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Platooning is a promising concept used within the Intelligent Transportation System to increase efficiency and safety of road transportation. It is based on periodically sharing the kinematic status of the platoon members to allow reducing inter-vehicle distances in a safe way. This coordination is automatic and depends heavily on the wireless channel. A common technique to improve the channel properties is to use Time-Division Multiple Access (TDMA) that organizes the access to the wireless medium in slots assigned exclusively to each vehicle. However, while platoons are physical and dynamic, the corresponding dynamic reconfiguration of a logical TDMA frame is non-trivial. In this paper we address this Cyber-Physical problem resorting to the RA-TDMAp protocol to track the dynamics of a platoon, specifically joining, merging and leaving. In our solution, we include an adequate admission control block, to verify whether joining or merging can be accepted, and we present the state-machine that handles the reconfiguration process. We validate our TDMA reconfiguration mechanism with simulations using the Plexe/Vein/OMNeT++ framework. We show the effectiveness of the proposed mechanisms which ensures a synchronized start of the platoon control with the TDMA frame reconfiguration.

Vehicular ad-hoc networks (VANET) enable vehicles to exchange information on traffic conditions, dynamic status and localization, to enhance road safety and transportation efficiency. A typical VANET application is platooning, which can take advantage of exchanging information on speed, heading and position to allow shorter inter-vehicle distances without compromising safety. However, the platooning performance depends drastically on the quality of the communication channel, which in turn is highly influenced by the medium access control protocol (MAC). Currently, VANETs use the IEEE 802.11p MAC, which follows a carrier sense multiple access with collision avoidance (CSMA/CA) policy that is prone to collisions and degrades significantly with network load. This has led to recent proposals for a time-division multiple access (TDMA)-based MAC that synchronize vehicles&rsquo, beacons to prevent or reduce collisions. In this paper, we take CSMA/CA and two TDMA-based overlay protocols, i.e., deployed over CSMA/CA, namely PLEXE-slotted and RA-TDMAp, and carry out extensive simulations with varying platoon sizes, number of occupied lanes and transmit power to deduce empirical models that provide estimates of average number of collisions per second and average busy time ratio. In particular, we show that these estimates can be obtained from observing the number of radio-frequency (RF) neighbours, i.e., number of distinct sources of the packets received by each vehicle per time unit. These estimates can enhance the online adaptation of distributed applications, particularly platooning control, to varying conditions of the communication channel.

Vehicular networks are a core component in Intelligent Transportation Systems (ITS) enabling communication among vehicles for collaborative applications. One example of such an application that may bring benefits in reducing travel time, fuel consumption and improving safety is platooning. This application coordinates a group of vehicles that travel together, doing automatic control of inter-distances and speeds [1]. A critical part of this application is the vehicle-to-vehicle (V2V) communication highlighting the importance of improving the channel quality. Existing ITS standards, namely WAVE (USA) and ITS-G5 (Europe), use IEEE 802.11p DSRC (Dedicated Short-Range Communication) [2] that relies on CSMA/CA distributed access arbitration. Despite the Collision Avoidance attribute collisions can still occur and the channel quality can degrade significantly in dense traffic environments.

Vehicular Ad-hoc Networks (VANETs) have a significant potential to enable new applications among multiple types of vehicles. In these networks, the Medium Access Control (MAC) plays an important role in providing an efficient communication channel. Currently, existing standards use the PHY and MAC of IEEE 802.11p, which is fully distributed and based on Carrier Sense Multiple Access with collision Avoidance (CSMA/CA), thus still prone to collisions. This has led to recent proposals for TDMA-based overlay protocols to prevent collisions, including one based on Reconfigurable and Adaptive TDMA (RA-TDMA). This protocol sets a TDMA round allocating slots to the nodes engaged in a given collaborative application, only, e.g., a platoon or a multimedia group. Then, other traffic, including from other TDMA rounds of concurrent applications can co-exist in space and time using the native CSMA/CA mechanism of IEEE 802.11p and the synchronization mechanism of RA-TDMA. In this paper, we do a qualitative comparison among TDMA protocols in VANETs showing the advantages of RA-TDMA. Early experimental results validate the capacity of RA-TDMA to support multiple concurrent rounds in a scalable manner.

Vehicular Ad-hoc Networks (VANETs) have a significant potential to enable new applications in the vehicular domain, some of which addressing traffic safety. In these networks, the Medium Access Control (MAC) plays an important role in providing an efficient communication channel. Currently, there are two protocols for ITS, proposed in the USA (IEEE WAVE) and in Europe (ETSI ITS-G5). Both cases use the PHY and MAC of IEEE 802.11p, the latter being fully distributed and based on CSMA/CA, thus still prone to collisions. This has led to recent proposals for TDMA-based overlay protocols to prevent collisions but leading to complex synchronization and scalability limitations. In this paper we propose enhancing IEEE 802.11p with an overlay protocol based on Reconfigurable and Adaptive TDMA. We specifically target multiple concurrent applications such as multiple platoons. Our proposal separates between the application level, with its own TDMA round and slots allocated to engaged nodes, e.g., a platoon, and the global level that manages multiple TDMA rounds in a mutually agnostic manner, thus, dynamic and scalable. We believe this is the first applications-oriented MAC protocol proposed for VANETS and we discuss its deployment and potential advantages in two typical uses cases, namely platooning and smart intersections.