Abstract
Reliable broadcast has long been a challenging problem for wireless communications due to unstable link connectivities. Recently, it has become an increasing concern for Vehicular Ad-hoc Networks (VANETs), especially for applications related to driving safety. It is suggested by both the Cooperative Intelligent Transport Systems (C-ITS) and Dedicated Short Range Communication (DSRC) standards that vehicles could broadcast Cooperative Awareness Messages (CAMs) to enhance traffic awareness and improve driving safety. Since each CAM contains vehicle's driving status and safety alerts such as geographic location, driving speed, overtaking warning, etc., frequent packet losses in the broadcast of CAMs may fail to provide traffic awareness around vehicles and detect traffic hazards when driving on roads.
Many reliable broadcast schemes have been proposed for wireless communications but few could be adopted to the dissemination of CAMs in VANETs. This is mainly due to the unique characteristics of VANETs such as dynamic network topology, fluctuations in link qualities, and short lifetime of CAMs. Each vehicle can only acquire local network information in such a harsh environment, which makes it challenging to achieve reliable and real-time dissemination of CAMs in the fully decentralised VANETs. This thesis focuses on designing vehicular communication protocols for decentralized broadcast of CAMs in VANETs, aiming to make reliable and real-time dissemination of CAMs from three aspects: reducing broadcast collisions, improving broadcast reliability, and reducing broadcast delay.
To reduce broadcast collisions, a new concept called bitmap is introduced to record each vehicle's receiving status of the most recently transmitted packets in the broadcast of CAMs. Then, a decentralized Bitmap based Collision Avoidance (BCA) scheme is proposed to detect and handle broadcast collisions in VANETs with just the local information of a vehicle. BCA enforces vehicles to exchange bitmaps with their neighboring vehicles and check whether these vehicles have received the most recently transmitted packets in the broadcast of CAMs. If a neighboring vehicle fails to receive a packet, BCA finds out whether the packet is lost due to broadcast collisions, and vehicles that have caused this broadcast collision will hop to other time slots for collision avoidance. BCA is evaluated with stress tests and scenario tests in simulations. Both kinds of tests show that BCA can resolve broadcast collisions quickly and reduce CAM losses in the broadcast.
To improve broadcast reliability, each vehicle piggybacks some received CAMs to help other vehicles recover their lost CAMs. As CAMs are piggybacked in each vehicle's routinely transmitted packets, it does not require extra retransmission to recover these lost CAMs. Based on this idea, two decentralized cooperative piggybacking schemes, Benefit based Piggybacking (BP) and Suggestion based Piggybacking (SP), are proposed. BP uses a benefit function to evaluate how many vehicles will recover their lost CAM if a CAM is piggybacked, and the CAM that can help more vehicles will be piggybacked first. To strengthen the cooperation in the recovery of lost CAMs, SP enforces vehicles to generate suggestions and negotiate which CAMs should be piggybacked until a consensus is achieved. BP and SP do not use central administration or global information, but simulation results show that they can achieve higher broadcast reliability and shorter time delay than most existing solutions due to the cooperation among vehicles.
To shorten broadcast delay, broadcast links are scheduled in coordination with vehicles' piggybacking strategy. This is motivated by the fact that the available CAMs for piggybacking are highly relevant to the broadcast schedule of vehicles. Based on this idea, a decentralized Piggybacking-Aware Link Scheduling (PALS) scheme is proposed, where the piggybacking of a CAM is scheduled closely after the generation of this CAM to shorten the time of recovery. Simulation results show that PALS can recover more CAMs by shortening time delay via link scheduling, yielding high Packet Delivery Ratio (PDR) and low time delay in the broadcast.