Today, road accidents represent a serious public problem and are one of the leading causes of people death and disability in many countries. For instance, in Canada, there were more than 2200 deaths and about 170,500 injuries as a result of traffic collisions in 2010. This severe social impact of traffic accidents comes in addition to the financial losses due to traffic congestion, as well as vehicle and property damage, which indicates an urgent need of efficient solutions to this socioeconomic problem of traffic accidents.
To reduce the risk and severity of a vehicle crash on road, wireless communications can be employed among vehicles moving in the vicinity of each other (vehicle-to-vehicle, or V2V), or among vehicles and specially deployed road-side units (RSU) (vehicle-to-RSU, or V2R). Communications so established in a distributed manner (i.e., without the need of any central controller), form what is generally known as a vehicular ad hoc network (VANET). Based on vehicle-to-vehicle and vehicle-to-RSU communications, a wide variety of VANET-based advanced safety applications can be realized, including lane change warning, highway merge assistance, in-vehicle signage, and cooperative forward collision avoidance. By deploying such VANET-based safety applications, studies done by the National Highway Traffic Safety Administration (NHTSA) at the United States Department of Transportation (USDOT) show that approximately 80% of the crash scenarios can be prevented, indicating a great potential of VANETs in enhancing road safety.
The majority (if not all) of the VANET safety applications require that each node (i.e., vehicle or RSU) broadcasts safety messages to all the surrounding nodes both periodically and also in case of an unexpected safety event. Some unexpected safety events may include a hard brake, an approaching emergency vehicle, or hazardous road condition detection. Failure or delay in delivering a safety message may shorten the reaction time available to a driver, thus resulting in undesired consequences. It is therefore desirable that each node successfully and timely delivers its safety messages to all the surrounding nodes. A medium access control (MAC) protocol for VANETs, such as VeMAC protocol to be further described herein, should support a reliable broadcast service, thus allowing such timely delivery of safety messages.
One solution proposed for MAC in VANETs is the IEEE 802.11p standard. This is a solution based on the legacy IEEE 802.11 standard (WiFi), which has been developed mainly for unicast communications. This standard has considerable limitations when used to support the broadcast-based safety applications in VANETs. For example, for unicast communications, a request-to-send/clear-to-send (RTS/CTS) handshaking mechanism is often used to mitigate the hidden terminal problem. However, for a broadcast packet (the term packet refers to a MAC layer protocol data unit) according to the IEEE 802.11p standard, no RTS/CTS exchange should be used and no acknowledgment (ACK) should be transmitted by any of the recipient of the broadcast packet (the packet type used to carry a safety message). This absence of RTS/CTS exchange results in a hidden terminal problem, which reduces the rate of successful packet delivery of the IEEE 802.11p broadcast service, especially with the lack of ACK packets. Another limitation of the IEEE 802.11p standard is that it employs enhanced distributed channel access (EDCA) scheme to support safety applications. Event-driven and periodic safety messages are likely to be assigned to the highest access categories (ACs) according to this scheme. Therefore, these safety messages will contend for the channel by using a small contention window (CW) size, as specified in the IEEE 802.11p standard. Although this small CW size allows the safety messages to be transmitted with small delays, it increases the probability of transmission collisions when multiple nodes within the same communication range simultaneously broadcast their safety messages. Additionally, unlike the unicast case, the CW size is not doubled when a collision happens among the broadcast safety messages, since there is no collision detection for the broadcast service due to the absence of CTS and ACK mechanism.
The forgoing creates challenges and constraints for establishing and maintaining vehicular communications. It is an object of the present invention to mitigate or obviate at least one of the above mentioned disadvantages.