOLPHIN, in terms of the packet collision rate, the transmission delay and the vehicle collision rate.

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R EFERENCES ¨ uner, [1] A. Avila, G. Korkmaz, Y. Liu, H. Teh, E. Ekici, F. Ozg¨ ¨ uner, K. Redmill, O. Takeshita, K. Tokuda, M. Hamaguchi, ¨ Ozg¨ U. S. Nakabayashi, and H. Tsutsui, “A complete simulator architecture for inter-vehicle communication based intersection warning systems,” Proceedings of the 8th International IEEE Conference on Intelligent Transportation Systems, pp. 461–466, 2005. ¨ uner, U. ¨ uner, K. Redmill, ¨ Ozg¨ [2] A. Dogan, G. Korkmaz, Y. Liu, F. Ozg¨ O. Takeshita, and K. Tokuda, “Evaluation of intersection collision warning system using an inter-vehicle communication simulator,” Proceedings of the 7th International IEEE Conference on Intelligent Transportation Systems, pp. 1103–1108, 2004. [3] J. H¨arri, F. Filali, C. Bonnet, and M. Fiore, “VanetMobiSim: Generating realistic mobility patterns for VANETs,” Proceedings of the 3rd International Workshop on Vehicular Ad Hoc Networks, pp. 96–97, 2006. [4] T. Hikita, T. Kasai, and A. Yoshioka, “Integrated simulator platform for evaluation of vehicular communication applications,” Proceedings of IEEE ICVES’08, Sept. 2008. [5] C. Young and B. Chang, “A highway traffic simulator with dedicated short range communications based cooperative collision prediction and warning mechanism,” Proceedings of the IEEE Intelligent Vehicles Symposium (IV’08), pp. 114–119, 2008. [6] B. Kerner, S. Klenov, and A. Brakemeier, “Testbed for wireless vehicle communication: A simulation approach based on three-phase traffic theory,” Proceedings of the IEEE Intelligent Vehicles Symposium (IV’08), pp. 180–185, 2008. [7] Network Simulator ns-2. [Online]. Available: http://www.isi.edu/ nsnam/ns/ [8] GloMoSim. [Online]. Available: http://pcl.cs.ucla.edu/projects/ glomosim/ [9] QualNet. [Online]. Available: http://www.qualnet.com/ ¨ uner, and E. Ekici, “Performance evaluation of inter¨ Ozg¨ [10] Y. Liu, U. section warning system using a vehicle traffic and wireless simulator,” Proceedings of the IEEE Intelligent Vehicles Symposium (IV’05), pp. 171–176, 2005. ¨ uner, U. ¨ uner, O. Takeshita, K. Redmill, Y. Liu, G. Korkmaz, ¨ Ozg¨ [11] F. Ozg¨ and A. Dogan, “A simulation study of an intersection collision warning system,” Proceedings of the International Workshop on ITS Telecommunications, Singapore, June 2004. [12] CSIM. [Online]. Available: http://www.mesquite.com/ [13] Draft Amendment for Wireless Access in Vehicular Environments (WAVE), The Institute of Electrical and Electronics Engineers (IEEE) Std. P802.11p, 2006. ¨ uner, “Black-burst-based multihop [14] G. Korkmaz, E. Ekici, and F. Ozg¨ broadcast protocols for vehicular networks,” IEEE Transactions on Vehicular Technology, vol. 56, no. 5, pp. 3159–3167, 2007. [15] K. Tokuda, M. Akiyama, and H. Fujii, “DOLPHIN for inter-vehicle communications system,” Proceedings of the IEEE Intelligent Vehicles Symposium (IV 2000), pp. 504–509, 2000. [16] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, The Institute of Electrical and Electronics Engineers (IEEE) Std. 802.11, 1999. [17] T. Rappaport, Wireless Communications: Principles and Practice. IEEE Press Piscataway, NJ, USA, 1996. [18] H. El-Sallabi, “Fast path loss prediction by using virtual source technique for urban microcells,” Proceedings of the IEEE Vehicular Technology Conference, pp. 2183–2187, 2000.

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