Researchers:Farzad Farnoud, Le Zhang,

Behnam Hassanabadi, Christine Shea

VANET Collision-Avoidance and Platooning using reliable MAC on DSRC

Project Code:F202

Supervisor:Dr. Shahrokh Valaee

Motivation and Background: In 2002 there were approximately 228,000 injuries and 3,000 deaths caused by motor vehicle accidents in Canada. The leading cause of these accidents is driver error, particularly slow driver reaction time. In a high-speed highway scenario, this slow reaction time can often lead to catastrophic multi-car pileups. In an effort to reduce the number of vehicular accidents on the road, intelligent transportation systems (ITS) are being developed. One promising intelligent transportation system uses wireless communication technologies to bring wireless access to the vehicular environment (WAVE). The Dedicated Short Range Communication (DSRC) standard is a wireless protocol that is in development to allow vehicle to vehicle (V2V) and vehicle to roadside (V2R) communication. This technology will allow vehicles to communicate with one another, and will create Vehicle Ad Hoc Networks (VANETs) on the road. VANETs will allow cars to send safety messages amongst one another to indicate the presence of accidents and other hazards. In order for these safety applications to run effectively, it is necessary to have a highly reliable Medium Access Control (MAC) layer, such that vital safety messages aren’t lost. In this project, we have built a simple VANET test bed using developmental DSRC devices from MARK IV Industries as the physical layer. We have then developed two safety-relevant prototype applications on this test-bed, which include Collision Avoidance and Vehicle Platooning. Finally we present the theoretical basis for a reliable VANET MAC scheme, as well as simulations.

Reliable Medium Access Control Scheme

Robotic Application Test Bed Physical Layer: DSRC

Collision-Avoidance Application

Vehicle Platooning Application

Dedicated Short-Range Communication (DSRC) was introduced in 2003 as a means to facilitate V2V and V2R communication. In this project, we have collaborated with MARK IV industries, and they have supplied us with experimental DSRC devices, which provide the physical layer of our test bed.

Theory:VANET broadcasting occurs under high-mobility with harsh channel conditions. In addition, most VANETs will operate under the developmental IEEE 802.11p (WAVE) standard. The WAVE standard uses 802.11 technology, which generates further issues, including a lack of acknowledgments (ACKs) and the Hidden Terminal problem. All of these issues result in an unreliable MAC, which will allow vital safety messages to be lost.

Simulations:

The proposed scheme uses a repetition-based broadcast using Optical Orthogonal Codes. Each frame is divided into L timeslots, and each vehicle is given a binary code, which represents the repetition pattern. For a given binary code, a 1 indicates a repetition (the packet should be rebroadcast) and a 0 indicates that no broadcast should be made.

We examine three different repetition-based schemes, with a focus on Optical Orthogonal Codes. The first scheme, Synchronous Fixed Retransmission (SFR), retransmits a packet w times per frame, where the w repetition slots are chosen randomly from the L available timeslots. In the next scheme, Synchronous p-Persistent Retransmission (SPR), each of the L timeslots decides to broadcast the packet with probability p, and decides to remain idle with probability 1-p. The final scheme uses Optical Orthogonal Codes (OOC) to decide the repetition pattern. OOC’s are desirable because they have a small cross-correlation. For any two codewords x and y in an optical orthogonal code C, with length L, the maximum cross correlation λ, will be:

Given a constant weight (the number of 1’s), w, it is possible to generate OOC codes with a specific maximum correlation. We can therefore choose this maximum correlation to be 1, and create a code with only one interfering slot amongst any two codewords. Each of these codewords can then be assigned to a different vehicle in the VANET. For a specific frame, any two vehicles will only have one interfering retransmission, thus greatly increasing the probability of successful transmission. A sample code is displayed below:

CyxL

iii

yxyx ,,1

1100001 0010101

0110010 1000110

0101100 0001011

1011000

Fig. 5: Simulation results for μp =0.3 and N = 61; left and right plots show Probability of Success vs. distance from the receiver for (L,w) = (64,6) and (94,8) respectively

Fig. 6: Simulation of different QoS Levels for μp =0.3 and N = 61; left and right plots show probability of success and delay vs. distance, respectively, for L = 64, whigh = 6, and wlow =4

For the simulations, a Rician fading channel model was used. For N vehicles within the reception range, each vehicle independently decides to broadcast its location with probability μp . The probability of success was simulated for each of the SFR, SPR, and OOC repetition schemes. In addition, QoS levels were simulated by randomly removing a 1 (repetition) from the lower priority vehicles’ codewords.

Set-up:A robotic test bed was developed to run and test our collision-avoidance and vehicle platooning applications. The design consists of the following:

two ER1 Robotics System test vehicles

two Windows XP laptops (attached to each vehicle)

two wireless Logitech game pads

two MARK IV DSRC units

two Cricket mote transmitters (mounted on the ceiling)

two Cricket mote receivers (attached to each vehicle)

The Cricket technology provided us with a location sensing subsystem. The cricket motes use RF and ultrasound technology to simulate an indoor GPS system. The two cricket transmitters were mounted on the ceiling against a wall, and the cricket receivers were placed on the vehicles. Each test vehicle was then able to determine its location.

The Logitech game pads were used to steer the test vehicles and turn on platooning mode.

Once the test-bed was functional, we developed a collision-avoidance safety application. The software was written in C++, and the basic algorithm is depicted on the right.

The vehicles transmit their current position to one another using DSRC, and if a collision is predicted, the vehicles motion is disabled in that direction.

Fig. 1 – A Cricket mote and Logitech Game pad

Fig. 3 – MARK IV DSRC Device

Fig. 4 – The complete robotic test bedFig. 2 – Collision Avoidance Algorithm

Another safety application that was developed and tested was vehicle platooning. Vehicle platooning is an intelligent cruise-control where one vehicle follows another vehicle at a safe distance while avoiding a crash. For our test bed, software was developed to cause a “platooning” vehicle to move to the location of the other test vehicle, but not hit it.