Safety vehicular communications
The main aim of this research is to provide safety vehicular communications. The VANETs comprise Vehicle-to-Vehicle and Vehicle-to-infrastructure communications. This is based on IEEE 802.11p standard. IEEE 802.11p also known as Wireless Access in Vehicular Environment (WAVE). The current state-of-art explains about the WAVE standards. There are several applications for a WAVE. To enhance high traffic safety Cooperative collision avoidance (CCA) is the key application discussed. There are several challenges which will influence the future of vehicular wireless access. VANETs must work reliably in order to gain maturity.
The research on wireless communication in vehicles has started in 1980s. At first Federal Communications Commission (FCC) introduced this concept to reduce the traffic fatalities in US. Factors like adaption of IEEE 802.11 standards played prominent role for the drastic increase in the wireless communication in vehicles for the past few years. Vehicular communication is necessary for road safety, to track the position of the vehicle, to increase the efficiency in the transportation and to avoid collisions. The two applicable solutions for wireless technologies are MANET and VANET. Vehicular ad hoc network (VANET) is one of the promising applications of mobile ad hoc networks (MANET). The word ad hoc is derived from a Latin phrase know as ‘for this purpose' or ‘to solve the specific problem/task'. The centralized control is not necessary for the MANET and hence it is known as self forming network. In the ad hoc network each node works as both router and data terminal. These nodes communicate with the other nodes by wireless medium in their radio range. VANETs are effective subset of MANETs. (Nathan Balon: 2006)
Although we have cellular networks for voice communication, they are not well suited for direct vehicle-to-vehicle and vehicle-to-infrastructure (road side units) communications. VANETs offer direct communication between vehicle-vehicle and vehicle-road side units. The two projects which are relevant to VANETs in the world: in Germany there is Fleet Net project and in Janpan ITS (Intelligent Transportation Systems) project. The main goals of this project are to provide necessary information about the surrounding environment to the driver in the vehicle in order to have easy, safe and efficient transportation. National governments are funded substantially this project and contributes licensed spectrum of 700MHz from the range of 5.8-5.9 GHz. 5.9GHz spectrum was termed as Dedicated Short Range Communication (DSRC) which is based on the IEEE 802.11a standards. There are several challenges for VANETs. The key challenge of VANET is that it has no communication coordinator. IEEE 802.11p and IEEE 609 Wireless Access in Vehicular Environments help us to meet the challenges. (Nathan Balon: 2006 & Hannes Hartenstein, Kenneth P. Laberteaux: 2008).
IEEE 802.11p introduced to support Intelligent Transportation System (ITS) and to add wireless access in vehicular environments to IEEE 802.11 standards. This includes the high exchange data rate between vehicle-to-vehicle and vehicle-to-infrastructure in the licensed ITS band range of 5.8-5.9 GHz. The main purpose of this standard is to provide set of specifications required to have interoperability between wireless devices trying to communicate with the rapidly changing communication environments and where transactions takes place with time frames which is minimum possible with infrastructure or ad hoc 802.11 networks. The specifications of 802.11p describe the functions and services required by WAVE stations to operate in the rapidly changing environment and defines WAVE signalling technique and interface functions which are controlled by IEEE 802.11 Media Access Control (MAC). (Wayne Fisher: 2004).
2.1.MEDIA ACCESS CONTROL (MAC):
The media access control layer is a sub layer of data link layer. The main objective of media access control layer is to access the shared medium in the wireless channel. (Nathan Balon: 2006). It prevents collision from the nodes within the transmitting range of each other sending at the same time. To restrict large number of collisions and to reliably send data, IEEE 802.11 protocols can be used with CSMA/CD acknowledgements. There are two protocols which are defined by 802.11. They are Distributed Coordination Function (DCF) and Point Coordination Function (PCF). The DCF protocol is easy to implement but there is no guarantee in quality of service. PCF protocol relies on central node to support the of the packets. The main functions of MAC sub layer are selection of suitable format for transporting the data, management of priorities, changing the type of sending channel, encryption and random acess. (Benard H.Walke).
3.Current state-of-art of vehicular wireless access:
The researcher or developers used wireless local area networks (WLAN or Wi-fi) for vehicular communications. The wide perspective applications for vehicles were not fulfilled by these wireless technologies. Later they found new environment for vehicles known as wireless access in vehicular environments (WAVE). The WAVE system initiated to establish the physical platform for both Intelligent Transportation Systems (ITS) and Vehicular infrastructure integration (VLL). The WAVE system provides warning messages to the driver when there is a chance to occur accidents which helps to improve driver safety. The WAVE system performs tracking of location and monitoring the speed of the neighbouring vehicle with the help of Global Positioning Systems (GPS). The WAVE prototype is developed to make its reality. The designers and researchers have developed several Dedicated Short Range Communications (DSRC) prototypes with the help of existing wireless technologies such as wi-fi, to the vehicular environment. The wi-fi systems were proved as a unstable systems for vehicular environments by both theoretical and practical analysis. The industry needs the dedicated standard protocols. At present, IEEE 802.11p group working on the development of physical and MAC protocols for the WAVE. (Weidong Xiang, Yue Huang and Sudhan Majhi: 2008). The design of a WAVE prototype involves the setup of the development in the graphic user interface (GUI). IEEE 802.11p is under development, Federal Communications committee (FCC) has assigned the range of 5.850-5.925 GHz for wireless access in vehicular environments. The system architecture and design of WAVE is as follows:
The study on WAVE system introduced several new research topics. Some of the challenging topics are mobile channel modelling, quick synchronizations, study of Doppler shift, fast channel tracking, capacity evaluating when there is multiple input and multiple output (MIMO), adaptive modulation, MAC protocols in novel networks. The main objective of this protocol is to address the functions of the above challenges.
The prototype should be user friendly with GUI touch screen and voice controlled command facility. It gives all the information with image/video based traffic monitor. The GUI is developed in the matlab environment. All the designed functions is available to the driver in two fold purposes way. The WAVE prototype algorithm includes all the challenging topics discussed above. To demonstrate the defined functions in the WAVE systems, a real time WAVE prototype is proposed.
The PHY/MAC layers protocol is based on IEEE 802.11 standard with distributed coordination function (DCF). But there is no guarantee in quality of service (QOS). The American society for testing and materials (AMTS) modified 802.11a standard for a better match with the vehicular environments. Based on this scenario, the IEEE 802.11p is
standardized. |This IEEE 802.11p is based on the orthogonal frequency division multiplexing (OFDM) PHY layer which uses 10MHz channels at the rate ranges 3 to 27Mb/s.
The IEEE 802.11p standard depends on the spectrum allocation of the dedicated short range spectrum (DSRC). The spectrum of 5.9GHz is used in vehicle-to-vehicle and vehicle-to-infrastructure communications.
From the above fig, the DSRC spectrum is divided in to seven 10MHz wide channels. Ch 178 is control channel (CCH) which is used for only safety communicatios. At the end of the spectrum band the two channels are for special uses. And the rest are service channels (sch) which can be used for both safety and non-safety purposes. The DSRC band is a free licensed spectrum. Because Federal communications committe (FCC) does not charge any cost for using this spectrum. This spectrum is free and restricted that can be used under regulations of the FCC with free of cost.
The control channel is used to give information about the service channels availability. If the vehicle gets the acknowledgement from the control channel about the service channel availability, then the vehicle switches to one of the service channel to use channel for the communication. Vehicular ad hoc networks are also termed as the Inter-vehicle communication (IVC), DSRC or WAVE. In the design of VANETs, medium access control (MAC) plays a key role. Although we have proposed techniques such as TDMA and SDMA, VANETs use carrier sense multiple access based (CSMA) on 802.11p standards. This reduces the cost and accepts random elements. The extension of SDMA is Adaptive space
division multiplexing (ASDM) which allows time slots based on a location of the vehicle. ASDM splits the roadway in to cells and the mapping function in it maps the cells to time slot. By maintaining adequate distance, this protocol gives certain quality of service (QOS) to the vehicles.
There are several challenges for wireless communications in vehicular environments/VANETs. There is no communication coordination can be assumed. Because of this handshaking protocols can be assumed for the communication to send the information to all the vehicles which is very complex. One more challenging factor is, that VANETs don't have privacy. Like the receiver can trust the source of information which might contradict the privacy of the source (sender). The IEEE 802.11 research is going on about the WAVE to meet all the challenges aroused.
The applications of Wireless vehicular access are classified as safety, transport efficiency and information or entertainment applications. The examples for each classification are as follows:
Cooperative forward collision warning which is used to avoid rear-end collision.
Traffic light speed advisory to assist driver according to the signal.
Remote wireless diagnosis to track the state of the vehicle for remote diagnosis.
There are eight high potential applications identified by the association vehicle safety communications (VSC). Warning during traffic signal violation, curve speed warning, electronic emergency brake light , pre-car sensing, warning when cooperative forward collision occurs, left turn assistant, warning when there is change in the lane, and stop sign movement assistant. These eight applications requires vehicle-to-vehicle communication and communication with the road side infrastructure. All the technical requirements shows the importance of one hop broadcasting communication. One vehicle transmits a packet of data and every vehicle is able to receive the packet can be treated as one hop neighbour.
This comes in to two ways. One is Event driven message and the other is periodic messages. Event driven messages sent when there is any dangerous situation. The periodic messages
informs the status of the neighbouring vehicles by detecting the position of the sending vehicle. The vehicle safety communications (VSC) requires periodic hop broadcasting as the forward collision warning requires 10 messages/sec frequency, with a maximum latency of 100ms and minimum range of 150 meters. The Car-car communication consortium (C2C-CC) analyzed navigation, optimal speed advisory by green light and lane merging assistants in the transport efficient applications. The C2C-CC provides reasonable level of security to trust the detected information. The entertainment applications is a different one which consists of fuel consumption management, important notifications, internet access are few applications.
COOPERATIVE COLLISION AVOIDANCE (CCA):
To enhance high traffic safety, the DSRC based wireless communication protocols can act effectively for the development of cooperative collision avoidance (CCA) application. \By using three car highway example the mechanism of CCA can be explained. Initially let us assume that all cars are moving at a fixed speed as shown in the fig below. At first the front car which is car0 initiates emergency deceleration due to the emergency event. Car1 gets decelerates when the driver in the car1 sees the brake light of car0. Similarly, the car2 driver does the same process when he sees the brake light of car1. The reaction time of the driver is 1.5s and car0 hits car1 at a distance of 120m. Now the conclusion is all the three cars end-up with the chain collision.
For the same process as mentioned, in this car0 sends the wireless collision warning messages (W-CWM) to all the cars available. In order to have complete coverage, this message is forwarded in multihop manner. Based on the W-CWM message reception, the driver reacts for decelerating without the tail of the light brake. From the fig it shows that car1 collides car0. The car2 can avoid collision when it receives W-CWM message. Hence by using wireless communication networks, there is possibility to design CCA which improves highway safety to avoid chain collisions. At the same time, fast warning messages delivery and reliability is crucial for CCA.
The list of various applications are
Entertainment: In vehicle with the help of GUI, streaming of media and the accessabilty of internet is available as a user-centric.
Remote operation: The operation of devices in remote locations like home or office e.g. switching on the heater are also Fixed-mobile applications.
Real-time weather: The vehicles can download current weather report and forecasts in a user friendly manner.
Asset tracking: vehicle reports its position and its items when it is in a Fixed-mobile way.
Road hazard detection, Map generation, Slot booking, Congestion information and Mobile commerce are few important applications in vehicular wireless access.
ENVISIONING THE FUTURE DEVELOPMENT OF WAVE:
There are many applications for vehicular networks for safety features such as collision avoidance and crash notification. In future we can see a combination of two initiatives for wireless access. Firstly, the Governments are going to begin distribution of converged networks along with the motor ways which is UK's National Roads Telecommunication Services (NRTS) to carry data from the CCTV to the roadside emergency phones. These networks can access both Government and commercial enterprises to offer special services built on vehicles travelling on the road networks. Secondly, to cover large area of the cities both public and private sectors will roll out wireless networks. Due to the competition among the providers the cost will drive down, since the cost of the deployment is low when compared to the vehicular networks. The band width is an important issue, at present the connectivity is less than 1 Mb/s. This is set to increase in future for its provision.
Apart from the advances in the wireless networks, modern vehicles contain many processors which are used for various purposes. We can expect vehicles to provide embedded computing platforms for the execution of user applications by decreasing the power requirements, size and cost. This will allow the applications distributed over the network which makes use of a distributed computing architecture.
Firstly, the IEEE 802.11p standards are described on which the vehicular wireless access is based. Then the current state-of-art is known which includes the WAVE standards based on IEEE 802.11 standards. In this we discussed about the channel allocation by using DSRC spectrum band. Later applications of the WAVE are given with examples as well as with detailed explanation. Finally we envisioned the future development of WAVE.
Finally, there are many challenges that will strongly influence the future of vehicular wireless access which is based on the IEEE 802.11 standards. Hence VANETs must work on the situations with reliability or it should be able to concentrate on which area it is not reliable. In future it gains level of maturity with the IEEE 802.11 ongoing research.