Safety Communication for Vehicular Networks: Context-Aware Congestion Control and Cooperative Multi-Hop Forwarding

Vehicular safety applications have the potential to make travel on our roads and highways much more safe. These applications require both reliable and up-to-date knowledge of the local neighbourhood, as well as reliable multi-hop propagation of safety alert messages.;Under the IEEE Wireless Access in Vehicular Environments (WAVE) standard, the envisioned platform for vehicular communication, the former is attained through an exchange of single-hop broadcast safety beacons in the control channel. However, congestion of these periodic broadcast safety packets remains an obstacle to the large-scale deployment of vehicular ad hoc networks (VANETs). Excessive offered load on the shared control channel results in a deterioration of network performance and a subsequent reduction in the safety level at the application layer. Existing methods focus on providing fairness in the resources allocated, but fail to account for the different network performance requirements of vehicles in different driving situations.;First, we address the problem of beaconing rate adaptation in response to network congestion and the driving context. We define a delay constraint profile for each communication link based on the driving context of the vehicles. We formulate the problem as a load minimization problem subject to a probabilistic constraint on the probability of violation of the delay requirements for each link in the network. We also develop an alternate formulation of the problem as a weighted network utility maximization (NUM) problem. We propose distributed algorithms to solve this problem in a decentralized manner and validate their performance through simulations. Some analytical results on their convergence are also provided.;Next, we turn to the problem of providing reliable multi-hop forwarding in vehicular networks. Cooperative vehicular multi-hop schemes achieve reliability using broadcast transmissions and multiple forwarding relays at each hop. However, packet duplication must be controlled to circumvent the broadcast storm problem. Existing multi-hop dissemination schemes do not account for the presence of periodic safety beacons on the shared safety channel. We propose a cooperative forwarding protocol for highway vehicular networks, which extends a reliable vehicular broadcast medium access control (MAC) protocol based on positive orthogonal codes (POC). Multiple cooperating relays act as a virtual relay and schedule their transmissions to correspond to a single POC codeword. The proposed method exploits spatial diversity while mitigating the effect of hidden terminals. By allocating separate POC-based schedules for multi-hop packets and the periodic broadcast of safety heartbeat packets, the proposed protocol reduces the interference between the two types of safety transmissions. The performance of the protocol is studied through analysis using a Markov model and validated via simulations.