Multihop Beaconing Forwarding Strategies in Congested IEEE 802.11p Vehicular Networks

Multihop propagation of situational information is a promising technique for improving beaconing performance and increasing the degree of situation awareness onboard vehicles. A possible way of achieving this is by piggybacking information on the beacon packets that are periodically sent by each vehicle in the network, as prescribed by the Dedicated Short-Range Communications standards and the European Telecommunications Standards Institute. However, prescribed limitations on beacon size imply that only information on a very small number of surrounding vehicles can be piggybacked in a beacon packet. In most traffic situations, this number is well below the typical number of vehicles within the transmission range, implying that multihop forwarding strategies must be devised to select which neighboring vehicle's information to include in a transmitted beacon. In this paper, we designed different multihop forwarding strategies and assessed their effectiveness in delivering fresh situational information to surrounding vehicles. Effectiveness is estimated in terms of both information age and the probability of experiencing a potentially dangerous situation-awareness blackout. Both metrics are estimated as a function of the hop distance from the transmitting vehicle and in the presence of different levels of radio channel congestion. The investigation is based on extensive simulations whose multihop communication performance is corroborated by real-world measurements. The results show that network-coding-based strategies substantially improve forwarding performance, as compared with a randomized strategy, reducing the average information age by up to 60% and the blackout probability by up to two orders of magnitude. We also consider the effect of multihop propagation of situational information on the reliability of a forward collision warning application and show that network-coding-based propagation yields a factor-3 improvement of reliability with respect to a randomized forwarding strategy and even higher improvements with respect to the case of no propagation.