Browsing by Author "Aslam, Aqsa"
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- Empirical Performance Models of MAC Protocols for Cooperative Platooning ApplicationsPublication . Aslam, Aqsa; Santos, Pedro M.; Santos, Frederico Miguel; Almeida, LuisVehicular 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’ 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.
- A Flexible TDMA Overlay Protocol for Vehicles PlatooningPublication . Aslam, Aqsa; Almeida, Luis; Santos, FredericoVehicular Ad-hoc Networks (VANETs) can enable a wide range of vehicle coordination applications such as platooning. A good use of the communication channel is paramount for an adequate quality of service. Currently, IEEE 802.11p is the standard used in VANETs and relies on CSMA/CA, which is prone to collisions that degrade the channel quality. This has led to recent proposals for TDMA-based overlay protocols that synchronize vehicles beacons to prevent or reduce collisions. In this paper, we propose RA-TDMAp that puts together properties of two previous works. On one hand, it allows the nodes in one platoon to remain synchronized even in the presence of interfering traffic, e.g. from other vehicles, by adapting the phase of the TDMA round to escape periodic interference. On the other hand, it reduces channel occupation by having just the leader transmitting with high power, to reach all the platoon at once, while the followers transmit with low power. The order of transmission is such that the leader gathers information from the whole platoon in just one round. We simulated RA-TDMAp in realistic conditions using the PLEXE simulation framework. We show the phase adaptation of the TDMA round and we compare RA-TDMAp to state of the art protocols tailored for platooning, with three networking metrics: channel busy ratio, collisions and safe time ratio, all of which confirm the superiority of RA-TDMAp.
- Impact of Platoon Size on the Performance of TDMA-Based MAC ProtocolsPublication . Aslam, Aqsa; Almeida, Luís; Santos, FredericoVehicular 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.
- Reconfiguring TDMA Communications for Dynamic Formation of Vehicle PlatoonsPublication . Aslam, Aqsa; Santos, Frederico; Almeida, LuísPlatooning 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.