Abstract-In this paper we introduce a new geocasting concept to target vehicles based on where they will be in the direct future, in stead of their current position. We refer to this concept as constrained geocast. This may be useful in situations where vehicles have interdependencies based on (future) maneuvers. We have developed a first version of such a protocol in the context of an automated merging application, and tested it using simulations. Results show that the protocol is able to meet the requirements of such applications. Compared to a common geo-broadcast protocol this protocol becomes more reliable as road traffic densities increase, but in other aspects the performance is so far lacking. Based on our experiences with implementing the protocol however we see plenty of room for further improvement. ooperative Adaptive Cruise Control (CACC) is a traffic efficiency system in which the speed of vehicles is automatically controlled in a cooperative matter using vehicular communication. Because of the short reaction time of CACC compared to human drivers, vehicles can drive relatively close together (<1s) forming so-called platoons. They are also able to react significantly faster to speed changes of their preceding vehicles, minimizing unnecessary braking and accelerating and in this way increasing overall traffic flow stability (see [1] ). The goals of CACC include increasing the capacity of the road network and decreasing vehicle emissions. We are currently involved in the Connect & Drive project which' goal is to design, implement and test a CACC system using short-range vehicle-to-vehicle (V2V communication. A requirement of any CACC system is that it must be able to support the merging of vehicles inside an existing platoon. Vehicles may for instance simply wish to join a platoon, or are forced to do so at a road narrowing or at a merging junction (see Fig. 1 ). The first and the second case concern situations in which vehicles drive in adjacent lanes, and can be executed using direct one-hop communication: when a vehicle inside a platoon receives a request from another vehicle to join the platoon it will create a so-called merging gap by gradually decreasing its speed, thereby increasing the headway to its preceding vehicle. When the merging gap is large enough the merging vehicle aligns with it and joins the platoon. Afterwards normal CACC operation is resumed. Creating a merging gap and aligning with it takes time, and this becomes problematic in the case of merging at a junction. Especially in metropolitan areas the merging vehicle will not be able to communicate directly with the platoon in which it wants to join until it has reached the merge area. Taking the length of the merge area into account, as well as realistic vehicle speeds, this will often not allow the merging vehicle to join the platoon. For this reason we require a communication system that is able to warn any vehicle inside a platoon in advance, using indirect multi-hop communication, that it needs to create a merging gap for a merging vehicle at a junction. (Abiding) geocast is a form of routing in which messages are routed through a network based on spatiotemporal constraints. Normally these constraints directly specify the destination region and destination time span, see Section II. In our case however the constraint is different, since the location of the vehicle that must create a gap for the merging vehicle is unknown. Stated generically the constraint that is used by the geocast protocol is as follows: "Any node 1 that will be within a certain region during a certain future time period is a destination node and should receive a message." 1 In this paper a node is defined as a network-entity, so any communicationcapable vehicle is also a node.
In REF , the cooperative adaptive cruise control (CACC) is developed in which the vehicles positions are estimated which results in high network overhead.
Constrained geocast to support Cooperative Adaptive Cruise Control (CACC) merging
2010 IEEE Vehicular Networking Conference
2010 IEEE Vehicular Networking Conference
Computer Science